Easykemistry

Monday, 13 May 2024

ISOMERISM at a glance

ISOMERISM IN ORGANIC COMPOUNDS

Isomerism is defined as the occurrence of two or more compounds with the same molecular formular but different molecular structures.

The different molecular structures are known as Isomers.

Isomers may belong to the same homologous series ( having the same functional group) or may belong to different homologous series (having different functional group)

Isomers with the same functional group, (i.e. belonging to the same homologous series) have similar chemical properties while isomers having different functional group (i.e. belonging to different homologous series) have different physical and chemical properties.

Example:- the formula C2H6O has two isomers

1. Ethanol an Alkanol with molecular formula CH3CH2OH has a melting point of 78 degree celçius and is a liquid at room temperature

H H
| |
H--C--C--OH
| |
H. H

and

2. an Alkoxy compound with molecular formula CH3OCH3 which is a gaseous at room temperature

H H
| |
H--C--O--C--H
| |
H H

TYPES OF ISOMERISM

(i) Structural isomerism

(ii) Stereo or geometric Isomerism

(iii) Optical isomerism

(I) STRUCTURAL ISOMERISM

This is the occurrence of two or more compounds with the same molecular formula but different molecular structures

TYPES OF STRUCTURAL ISOMERISM

(a) Chain isomerism: in this case, the isomer differs in the way the carbon atoms are arranged in the molecule. Eg

CH3CH2CH2CH3 CH3CHCH3
n-butane |
CH3
2-methylpropane (isobutane)

(‘n’ stands for the normal compound while ‘iso’ means the isomer of the normal compound).

NOTE: - Isomerism begins from the 4th member of the alkane homologous series, that is, butane, and as the number of carbon atoms increases the number of isomers also increases. For example, butane has two isomers normal-butane and 2-methyl propane, pentane (C5H12) has three isomers: n-pentane, 2-methylbutane and 2,2-dimethylpropane.

I. CH3CH2CH2CH2CH3. (n-pentane)

CH3
|
II. CH3CHCH2CH3 (2-methylbutane)

III. CH3
|
CH3-C-CH3
|
CH3 2,2-dimethylpropane

Three isomers for pentane

Another example is hexane (C6H14)

I. CH3CH2CH2CH2CH2CH3

n-hexane

CH3
|
II. CH3CHCH2CH2CH3 2-methylpentane

III. CH3
|
CH3CH2CHCH2CH3 3-methylpentane

CH3
|
IV. CH3CCH2CH3 2,2-dimethylbutane
|
CH3

CH3
|
V. CH3CHCHCH3 2,3-dimethylbutane
|
CH3

Hexane has 5 isomers

(b) position isomerism: Position isomers are those that have a substituent in different position on the same carbon skeleton. Example

CH3CH2CH2OH CH3CHCH3
propan-1-ol |
OH
propan-2-ol

(c) Functional Group isomerism: These are isomers having the same molecular formular but different functional groups. They belong to different homologous series.

e.g.
H
/
(i) CH3CH2C CH3-CCH3
\\ ‖
O O
Propanal (an aldehyde) propan-2-one (a ketone)

(ii) CH3CH2OH CH3-O-CH3

Ethanol ( Alkanol) Methoxymethane (dimethyl ether) (Alkoxy compounds)

(2) STEREO ISOMERISM OR GEOMETRIC ISOMERISM

In this type of isomerism, compounds have the same molecular formular but differ only in the way their atoms are arranged in space.

TYPES STEREO ISOMERISM

(a) Geometrical isomerism: This type of isomerism is found in compounds having either a double bond, triple bond or a ring structure. These multiple bonds prevent the free rotation about a carbon=carbon atom. that is, the carbon atoms are not cylindrically symmetrical. There are two forms of geometric isomers, the Cis and Trans isomer. For example, but-2-ene has two geometric isomer which are trans-but-2-ene and cis-but-2-ene as shown below.

when naming geometric isomers, you look at the substituents, where the substituents (group other than Carbon atoms) are on the same side of the double bond, that isomer is named the Cis isomer and when the substituents are on opposite sides of the double bond, we put a trans- isomer, but the names are the same

H CH3 H H
\ / \ /
C=C C=C
/ \ ∕ ∖
CH3 H CH3 CH3

Trans-but-2-ene Cis-but-2-ene

      H Br H H
\ / \ /
C=C C=C
/ \ / \
Br              H Br Br
Trans-1,2-dibromoethene Cis-1,2-dibromoethene

C1OOH. C4OOH H COOH
\ / \ /
C2=C3 C=C
∕ ∖ / \
COOH H H H
Trans-butenedioic acid Cis-butenedioic acid

(b) Optical isomerism: Optical isomers are isomers that have the same molecular and structural formular but cannot be superimposed on each other. In other words, an optical isomer is one which is not super-imposable on its mirror image. An optical isomer has at least one carbon atom which has four different groups or atoms attached to it. Such a carbon atom (surrounded by four different atoms /group) is called chiral carbon or chiral center. E.g. 2-hydroxylpropanoic acid (Lactic acid).

CH3
|
HO — *C— H
|
COOH

The Asterix C-atom is called a chiral C-atom because it is surrounded by four (4) different groups.

In glucose, there are several chiral carbon atoms.

H
∣
C = O
|
H — *C —OH
|
OH —*C —H
|
H — *C— OH
|
H —*C —OH
|
H — C—H
|
OH

Optical isomers are also called Enantiomers.

Isomers that belong to the same homologous series (same functional group) have similar chemical properties but may have different physical properties. But isomers which belong to different homologous series (different functional group) have different chemical and physical properties.

