Easykemistry

Wednesday, 18 September 2024

OXIDATION NUMBER at a glance

Oxidation number (O.N) of an element is the charge on an atom of the element whether it is by itself or bonded to another atom. It indicates the number of electrons the atom has gained or lose at that moment. That is, it is the charge an element will have if electrons were transferred to or from it. It is usually zero (0) for an element in the uncombined state. It is also referred to as the oxidation state of the element

NOTE: - The sign or charge for O.N is written before the number (–2) but it is written after the number for an ionic charge i.e O^2–

Rules for calculating oxidation number

The following rules are applied when assigning an oxidation number or calculating the oxidation number of an element thus

1. The O.N of oxygen is always equal to -2 except in peroxides (–1)

2. O.N of hydrogen is always equal to plus one (+1) except again in metallic hydrides (–1).

3. The O.N for an element in the elemental (or ground) state O.N = O (zero) e.g.

Na = O, Cl₂ = O, O₂ = O etc.

4. For an for a simple ion is equal to the charge on it for example

Na+ = +1, Cl^– = –1, O^2– = -2

5. Oxidation number of a radical is equal to the charge on it e.g CO₃^2-= –2, NO₃^-=-1, SO₄^2-=-2

3 The O.N of a compound is equal O (because it is the sum of the e.g.

H₂O = O,

i.e (O.N of H × no of H atoms) + (O.N of O) =

(+1 x 2) + ( -2 x1)

2 -2 = 0

NaOH = (+1 x1) + (-2 x 1) + (1 x 1)

+1 - 2+ 1

+2-2=0

Rules for specific groups in the PT.

I For group 1A elements (comprising Li,Na,K, e.t.c) their O.N = +1

II For group 2A elements (comprising Be, Mg, Ca e.t.c) their O.N = +2 in all compounds

III For group 3 elements (B, Al, e.t.c) their O.N = +3 especially in their binary compounds.

IV For group 5 = -3

V For group 6 = -2 except Oxygen (O) in peroxides).

VI For group 7 = –1 respectively especially in their binary compounds

Determination of the oxidation number of an element

1 Find the O.N of the underlined elements in the following

a). ZnCl₂ b). SO₃ c). NO₃^- d). Ca²⁺

Solution

To determine the O.N of the underlined elements, we must follow the general rules for calculating O.N of an element.

a). ZnCl₂: The O.N of a compound is zero, i.e. ZnCl₂= O.

Since Cl is a group 7(A) element and ZnCl₂ is a binary compound then the O.N of Cl is –1, therefore, the O.N of Zn is

(O.N of Zn) + (O.N of Cl ´ 2) = 0
x + (–1 ´ 2) = 0
x – 2 = 0
x = +2

b) SO₃

Solution

(O.N of S) + (O.N of 0 ´ 3) = 0
x + (–2 ´ 3) = 0
x – 6 = 0
x = +6

Trioxosulphate(IV) ion

c) NO₃^-

Solution

The O.N of a radical is equal to the charge on it, hence

NO₃^- = –1
that is,
(O.N of N) + (O.N of 0x3) = –1

x + (–2x3) = –1
x – 6 = –1
x = +6 – 1
= +5

Trioxonitrate (V) ion

d)Ca²⁺ The O.N of an ion is the charge on it, i.e., Ca²⁺ = +2 Calcium = ion

Uses of oxidation number

Oxidation number is used for the

1. It is used in the IUPAC (International Union of Pure and Applied chemistry) system of naming compounds e.g. H₂SO₄: Tetraoxosulphate(VI) acid

2. It is used to know the oxidation state or number of an element in a compound

OBJECTIVE QUSETION

1. Which species undergoes reduction in the reaction represented by the equation below?

H₂S_(aq)+2FeCl_3(aq)→S_(s)+ 2HCl +3FeCl₂

(a) Fe³⁺

(b). H₂S

(c). Cl^–

(d) S

2. Oxidation is a reaction which involves the following except

(a). Loss of electrons

(b). Increase in oxidation number

(c). Gain of oxygen

(d). addition of hydrogen

The O.N of the following underlined elements are

3. Na₂SO₄

(a) +4,

(b) -2,

(d) –5

4. Al (H₂O)₆]³⁺

(a) +3

(b) –3

(d) –6

5. K₂Cr₂O₇

(a) +5

(b) +4

(d) +6

6. Mn

(a) +6

(b) +7

(d) +3

THEORY QUESTION

2. Find the oxidation numbers of the following underlined elements.

(a) K₂Cr₂O₇ (b) KMnO₄ (c) HNO₃

(d). S^2- (e). Cl^- (f). Cr₂

2.a State two applications of oxidation numbers

b. What is the oxidation state of manganese in each of the following species?

i. MnO₂ ii MnO₄^- iii. MnCl₂

Thursday, 12 September 2024

SODIUM at a glance

Sodium: is found in group 1 period III on the periodic table. It has an atomic number of 11 and an atomic mass of 23.

It does not occur as a free element in nature because it is very reactive. However, it is found mainly in the combined state as sodium chloride in sea water, and as rock salt (Halite) in underground deposits.

