Quiz 4 of 12

2018 Chemistry WAEC Theory Past Questions

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Question 1

(a) (i) Define the term fermentation

Answer: Fermentation is the conversion of glucose or sugar to ethanol {alcohol} and release of carbon(iv) oxide as a by-product. It can also be defined as the decomposition or breakdown of glucose {starch or carbohydrate} to ethanol {alcohol} and release of carbon (iv) oxide.

(ii) Name the catalyst that can be used for this process

Answer: Yeast/Zymase  

 

(b) Name two factors which determine the choice of an indicator for an acid-base titration

Answer:

 

(c) Consider the following reaction equation:

Fe  +  H2SO4  →   FeSO4  +  H2

Calculate the mass of unreacted iron when 5.0g of iron reacts with 10cm3 of 1.0mol/dm3 H2SO4 [Fe = 56.0]

Solution

1 mole of H2SO4 = 1 mole of Fe

1000cm3 → 56g

10cm3   → x g

= \( \frac{56}{1000}\scriptsize \: \times \: 10\)

= 0.56g

Mass of unreacted Fe = 5 - 0.56g

= 4.44g

 

(d) Name one: (i) Heavy chemical used in electrolytic cells

Answer: Tetraoxosulphate (vi) acid (ii) Fine chemical used in textile industries

Answer: Dies  

 

(e) Explain briefly how a catalyst increases the rate of a chemical reaction.

Answer: A catalyst provides an alternative pathway for the reaction to occur by lowering the activation energy.

 

(f) (i) Write the chemical formula for the product formed when ethanoic acid reacts with ammonia

Answer: CH3COONH4

(ii) Give the name of the product formed in (f)(i)

Answer:  Ammonium ethanoate

 

(g) List three properties of aluminum that makes it suitable for the manufacture of drinks cans

Answer: 

 

(h) State two industrial uses of alkyl alkanoates

Answer: 

 

(i) Name two steps involved in the crystallization of a salt from its solution

Answer: 

 

(j) List two effects of global warming

Answer: 

Question 2

(a)(i) State the collision theory of reaction rates.

Answer: Collision theory states that atoms, molecules, ions, particles, or reactants involved in a chemical reaction must collide before the reaction can take place.

(ii) Using the collision theory, explain briefly how temperature can affect the rate of a chemical reaction.

Answer: When the temperature is increased or decreased, the reacting molecules gain or lose kinetic energy and move faster or slower.  

(b)(i) Sketch a graphical representation of Charles' law.

Answer: 

OR    

(ii) Calculate the volume of oxygen that would be required for the complete combustion of 2.5 moles of ethanol at s.t.p. [molar volume at s.t.p = 22.4 dm3]

Solution:

Given: moles of ethanol = 2.5 ; molar volume at s.t.p = 22.4dm3

Find: volume of oxygen?

C2H5OH(aq)  +  3O2(g) 3H2O(l)  + 2CO2(g)

From the equation, 1 mole of ethanol requires 3 x 22.4dm3 of oxygen

  2.5 moles =  \( \frac{2.5 \: \times \: 3 \: \times \: 22.4}{1} \)

= 168.0dm3

 

(c)(i) Define esterification.

Answer: Esterification is the reaction between an alkanol and alkanol acid to produce an ester {alkanoate} and water in the presence of a mineral acid

(ii) Give two uses of alkanoates.

Answer:

(iii) Give the products of the alkaline hydrolysis of ethyl ethanoate.  

Answer:  Ethanol and sodium athanoate or potassium ethanoate C2H5OH and CH3COONa or CH3COOK

 

(d) A tin-coated plate and a galvanized plate were exposed for the same length of time.

(i) Which of the two plates corrodes faster

Answer: Tin-coated plate corrodes faster

(ii) Explain briefly your answer in 2(d)(i)

Answer:  Zinc is more reactive than tin i.e. higher in electrochemical series than tin. When the tin-coated plate is exposed, the plate goes into the solution {corrodes}.

In Zinc-coated {galvanized} plate, it is the zinc that gets corroded, hence, protecting the plate better prevents the zinc-coated plate from corroding.

