WAEC Chemistry 2018/2019 Specimen & Practicals
Read below for WAEC Chemistry Specimen and Practicals 2018/19
Question 1
1. (a) (i) Define ionic bond
(ii) What type of bond(s) exist(s) in:
I. magnesium oxide;
II.ammonium ion? [4 marks]
(b) Determine the oxidation number of sulphur in Na2S2O3. [3 marks]
(c) State Faraday’s first law. [2 marks]
(d) Give one example each of:
(i) acid salt
(ii) base salt. [2 marks]
(e) Name the type of energy change that occurs in each of the following processes;
(i) I2(s) ---------> I2(g);
(ii) C1(g) + e- ------> C1-(g). [2 marks]
(f) State the effect of each of the following aqueous solutions on litmus paper:
(i) Na2SO4(aq);
(ii) AlC13(aq) [2 marks]
(g) Define the term efflorescence [2 marks]
(h) Give two uses of activated charcoal [2 marks]
(i) State one use of each of the following processes in the chemical industry:
(i) hydrogenation of vegetable oil;
(ii) cracking;
(iii) esterification. [3 marks]
(j) Calculate the amount of silver deposited in moles when 10920
coulombs of electricity is passed through a solution of a silver salt.
[Faraday constant = 96500 C mol-1] [3 marks]
OBSERVATION: This was a compulsory question. Majority of the candidates that answered
this question did fairly.
In part (a), majority of the candidates could not define ionic bond correctly.
Most of the candidates stated the type of bond existing in magnesium oxide as ionic/electrovalent but failed to state the bond in ammonium ion as covalent.
In part (b), some candidates were not able to determine the oxidation number of sulphur in Na2S2O3. Many however did but carelessly omitted the positive sign required for oxidation states.
In part (c), majority of the candidates defined Faraday’s first law of electrolysis however, some candidates could not differentiate between quantity of electricity and current thereby made mistake in their definitions.
In part (d), majority of the candidates gave examples of acid salt but many candidates could not give example of basic salt.
In part (e), majority of the candidates could not identify the type of energy change that occurred.
In part (f), majority of the candidates stated correctly the effect of Na2SO4(aq) on litmus paper but many failed to realize the hydrolyzing effect of water of AlCl3 to produce an acidic solution and therefore stated that it is neutral to litmus thereby loosing the marks allocated to that question.
In part (g), some candidates gave the correct definition of efflorescent but most of the candidates gave the definition for efflorescent substance instead.
In part (h), some of the candidates could not give uses of activated charcoal. However, very few candidates were able to give the correct uses of activated charcoal.
In part (i), majority of the candidates were able to give one use for each of the stated processes.
In part (j), majority of the candidates were not able to calculate the amount in moles of silver deposited by passing 10920C or electricity through a solution of silver salt. The expected answers include:
(a) (i) Ionic bond is the electrostatic force of attraction between
oppositely charged ions / cations and anions.
OR
Ionic bond is defined as the type of bond that is formed
from / by / through the transfer of electrons from one
atom of an element (metal) to an atom of another (non metal).
(ii) (I) - ionic / electrovalent
(II) - coordinate (covalent) / dative covalent
(b) (i) Na2S2O3
2 x Na + 2 x S + 3 x O = 0
2 x 1 + 2S + 3 (-2) = 0
2 + 2S – 6 = 0
2S = 4
S = +2 (c) Faraday’s first law states that the mass of an element discharged during electrolysis is directly proportional to the quantity of electricity passed
Accept equation Q M where Q is quantity of electricity passed
and M is mass deposited.
(d) (i) NaHSO4, NaHCO3, KHSO3 etc
(ii) Mg(OH)Cl, Zn(OH)Cl, Zn(OH)NO3 etc
(e) (i) Sublimation energy
(ii) Electron affinity
(f)(i) Na2SO4(aq) No effect on litmus paper/neutral to litmus
(ii) AlCl3(aq) Turns blue litmus paper red/acidic to litmus
Question 2
(a) (i) Define in terms of electron transfer
I. oxidizing agent;
II. reducing agent.
(ii) Write a balanced equation to show that carbon is a reducing agent.