It is extracted from fused sodium chloride by electrolysis using the Dawn's Cell. a little amount of CaCl2 is added to lower the melting point (from about 801 to 600)

Chemistry of the reaction

at the cathode

At the cathode: - the sodium ions migrate to the cathode where they gain an electron each to become reduced to metallic sodium

Na⁺_(l) + e- → Na_(s)

At the anode: - the chloride ions migrate to the anode where they loss their excess charge/electrons and become reduced to atomic chlorine

Cl^- → Cl + e^-

the chlorine atom combines with another chlorine atom and is discharged as chlorine gas

Cl + Cl → Cl_2(g)

Properties of Sodium:

i. Sodium is soft and can easily be cut with a knife.

ii. It has a of 0.968g/cm³

iii. It has a silvery-white appearance.

iv. It has a low melting point.

v. It has a high boiling point.

v. Sodium is a good conductor of electricity.

Chemical Properties

i) Reaction with air or oxygen: - Sodium metal tarnishes on exposure to air

4Na_(s) + O_2(g) →2Na₂O_(s)

Na₂O_(s) + H₂O_(l) → 2NaOH (s)

NaOH_(aq) + CO₂→ Na₂CO₃

In excess air or oxygen, it burns with a golden/bright yellow flame to yield sodium peroxide Na₂O₂,

2 Na_(s) + O_2(g)→ Na₂O_2(s)

In limited supply of air sodium oxide (Na₂O) is formed.

Na_(s) + O_2(g)→ 2Na₂O_(s)

Because of its reactivity sodium is stored under paraffin oil or other organic solvents like naphtha or toluene.

ii. Reaction with water: - It reacts violently with cold water to yield sodium hydroxide and hydrogen with large amount of heat.

Na_(s) + H₂O_(l) → NaOH_(aq) + H_2(g)

iii. Reaction with acids: - It reacts explosively to form a salt and hydrogen gas

Na_(s) + HCl_(aq) →NaCl_(aq) + H_2(g)

Na_(s) H₂SO_4(aq) → Na₂SO_4(aq) + H_2(g)

This reaction is highly dangerous and should not be carried out in the school laboratory.

Reaction with non-metals: - sodium combines directly with the following non-metals when heated to form binary compounds.

Na_(s) + S_(s) →Na₂S_(s)

Na_(s) + H₂ → NaH_(s)

Na_(s) +P_(s) → Na₃P_(s)

Na_(s) + Cl_(g) → NaCl_(s)

Sodium does not react with carbon, boron and nitrogen

Reaction with mercury: - Sodium forms various stable mixture with mercury known as sodium amalgam of varying composition such as NaHg, Na₂Hg, Na₃Hg etc.

Sodium amalgam reacts with water to yield hydrogen.

Na_(s) + Hg_(l) →NaHg_(l)

v. Reaction with ammonia: - Sodium reacts with ammonia to form sodamide and hydrogen gas.

Na_(s) + NH_3(g)→ NaNH_2(s) + H_2(g)

As a reducing agent: - Sodium act as a strong reducing agent. It reduces some metallic chlorides and oxides to their metals.

Na_(s) + BeCl_2(s) → NaCl_(s) + Be_(s)

Test for sodium ions

i). Flame test: when sodium compounds give a bright or golden yellow flame when burnt in a non-luminous flame

Uses of Sodium:

-i). Sodium is used in the manufacture of other compounds like sodamide, sodium peroxide.

ii). Sodium alloys like NaK(sodium-potassium alloy), are used as coolant in nuclear reactors.

iii). Sodium vapor lamps are commonly used for street lighting

iv) It is used in the manufacture of tetraethyl lead (C₂H₅)₄Pb, which is used as an antiknock agent in petrol.

v) It is used as a laboratory reagent (Lassaigne's extract).

vii). It is used for producing amalgams used as reducing agents.

viii) Sodium used as a catalyst in the preparation of artificial rubber and also as a deoxidizer in the preparation of light alloys.

COMPOUNDS OF SODIUM

Sodium compounds are generally white crystalline salts and are mostly soluble in water.

1. Sodium chloride (NaCl): (table salt) it is found naturally in sea water and in underground deposits as rock salt.

Properties

- It is a white anhydrous crystalline solid

- It has a melting point of 801⁰C and a boiling point of 1420⁰C.

- The pure form is not deliquescent.

Uses

1. It is used as a food preservative.

2. It is used as an important raw material for the manufacturing of Na, NaOH, Cl₂, Na₂CO₃, NaClO₃ and other compounds.

3. It is used for salting out soap

4. It is used in glazing earthenware

5. It is used in regenerating water softener.

2. Sodium hydroxide (NaOH): It is a white crystalline solid, made into flakes or pallets

Properties

-i). It is a white crystalline solid

ii). It is highly deliquescent

iii) It has a melting point of 320⁰C without decomposing.

iii). It dissolves in water to give a strong alkaline solution with the evolution of heat

Chemical properties

With acids: - NaOH produce salt and water.

2NaOH_(aq) + H₂SO_4(aq) → Na₂SO_4(aq) + 2H₂O_(l)

With acidic oxides: - It form sodium salt. E.g.

NaOH_(aq) + SO_2(g) → NaHSO_3(aq)

With ammonium salts: - When heated with an ammonium salt, ammonia gas is liberated.

NaOH_(aq) + NH₄Cl_(s)→ NaCl_(aq) + H₂O_(l) + NH_3(g)

With metals – Al and Zn are amphoteric; they combine with excess NaOH to form the alluminate (III) and respectively with hydrogen gas.

2Al_(s) + 2NaOH_(aq) + 6H₂O_(l) → 2NaAl(OH)_4(aq) + 3H_2(g)

sodium aluminate (III)

Zn_(s) + 2NaOH_(aq) + 2H₂O_(l) → Na₂Zn(OH)_4(aq) + H_2(g)

sodium zincate (II)

Therefore, Aluminium or Zinc containers should not be used to store NaOH.