Question 3

(a)(i) Draw the structure of the sixth member of the alkenes. (ii) Calculate the relative molecular mass of the sixth member of the alkene.

Solution:

C7H4 = (12 x 7)  +  (1 x 4)

= 84 + 14 = 98

(iii) State one difference between cracking and reforming in the petroleum industry. [H = 1, C = 12]   Answer: 

Cracking Reforming
  • Involves breaking large molecules of petroleum fractions into smaller molecules
Involves rearrangement of atoms in molecules of petroleum fractions to obtain branched and cyclic hydrocarbons
  • Used to increase the quantity of petrol
Used to improve the quality of petrol
  • Can be achieved thermally or catalytically
Occurs in presence of a catalyst

 

(b)(i) Define the term enthalpy of neutralization.

Answer: Enthalpy of neutralization is the heat evolved when an acid reacts with a base to form one mole of water.

(ii) Describe briefly how the enthalpy of neutralization of the reaction of dilute hydrochloric acid and aqueous potassium hydroxide could be determined.  

Answer: Equimolar solutions of hydrochloric acid and potassium hydroxide are prepared separately. A known volume of the acid is placed on a calorimeter and the temperature is recorded.

The same volume of the potassium hydroxide solution at the same temperature is poured into the calorimeter and the mixture is stirred gently. The maximum temperature of the mixture is recorded.

Heat lost in the reaction = heat gain by the solution formed

Heat of neutralization = mass of solution x specific heat capacity of water x change in temperature.

Prepare equimolar solution of HCI and NaOH separately. Measure an equal volume of acid and alkali into two separate beakers. Take the temperature of each of the solutions i.e. t1 and t2

Find the average of the two temperatures

\( \frac{ t_{1} \: + \: t_{2}}{2} \)

Heat lost in the reaction = t4 – t3

∆H neutralization = mass of solution x specific heat capacity of water x change in Temperature

 

(c) An electrochemical cell is constructed with copper and silver electrodes.

(i) State which of the electrodes will be the:

I. anode

Answer: Copper electrode (Anode)

II. cathode

Answer: Silver electrode (Cathode)

(ii) Give the reason for your answer in 3(c)(i).

Answer: Silver has a more positive standard electrode potential than copper. Copper is more electropositive than silver; copper is higher in the electrochemical series.

(iii) State the type of reaction occurring at each electrode.

Answer:

Anode – oxidation {occurs at the copper electrode}

Cathode – reduction {occurs at the silver electrode}

(iv) Write a balanced equation for the overall cell reaction.  

Answer: Cu(s)  + 2A+(aq) → Cu2+(aq)  +  2Ag(s)

 

(d)(i) Name the compound formed when iron is exposed to moist air for a long time.

Answer: Hydrated iron(iii) oxide

(ii) Write a balanced chemical equation for the reaction in 3(d)(i).

Answer:

4Fe(s)  +  3O2(g)  +  2 x H2O(l) → 2Fe2O3 , x H2O

4Fe(g) + 3O2(g)  +  2H2O → 2Fe2O3 . H2O

(iii) Name one ore of iron.

Answer:

Question 4

(a)(i) Draw and label a diagram for the laboratory preparation of a dry sample of sulphur(IV)oxide. (ii) Write a balanced chemical equation for the reaction in (a)(i).

Answer:

Na2SO3(aq)  +  2HCl(aq) → 2NaCl(aq)  +  H2O(l)  +  SO2(g)

OR

K2SO3(aq)  +  2HCl(aq)  → 2KC(aq)  + H2O(l)  +  SO2(g) 

OR

Cu(s)  +  2H2SO4(aq) → CuSO4  +  2H2O(l)  + SO2(l)

 

(iii) State the precaution that must be taken in the preparation of the gas stated in (a)(i).

Answer: It should be prepared in a fume chamber or cupboard

 

(iv) Give a reason why the precaution stated in (a)(ii) must be taken.

Answer: Because it is poisonous 

 

(b)(i) State Dalton's law of partial pressures.