(iii) State the change in oxidation number of the specie that reacted with carbon in 2 (a)(ii). [5 marks]
(b) A gas X has a vapour density of 32. It reacts with sodium hydroxide solution to form salt and water only. It decolourizes acidified potassium tetraoxomanganate (VII) solution and reacts with H2S to form sulphur. Using the information provided:
Identify gas X;
state two properties exhibited by X;
give two uses of X. [5 marks]
(c) Consider the following substances:
(i) sodium;
(ii) lead (II) iodide;
(iii) hydrogen;
(iv) magnesium;
(v) oxygen.
Which of the substances
(i) conducts electricity?
(ii) is produced at the cathode during electrolysis of H2SO4(aq)?
(iii) corresponds to the molecular formula AB2 ?
(iv) is an alkaline earth metal? [5 marks]
(d) (i) Define the term salt.
(ii) Mention two types of salt.
(iii) Give an example of each of the salts mentioned in 2(d)(ii) above. [6 marks]
(e) In a neutralization reaction, dilute tetraoxosulphate (VI) acid completely reacted with sodium hydroxide solution.
(i) Write a balanced equation for the reaction.
(ii) How many moles of sodium hydroxide would be required for the complete neutralization of 0.50 moles of tetraoxosulphate (VI) acid? [4 marks]
OBSERVATION: This question was attempted by majority of the candidates and the performance was fair.
In (a)(i), candidates correctly defined oxidizing agent and reducing agent in terms of electron transfer as follows:
(i) Oxidizing agent is a substance which accepts electrons/is an electron acceptor.
(ii) Reducing agent is a substance which donates electrons/is an electron donor.
In (a)(ii) and (iii),most of the candidates could neither write a balanced equation to show that carbon is a reducing agent nor state the change in oxidation number of the specie that reacted with carbon. The expected answers from candidates were:
(ii) 2CuO(s) + C(s) → 2Cu(s) + CO2(g)
H2O(g) + C(s) → 2CO(g) + H2(g)
CO2 (g) + C(s) → 2CO(g)
(iii) Cu in CuO from +2 to O
H in H2O from +1 to O
C in CO2 from + 4 to +2
In (b), candidates correctly identified gas X, stated two properties exhibited by X and gave two uses of X as follows:
(i) X is sulphur (IV) oxide/sulphur dioxide
(ii) - heavier than air
- Acidic
- Reducing/oxidizing agent
- Colourless (poisonous) gas with irritating smell
(iii) Used for bleaching
For the manufacture of H2SO4
- As germicide and fumigant
- For preservation
- As refrigerant e.t.c.
In (c)(i) – (iv), most candidates were able to correctly give their responses as follows:
(i) sodium/magnesium
(ii) hydrogen
(iii) lead (II) iodide
(iv) magnesium
In (d)(i), candidates correctly defined salt as a compound formed when all or parts of the hydrogen of an acid is replaced by metal or ammonium ion.
They were also able to correctly mention two types of salt with corresponding example in (d)(ii) and (iii) as follows:
(ii) Normal salt
Acid salt
Basic salt
Double salt
Complex salt
(iii) NaCl/ZnSO4/ KHSO4/ NaH2PO4/ Zn(OH)Cl/Mg(OH)NO3/NH4Fe(SO4)2. 6H2O
In (e)(i), candidates correctly wrote a balanced equation for the reaction between dilute tetraoxosulphate (VI) acid with sodium hydroxide solution as follows:
H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O (1)
In (e)(ii), candidates correctly determined the number of moles of sodium hydroxide that would be required for the complete neutralization of the given tetraoxosulphate (VI) acid thus:
From the reaction 1 mole of H2SO4 ≡ 2 moles of NaOH
∴ 0.50 moles of H2SO4 ≡ 2 x 0.5 of NaOH
1
= 1 mole of NaOH
Question 3
(a) Consider the following reaction sequence.
(i) What process leads to the formation of K?
(ii) Write the formula of K.
(iii) Write the structural formula of L and name L.
(iv) Name An.
(v) Write the structure of M and name M. [7 marks]
(b) (i) What are carbohydrates?