As a precipitating agent: - NaOH solution is most times used to precipitate insoluble hydroxides. E.g

Zn²⁺_(aq)+ 2OH^-_(aq) → Zn(OH)_2(s)

Pb²⁺_(aq)+ 2OH^-_(aq) → Pb(OH)_2(s)`

Zn(OH)2, Al(HO)3, Sn(OH)2, and Pb(OH)2, are amphoteric and will react excess sodium hydroxide to form complex salts. E.g

Zn(OH)_(s)+ 2NaOH_(aq)→ Na₂Zn(OH)_4(aq)

With non-metals: NaOH reacts with various non-metals to form sodium salts.

Reaction with glass – High concentrations of NaOH attack glass to form sodium trioxosilicate (IV). Hence, glass stoppers are not used to cover reagent bottles containing concentrated sodium hydroxide or burette because they would become stuck. This is called etching.

Uses of NaOH

1. it is used as a strong alkali

2. it is used as an analytical and precipitating reagent

3. it is used for absorbing CO₂

4. it is used for making soap, rayon (artificial silk),

5. I is used for making paper

6. it is used for making various compounds like sodium trioxochlorate (V), sodium methanoate and phosphine.

7. it is used for purification of bauxite

8. it is used petroleum refining.

9. it is used for the bleaching of cotton textiles.

3.Sodium tetraoxosulphate (IV) (Na₂SO₄₎

Properties

It occurs both in the anhydrous form called saltcake or as a decahydrate form known as Glauber’s salt which is efflìorescent.

Uses Of Na₂SO₄

i. It is used as a purgative

ii. In producing of sodium sulphide

Iii it is in the manufacture of wood pulp, glass, and detergents

4. Sodium bicarbonate (NaHCO₃): (baking soda): - A white it is utilized in cooking and as a leavening agent in baking.

- Sodium carbonate (Na₂CO₃): Also called soda ash

Properties

-Na₂CO₃ in form of soda ash (i.e. anhydrous Na₂CO₃) is a fine white powder, while washing soda (Na₂CO₃.10H₂O) is translucent and crystalline.

i. They both dissolve in water to form an alkaline solution by hydrolysis.

ii. Washing soda is efflorescent

iii. It does not decompose on heating

iii. t reacts with acid to liberate CO₂

Uses of Na₂CO₃

i. It is used in the industrial manufacturing of glass

ii. It is used as a water softener

iii. It is used in manufacturing of detergent

iv. It is used in the manufacturing NaOH, borax, waterglass, soap and paper

iv. It is used in laboratory to standardize acids and as an analytical reagent.

vi. it is used in glass production,

vi. It is used as a pH regulator in various industrial processes

SOLVAY PROCESS: This is the industrial preparation of NaCO3

The raw materials are sodium chloride, ammonia gas and limestone. The reactions are as follows

1.The ammonia gas in brine (conc. sodium chloride) to give a mixture known as ammoniacal brine.

ii. This mixture is then allowed to trickle down a Solvay tower as a stream of carbon (IV) oxide is forced up the tower. It reacts with the ammonia in the mixture to yield ammonium hydrogen trioxocarbonate (IV) (NH₄HCO₃).

i). NH_3(g) + CO_2(g) +H₂O →NH₄HCO_3(aq)

The NH₄HCO₃ reacts with the sodium chloride to give sodium hydrogen trioxocarbonate (IV) (NaHCO₃).

ii). NH₄HCO_3(aq) + NaCl_(aq) →NaHCO_3(s) + NH₄Cl_(aq)

The sodium hydrogen trioxocarbonate (IV) is slightly insoluble in water and so precipitates out as a white sludge.

The NaHCO₃ is then filtered, rinsed and heated to give anhydrous sodium trioxocarbonate (IV) (soda ash), steam and carbon (IV) oxide

iii). NaHCO_3(s) →Na₂CO_3(s) + H₂O_(l) + CO_2(g)

The anhydrous Na₂CO_3(s) (soda ash) is redissolved in hot water and recrystallize to give the pure hydrated compound (Na2CO₃.10H₂O) called washing soda

iv). Na₂CO_3(s) + 10H₂O_(l) →Na₂CO₃.10H₂O_(s)

some highlights of the process

-Perforated dome-shaped baffle-plates are incorporated into the Solvay tower to slow down the flow rate of the ammoniacal brine so as to allow for proper contact between the ammoniacal brine and the carbon (IV) oxide as well as increase the surface area of reaction

-The concentrated sodium chloride also serves as a carrier for the ammonia gas.

Importance and Economics of the reaction: -

i. All the raw materials required in the Solvay process are quite cheap and are also readily available.

ii. Almost all the carbon (IV) oxide generated during the process from the decomposition of the NaHCO_3(s) is recycled, making the process quite economical.

iii. The sodium chloride solution is obtained from sea water or from rock salt deposits,

iv. the carbon (IV) is got from limestone found in rich deposits around.

CaCO_3(s) →CaO_(s) + CO_2(g)

The calcium oxide (CaO) is then reacted with the ammonium chloride to generate ammonia gas, which is also recycled back into the, producing calcium chloride as a by-product from the process.

CaO_(s) + NH₄Cl_(aq)→ CaCl₂ +H₂O+ NH_3(g)

5. Sodium nitrate (NaNO₃): It is a white crystalline solid produced when sodium hydroxide reacts with trioxonitrate (V) acids.

Properties

I). it is a white crystalline solid

ii). It has a melting point of 310⁰C and decomposes on further heating.