OBJECTIVE QUESTIONS

1. Isomerism is the phenomenon in which compounds have

A. same molecular mass only

B. same empirical formula only

C. same molecular formula but different structures

D. different molecular formula but same structure

2. Which of the following pairs are isomers?

A. CH₄ and C₂H₆

B. C₂H₆ and C₃H₈

C. C₄H₁₀ and C₄H₁₀

D. C₂H₄ and C₂H₆

3. Butane and 2-methylpropane are examples of

A. position isomers

B. functional isomers

C. chain isomers

D. geometric isomers

4. The two main classes of isomerism are

A. structural and stereoisomerism

B. chain and position

C. optical and functional

D. alkane and alkene

5. Which compound shows chain isomerism?

A. C₂H₆

B. C₃H₈

C. C₄H₁₀

D. CH₄

6. Intermediate Level

Position isomerism occurs due to different

A. molecular formula

B. carbon chain length

C. functional groups

D. positions of functional group or substituent

7. Ethanol and dimethyl ether are

A. chain isomers

B. position isomers

C. functional group isomers

D. optical isomers

8. Which of the following shows functional isomerism?

A. Propane and propene

B. Ethanol and ethanoic acid

C. Propanal and propanone

D. Butane and butene

9. Geometric isomerism is also known as

A. structural isomerism

B. optical isomerism

C. cis–trans isomerism

D. functional isomerism

10. Which compound can show geometric isomerism?

A. Ethane

B. Propane

C. But-2-ene

D. Methane

11. Which of these has the highest number of chain isomers?

A. C₃H₈

B. C₄H₁₀

C. C₅H₁₂

D. C₂H₆

12. Optical isomerism is caused by

A. double bonds

B. presence of aromatic ring

C. chiral carbon atom

D. branching

13. The pair CH₃CH₂CHO and CH₃COCH₃ are

A. chain isomers

B. position isomers

C. functional isomers

D. geometric isomers

14. Which statement is true about isomers?

A. They always have the same boiling point

B. They always have identical properties

C. They may have different physical and chemical properties

D. They must belong to different homologous series

15. The number of structural isomers for C₄H₈ (alkenes only) is

A. 1

B. 2

C. 3

D. 4

THEORY QUESTIONS

1.(a)(i)Define isomerism.

(ii). What are isomers?

(iii). State two differences between structural and stereoisomerism.

1(b)(i). Explain chain isomerism with one example.

(ii)Explain position isomerism with one example.

(iii). What is functional group isomerism?

1(c)(i). Define geometric isomerism.

State the condition necessary for geometric isomerism.

What is optical isomerism?

Give two examples of compounds that show functional isomerism.

Structured Questions

(a) Define structural isomerism

(b) List three types of structural isomerism

(a) What is chain isomerism?

(b) Draw the structural formulas of the isomers of C₄H₁₀

(a) Explain functional isomerism

(b) Show the structures of ethanol and dimethyl ether

(a) Distinguish between cis and trans isomers

(b) Use but-2-ene to illustrate

Draw and name all the structural isomers of C₅H₁₂.

Long Essay Questions

(a) Define isomerism

(b) Describe the different types of structural isomerism

Explain geometric isomerism using suitable diagrams and discuss its importance in chemistry.

Write short notes on:

(i) Chain isomerism

(ii) Position isomerism

(iii) Functional group isomerism

(iv) Optical isomerism

List the structural isomers of C₄H₈ and classify them.

Describe the differences between structural and stereoisomerism with examples.

Friday, 19 April 2024

HYDROCARBONS at a glance

Organic Chemistry is the study of Carbon and its compounds.

Organic compounds are compounds containing Carbon, hydrogen and any one or two other elements like Sulphur, Nitrogen, Oxygen, Phosphorus and even metals like Sodium and potassium.

General Properties of Organic Compounds

1.The solid ones have low melting and low boiling points.

2. They are soluble in non-polar solvents.

3. They are thermally unstable ( i.e easily decompose/ breakdown on heating)

4. They combustible

5. They are mostly covalent in nature.

HYDROCARBONS

Hydrocarbons are compounds containing only two elements, that is, Hydrogen and Carbon.

There are three main families of hydrocarbons

****The Alkanes

***

*The Alkenes and

****The Alkynes

Sources of hydrocarbons

There are three main sources of hydrocarbons,

i. Crude oil or petroleum

ii. Natural gas

iii. Coal

I. Crude oil/petroleum: - Crude oil is a viscous dark liquid that is found in deposits underground. It is formed from animal remains and marine vegetation, algae e.t.c when these are subjected to pressure, heat and bacterial activity they gradually change to this dark viscous liquid known as crude oil.

Crude oil exploration and drilling

Crude oil-bearing rocks and areas are located and explored by the following processes.

i. Taking an aerial photograph of the place.

ii. Examining and testing the surface rocks.

iii. Drilling large holes/core into the ground

iv. Exploding dynamite in the deep holes drilled into the earth's crust taking recordings and reflections of shock waves resulting from the explosions in the rocks.

II. Natural gas: - Natural gas is a naturally occurring hydrocarbon mixture composed mainly of methane (CH₄ about 65% -90%)). Along with methane are other hydrocarbons like ethane, propane and butane.

It is formed from the decomposition and compression of organic matter, like dead plants and animals, over thousands of years and is usually found in deep underground deposits.

It is extracted from these underground deposits (reservoirs) by drilling pipes into the reservoirs. It is then separated and purified into its various components by fractional distillation.

Uses

i. it is used mainly for heating as fuel,

ii. it is used for generating electricity

iii. it used as fuel for transportation in the form of liquified natural gas

Even though we consider natural gas as a relatively clean fossil fuel, its extraction, transportation and various uses can still affect our environment (such as water pollution, methane emissions and land disturbance).

