Practice exam-style questions

© Oxford University Press 2020 1 Chemistry for Queensland Units 3 & 4 Teacher obook assess ISBN 9780190313487 Permission has been granted for this page to be photocopied within the purchasing institution only.

Student book questions

Unit 3 Equilibrium, acids and redox reactions

Pages 212–215

Practice exam-style questions

Multiple choice

1 Which of the following is a redox reaction?

A 2Al(s) + 3Cl2(g) 2AlCl3(s)

B Pb2+(aq) + O2–(aq) PbO(s)

C KOH(aq) + NaNO3(aq) NaOH(aq) + KNO3(aq)

D CH3CH2OH(aq) + HCOOH(aq) CH3COOCH3(aq) + H2O(l)

The following reaction relates to questions 2 and 3.

2NOCl(g) ⇌ 2NO(g) + Cl2(g)

H = +75.5 kJ mol-1

2 What is the equilibrium expression for the reaction?

A

22 NO + Cl

2 NOCl

B

2

2

2

NO + Cl

NOCl

C

2

2

2

NO Cl

NOCl

D

22 NO + Cl

2 NOCl


3 Consider the following equilibrium graph. What change was made to the system at time t?

A The temperature of the system was increased.

B Additional NO was added to the system.

C A catalyst was added.

D The volume of the system was halved.

4 The concentration of ethanoic acid (pKa 4.75) in vinegar is approximately 1.0 M. What is the pH of vinegar?

A 4.8

B 2.4

C 0.004

D 5.4


5 Lactic acid (pKa 3.88), oxalic acid (pKa 1.23) and malic acid (pKa 3.40) are all weak acids found in food products. The order of strongest to weakest acid is:

A lactic acid oxalic acid malic acid

B lactic acid malic acid oxalic acid

C oxalic acid malic acid lactic acid

D oxalic acid lactic acid malic acid

6 In a galvanic cell, what is the most important purpose of the salt bridge?

A To transfer electrons between cells and therefore carry the electricity that is generated by the cell

B To connect the two half-cells to ensure that the cell is a complete circuit

C To control the flow of current in the cell so that the cell does not produce excessive amounts of energy

D To transfer ions that carry charge between cells in order to balance the build-up of positive and negative charges in half-cells

7 Consider the cell diagram and determine which option A–D is correct.


Oxidation Reduction E° (V)

A H2O2 O2 + 2H+ + 2e– H2O2 + 2H+ + 2e– 2H2O 1.09

B O2 + 2H+ + 2e– H2O2 2H2O H2O2 + 2H+ + 2e– 1.09

C H2O2 + 2H+ + 2e– 2H2O H2O2 O2 + 2H+ + 2e– 2.45

D 2H2O H2O2 + 2H+ + 2e– O2 + 2H+ + 2e– H2O2 2.45

8 The lead acid battery in a car is a rechargeable battery and therefore operates as both a galvanic and electrolytic cell. The half-equations occurring within the battery are:

PbSO 4(s) + 2e – ⇌ Pb(s)+SO 4 2–(aq) E° = –0.36 V

PbO 2(s)+SO 4 2–(aq)+4H +(aq)+2e – ⇌ PbSO 4(s)+2H 2O(l) E° =1.69 V

The half-equation that occurs at the anode during recharge is:

A PbSO4(s) + 2e– Pb(s) + SO42–(aq)

B Pb(s) + SO42–(aq) PbSO4(s) + 2e–

C PbO2(s) + SO42–(aq) + 4H+(aq) + 2e– PbSO4(s) + 2H2O(l)

D PbSO4(s) + 2H2O(l) PbO2(s) + SO42–(aq) + 4H+(aq) + 2e–


Short answer

9 Sulfur dioxide gas and chlorine gas were mixed at a particular temperature to produce an equilibrium mixture:

SO 2(g) + Cl 2(g) ⇌ SO 2Cl 2(g) ΔH = –67kJ mol –1

For a particular experiment, the concentrations of the three substances are plotted against time in the following graph.

a Construct an expression for the equilibrium constant, K, of the reaction.

b Determine at which time intervals the reaction was at equilibrium.

c Calculate the value of K at the 2-minute and 9-minute marks.

d Identify one possible reason why the values of K calculated in part c are different.


e Determine what change was made to the system at:

i 3 minutes

ii 7 minutes

iii 10 minutes.