Uses

i) it is used primarily as a nitrogenous fertilizer

ii) In making trioxonitrate (V) acid, potassium trioxonitrate (V) and sodium dioxoxnitrate (III).

iii). It is used in the production of explosives and glass.

Objective questions

THEORY QUESTIONS

1. Explain with equations where appropriate the functions of the following substances in the Solvay Process (i) limestone (ii). ammonia (iii). brine.

2. Calculate the mass of sodium trioxocarbonate (IV) produced by the complete decomposition of 16.8g of sodium hydrogen trioxocarbonate (IV). [ H=1, O=16, Na=23, S=33]

Monday, 9 September 2024

ALKYNES at a glance

UNSATURATED HYDROCARBON (ALKYNES)

Alkynes are a homologous series of unsaturated hydrocarbons containing at least one triple bond. It has a functional group of (≡) and general molecular formular of C_nH_2n-2 where n= 1,2,3, ... n for successive members of the group.

The first member of the alkyne family is ethyne (acetylene).

Alkynes are named by replacing ending –ane of the corresponding alkane with –yne.

NOTE: Since alkynes contain triple bonds between C≡C therefore n=1 is not visible.

When n=	General Molecular Formulae C_nH_2n-2	Name
2.	C₂H_2x2-2= C₂H₂	Ethyne
3.	C₃H_2x3-2= C₃H₄	Propyne
4.	C₄H_2x4-2= C₄H₆	Butyne
5.	C₅H_2x5-2= C₅H₈	Pentyne
6.	C₆H_2x6-2= C₆H₁₀	Hexyne
7.	C₇H_2x7-2= C₇H₁₂	Heptyne
8.	C₈H_2x8-2= C₈H₁₄	Octyne
9.	C₉H_2x9-2= C₉H₁₆	Nonyne
10.	C₁₀H_2x10-2= C₁₀H₁₈	Decyne
11.	C₁₁H_2x11-2= C₁₁H₂₀	Undacyne
12.	C₁₂H_2x12-2= C₁₂H₂₂	Dodecyne
13.	C₁₃H_2x13-2= C₁₃H₂₄	Tridecyne
14.	C₁₄H_2x14-2= C₁₄H₂₆	Tetradecyne
15.	C₁₅H_2x15-2= C₁₅H₂₈	Pentadecyne
16.	C₁₆H_2x16-2= C₁₆H₃₀	Hexadecyne
17.	C₁₇H_2x17-2= C₁₇H₃₂	Heptadecyne
18.	C₁₈H_2x18-2= C₁₈H₃₄	Octadecyne
19.	C₁₉H_2x19-2= C₁₉H₃₆	Nonadecyne
20.	C₂₀H_2x20-2= C₂₀H₃₈	Icosyne/Eiocosyne

MOLECULAR STRUCTURES OF ALKYNES

N	ALKYNES	STRUCTURAL FORMULAR	MOLECULAR FORMULAR
2.	C₂H₂ Ethyne	H-C≡C-H	HC≡CH
3.	C₃H₄ Propyne	H H-C-C≡C-H H	CH₃C≡CH
4.	C₄H₆ Butyne	H H H-C-C-C≡C-H H H	CH₃CH₂C≡CH
5.	C₅H₈ Pentyne	H H H H-C-C-C-C≡C-H H H H	CH₃(CH₂)₂C≡CH
6.	C₆H₁₀ Hexyne	H H H H H-C-C-C-C-C≡C-H H H H H	CH₃(CH₂)₃C≡CH
7.	C₇H₁₂ Heptyne	H H H H H H-C-C-C-C-C-C≡C-H H H H H H	CH₃(CH₂)₄C≡CH
8.	C₈H₁₄ Octyne	H H H H H H H-C-C-C-C-C-C-C≡C-H H H H H H H	CH₃(CH₂)₅C≡CH
9.	C₉H₁₆ Nonyne	H H H H H H H H-C-C-C-C-C-C-C-C≡C-H H H H H H H H	CH₃(CH₂)₆C≡CH
10.	C₁₀H₁₈ Decyne	H H H H H H H H H-C-C-C-C-C-C-C-C-C≡C-H H H H H H H H H	CH₃(CH₂)₇C≡CH

NOMENCLATURE OF ALKYNES

The nomenclature of alkynes is similar to that of alkenes in many respects as shown in the structures below. The only difference lies on the type of bonds, in alkenes (double bond) and alkynes (triple bond).

(i) CH₃-CH₂-C≡CCH₃

Pent-2-yne

(ii) CH₃CH₂CH₂C≡CCH₃

hex-2-yne

C1H₃ C1H₃
            |                                                        |
(iii)       C2HC3≡C4C5H₃ (iv) C2H₂-C3≡C4-C5H₂
            |                                                                  |
CH₃ C6H₃
4-methylpent-2-yne hex-3-yne

CH₃ CH₃ CH₃
_||             |
(v)        CH₃CHC≡CCHCH₃ (vi)       C1H₃C2-C3≡C4-C5-C6H₃
|                                                  |              |
CH₃                                           CH₃ CH₃
  2,5-dimethylhex-3-yne 2,2,5,5-tetramethylhex-3-yne

          CH₃
|
(vii)      CH₃CH-C ≡CC-CH₂CH₃(viii)     CH₃C≡CCH₂
|              |                                                             |
CH₃ CH₃ CH₃
   2,5,5-trimethylhept-3-yne pent-2-yne

6-ethyl,3,3-dimethyldec-1,6-diyne 8-ethyl, 8-methylnon-1,3,5-triyne

   Cl
                                                                                      |
(xi)       CH₃C≡CCHCH₃ (xii)      CH₃-C-C≡CH
                        |                                                          |
Cl Cl
  4-chloropent-2-yne                                         3,3-dichlorobut-1-yne