III. Coal: - Coal is a solid fossil fuel that is mainly composed of carbon, as well as various amounts of other elements such as hydrogen, oxygen, sulfur, and nitrogen (see carbon and its compounds)

Fractional Distillation Of Petroleum

Unless it is refined, crude oil will just be a viscous liquid with very limited use. But when it undergoes fractional Distillation, it is separated into various compounds.

Fractions from fractional Distillation of Petroleum

Table: -showing various petroleum fractions and their uses

	Fractions	Number of C-atoms	Uses
1.	Petroleum gas	C₁ - C₄	Used mainly as fuel and manufacturing of other organic compounds like chloromethane
2.	Ether	C₄ – C₆	Used as solvents
3.	Petrol	C₇ -C₁₀	Used mainly as fuel for motor car engines
4	Kerosene	C₁₀ – C₁₈	Fuel for cooking, for lighting and used for jet engines
5	Diesel oil	C₁₈ – C₂₅	It is used as a raw material for cracking process, used as fuel for diesel engines
6	Lubricating oils	C₂₀ – C₃₅	Used for i. oiling / lubricating moving parts of machines, engines and generators ii. Raw material for making candles, hair cream and body creams products
7.	Bitumen	Above C₃₅	Used for road construction

Classification of Hydrocarbons

Hydrocarbons are classified into

I. Aliphatic and

II. Aromatic Hydrocarbons

ALIPHATIC HYDROCARBONS: - These are straight chain, branched chain or cyclic hydrocarbons that are saturated or unsaturated.

Examples of aliphatic hydrocarbons are (CH_₃CH_₂CH_₃) propane, (CH₃CH₂CH=CH₂) butane, pentyne CH₃CH₂CH₂C≡CH)

Straight Chain hydrocarbons

        H H H
׀   ׀ ׀
H – C – C – C −H
׀ ׀ ׀
H    H   H
Propane

H H H
׀ ׀ ׀
H−C – C – C= C– H
׀ ׀ ׀
H H H
Butene

H H H
׀ ׀ ׀
H – C – C– C –C ≡ C –H
׀ ׀ ׀
H H H

Pentyne

Branched chain hydrocarbon

H H H
׀ ׀ ׀
H− C – C – C −H
׀ ׀
H H
H – C – H
׀
H

2-methyl propane

Cyclic hydrocarbon
H
׀
C
∕ \
H − C − C – H

Cyclopropane

AROMATIC HYDROCARBON: - These are hydrocarbons that contain the benzene ring (C₆H₆) in its structure. Aromatic Hydrocarbons are unsaturated.

benzene

Terminologies in Organic Chemistry.

I. CATENATION- this is the ability of an element to form single and multiple bonds with itself and other elements.

2. OCTANE NUMBER: - This is the measurement of the efficiency of combustion of petrol in car engines.

It is based on the proportion of branched chain alkanes to straight chains in a given fuel.

For instance, a sample of petrol that contains a high proportion of normal (straight chain) heptane than 2,2,4 trimethylpentane has a low octane number of below 50.

3. HYBRIDIZATION: - This is the mixing of orbitals to get the same number of hybrid orbitals.

4. HOMOLOGOUS SERIES: - This is a family of organic compounds that conforms to a general molecular formular, and successive members differs by a -CH2- group.

Examples of homologous series include the alkanes, alkenes and alkynes

Characteristics of a Homologous Series

i. Members differs by a -CH2- group

. Members have the same general molecular formular

iii. Members have similar chemical properties

iv. Members have the same general method of preparation

v. There is a gradual change in the physical properties of members as you go down the group.

ALKYL GROUP: - This is a homologous series of hydrocarbons formed by the loss of a hydrogen atom by a corresponding alkane.

The following table shows the first five alkyl members and their corresponding alkane.

Alkyl group	Corresponding Alkane	Formula
CH3	CH4	H ׀ H – C – ׀ H
C2H5	C2H6	H H ׀ ׀ H – C – C – ׀ ׀ H H
C3H7	C3H8	H H H ׀ ׀ ׀ H – C – C – C − ׀ ׀ ׀ H H H
C4H9	C4H10	H H H H ׀ ׀ ׀ ׀ H – C – C – C – C − ׀ ׀ ׀ ׀ H H H H
C5H11	C5H12	H H H H H ׀ ׀ ׀ ׀ ׀ H – C – C – C – C – C − ׀ ׀ ׀ ׀ ׀ H H H H H

The alkyl groups are generally represented by the letter R

FUNCTIONAL GROUP: - A functional group is an atom, a bond or a group of atoms that is present in all the members of a homologous series, and it determines the chemical properties of the family.

The table below shows some functional groups and their names as sufixes and prefixes

Functional groups

Name as Suffix

Name as Prefix

−OH

R-OH

-Ol

Hydroxy-

− COOH

O
∕∕
— C
∖
H

-Oic acid

−COOR

O
∕
− C
\
OR

Alkanoate

− CONH2

O
∕
− C
\
NH2

-amide

−C=O

O
∕∕
−C
\

-One

-oxo-

-CHO

O
∕∕
−C
\
H

-al

Oxo-

−NH2

−N

-amine

amino

−CN

−C≡N

-carbonitrile

cyano

−Cl

-Chloro

−Br

-bromo

−I

-iodo

−

-ane

-ene

≡

-yne

The three main hydrocarbons (that is, the alkane, the alkene and the alkyne are discussed separately in different posts.