Answer: It states that for a mixture of gases which do not react, the total pressure is equal to the sum of the partial pressures of the individual gases.

(ii)The volume of a sample of methane collected over water at a temperature of 120C and a pressure of 700 mmHg was 30cm3. Calculate the volume of the dry gas at s.t.p.

[Standard vapour pressure of water at 120C is 10mmHg]

Solution:

Given: Methane: Volume = 30cm3 ; temperature = 120C. pressure = 700 mmHg

Saturated Vapour pressure of water at 120C = 10mmHg

Find: Volume of dry gas?

\( \frac{P_1V_1}{T_1} = \frac{P_2V_2}{T_2}\)

P1 = (700 - 10) mmHg = 690 mmHg

T1 = 12 + 273 = 28k

where T2 = 273

V2 = \( \frac{P_1V_1T_2}{P_2T_1} \)

= \( \frac{690 \: \times \: 30 \: 273}{760 \: \times \: 285} \)

= 26.10cm3

 

(c)(i) Write an equation for the reaction between chlorine and water.

Answer: Cl2  +  H2O   HCl  +  HClO

(ii) Why does litmus paper turn red when put in the resulting solution in (c)(i)?  

Answer: H+ ions are released. HCl produced is acidic. HClO produced is acidic.  

 

(d)(i) State the trend in the boiling points of chlorine, bromine, and iodine.

Answer: The boiling point of iodine is higher than that of bromine which itself is higher than that of chlorine in turn. 

Boiling points increase from chlorine to iodine

(ii) Explain briefly why water has a higher boiling point than ammonia.

Answer: Water has a stronger hydrogen bond than ammonia because oxygen has two lone pairs of electrons but nitrogen has just one pair of electrons as an atom, oxygen is more electronegative than nitrogen.

Question 5

(a)(i) State two industrial uses of hydrogen.

Answer: 

(ii) Consider the equation below.

\( \scriptsize Mg(HC0_{3})_{2(aq)} \: \rightarrow \: MgCO_{3(g)} \: + \:  H_2O_{(l)} \: + \: CO_{2(g)} \)

(i) State the type of hardness of water being removed as shown by the above equation.

Answer: Temporary hardness

(ii) Give two disadvantages of the hardness of water.

Answer:

 

(b)(i) In the extraction of aluminum by electrolysis, graphite electrodes are used. State the disadvantages of using this type of electrode.

Answer: At high temperatures, oxygen gas produced at the anode reacts with the graphite electrodes to form carbon (iv) oxide.

(ii) Calcium oxide reacts with water to form slaked line:

I. Write a balanced equation for this reaction;

Answer: CaO + H2O   →  Ca(OH)2

II. State one use of slaked line.

Answer: 

(c)(i) What is meant by saponification?

Answer: Saponification is the process in which fat or oil is boiled with alkali to produce soap {and propane -1, 2, 3 - triol}. It is the alkaline hydrolysis of fats and oils to produce soap.

(ii) List the raw materials needed for the manufacture of soap.

Fat and alkali are the two primary raw materials needed to manufacture soap. Depending on the intended application of the soap, sodium hydroxide or potassium hydroxide is generally used as an alkali.

(iii) Name the main by-product obtained from the manufacture of soap.

Answer: Propane -1, 2, 3 – triol  

 

(d) With the aid of chemical equations explain briefly how iron is extracted in the blast furnace using iron ore, coke, and limestone as raw materials at the: (i) bottom of the furnace

Answer: The O2 in the hot air combines with the carbon {coke} to produce carbon (iv) oxide which reduces to carbon (ii) oxide

C + O2 → CO2

CO2  + C  → 2CO

(ii) middle of the furnace

Answer: The CaCO3 decomposes under great heat to form CO2 and CaO

OR

CaCO3 decomposes to remove the impurities {S1 O2}

CaCO3 → CaO + CO2

CaO  + Si O2 → CaSiO3

(iii) top of the furnace.