(xiii)     CH₃CHC≡CC≡CCHCH₃ (xiv)     CH₃CHC≡CCHC≡CH
                   |    |                                          |            |
Cl Br Cl         Cl
  2-bromo, 7-chlorooct-3,5-diyne                     3,6-dichlorohept-1,3-diyne

H
                      |
H-C-H
                      |
H H H     |
              |   | | |
(xv)  H-C6-C5-C4-C3C2≡C1H
            |   | | |
H H H |
                       |
H-C-H
                    |
H
  3,3-dimethylhex-1

LABORATORY PREPARATION OF ETHYNES (ALKYNES)

Ethyne is prepared in the laboratory by adding cold water into calcium dicarbide (CaC₂). Much heat evolved and sand is placed beneath the flask to protect the flask from breakage. Ethyne is collected over water. The main impurity, phosphine, PH₃ is absorbed by the acidified CuSO₄ solution.

CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂.

Ethyne

PHYSICAL PROPERTIES OF ETHYNE

1. It is colourless gas

2. It has sweet smell when pure

3. Almost insoluble in water

4. It is neutral to litmus

5. It is strongly exothermic

CHEMICAL PROPERTIES OF ETHYNE

Alkynes such as ethyne also undergoes addition reaction – a reaction in which one molecule of a compound is simply added on to the alkynes at the position of the carbon – carbon triple bond (C≡C) and this is converted to carbon – carbon single bond (C-C) that is, the alkanes. Examples of addition reaction are:

1. Reaction of ethyne with hydrogen in the presence of nickel as a catalyst

Ni
CH≡CH + 2H₂ → CH₃CH₃
ethyne ethane

2. Reaction of ethyne with bromine to produce 1,1,2,2-tetrabromoethane. The reddish brown colour of bromine is destroyed.

CH≡CH + 2Br₂ → CHBr₂-CHBr₂

3. Reaction of ethyne with chlorine to produce hydrogen chloride

CH≡CH + Cl₂ → 2C+ 2HCl

4. Reaction of ethyne with oxygen or combustion reaction of ethyne (alkynes) to produce carbon(iv)oxide and water

2CH≡CH + 5O₂ →4CO₂ + 2H₂O

5. Polymerization reaction of ethyne to produce benzene.

3C₂H_{2 →} C₆H₆

6. Reaction of ethyne with water in the presence of dilute H₂SO₄ and mercury as a catalyst to produce ethanal

CH≡CH + H₂O →CH₃CHO

7. Reaction of ethyne with KMnO₄ to produce 1,2-ethan-diol (glycol)

            CH≡CH +KMnO₄ →CH₂-CH₂
                                 | |
OH   OH
1,2-ethan-diol

USE OF ETHYNE

1. In oxy-acetylene flame for welding and cutting of metals

2. In oxy-acetylene torch

3. In preparation of acetic acid

4. as a starting material for making polyvinylchloride (PVC) which is used in electrical insulation and water proofing.

TESTS TO DISTINGUISHED BETWEEN ALKANES, ALKENES AND ALKYNES.

The following test can be performed to distinguished clearly the different classes of hydrocarbons, that is, the alkanes, alkenes and alkynes.

All alkanes are saturated compounds while both alkenes and alkynes are unsaturated.

TEST 1:

To the suspected hydrocarbons, add an acidified solution of KMnO₄ or K₂Cr₂O₇ solution. Alkanes have no effect in any of these solutions while both alkenes and alkynes decolorized. Acidified KMnO₄ solution changes from purple to colourless, while K₂Cr₂O₇ changes from orange to green.

TEST 2: Add the solution of Ammonical copper (I) chloride to the suspected hydrocarbons. it will form a yellowish or reddish –brown precipitate with terminal alkynes (alkynes with the triple bond in front of or behind the first C- atom). Alkanes and alkenes show no reaction.

2NH₄OH_(aq)+ 2CuCl + C₂H₂→ CuC₂+ 2NH₄Cl + 2H₂O

TEST: To the suspected hydrocarbon, add solution of Ammonical silver tronitrate (v). Alkanes and alkenes have no effect, but alkynes form a yellowish precipitate.

2NH₄OH + 2AgNO₃ + C₂H₂ → 2AgC + 2NH₄NO₃ + 2H₂O

OBJECTIVE QUESTION

1.When alkynes are hydrogenated completely, they produce compounds with the general molecular formula

a. CnHn

b. CnH2n+2

c. CnH2n

d.CnH2n-2

Sunday, 25 August 2024

RADIOACTIVITY at a glance

TOPIC: RADIOACTIVITY AND NUCLEAR ENERGY

Radioactivity

Radioactivity is the spontaneous emission of radiation by an element. Such an element is called a radioactive element.

Discovery of radioactivity

Radioactivity was discovered by Henri Becquerel, a Physicist, and the phenomenon was named by Marie Curie who also discovered the elements thorium, polonium and radium. Presently more than 40 naturally occurring radioactive elements have been discovered.

Radioactive reactions are also called nuclear reactions

Differences between nuclear reactions and ordinary chemical reaction.

