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 (CH₃CH₂CH₃) 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:

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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

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

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

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

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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

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

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

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

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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

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

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

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

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

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

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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

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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 

3.

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.

b. 

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?

(c) How does a catalyst affect the activation energy of a reaction?

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