OBJECTIVE QUESTIONS

Here are 20 WAEC & NECO–standard objective (multiple-choice) questions on Hydrocarbons, suitable for exams and revision:

Objective Questions on Hydrocarbons

1. Hydrocarbons are organic compounds that contain
A. carbon and oxygen only
B. carbon, hydrogen and oxygen
C. carbon and hydrogen only
D. hydrogen and oxygen only

2. Which of the following is a saturated hydrocarbon?
A. Ethene
B. Ethyne
C. Benzene
D. Ethane

3. The general formula of alkanes is
A. CₙHₙ
B. CₙH₂ₙ
C. CₙH₂ₙ₊₂
D. CₙH₂ₙ₋₂

4. Which hydrocarbon undergoes substitution reaction?
A. Alkanes
B. Alkenes
C. Alkynes
D. Aromatic hydrocarbons

5. Ethene belongs to the homologous series known as
A. alkanes
B. alkenes
C. alkynes
D. aromatics

6. Which of the following is an unsaturated hydrocarbon?
A. Methane
B. Propane
C. Butene
D. Pentane

7. The functional group present in alkenes is
A. C–C
B. C≡C
C. C=C
D. –OH

8. Which reagent is used to test for unsaturation in hydrocarbons?
A. Limewater
B. Bromine water
C. Fehling’s solution
D. Sodium hydroxide

9. The process of breaking large hydrocarbon molecules into smaller ones is called
A. polymerization
B. fermentation
C. cracking
D. substitution

10. Which hydrocarbon has the molecular formula C₂H₂?
A. Ethane
B. Ethene
C. Ethyne
D. Benzene

11. Members of a homologous series have the same
A. molecular mass
B. physical properties
C. chemical properties
D. number of carbon atoms

12. Which of the following hydrocarbons is aromatic?
A. Ethane
B. Ethene
C. Ethyne
D. Benzene

13. The main source of hydrocarbons is
A. coal
B. natural gas
C. petroleum
D. water

14. Which hydrocarbon will decolourize bromine water?
A. Methane
B. Ethane
C. Propane
D. Ethene

15. The homologous series with general formula CₙH₂ₙ₋₂ is
A. alkanes
B. alkenes
C. alkynes
D. aromatics

16. What type of reaction occurs when ethene reacts with hydrogen?
A. Substitution
B. Addition
C. Elimination
D. Oxidation

17. Which property generally increases down a homologous series?
A. Chemical reactivity
B. Solubility in water
C. Boiling point
D. Unsaturation

18. The complete combustion of hydrocarbons produces
A. carbon monoxide and water
B. carbon dioxide and water
C. carbon and water
D. hydrogen and carbon dioxide

19. Which hydrocarbon is used in the manufacture of polythene?
A. Methane
B. Ethane
C. Ethene
D. Propane

20. Which statement about hydrocarbons is correct?
A. All hydrocarbons are unsaturated
B. All hydrocarbons contain oxygen
C. Hydrocarbons can be saturated or unsaturated
D. Hydrocarbons do not burn in air

1. Octane number is highest in petrol containing a high proportion of

a. heptane

b. octane

c. 2-methylpentane

d. 2,2,4-trimethylpentane

2. Compounds containing only hydrogen and carbon are called

a. hydrocarbons

b. alkanes

c. isomers

d. organic

Sunday, 14 April 2024

Rates of Chemical Reactions

During a chemical reaction, reactants collide with one another to form products, and the formation of these products do not occur at the same rates. Hence

Rate of a chemical reaction can be defined as the number of moles of reactants that are converted, or products that are formed per unit time.

Mathematically

Rate = mass in grammes

time taken

For instance, if 6g of zinc metal is placed in dilute tetraoxosulphate (VI) acid and it takes 3 mins to completely react, then the rate of chemical reaction is given by.

Rate = 6g = 2g/mins

3min

That is, 2g of the zinc was converted to ZnCl₂ per minute

Factors used for Measuring Rates of Reaction

The rate of a chemical reaction can be measured or can be determine by any one of the following.

i. decrease in mass of reactants.

ii. increase in volume of a gaseous product

iii. change in pH

iv. change in colour intensity

v. Change in pressure

These are all measurable factors.

Factors Affecting the Rate of Chemical Reactions.

The following factors will a reaction to be fast or slow.

i. Nature of reactants

ii. Temperature

iii.. Concentration

iv. Pressure

v. Presence of light

vi. Surface area

vii. Presence of a catalyst

Before we discussed how each of these factors will affect the rate of a chemical reaction. It is important that we understand the concept of collision theory.

Collision theory assumes that for a chemical reaction to occur, there must be Collision between reactants particles, and these Collisions must be effective.

A collision is said to be effective when it leads to formation of products

What this concept is actually implying is that all collisions do not lead to the formation of a product, only the ones that are effective. So, the various factors that affect chemical reactions are factors that actually increase the number of effective collisions.

I. Effect of Nature of reactants: - this is one of the factors that affects the rate of a reaction, since different elements/ compounds behave differently.

The reactivity of metals varies as you go down the activity series and so when metals react with acids for example the reactions are not the same.

Example by virtue of their nature (their high reactivity) sodium and potassium will react explosively with dilut acids while metals like zinc and iron will react moderately with dilute acids.

II. Effect of Temperature: - An increase in temperature will cause reactants particles to gain more kinetic energy leading to an increase in effective collision and hence, an increase in the rate of reaction. An increase in temperature will also lead to an increase in the energy of the system.

III. Effect of Concentration: - An increase in the concentration of the reactants will lead to an increase in the number of reactants per unit area (the reactants becomes closer) thus, leading to overcrowding and increase in the effective collision of the reactants and hence an increase in the rate of the reaction.

IV. Effect of pressure: - An increase in pressure will lead to a decrease in volume that is, a decrease in the intermolecular space between the reactants particles leading to an increase in the effective collision and hence an increase in the rate of reaction. And vice versa

V. Effect of presence of light: - Some reactions are photochemical, that is, affected by light. When such reactions are exposed to light the reactants, particles become more activated and collide more increasing the number of effective collisions and hence an increase in the rate of reaction.