	Nuclear reaction	Chemical reaction
1.	It involves the nucleus (nuclear particles) of the atom	It involves the valence electrons of the atom
	Large amount of energy change is involved	It involves low amount of energy change
	It involves the decomposition of the nucleus	It involves the transfer, loss, gain and sharing of electrons
	The rates of reactions are not affected by temperature and pressure	The rates of reactions are affected by temperature and pressure
	Does not involve the breaking or forming of bonds involved	It involves the breaking of old bonds and formation of new ones
	The reactions are irreversible	They could be reversible or irreversible

TYPES OF RADIATION

There are three major types of Radioactive radiation

1. alpha(α-) radiation

2. beta(β-) radiation and

3. gamma(γ) radiation/rays.

1. Alpha rays/radiation

These are fast moving stream of positively charged particles, that resembles the helium nucleus.

Properties of alpha rays

1. They are positively charged particles

2. They have low penetrating power (they can be stopped by a thin sheet of paper)

3. They have strong ionizing power (ionizing any gas through which they pass).

4. They cause fluorescence in some materials e.g ZnS

5. They have a mass number of 4 and atomic number of 2

2.Beta Rays

These are very fast moving streams of electrons.

They have velocities close to that of visible light.

Properties

1. They are negatively charged.

2. They have higher penetrating power than alpha particles ( can be blocked by a thin sheet of aluminum foil)

3. They have lower ionizing power than alpha particles.

4. They cause florescence in substances like anthracence but not zinc sulphide.

5. They are deflected toward the positive plates in an electrostatic field

Gamma Rays

Gamma rays/radiations are electromagnetic waves similar to visible light but with shorter wavelengths.

Properties

1. They have the highest penetrating power of the three particles ( they can be blocked by a thick block of Lead)

2. They travel with the speed of light.

3. They are not affected by an electrostatic field.

4 They do not have charge nor mass.

5. They have the least ionizing power among the three particles.

Summary of the properties of alpha, beta and gamma rays

Alpha ray

Beta ray

Gamma ray

Nature

Electrical Charge

Mass

Velocity

Relative penetration

Absorber

Helium nuclei ⁴₂He

4 units

About ¹/₂₀ the speed of light

Thin paper

Electron ^o_-1e

-1

¹/₁₈₄₀ units

Varies (from 3 – 99% of the speed of light)

100

Metal plate

Electromagnetic radiation

No charge

No mass

Speed of light

10,000

Large lead Block

Other types of radiations include

X – rays

X – rays are electromagnetic waves, that resembles visible light, but with a shorter wavelength. They are produced when fast moving electron knock out electrons from the inner shells of metal atoms and electrons from a higher/outer shell then falls down to replace the knocked – out electrons. This movement of electrons is accompanied by the emission of x – rays.

Properties and uses of x – rays

1. X – rays can penetrate readily through most solid substances such as wood, foils, flesh and paper, which are opaque to visible light.

Hard x – rays have greater penetrating power than soft x – rays.

2. Soft x – rays are used in medicine to photograph human parts while hard x – rays are used for destroying cancerous cells.

3. X – rays are used to study the arrangement of particles in crystal lattices in chemistry and in big organic molecules like proteins.

DETECTION OF RADIATIONS

Different types of devices have been developed over the years for detecting radiation. The most commonly used detectors are:

i. Geiger-Muller counter,

ii. the scintillation counter and

iii. photographic plate or solution,

iv. Diffusion cloud chambers

Others include:

4. The pulse (wulf) electroscope

5. Solid state detector

6. Photographic plate or solution

7. Dekatron counter.

When a certain quantity of a radioactive material disintegrates spontaneously the word decay is used. During the decay, there is usually an emission of alpha – or beta particles. Sometimes, gamma rays also accompany the emission of these particles. The outcome of the disintegration is that the parent nucleus (nucleus undergoing disintegration) undergoes a change in the atomic number and become the nucleus of a different element.

TYPES OF DECAY

Alpha Decay: When the nucleus of an atom loses an α-particle. i.e. a helium nucleus (⁴₂He) during disintegration, the atomic number of the atom is reduced by two units and its mass number by four units, e.g.

when an atom of uranium 238, loses an alpha particle, it will become an atom of thorium Tℎ. The nuclear reaction is written thus:

²³⁸₉₂U →²³⁴₉₀Th + ⁴₂He (α-partcle)

The atom undergoing decay is called the parent cell while the new element (atom) formed is called the daughter cell.

Beta Decay: this kind of radioactive change is equivalent to the splitting of a neutron in the atomic nucleus of the radioactive element into an electron and a proton. The proton remains in the nucleus while the electron is expelled out of the nucleus as beta particles. The mass number does not change but the atomic number increases by one.

For example, if a thorium nucleus Th emits a β particle, its nucleus changes into a nucleus of the element protactinium whose mass number is the same as that of thorium but whose atomic number is increased by one unit from 90 to 91.

²³⁴₉₀Tℎ →²³⁴₉₁Pa + ⁰_-1β-particle

Gamma Decay: Gamma decay involves only the release of large amount of energy and does not involve change in the mass number or atomic number. It always accompanies an α or β decay.

α + β-particle + γ-rays.

Radioactive decay series

The nucleus of the daughter element formed most times by transmutation is itself unstable, and will undergo further disintegration after some time. This time may vary from few microseconds to millions of years and it is known as HALF-LIFE. This series of changes may occur until a stable nucleus is finally formed. Examples is the uranium series, the thorium series and actinium series, each of which is named after the longest-lived element in the series.

Element and radiation emitted Isotope Half-life

Uranium

↓ α 4.5×106years

Thorium

↓ β ℎ 24.1 days

Protactinium

↓ β 1.18 minutes

Uranium

↓ α 2.5×105years

Thorium

↓ α ℎ 8.0×104years

Radium

↓ α 1622 years

Radon

↓ α 3.8 days

Half Life as a Factor of Stability of Nucleus

The half-life of a radioactive element is the time taken for half of the total number of atoms in a given sample of the elements to decay.