10 Chlorine gas is generated in an equilibrium reaction between hydrochloric acid and oxygen:

4HCl(g)+O 2(g) ⇌ 2H 2O(g)+2Cl 2(g)

ΔH = –128kJ mol –1

a Construct an equilibrium expression for the reaction if its equilibrium constant has a value of 2.46.

b Initially, 20 moles of HCl and 6 moles of O2 are injected into a 2 L vessel. At equilibrium, the concentration of O2 was measured to be 1 M. Calculate Qc of the reaction at 30 seconds.


c Determine what you can conclude about the equilibrium position of the reaction mixture based on the Qc value that you have calculated.

d Use your answer to part b to infer the direction the reaction would need to move in order to reach equilibrium. Explain your answer.

e Using Le Châtelier’s principle, explain the effect of increasing the temperature of the system.

f Using Le Châtelier’s principle, explain the effect of decreasing the volume of the system.

11 Blood contains a natural carbonic acid/ bicarbonate buffer. Carbon dioxide is absorbed into the blood. A simplified representation of this process is provided in the equilibrium equation:

CO 2(g)+2H 2O(l) ⇌ H 3O +(aq)+HCO 3–(aq)

FIGURE 1 Carbon dioxide gets absorbed in the blood.


a Explain how a buffer can stabilise the pH of a solution.

b When blood is oxygenated, the concentration of CO2 is approximately 1.3 × 10–5 M and pH is 7.4.

i Calculate the concentration of H3O+.

ii Calculate Ka for the equation.

c Muscles metabolise fats and sugars and produce energy, both CO2 and H+ produced as by-products. Explain what would happen to a person who regularly exercises excessively.

12 At 0°C, the concentration of hydroxide ions in water is 2.95 × 10–7 M.

a Construct the ionisation equation for water.

b Calculate the pOH of the water and therefore the pH.


c Calculate the concentration of H+.

d Calculate Kw.

e Calculate the pH.

f The self-ionisation of water is an endothermic process. Explain what happens to the pH of the water as temperature increases.

13 A household cleaner containing hydrochloric acid is analysed in a laboratory to determine whether the concentration of the acid is stated correctly on the packaging. The label claims that it contains 30% w/w HCl. The concentration of hydrochloric acid is determined by an acid–base titration using a standard solution of sodium carbonate.

a Deduce why sodium carbonate is considered to be a good standard solution.


b To analyse the cleaner, 10.0 g is dissolved in a 250.00 mL volumetric flask. 10.00 mL aliquots of this solution are pipetted into conical flasks and titrated against 0.115 M sodium carbonate. The data in the table outlines the titres of sodium carbonate obtained from the titration.

Initial volume (mL) Final volume (mL) Titre volume (mL)

Flask 1 0.00 21.43 21.43

Flask 2 21.43 41.35 19.92

Flask 3 10.56 30.58 20.02

Flask 4 15.21 35.27 20.06

i Construct the balanced chemical equation for the reaction.

ii Calculate the amount of HCl, in mol, in the 10.00 mL aliquots.

iii Calculate the amount of HCl, in mol, in the 250.00 mL volumetric flask.

iv Calculate the mass of the HCl in the cleaner.

v Calculate the concentration of HCl in % m/m.


vi Compare the experimental concentration to the one stated on the cleaner. Determine what conclusions you can make.

c Explain the colour change that would be observed with a methyl orange indicator.

d After the experiment, a student realises that their partner had rinsed the burette with water and then filled it with standard solution before beginning the titration. Justify the effect of this error on the final concentration of HCl calculated in the experiment.

14 Construct a galvanic cell that can be used to plate silver onto a cheaper metal. Your choice of materials must be safe enough to use in a high school laboratory.

a Determine and label the:

– solutions used in the two half-cells and whether they undergo oxidation or reduction

– two half-equations and the balanced overall equation, all must include appropriate states

– materials used as the anode and cathode and the polarity of each electrode

– salt bridge and a suitable chemical, identifying the movement of ions in the bridge

– direction of electron flow.

b Justify your choice of material for each electrode, using the electrochemical series and your knowledge of oxidants and reductants.

c Justify your choice of chemical for the salt bridge.


The following diagram relates to questions 7 and 8.

Potassium was discovered in 1807 by English scientist Sir Humphrey Davy. Davy passed a current through

a molten solution of potassium hydroxide and observed the production of a gas at one electrode and beads of

a shiny metal at the other. The metal was identified as potassium.

15 a Identify the missing components below.

Polarity of electrode Half-equation

Anode

Cathode

b Construct the overall reaction equation.

c Determine what voltage is required to run the cell.

d Deduce what issue is involved with not including a porous membrane between the electrodes.


16 The cell was recreated using a 1 M aqueous solution of KOH at 25°C and 1 atm.

a Identify the missing components below.

Polarity of electrode Half-equation

Anode

Cathode

b Construct the overall reaction equation.

c Determine what voltage is required to run the cell.

d Determine the effect of running the same aqueous cell at 2 M concentrations. Justify your answer using the electrochemical series.

e A student suggests that it is not possible to produce solid potassium using aqueous electrolysis. Critique their statement.

Practice exam-style questions

Documents