VI. Effect of a catalyst: - A catalyst increases the rate of a reaction by creating a different pathway with a lower activation energy.

ACTIVATION ENERGY: see definition in chemistry.

For a collision to be effective, the reactants must possess the minimum amount of energy needed to overcome the energy barrier (activation energy for the reaction) for that reaction. The higher the activation energy, the slower the rate of reaction; positive catalyst helps to lower the activation energy.

Energy Profile Diagram

Energy profile diagram is a diagram / graph that shows the pathway of a chemical reaction. whether the reaction is exothermic or endothermic.

Figure1: Energy profile diagram of an exothermic reaction.

The reactants have higher energy than the products, showing that heat is released during the reaction. The peak represents the activated complex, while Ea is the activation energy and ΔH is the heat of reaction.

Figure2: Energy profile diagram of an endothermic reaction.

The products have higher energy than the reactants, indicating that heat is absorbed during the reaction. The highest point is the activated complex, Ea is the activation energy, and ΔH represents the heat of reaction.

Rate Curve

A rate Curve is the graph which shows the rate of a reaction. It is a graph of reaction against time.( that is, change in concentration against time, decrease in mass against time, e.t.c)

The slope or the gradient of the curve is steep at the beginning because the reaction is fastest ( since , it becomes less steep as the reaction progresses and slows down, then it finally becomes horizontal.

The point at which the graph becomes horizontal indicates the end point of the reaction, when one of the reactants is completely used up

figure 1: A typical rate curve showing how the mass of a reactant decreases with time during a chemical reaction. The steep part of the curve shows a fast reaction at the beginning, while the flatter part shows the reaction slowing down as the reactants are used up.

Figure 2: A graph showing how the volume of a gaseous product increases with time during a chemical reaction. The curve rises steeply at first, indicating a fast rate of gas production, and then levels off as the reaction nears completion.

ORDER Of REACTIONS

1. A first-order reaction is a chemical reaction in which the rate depends on the concentration of only one reactant raised to the power one.

In simple terms:
The speed of the reaction is directly proportional to the concentration of one reactant.

Rate ɑ [A]
or

{Rate} = k[A]

Where:

(A) = reactant
(k) = rate constant

What this means

If the concentration of the reactant is doubled, the rate of reaction also doubles.
If the concentration is halved, the rate is halved.

Example

The decomposition of hydrogen peroxide:

H₂O₂ → H₂O + O₂

The rate depends only on the concentration of (H₂O₂), so it is a first-order reaction

A first-order reaction has a constant half-life — the time taken for half of the reactant to be used up does not change, no matter the starting concentration.

Example of a First-order reaction)

The half-life of a first-order reaction is 10 minutes.
If the initial concentration of the reactant is 80 mol dm⁻³, find the concentration after 20 minutes.

Solution

For a first-order reaction, the half-life is constant.

In 10 minutes → concentration becomes half

80 →40

In another 10 minutes (total = 20 minutes) → it halves again

40 → 20

Answer

The concentration after 20 minutes is 20 mol dm⁻³.

A second-order reaction is a chemical reaction in which the rate depends on the square of the concentration of one reactant, or on the product of two reactants.

It can be written as:

Rate = k[A]²
or
Rate = k[A][B]

what this means is that

If the concentration of a reactant is doubled, the rate increases four times.
If the concentration is tripled, the rate increases nine times.

Simple example

The reaction between nitrogen dioxide molecules:

2NO₂ →2NO + O₂

This is second order because the rate depends on ([NO₂]²).

Another example of a 2nd order reaction is

The reaction between hydrogen and iodine:

H₂ + I₂ →2HI

This reaction is second order because experiments show that its rate depends on the product of the concentrations of both reactants:

Rate = k[H₂][I₂]

It means:

If the concentration of H₂ is doubled, the rate doubles.
If the concentration of I₂ is also doubled, the rate doubles again. So the overall rate becomes four times faster.

The reaction is second order because the rate depends on the concentrations of two reacting substances.

waec/neco keypoint

A second-order reaction does not have a constant half-life — its half-life changes as the concentration changes.

OBJECTIVE QUESTIONS

1. Which of the following best describes the rate of a chemical reaction?

A. The amount of heat produced
B. The speed at which reactants are used up
C. The colour change in a reaction
D. The total energy of the reaction

2. Which of the following factors does NOT affect the rate of a chemical reaction?

A. Temperature
B. Pressure
C. Concentration
D. Density

3. An increase in temperature increases the rate of reaction because

A. molecules become heavier
B. molecules move faster and collide more
C. the volume of the gas increases
D. the pressure decreases

4. Which of the following will increase the rate of reaction between zinc and hydrochloric acid?

A. Using dilute acid
B. Using powdered zinc
C. Lowering the temperature
D. Using large zinc pieces

5. A catalyst increases the rate of a chemical reaction by

A. increasing the temperature
B. creating a different pathway with a lower activation energy
C. increasing the pressure
D. increasing the concentration

6. Which of the following is an example of a catalyst?

A. Copper (II) sulphate
B. Manganese (IV) oxide
C. Sodium chloride
D. Water

7. In a reaction between a solid and a liquid, the rate of reaction is increased by

A. decreasing the surface area of the solid
B. increasing the size of the solid
C. increasing the surface area of the solid
D. reducing the concentration of the liquid

8. Which of the following will slow down a chemical reaction?

A. Increasing temperature
B. Increasing concentration
C. Adding a catalyst
D. Lowering temperature

9. The rate of reaction between magnesium and dilute hydrochloric acid can be measured by

A. change in colour
B. volume of gas produced
C. mass of magnesium used
D. number of bubbles formed

10. Which graph best represents a fast reaction?

A. One that rises steeply
B. One that rises slowly
C. One that is flat
D. One that slopes downward

11. Which of the following graphs best represents an exothermic reaction?
A. Products higher than reactants
B. Products lower than reactants
C. Reactants and products at same level
D. No activation energy

