Chemical ReactionsCambridge IGCSE Chemistry: Topic test

20 questions, 54 marks

Cambridge IGCSE Chemistry

Chemical Reactions topic test

Total 54 marks

Name

Class

Date

1
A student lights a candle made of solid wax (a hydrocarbon). Near the wick, some solid wax melts to a liquid and evaporates; the wax vapour then burns in the flame, producing carbon dioxide and water vapour, while the melted liquid wax that drips down the side of the candle cools and resolidifies.
(a)
Which row correctly classifies the two changes described: the wax melting/resolidifying down the side of the candle, and the wax vapour burning in the flame?
[1 mark]
AMelting/resolidifying: physical change; burning: chemical change
BMelting/resolidifying: chemical change; burning: physical change
CBoth changes are physical changes
DBoth changes are chemical changes
(b)
Which statement explains why burning the wax vapour is a chemical change but melting the wax is a physical change?
[1 mark]
ABurning only changes the state of the wax, whereas melting forms a new substance
BBurning forms new substances with different properties, whereas melting only changes the state of the same substance
CBoth changes form new substances, but only burning is irreversible
DMelting releases energy to the surroundings, which makes it a chemical change
(c)
State one difference in general between physical changes and chemical changes, illustrating your answer using the two changes in this scenario.
[2 marks]
Total for question 1: 4 marks
2
A group of students investigates the reaction between magnesium and dilute sulfuric acid by collecting the hydrogen gas produced in a gas syringe. In experiment 1, they use a 4 cm strip of magnesium ribbon; in experiment 2, they repeat the reaction with the same mass of magnesium as a fine powder, keeping the acid concentration, volume and temperature the same in both experiments. The powdered magnesium reaction finishes producing gas in less time than the ribbon.
(a)
Why does the powdered magnesium react faster than the ribbon?
[1 mark]
AThe powder has a smaller total surface area, so fewer collisions are needed to complete the reaction
BThe powder has a lower activation energy than the ribbon
CThe powder has a larger total surface area exposed to the acid, so there are more frequent collisions between acid particles and magnesium particles
DThe powder reacts with a different concentration of acid than the ribbon
(b)
Which method would allow the students to measure the rate of this reaction by monitoring the gas produced over time?
[1 mark]
ARecording the mass of the flask and its contents only once, at the very end of the reaction
BMeasuring the temperature of the acid before the reaction begins
CMeasuring the length of the magnesium ribbon before it is added to the acid
DRecording the total volume of gas collected in the gas syringe at regular time intervals
(c)
Using collision theory, explain why increasing the surface area of the magnesium increases the rate of reaction.
[2 marks]
Total for question 2: 4 marks
3
A chemical company manufactures sulfuric acid using the Contact process. In one stage, sulfur dioxide reacts with oxygen in a reversible reaction over a vanadium(V) oxide catalyst inside a closed reactor: 2SO2(g) + O2(g) <=> 2SO3(g). The reactor is operated at 450 degrees C and about 200 kPa (2 atm).
(a)
State what is meant by a reversible reaction being 'at equilibrium', and explain how the students could tell from measurements in the closed reactor that the reaction had reached equilibrium.
[3 marks]
(b)
Explain, in terms of rate of reaction and position of equilibrium, why a temperature of 450 degrees C rather than a much lower temperature is used in the Contact process, even though the forward reaction is exothermic.
[4 marks]
Total for question 3: 7 marks
4
A student investigates the reaction between acidified potassium manganate(VII) solution (purple) and iron(II) sulfate solution. When mixed, the iron(II) ions are oxidised to iron(III) ions and the purple colour of the manganate(VII) ions fades as they are reduced to colourless manganese(II) ions. The student repeats the experiment using iron(II) sulfate solutions of different concentrations, timing how long the purple colour takes to fade completely each time.
(a)
Identify the oxidising agent and the reducing agent in this reaction, and explain, in terms of electron transfer and oxidation number, why this is classified as a redox reaction.