For example, the half-life of radium-226 is 1622 years. This means that if we have one thousand radium atoms at the beginning, then at the end of 1622 years, 500 atoms would have disintegrated, leaving 500 undecayed radium atoms. In the next 1622 years, half the remaining atoms i.e. 250 would have disintegrated, leaving 250 undecayed atoms of radium and so on.

The stability of an atomic nucleus is related to the ratio of neutrons to protons in the nucleus. Atoms with neutron-proton ratio less than 1 or greater than 1.5 tend to be unstable and undergo radioactive radiation.

Nuclear Chemistry and Reactions

Nuclear chemistry is concerned with reactions that involve the nucleus. When nuclear particles like proton, neutrons, alpha and beta particles as well as electrons and gamma rays are used as bullets for bombarding the nuclei of some atoms, one or two of the following events may take place:

a) Artificial radio isotopes may be created

b) Atomic transmutation may occur

This transmutation is the process by which radioactive elements change into different elements.

Artificial Transmutation

Because large amount of heat is released from radioactive disintegration, scientists, devised a means of tapping, harnessing, controlling, and using this large amount of energy. This results in artificial transmutation since the natural transmutation is not subject to human control i.e. spontaneous. In 1919, Ernest Rutherford succeeded in transmuting a nitrogen isotope into an oxygen isotope by bombarding the nitrogen isotope with alpha particles.

147N+ ⁴₂α → ¹⁷₈O + ¹₁P

^Proton

Many atomic transmutations can be carried out by bombarding different elements with fast-moving atomic particles like neutrons, protons, deuterons (nuclei of deuterium atoms) and alpha particles. The neutron is a very effective bombarding particle because it is heavy and has no charge.

Generally, when light elements are bombarded by neutrons, new elements are formed by ejecting charged particles.

But, if the nucleus being bombarded is heavy, it captures the neutron to produce an isotope of the original

element. +

Nuclear Fission

Nuclear fission is a process in which the nucleus of a heavy element is split into two nuclei of nearly equal mass with a release of energy and radiation.

Nuclear Fusion

Nuclear fusion is a process in which two or more light nuclei fuse or combine to form a heavier nucleus with the release of energy and radiation.

It is the opposite of nuclear fission. The energy released during nuclear fusion is greater than the energy released during nuclear fission because of the differences in the binding energies of the isotope.

This principle is applied in the making of hydrogen bombs. The sun and stars also obtain their energy from nuclear fusion reactions.

1n 1939, a German nuclear scientist (Halm) discovered that atomic nucleus of isotopes of uranium could absorb a neutron and then break into two (atomic fission) e.g.

He noticed that the total products of fission have a slightly different mass from that of the total initial material. This difference in mass is radiated as energy, and the amount of energy is given by Einstein in the equation E = mc²

where E= energy in joules,

m= mass in Kg

and c = velocity of light in s-1.

A large amount of energy is released during nuclear fission This energy released can now be harnessed during nuclear reactions and are used in power stations to generate electricity and nuclear submarines.

Uses Of Radioactive Isotopes

1. Medical Uses:

a. It is used for the treatment of cancer, radioactive isotopes like

(i). cobalt 60 for treating tumors and destroying cancerous cells

(ii). Iodine – 131 for treating cancer of the thyroid gland and

(iii). phosphorus -32 for treating leukemia

b. Alpha rays are used in the sterilization of hospital equipment and in the pasteurization of foods.

2. Agricultural purposes: radiation is used in insect and pest control. This is done by sterilizing the male pupae of an undesirable insect by radiation, so that sterile male adults are produced. They are then released in large numbers to mate with the native female adults which will lay unfertilized eggs.

3. Radioactive tracers: due to continuous emission of radiation by a radioactive isotope it is possible to trace its position and also to measure its concentration using a sensitive counter. And so when a small quantity of a radioactive isotope is mixed with its stable isotope it is possible to follow its path through a series of changes or reactions . This technique has been used to study many metabolic processes in both plants and animals e.g.

(i). the uptake of iodine by the thyroid gland using iodine-131

(ii). photosynthesis using carbon-14 in the form of carbon (IV)oxide.

(iii). In medicine for tracing the movement of materials or food through the alimentary canal.

4. It is used in generating electricity from nuclear reaction occurring in nuclear power stations/ plants.

5. In Radiological Dating: Radioactive carbon-14 isotope (with half-life of about five thousand years) is used for determining the age of organic materials. The process involves measuring the amount of carbon-14 present in the specimen and the age is determined from this value and the half-life of the isotope.

The presence of very long – lived radioisotope ( e.g uranium 238 with a half-life of 4.5×10⁹ years) present in the earth crust is utilized by geologists to estimate the age of rocks and archaeological objects. It is also used in distinguishing genuine antiquities from fake ones.

OBJECTIVE QUESTIONS

1. Equation? ²³⁴₉₀Th → ^y_xPa + β- particle

X Y

a. 91 234

b. 92 235

c. 89 235

d. 88 238

2. A radioactive solid is best stored

a. Under paraffin oil

b. Under ultraviolet light

c. In a cool, dark cupboard

d. In a box lined with lead

Briefly discuss the following

- Atomic pile and atomic bomb

- Binding energy

- Nuclear energy for power

Friday, 23 August 2024

ISOTOPY at a glance

ISOTOPY /ISOTOPES

Isotopy is the existence of atoms of the same element having the same atomic number but different mass number or atomic mass.