Answer: Iron (iii) oxide is reduced to metallic iron {Fe}

Fe2O3  +  3CO →  2Fe  + 3CO2

OR

Fe3SO4  +  4CO → 3Fe  +  4CO2

Question 1

(a) (i) Define the term fermentation

Answer: Fermentation is the conversion of glucose or sugar to ethanol {alcohol} and release of carbon(iv) oxide as a by-product. It can also be defined as the decomposition or breakdown of glucose {starch or carbohydrate} to ethanol {alcohol} and release of carbon (iv) oxide.

(ii) Name the catalyst that can be used for this process

Answer: Yeast/Zymase  

 

(b) Name two factors which determine the choice of an indicator for an acid-base titration

Answer:

 

(c) Consider the following reaction equation:

Fe  +  H2SO4  →   FeSO4  +  H2

Calculate the mass of unreacted iron when 5.0g of iron reacts with 10cm3 of 1.0mol/dm3 H2SO4 [Fe = 56.0]

Solution

1 mole of H2SO4 = 1 mole of Fe

1000cm3 → 56g

10cm3   → x g

= \( \frac{56}{1000}\scriptsize \: \times \: 10\)

= 0.56g

Mass of unreacted Fe = 5 - 0.56g

= 4.44g

 

(d) Name one: (i) Heavy chemical used in electrolytic cells

Answer: Tetraoxosulphate (vi) acid (ii) Fine chemical used in textile industries

Answer: Dies  

 

(e) Explain briefly how a catalyst increases the rate of a chemical reaction.

Answer: A catalyst provides an alternative pathway for the reaction to occur by lowering the activation energy.

 

(f) (i) Write the chemical formula for the product formed when ethanoic acid reacts with ammonia

Answer: CH3COONH4

(ii) Give the name of the product formed in (f)(i)

Answer:  Ammonium ethanoate

 

(g) List three properties of aluminum that makes it suitable for the manufacture of drinks cans

Answer: 

 

(h) State two industrial uses of alkyl alkanoates

Answer: 

 

(i) Name two steps involved in the crystallization of a salt from its solution

Answer: 

 

(j) List two effects of global warming

Answer: 

Question 2

(a)(i) State the collision theory of reaction rates.

Answer: Collision theory states that atoms, molecules, ions, particles, or reactants involved in a chemical reaction must collide before the reaction can take place.

(ii) Using the collision theory, explain briefly how temperature can affect the rate of a chemical reaction.

Answer: When the temperature is increased or decreased, the reacting molecules gain or lose kinetic energy and move faster or slower.  

(b)(i) Sketch a graphical representation of Charles' law.

Answer: 

OR    

(ii) Calculate the volume of oxygen that would be required for the complete combustion of 2.5 moles of ethanol at s.t.p. [molar volume at s.t.p = 22.4 dm3]

Solution:

Given: moles of ethanol = 2.5 ; molar volume at s.t.p = 22.4dm3

Find: volume of oxygen?

C2H5OH(aq)  +  3O2(g) 3H2O(l)  + 2CO2(g)

From the equation, 1 mole of ethanol requires 3 x 22.4dm3 of oxygen

  2.5 moles =  \( \frac{2.5 \: \times \: 3 \: \times \: 22.4}{1} \)

= 168.0dm3

 

(c)(i) Define esterification.

Answer: Esterification is the reaction between an alkanol and alkanol acid to produce an ester {alkanoate} and water in the presence of a mineral acid

(ii) Give two uses of alkanoates.

Answer:

(iii) Give the products of the alkaline hydrolysis of ethyl ethanoate.  

Answer:  Ethanol and sodium athanoate or potassium ethanoate C2H5OH and CH3COONa or CH3COOK

 

(d) A tin-coated plate and a galvanized plate were exposed for the same length of time.

(i) Which of the two plates corrodes faster

Answer: Tin-coated plate corrodes faster

(ii) Explain briefly your answer in 2(d)(i)

Answer:  Zinc is more reactive than tin i.e. higher in electrochemical series than tin. When the tin-coated plate is exposed, the plate goes into the solution {corrodes}.

In Zinc-coated {galvanized} plate, it is the zinc that gets corroded, hence, protecting the plate better prevents the zinc-coated plate from corroding.