[6 marks]
(b)
The student finds that at a higher concentration of iron(II) sulfate solution, the purple colour fades in a shorter time. Using collision theory, explain why increasing the concentration of iron(II) sulfate increases the rate of this redox reaction, and describe how the student could use her results to compare the rates of the different concentrations fairly.
[6 marks]
Total for question 4: 12 marks
5
A student adds excess powdered zinc metal to blue copper(II) sulfate solution in a test tube and stirs. Over several minutes, the blue colour of the solution fades and a reddish-brown solid coating forms on the surface of the unreacted zinc.
(a)
Which row correctly identifies what happens to the zinc and the copper(II) ions during this reaction?
[1 mark]
AZinc is oxidised (loses electrons); copper(II) ions are reduced (gain electrons)
BZinc is reduced; copper(II) ions are oxidised
CBoth zinc and copper(II) ions are oxidised
DBoth zinc and copper(II) ions are reduced
(b)
What happens to the oxidation number of copper during this reaction?
[1 mark]
AIt stays the same, at +2
BIt decreases from +2 to 0
CIt increases from 0 to +2
DIt decreases from +2 to +1
(c)
Explain why this reaction is classified as a redox reaction, referring to electron transfer.
[2 marks]
Total for question 5: 4 marks
6
A student adds a small amount of powdered manganese(IV) oxide to hydrogen peroxide solution in a flask connected to a gas syringe, and collects the oxygen gas produced. She repeats the experiment with the same volume and concentration of hydrogen peroxide solution but without adding any manganese(IV) oxide. The reaction with manganese(IV) oxide produces the same total volume of gas but in a much shorter time; the mass of manganese(IV) oxide is unchanged at the end.
(a)
What is the role of manganese(IV) oxide in this reaction?
[1 mark]
AIt is a reactant, since it is used up during the reaction
BIt is a product, since it forms during the reaction
CIt is a catalyst, since it increases the rate of reaction and is chemically unchanged at the end
DIt has no effect on the rate of the reaction
(b)
How does adding manganese(IV) oxide affect the activation energy of the decomposition of hydrogen peroxide?
[1 mark]
AIt increases the activation energy
BIt has no effect on the activation energy, only on the products formed
CIt removes the need for any activation energy
DIt decreases the activation energy, by providing an alternative reaction pathway
(c)
Using collision theory, explain why adding manganese(IV) oxide increases the rate of this reaction.
[2 marks]
Total for question 6: 4 marks
7
A student researching industrial chemistry reads that ammonia is manufactured by reacting nitrogen and hydrogen in a reversible reaction inside a reactor operating at high pressure: N2(g) + 3H2(g) <=> 2NH3(g). She wants to know how the percentage yield of ammonia would change if the reactor pressure were increased while the temperature stays the same.
(a)
State the effect of increasing the pressure on the position of equilibrium in this reaction, and explain your answer in terms of the number of gas molecules on each side of the equation.
[3 marks]
(b)
The reactor actually operates at about 200 atm rather than at a much higher pressure, even though a higher pressure would give an even greater percentage yield. Explain why 200 atm is used as a compromise, considering cost and safety as well as yield.
[4 marks]
Total for question 7: 7 marks
8
A student heats solid ammonium chloride strongly in a test tube. It appears to disappear as it decomposes into ammonia gas and hydrogen chloride gas: NH4Cl(s) <=> NH3(g) + HCl(g). Near the cooler mouth of the test tube, a white solid reappears as the gases recombine on cooling.
(a)
Explain why the decomposition of ammonium chloride on heating is a chemical change, and explain, using the idea of a reversible reaction, why a white solid reappears near the cooler mouth of the tube even though no new substance was added.
[6 marks]
(b)
Predict and explain what would happen to the position of equilibrium, and to the amount of white solid formed at different points along the tube, if the student repeated the experiment using a much longer test tube with a wider temperature range along its length, referring to the effect of temperature on this reversible reaction.
[6 marks]
Total for question 8: 12 marks

End of questions