The different atoms are called isotopes. Hence

Isotopes are atoms of a particular element with the same atomic number but different mass number.

Examples of isotopes are

i. ³⁵₁₇Cl and ³⁷₁₇Cl

ii. ¹⁵₈O, ¹⁶₈O, ¹⁷₈O and ¹⁸₈O

iii. ¹₁H, ²₁H and ³₁H

Relative Abundance: - when atoms exist naturally in isotopic mixture, they do so in a particular amount, when this amount is expressed in percentage, it is known as relative abundance.

For example, chlorine is found always in two isotopic mixtures out of 100, 75 is Cl-35 and 25 is Cl-37. so, 75 and 25 when expresses in percentage are the relative abundance of the two isotopes.

The relative abundance of an element is the amount (expressed in %) in which a particular isotope occurs in nature

Examples of isotopes are

i. ³⁵₁₇Cl with relative abundance of 75% and ³⁷₁₇Cl with relative abundance of 25%.

ii. ¹⁵₈O with relative abundance of 0.17, ¹⁶₈O with relative abundance of 99.76%, ¹⁷₈O with abundance of 0.05% and ¹⁸₈O with abundance of 0.02%

RELATIVE ATOMIC MASS: - The relative atomic mass of an element is the number of times the average mass of one atom of the element is as heavy as 8one-twelfth the mass of carbon-12.

DETERMINATION OF RELATIVE ATOMIC MASS OF AN ELEMENT GIVEN THE RELATIVE ABUNDANCE

The relative abundance of an element can be calculated from their various isotopic masses, and their relative abundance as follows: -

1. Naturally occurring exist in two isotopic mixtures ³⁵₁₇Cl with relative abundance of 75% and ³⁷₁₇Cl with relative abundance of 25% respectively. Calculate the relative atomic mass of Cl

SOLUTION

75 x 35 + 25 x 37 =
100 100

26.25 + 9.25
= 35.5

2. An element Q has two isotopes ⁶³₂₉Q and ⁶⁵₂₉Q with relative abundance of 70% and 30% respectively. Calculate the relative atomic mass of Q

SOLUTION

70 x 63 + 30 x 65 =
100 100
44.2 + 19.5
= 63.70

3. X is an element which exists as an isotopic mixture containing 90% of ³⁹₁₉Xand 10% of ⁴¹₁₉X

a. How many neutrons are present in ⁴¹₁₉X isotope

b. Calculate the mean relative atomic mass of X

Solution

a. Neutrons in ⁴¹₁₉X

= 41-19 = 22

b. R.A.M =

               90 x 39 + 10 x 41 =
  100           100

35.10 + 4.10

= 39.2

CALCULATIONS

1. The following are more examples of calculations of relative atomic masses of elements.

2. An element Y exist in two isotopic forms ³⁹₁₈R and ⁴⁰₁₈R in the ratio 3:2 respectively. What is the relative atomic mass of the element?

SOLUTION

First you add the ratio together, that is, 3+2 =5

R.A.M of Y
= 3 x 39 + 2 x40 =
5 5

0.6 x 39 + 0.4 x 40 =
23.4 + 16 =
= 39.4

3. An element with relative atomic mass 16.2 contains two isotopes ¹⁶₈R with relative abundance 90% and ^m₈R with relative abundance 10%. What is the value of m?

SOLUTION

16.2 = 90 x 16 + 10 x m
100 100
16.2 = 0.9 x 16 + 0.1m
16.2 = 14.4 + 0.1m
16.2 – 14.4 = 0.1m
1.8 = 0.1 m
m = 1.8 = 18
0.1
The value of m is 18

OBJECTIVE QUESTIONS

1. The atomic number of an element is precisely

(a) the number of protons in the atom

(b) the number of electrons in the atom

(d) the number of protons and neutrons in an atom

2. An atom can be defined more accurately as

(a) the smallest indivisible particle of an element that can take part in a chemical reaction

(b) the smallest part of an element that can take part in a chemical reaction

(d) is the simplest unit of an element

3. The mass number is

(a) proton number + neutron number

(b) electron number + proton number

(d) electron number + atomic number

4. Calculate the relative atomic mass of an element having two isotopes ¹⁰⁷Ag and ¹⁰⁹Ag in the ratio 1:1

(a) 106

(b) 107

(d) 109

5. An element X has two isotopes ^18.8X and ^15.8X in the proportion of 1:9 respectively. Find the relative atomic mass of X

(a) 16.1

(b) 13.6

(d) 17.0

THEORY

1. (a) Define the term isotopy.

(b) Determine the number of electrons, protons and neutrons in each of the following:

i.³⁹₁₉K ii. ⁶³₂₆Cu iii. ²³₁₁Na

2. If an element R has isotopes 60% of ¹²₆Qand 40% ^x₆Q and the relative atomic mass is 12.4, find x.

3. Consider the atoms represented below: ^q_rX and ^s_rX

a. State the relationship between the two atoms.

b. What is the difference between them?

c. Give two examples of other elements which exhibit the phenomenon illustrated.

4. State the number of electrons, protons and neutrons present in the following atoms/ions

(a) ₂₀Ca

(b) ₁₆S^2-

(d) ₁₅P

THEORY QUESTIONS

1 a.(i) Define Isotopy

(ii). Element 3315Z and 3113Z occur in the ratio 1:3

Calculate the relative atomic mass of Z
Give a reason why the relative stomach mass of Z is not a whole number

easykemistry

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ISOTOPY at a glance