Question 3

(a)(i) Draw the structure of the sixth member of the alkenes. (ii) Calculate the relative molecular mass of the sixth member of the alkene.

Solution:

C7H4 = (12 x 7)  +  (1 x 4)

= 84 + 14 = 98

(iii) State one difference between cracking and reforming in the petroleum industry. [H = 1, C = 12]   Answer: 

Cracking Reforming
  • Involves breaking large molecules of petroleum fractions into smaller molecules
Involves rearrangement of atoms in molecules of petroleum fractions to obtain branched and cyclic hydrocarbons
  • Used to increase the quantity of petrol
Used to improve the quality of petrol
  • Can be achieved thermally or catalytically
Occurs in presence of a catalyst

 

(b)(i) Define the term enthalpy of neutralization.

Answer: Enthalpy of neutralization is the heat evolved when an acid reacts with a base to form one mole of water.

(ii) Describe briefly how the enthalpy of neutralization of the reaction of dilute hydrochloric acid and aqueous potassium hydroxide could be determined.  

Answer: Equimolar solutions of hydrochloric acid and potassium hydroxide are prepared separately. A known volume of the acid is placed on a calorimeter and the temperature is recorded.

The same volume of the potassium hydroxide solution at the same temperature is poured into the calorimeter and the mixture is stirred gently. The maximum temperature of the mixture is recorded.

Heat lost in the reaction = heat gain by the solution formed

Heat of neutralization = mass of solution x specific heat capacity of water x change in temperature.

Prepare equimolar solution of HCI and NaOH separately. Measure an equal volume of acid and alkali into two separate beakers. Take the temperature of each of the solutions i.e. t1 and t2

Find the average of the two temperatures

\( \frac{ t_{1} \: + \: t_{2}}{2} \)

Heat lost in the reaction = t4 – t3

∆H neutralization = mass of solution x specific heat capacity of water x change in Temperature

 

(c) An electrochemical cell is constructed with copper and silver electrodes.

(i) State which of the electrodes will be the:

I. anode

Answer: Copper electrode (Anode)

II. cathode

Answer: Silver electrode (Cathode)

(ii) Give the reason for your answer in 3(c)(i).

Answer: Silver has a more positive standard electrode potential than copper. Copper is more electropositive than silver; copper is higher in the electrochemical series.

(iii) State the type of reaction occurring at each electrode.

Answer:

Anode – oxidation {occurs at the copper electrode}

Cathode – reduction {occurs at the silver electrode}

(iv) Write a balanced equation for the overall cell reaction.  

Answer: Cu(s)  + 2A+(aq) → Cu2+(aq)  +  2Ag(s)

 

(d)(i) Name the compound formed when iron is exposed to moist air for a long time.

Answer: Hydrated iron(iii) oxide

(ii) Write a balanced chemical equation for the reaction in 3(d)(i).

Answer:

4Fe(s)  +  3O2(g)  +  2 x H2O(l) → 2Fe2O3 , x H2O

4Fe(g) + 3O2(g)  +  2H2O → 2Fe2O3 . H2O

(iii) Name one ore of iron.

Answer:

Question 4

(a)(i) Draw and label a diagram for the laboratory preparation of a dry sample of sulphur(IV)oxide. (ii) Write a balanced chemical equation for the reaction in (a)(i).

Answer:

Na2SO3(aq)  +  2HCl(aq) → 2NaCl(aq)  +  H2O(l)  +  SO2(g)

OR

K2SO3(aq)  +  2HCl(aq)  → 2KC(aq)  + H2O(l)  +  SO2(g) 

OR

Cu(s)  +  2H2SO4(aq) → CuSO4  +  2H2O(l)  + SO2(l)

 

(iii) State the precaution that must be taken in the preparation of the gas stated in (a)(i).

Answer: It should be prepared in a fume chamber or cupboard

 

(iv) Give a reason why the precaution stated in (a)(ii) must be taken.

Answer: Because it is poisonous 

 

(b)(i) State Dalton's law of partial pressures.

Answer: It states that for a mixture of gases which do not react, the total pressure is equal to the sum of the partial pressures of the individual gases.