12. The peak of an energy profile diagram represents the
A. reactants
B. products
C. activated complex
D. catalyst

13. The minimum energy required for a reaction to occur is called
A. enthalpy
B. heat of reaction
C. activation energy
D. reaction rate

14. In an endothermic reaction, the heat of reaction (ΔH) is
A. zero
B. negative
C. positive
D. constant

15. If a reaction produces a gas and the mass of the reaction mixture decreases with time, this is because
A. heat is absorbed
B. gas escapes
C. the solid melts
D. the reaction stops

16. A steep slope on a rate of reaction graph indicates
A. a slow reaction
B. no reaction
C. a fast reaction
D. equilibrium

17. Which of the following will increase the rate of a chemical reaction?
A. Decreasing temperature
B. Removing a catalyst
C. Increasing concentration
D. Decreasing surface area

18. In an exothermic reaction, the products are
A. more energetic than the reactants
B. less energetic than the reactants
C. equal in energy to the reactants
D. unstable

19. The heat energy absorbed or released during a reaction is represented by
A. Ea
B. ΔH
C. K
D. pH

20. On a volume of gas versus time graph, the reaction is complete when
A. the curve is steep
B. the curve is straight
C. the curve becomes horizontal
D. the volume decreases

21. The rate of a reaction is given by the expression
{Rate} = k[A]^2
If the concentration of (A) is increased from 0.2 mol dm⁻³ to 0.4 mol dm⁻³, how does the rate of reaction change?

A. It doubles
B. It triples
C. It increases four times
D. It remains the same

22. A finely divided form of a metal burn more readily in air than the rod form because the rod has (a) higher molar mass (b). smaller surface area (c) protective oxide coating (d) different chemical properties

THEORY QUESTIONS.

1.(a)(i) Define rate of a chemical reaction?

ii). mention three factors that can affect the rate of a chemical reaction

iii). state the collision theory

b). A sample of carbon is burnt at a rate of 0.50gper second for 30 minutes to generate heat.

(i). write a balanced equation for the reaction

(ii). determine the I. volume of carbon (IV) oxide produced at s.t.p. II. moles of oxygen used up in the process at s.t.p. [ C= 12.0, O= 16.0, Molar volume Vm = 22.4dm³

c. State how each of the following affects the rate of chemical reactions (i) surface area (ii) catalysts [waec]

2.a(I) What is meant by the rate of chemical reaction?

(ii). Explain in terms of collision theory, the effect of temperature increase on reaction rate.

(b) When hydrogen peroxide is exposed to air, it decomposes (I) write an equation for the reaction (ii). Outline an experiment to illustrate the effect of a named catalyst on the rate of decomposition.

(iii).

3a(i) Sketch an energy profile diagram to show the effect of a catalyst on the reaction rate, given that it is exothermic.

b. The graph below is the ratio curve for the following reaction carried out in an open vessel

MgCO3(aq) + 2HCl → MgCl2(aq) +H2O(l) + CO2(g)

(i) For how long the reaction occurred

(ii) why was there a loss in mass?

(iii) State whether the reaction rate was fastest at the beginning, the middle or towards the end of the reaction. Give reason for your answer.

(iv) List three reaction conditions that can affect the slope of the curve.(waec)

4(a)In an experiment, excess 0.050mol/dm3 HCl was added to 10g of granulated zincin a beaker. Other conditions remaining constant state how the reaction rate would be affected in each case if the experiment was repeated using

(i) 1.0mol/dm₃ HCl

(ii) 8.0g of granulated zinc

(iii) 10g of zinc dust

(iv) a higher volume of 0.5mol/dm₃ HCl

(v) a reaction vessel dipped in crushed ice

(vi) equal volumes of water and 0.50mol/dm3 HCl.

5. (a) What is meant by the rate of a chemical reaction?

(b) State three factors that affect the rate of a chemical reaction.

6. Explain how each of the following affects the rate of a chemical reaction:

(a) Temperature
(b) Concentration
(c) Surface area

7. (a) What is a catalyst?

(b) Give two examples of catalysts.
(c) State two characteristics of a catalyst.

8. A piece of calcium carbonate reacts with dilute hydrochloric acid.

(a) Write the balanced chemical equation for the reaction.
(b) State two ways in which the rate of the reaction can be increased.
(c) Explain one of the ways stated in (b).

9. (a) Explain why powdered zinc reacts faster with dilute hydrochloric acid than a lump of zinc.

(b) What is meant by activation energy?

10. During a chemical reaction, the volume of gas produced was measured with time.

(a) What information can be obtained from such a graph?
(b) How can you tell from the graph that the reaction is fast?

11. State two industrial processes where catalysts are used and name the catalyst used in each case.

(b) Explain why an increase in pressure increases the rate of reaction between gases.

(12). (i) Draw the energy profile diagram for the reaction H2(g) + I2(g) → 2HI(g) △H = -13kJmol-1 if the concentration of HI(g) increases from 0.001 to 0.002 mol/dm3 in 80 secs what is the rate of the reaction

Tuesday, 14 March 2017

LAWS AND PRINCIPLES

LAWS AND PRINCIPLES

Aufbau principle: - when building up atoms one proton is added at a time to the nucleus and one electron around it to obtain the ground state electron configuration of the element.

AVOGADRO’S LAW: - States that equal volume of all gases at the same temperature and pressure contain the same number of molecules

BOYLE’S LAW: - States that the volume of a given mass of gas is inversely proportional to its pressure provided the temperature remains constant.

Mathematically Vἀ1/P T= constant

CHARLES LAW: - States that the volume of a given mass of gas is directly proportional to it temperature provided the pressure remains constant.