(ii) Give one example each of a
> Monosaccharide;
> Disaccharide;
> Polysaccharide. [5 marks]
(c) Consider the following structure of a simple sugar.
H
|
C = O
|
HO - C - OH
|
HO - C - H
|
H - C - OH
|
H - C - OH
|
CH2OH
(i) Which functional group makes the compound a reducing agent?
(ii) State what would be observed when
(I) the compound is mixed with Fehling’s solution and boiled;
(II) few drops of concentrated H2SO4 is added to the sample of the compound.
(iii) Write an equation for the reaction in 3(c)(ii)(II). [6marks]
(d) A hydrocarbon Z with molecular mass 78 on combustion gave 3.385 g of CO2 and 0.692 g of H2O. Determine the molecular formula of Z.
[H = 1, C = 12, O = 16] [7 marks]
OBSERVATION: This question was attempted by few candidates and the performance was good.
In (a), candidates correctly responded to the questions thus:
(i) Dehydration
(ii) C2H4
(iii) H H
| | ethane – 1,2-diol/ethylene glycol
H – C – C – H
| |
OH OH
(iii) Polyethene/polythene.
In (b)(i), candidates correctly stated that carbohydrates are (naturally occurring) organic compounds containing carbon, hydrogen and oxygen with the hydrogen and oxygen present in the ratio of 2:1 (as in water).
In (b)(ii), they correctly gave example of each of monosaccharide, disaccharide and polysaccharide thus:
I. Monosaccharide: glucose/fructose
II. Disaccharide: sucrose/lactose/maltose
III. Polysaccharide: starch/glycogen/cellulose
H
|
In (c)(i), candidates gave C = O /CHO as the functional group that makes the compound a reducing sugar.
In (c)(ii), candidates correctly stated what would be observed in I and II thus:
Brick red precipitate
Black/charred mass (of carbon is observed)
In (c)(iii), most candidates correctly wrote an equation for the reaction in 3(c)(ii) (II) as follows:
C6H12O6 → 6C + 6H2O
In (d), majority of the candidates correctly determined the molecular formula of Z thus:
mass of hydrogen in Z = 2 x 0.692 = 0.077g
18
mass of carbon in Z = 12 x 3.385 = 0.923g
44
C H
0.923 0.077
12 1
0.077 0.077
0.077 0.077
0.077 0.077
1 1
Empirical formula of Z = CH.
(CH)n = 78
13n = 78
n = 78 = 6
13
Molecular formula of Z = C6H6
Question 4
(a) (i) Define covalent bond.
(ii) Give two properties of covalent compounds
(iii) With the aid of a diagram, show how ammonia molecule is formed.
(iv) Illustrate with a diagram the formation of ammonium ion.
(v) What type of bond(s) exist(s) in
Ammonia,
Ammonium ion? [ 1H, 7 N] [10 marks]
(b) (i) Write three subatomic particles with their corresponding relative masses.
(ii) Name the possible states in which water can exist. [9 marks]
(c) (i) State Graham’s law of diffusion.
(ii) Arrange the following gases, He, CH4 and N2 in order of increasing rates of diffusion. Give a reason for the order.
[ H = 1, He = 4, C = 12, N = 14 ] [4 marks]
(d) Draw the structures of the following compounds:
(i) 2, 3-dimethylbutane;
(ii) 1, 4-dibromocyclohexane. [2 marks]
OBSERVATION: The question was attempted by majority of the candidates and the performance was fair.
In (a)(i), most candidates correctly defined covalent bond however, some of them lost marks because the definition given was not complete. The expected response from candidates was that covalent bond is a bond between two atoms in which each of the atoms contributes to the shared pair of electrons.
In (a) (ii), candidates correctly gave two properties of covalent compounds from the following:
Non- conductors of electricity.
Insoluble in water/soluble in non-polar solvents.
Have low melting/boiling point.
In (a)(iii), most candidates could not correctly show with the aid of a diagram how ammonia molecule is formed. The required answer to the question was
The candidates were also unable to correctly illustrate the formation of ammonium ion with a diagram in (a)(iv).
The expected diagram was as follows:
In (a)(v), candidates correctly stated that the type of bond which exists in each of ammonia and ammonium ion were covalent bond and covalent bond/dative or coordinate covalent bond respectively.