(ii)The volume of a sample of methane collected over water at a temperature of 120C and a pressure of 700 mmHg was 30cm3. Calculate the volume of the dry gas at s.t.p.

[Standard vapour pressure of water at 120C is 10mmHg]

Solution:

Given: Methane: Volume = 30cm3 ; temperature = 120C. pressure = 700 mmHg

Saturated Vapour pressure of water at 120C = 10mmHg

Find: Volume of dry gas?

\( \frac{P_1V_1}{T_1} = \frac{P_2V_2}{T_2}\)

P1 = (700 - 10) mmHg = 690 mmHg

T1 = 12 + 273 = 28k

where T2 = 273

V2 = \( \frac{P_1V_1T_2}{P_2T_1} \)

= \( \frac{690 \: \times \: 30 \: 273}{760 \: \times \: 285} \)

= 26.10cm3

 

(c)(i) Write an equation for the reaction between chlorine and water.

Answer: Cl2  +  H2O   HCl  +  HClO

(ii) Why does litmus paper turn red when put in the resulting solution in (c)(i)?  

Answer: H+ ions are released. HCl produced is acidic. HClO produced is acidic.  

 

(d)(i) State the trend in the boiling points of chlorine, bromine, and iodine.

Answer: The boiling point of iodine is higher than that of bromine which itself is higher than that of chlorine in turn. 

Boiling points increase from chlorine to iodine

(ii) Explain briefly why water has a higher boiling point than ammonia.

Answer: Water has a stronger hydrogen bond than ammonia because oxygen has two lone pairs of electrons but nitrogen has just one pair of electrons as an atom, oxygen is more electronegative than nitrogen.

Question 5

(a)(i) State two industrial uses of hydrogen.

Answer: 

(ii) Consider the equation below.

\( \scriptsize Mg(HC0_{3})_{2(aq)} \: \rightarrow \: MgCO_{3(g)} \: + \:  H_2O_{(l)} \: + \: CO_{2(g)} \)

(i) State the type of hardness of water being removed as shown by the above equation.

Answer: Temporary hardness

(ii) Give two disadvantages of the hardness of water.

Answer:

 

(b)(i) In the extraction of aluminum by electrolysis, graphite electrodes are used. State the disadvantages of using this type of electrode.

Answer: At high temperatures, oxygen gas produced at the anode reacts with the graphite electrodes to form carbon (iv) oxide.

(ii) Calcium oxide reacts with water to form slaked line:

I. Write a balanced equation for this reaction;

Answer: CaO + H2O   →  Ca(OH)2

II. State one use of slaked line.

Answer: 

(c)(i) What is meant by saponification?

Answer: Saponification is the process in which fat or oil is boiled with alkali to produce soap {and propane -1, 2, 3 - triol}. It is the alkaline hydrolysis of fats and oils to produce soap.

(ii) List the raw materials needed for the manufacture of soap.

Fat and alkali are the two primary raw materials needed to manufacture soap. Depending on the intended application of the soap, sodium hydroxide or potassium hydroxide is generally used as an alkali.

(iii) Name the main by-product obtained from the manufacture of soap.

Answer: Propane -1, 2, 3 – triol  

 

(d) With the aid of chemical equations explain briefly how iron is extracted in the blast furnace using iron ore, coke, and limestone as raw materials at the: (i) bottom of the furnace

Answer: The O2 in the hot air combines with the carbon {coke} to produce carbon (iv) oxide which reduces to carbon (ii) oxide

C + O2 → CO2

CO2  + C  → 2CO

(ii) middle of the furnace

Answer: The CaCO3 decomposes under great heat to form CO2 and CaO

OR

CaCO3 decomposes to remove the impurities {S1 O2}

CaCO3 → CaO + CO2

CaO  + Si O2 → CaSiO3

(iii) top of the furnace.

Answer: Iron (iii) oxide is reduced to metallic iron {Fe}

Fe2O3  +  3CO →  2Fe  + 3CO2

OR

Fe3SO4  +  4CO → 3Fe  +  4CO2