Mathematically V ἀ T P=constant

COLLISION THEORY: - This theory assumes that for a chemical reaction to occur, there must be effective collision between reactants particles

COULOUMB’S LAW: - state stat the electrostatic force associated with two charges A and B is directly proportional to the product of their magnitudes and inversely proportional to the square of the distance between them

DALTON’S ATOMIC THEORY: - States that

I. All element is made up of small indivisible particles called atoms

II. Atoms can neither be created nor destroyed during the cause of a chemical reaction to

III. Atoms of a particular element are exactly alike in all respect but are different from atoms of other elements.

IV. When atoms combine together, they do so in simple whole number ratio to one another

DALTON’S LAW OF PARTIAL PRESSURE: - of partial pressure; states the in a mixture of gases that do not react chemically together, the total pressure exerted by the gas is the sum of the individual gases that make up the mixture.

P_TOTAL =P₁ +P₂+P₃+.......+P_n

FARADAY’S FIRST LAW OF ELECTROLYSIS: - States that the mass of an element deposited or liberated at an electrode during electrolysis is directly proportional to the quantity of electricity passing through the electrolyte.

Mathematically m ἀ Q

FARADAY’S FIRST LAW OF ELECTROLYSIS: State that the mass of an element discharged during electrolysis is directly proportional to the quantity of electricity passing through the electrolyte.

FARADAY’S SECOND LAW OF ELECTROLYSIS: - states that if the same quantity of electricity is passed through solutions of different electrolyte the number of moles of each element discharged is inversely proportional to the charge on the ion

GAY –LUSSACS LAW of combining volume: - state that when gases combine, they do so in volumes which are in simple ratio to one another and to the volume of the product if gaseous provided temperature and pressure remain constant.

GRAHAM’S LAW OF DIFFUSION: - States that the rate of diffuse on of a gas is inversely proportional to the square root of its vapour density provided temperature and pressure remains constant.

HESS’S LAW OF CONSTANT HEAT OF SUMMATION: - This law states that the total enthalpy change of a chemical reaction is constant regardless of the route by which the chemical change occurs, provided that the conditions at the start of a reaction are the same as the final conditions

HUND’S RULE of maximum multiplicity: - state that when electrons fill degenerate orbitals they go in singly first before pairing occurs.

LE’CHATERLIER’S PRINCIPLE: - states that if an external constraint (such as a change in temperature, pressure or concentration) is imposed on a system in equilibrium the equilibrium will shift so as to cancel /annul the effect of the change.

LAW OF CONSERVATION OF MASS: - State that mater can neither be created nor destroyed but can be converted from one form to another.

LAW OF CONSTANT COMPOSITION: - States that all pure samples of a given chemical compound contain the same element combined in the same proportion by mass.

LAW OF DEFINITE PROPORTION: -See law of constant composition

LAW OF MULTIPLE PROPORTIONS: - States that if two elements A and B combine to form more than one compound, the various masses of A that combines with a fixed mass of B are in simple ratio with one another

MARKOVNIKOFF'S RULE: - states that when a hydrogen halide is adding to an already substituted alkene, the hydrogen atom from the halide will attach itself to the Carbon atom with the highest number of hydrogen atoms.

PAULI’S EXCLUSSION PRINCIPLE: - This principles states that 'No two electrons in the same orbital of an atom should have the same set of four quantum numbers'.

DEFINITIONS IN CHEMISTRY

DEFINITION

AN ACID

Arrhenius definition: An acid is a substance which produces hydrogen ion (H⁺) as the only positive ion in solution.

HCl → H+ + Cl-

H2O → H+ + OH-

Bronsted -Lowry’s definition: An acid is a proton donor.

Lewis's acid: An acid is any species that accepts an electron pair.

ACID-ANHYDRIDE: is the oxide of a nonmetal which dissolves in water to yield the corresponding acid.

ABSOLUTE-ZERO-TEMPERATURE: This is the lowest temperature in the Kelvin scale in which the volume of a gas is assumed to be zero.

ACTINADES: The Period 7 elements that forms the second inner transition series (5f block). It includes thorium (Th;Z=90) through lawrencium (Lr; Z=103)

ACTIVATED COMPLEX: It is the highest energy level in a chemical reaction where the reactants are no longer reactants but are not yet products.

ACTIVATION ENERGY: this is the minimum amount of energy reactant particles must possess before a chemical reaction can occur. OR it is the energy barrier that reactants must overcome to become products

ACTIVITY: The change in number of nuclei (N) of a radioactive sample divided by the change in time (t)

ALKANES: A hydrocarbon with the general molecular formular CnH2n+2. having only C-C single bonds

ALKENES: A hydrocarbon that contains at least one carbon-carbon double bond (C=C bond)with the general formula CnH₂n.

ALKYNES: A hydrocarbon that contains at least one carbon-carbon triple bond (C=C ) with general formula CnH_2n-2

ALLOTROPY: This is when an element occurs in different form but in the same physical state.

ALLOTROPES: These are different forms of an element occurring in the same physical state.

ALPHA (Ἀ) DECAY: A radioactive process in which an alpha particle is emitted from the nucleus of a radioactive element.

ALPHA PARTICLE (ἀ or ⁴₂He). A positively charged particle, identical to a helium nucleus.

ANION: A negatively charged ion e.g Cl^-, O^2-, N^3- .

ANODE: The electrode at which oxidation occurs in an electrochemical cell or an electrolytic cell.

ATOM: this is the smallest particle of an element that can take part in a chemical reaction.

ATOMIC MASS: This is the sum of the protons and neutrons in an atom of an element.

ATOMIC NUMBER: This is the number of protons in one atom an element.

BETA (β) DECAY: This involves the emission of a beta particle by a radioactive isotope.