In (b)(i), candidates could not correctly write the three subatomic particles with their corresponding relative masses as they did not understand the demand of the question.
The expected answers were as shown in the table below.
PARTICLE RELATIVE MASSES
Proton 1
Neutron 1
Electron 1
1840
In (b)(ii), candidates correctly named the possible states in which water can exist as, solid, liquid and gaseous.
In (c)(i), most candidates correctly stated Graham’s law of diffusion however, some of them lost marks as they left the conditions under which the law holds while others wrote density instead of vapour density. The expected response from candidates was at constant temperature and pressure, the rate of diffusion of a gas is inversely proportional to the square root of its vapour density/molecular mass.
In (c)(ii) only few candidates correctly arranged the gases in increasing order of rates of diffusion with a reason as follows: N2, CH4, He: the smaller the molar mass, the faster the rate at which the gas diffuses or the larger the molar mass, the slower the rate at which gas diffuses.
In (d), most candidates could not draw correctly the structures of the organic compounds. The expected structures from candidate were as drawn below.
H
|
H - C - H
H - H | H
| | | |
H -C - C - C - C - H
| | | |
H | H H
H – C – H
|
H
2,3 -dimethylbutane 1,4-dibromocyclohexane
Question 5
(a) (i) What are acidic oxides?
(ii) Give one example of each of the following oxides:
I. acidic oxide;
II. basic oxide;
(III) amphoteric oxide;
(III) neutral oxide. [5 marks]
(b) (i) Define each of the following terms:
(I) Heat of combustion;
(II) Heat of neutralization.
(ii) Write an equation to illustrate each of the terms in 7(b)(i) above.
(iii) Given that the standard heat of combustion of butane (C4H10) is + 5877 kJmol-1, calculate the heat of combustion of 14.5 g butane.
[H = 1, C = 12] [10 marks]
(c) (i) Name two allotropes of sulphur.
(ii) State one difference between the two allotropes. [3 marks]
(d) (i) Give two characteristics of noble gases.
(ii) State one use each of
(I) He;
(II) Ar. [4 marks]
(e) State what is observed on warming ammonium trioxonitrate (V) with sodiumhydroxide. [2 marks]
OBSERVATION: This question was not popular with the candidates but the performance of those who attempted it was fair.
In (a)(i), most candidates gave an incomplete definition of acidic oxide by stating that they are oxides of non metals without stating that they dissolve in water to form acidic solutions hence lost the marks. However, they correctly gave one example each of acidic oxide, basic oxide, amphoteric oxide and neutral oxide as follows:
- Acidic oxides: SO2/CO2/ NO2, etc.
- Basic oxides: CaO/Na2O/K2O, etc.
- Amphoteric oxides: Al2O3/ZnO/PbO, etc.
- Neutral oxides: CO/N2O, etc.
In (b)(i), most candidates correctly defined each term thus:
(I) Heat of combustion is the heat change when one mole of a substance is completely burnt in oxygen/burnt in excess oxygen.
(II) Heat of neutralization is the heat change when one mole of water is produced as a result of a reaction between an acid and alkali in dilute solution/the heat change when one mole of H+ from an acid reacts with one mole of OH- from an alkali to form one mole of H2O.
In (b)(ii) majority of the candidates could not write an equation to illustrate each of the terms hence lost the marks. The correct equation were as follows:
I. C(s) + O2(g) → CO2 (g) ∆HcÓ¨
II. NaOH (aq) + HCI(aq) → NaCI(aq) + H2O(1) ∆HnÓ¨
In (b)(iii) candidates correctly calculated the standard heat of combustion of butane thus:
Molar mass of = 12 x 4 + 1 x 10 = 48 + 10 = 58gmol-1
58 g of C4H10 5877kJmol-1
∴ 14.5 of C4H10 = 5877 x 14.5 kJmol-1
58
= + 1469.25 kJ
In (c)(i) and (ii) candidates correctly named two allotropes of sulphur and stated one difference between the two allotropes.
(1) Rhombic
Monoclinic
(II)
Rhombic
Monoclinic
Yellow translucent crystal
Transparent amber crystal
Stable at temperature below 900
Unstable at temperature below 900
Higher density than monoclinic
Lower density than rhombic
Lower melting point
Higher melting point
Octahedral shaped
Needle shaped.
In (d)(i), candidates correctly gave two characteristics of noble gases from the following:
- They are monoatomic gases
- They are chemically inert or unreactive
- Have low melting/boiling points.
However in (d)(ii), candidates could not correctly give one use of each of Helium and argon.
The expected answers from candidates were
He – filling balloons
Ar – arc welding
In (e), most candidates correctly stated that ammonia gas was given off on warming ammonium trioxonitrate (V) with sodium hydroxide.
Question 6
(a) (i) Define hard water.
(ii) Name two substances responsible for hardness in water.
(iii) Give two methods for the removal of hardness in water. [6 marks]
(b) (i) What are the raw materials required for the manufacture of
tetraoxosulphate (VI) acid by the contact process?
(ii) Write an equation for the reaction that requires a catalyst in the contact process.
(iii) State the catalyst used in 8(b)(ii). [5 marks]
(c) (i) Give two uses of sodium tetraoxosulphate (VI)
Consider the reaction represented by the following equation:
exposure to air
Na2SO4 (aq) IOH2O (s) → Na2SO4 H2O + 9H2O (g)
(ii) What name is given to this type of reaction?
(iii) Calculate the solubility of Na2CO3 at 25oC, if 30.0cm3 of its saturated
solution at that temperature gave 1.80 g of the anhydrous salt.
[ C = 12, O = 16, Na = 23 ] [7 marks]
(d) (i) Define the term activated complex.
(ii) State one reason why a collision may not produce a chemical reaction.
(iii) The formation of water gas is represented by the following equation.
C(s) + H2 O(g) → CO (g) H2 (g) ΔH = + 131 kJmol-1
Draw an energy profile diagram for the reaction showing the
I. activated complex
II. enthalpy of reactants. [7 marks]
OBSERVATION: The question was attempted by most candidates and the performance was good.
In (a)(i), candidates correctly defined hard water as water which will not readily form lather with soap.
In (a)(ii) candidates correctly named two substances responsible for hardness in water from among the following:
Calcium tetraoxosulphate (VI)
Calcium hydrogentetraoxosulphate (IV)
Magnesium tetraoxosulphate (VI)
Magnesium hydrogentetraoxosulphate (IV)
In (a) (iii), candidates gave two methods for the removal of hardness in water from among the following:
- Heating the water to boiling/distillation
- Addition of calculated amount of calcium hydroxide [Ca(OH) 2]
- Addition of washing soda crystals to the water [Na2CO3.10H2O]
- ion exchange resin/permutit method.
In (b)(i), candidates correctly gave the raw materials for the manufacture of tetraoxosulphate (VI) acid by the contact process as sulphur and air/oxygen.
In (b)(ii) and (iii), candidates correctly wrote an equation for the reaction that requires a catalyst in the contact process and stated the catalyst used.
The equation is
2SO2 + O2 » 2SO3(g)
and the catalyst employed is V2O5/ vanadium (V) oxide.
In (c)(i) most candidates were able to correctly give two uses of sodium tetraoxosulphate (VI) from among the following:
- Water softener
- In glass/paper manufacture
- As analytical reagent
- As purgative
- In the manufacture of detergents
In (c)(ii) most candidates correctly gave the name of the type of reaction that was represented by the given equation.
In (c)(iii) candidates correctly calculated the solubility of Na2SO3 at 25°C.
In (d)(i), candidates did not know that activated complex is the intermediate (complex) that reactants must attain in order to form a product or a temporary species formed by reactant molecules as a result collision before the products are formed, instead they gave the definition of activation energy.
In (d)(ii), most candidates could not state one reason why collision may not produce a chemical reaction. They did not know that if the energy of the colliding particles is less than the activation energy or the reactants are not properly aligned, chemical reaction may not occur inspite of collision.
In (d)(iii), candidates drew the energy profile diagram for the endothermic reaction but most of them lost marks because they indicated the enthalpy of reaction instead of the enthalpy of reactants. The correct energy profile diagram is as drawn below.