BOILING POINT OF A LIQUID -: This is the temperature at which the saturated vapour pressure of the liquid is equal to the atmospheric pressure.

CATALYST: This is a substance that alters the rate of a chemical reaction, but itself is not affected by the reaction.

CARBOHYDRATES:- These are organic compounds that contain carbon, hydrogen and oxygen, where the hydrogen and oxygen are in the ratio of 2:1

CATHODE :- this is the electrode where reduction occurs in an electrochemical cell or an electrolytic cell.

CHEMICAL CHANGE: this is a change that is not easily reversible and in which a new substance is formed.

CONCENTRATION: this is the amount of a solute in moles per dm³ of solution.

COMBINATION REACTION: This is when two or more elements combine together to form a single compound example

Fe_{(s) +} S_(s) → FeS_(s)

COVALENT BONDING: this is a type of bonding that involves the sharing of electron pair between two atoms ( usually non-metals)

Cracking: this is the breaking down of higher molecular mass alkanes to lower molecular mass alkanes and alkenes.

C₁₀H₂₂ → C8H18 + C2H4

DEGENERATE ORBITALS: These are orbitals that are at the same energy level examples include the p-orbital, the d-orbital and the f-orbital.

EFFLORESCENCE: this is the phenomenon whereby some salts when exposed to the atmosphere loses part or all of their water of crystallization.

EFFLORECENT SALTS; these are salts which loses part or all of their wate of crystallization when exposed to the atmosphere.

ELECTRON AFFINITY: This is the energy evolved when an electron enters an atom in the gaseous state to form a mole of gaseous negative ions.

ELECTROLYSIS: - This is the decomposition of a compound brought about by the passage of a direct current through either a solution of the compound or its molten form

ELECTRONEGATIVITY: Is defined as the power of an atom to attract (shared pair or bonded) electrons in a molecule.

ELECTROVALENT BOND: this is a type of bond that involves the transfer of electrons from one atom (metal) to another (non-metal). OR

it is the electrostatic force of attraction between oppositely charged ions / cations and anions.

EMPIRICAL FORMULA: - This is the simplest formula of a compound; it shows the elements present and the ratio to which they are combined.

ENDOTHERMIC REACTION: This is a reaction during which heat is absorbed from the surrounding.

END POINT: This is the point at which the chemical reaction is complete during titration. The end point is detected with the help of an indicator.

ENTHALPY: This is the total heat content of a body/substance

ENTROPY:- This the degree of disorder or randomness of a system.

ESTERIFICATION REACTION: This is the reaction between an alkanoic acid and an alkanol to form an alkanoate (ester) and water only.

RCOOH + R'OH → RCOOR' + H2O

EXOTHERMIC REACTION: This is a reaction during which heat is given off to the surrounding.

FERMENTATION:- This is the breakdown of carbohydrates into ethanol and carbon (IV) oxide by the action of micro-organism.

FUNCTIONAL GROUP: - A functional group may be a bond, an atom or a radical it is common to a homologous series and it also determines the chemical property of the series.

HALF-LIFE: - This is the time taken for half the total number of atoms in a given sample of a radioactive substance to undergo decay.

HOMOLOGOUS SERIES: - This is a family of organic compounds that have a regular structural pattern where successive members differs by a -CH2- group.

HYBRIDIZATION:- This is mixing of two or more orbitals to form a set of new and equal number of hybrid orbitals. the three most common types of hybridization are the sp3, the sp2 and the sp hybridization

HYGROSCOPY: This is a phenomenon whereby some substances when exposed to the atmosphere absorbs moisture and only become sticky or wet.

ION: This is a charged atom (particle) or group of atoms carrying a charged.

IONISATION ENERGY: - This is the energy required to remove an electron from an atom to form a mole of gaseous ions.

IONIC BOND: see electrovalent bond.

ISOMERS: these are organic compounds with the same molecular formula but different structural formula.

ISOMERISM: This is the occurrence of two or more organic compounds with the same molecular formula but different structural formula.

ISOTOPES: These are atoms of the same element with the same atomic number but different mass number.

ISOTOPY: This is the occurrence of atoms of the same element with the same atomic number but different mass number.

MASS CONCENTRATION: This is the concentration expressed in g/dm3

MOLAR CONCENTRATION: This is the concentration expressed in mols/dm3

MOLAR MASS: - This is the mass of one mole of a substance. the unit is g/mol.

MOLAR SOLUTION: These is a solution that contains one mole (1mol) or the molar mass of a substance in 1dm3 of solution

MOLECULAR FORMULA: -This is the actual formula of a compound; it shows the exact number of atoms present in one molecule of compound.

OXYDATION: This is the loss of electrons from a substance.

OXYDIXING AGENT: A substance that accepts/gains electrons.

PHYSICAL CHANGE: this is a change that is easily reversible and in which no new substance is formed.

POLLUTION:- This is the release of harmful substances into the atmosphere in concentration that is harmful to man and his environment.

POLYMERIZATION: This is the process whereby two or more small molecules (monomers) link together to form large molecules (polymer).

REDUCTION: This is the gain of electrons

REDUCING AGENT: A substance that donate/loss electrons

SATURATED SOLUTION: - This is a solution that contains as much solute particles as it can hold at a particular temperature in the presence of undissolved solute particles.

STANDARD SOLUTION: This is a solution with a known concentration.

SUPER SATURATED SOLUTION: - This is a solution that contains More than the solute it can normally hold a particular temperature.

SOLUBILITY: - Solubility of a solute in a solvent is the amount of solute in moles or grams that can saturate 1dm3 of solvent (water) at a particular temperature.

VULCANIZATION:- Vulcanization is a process whereby natural rubber is heated with sulphur under pressure in order to obtain a harder, tougher and more elastic rubber

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