IGCSE Chemistry Chemical reactions topical questions

For each change below, state whether it is a physical change or a chemical change and give a reason.

1. (a)Ice melting[1]
2. (b)An iron nail forming rust[1]
3. (c)Water boiling[1]

Zinc granules react with dilute sulfuric acid. State one change the student could make to increase the rate, and explain your answer.

Green copper(II) carbonate is heated in a test tube. It turns black and gives off a gas that turns limewater cloudy.

1. (a)Name the gas given off.[1]
2. (b)Name the black solid left in the tube.[1]
3. (c)State whether this is a physical or a chemical change.[1]
4. (d)Give a reason for your answer to part (c).[1]

Marble chips react with dilute hydrochloric acid and the volume of carbon dioxide is recorded every 20 s.

1. (a)State what the gradient of the graph tells you about the rate of reaction.[1]
2. (b)The experiment is repeated with a more concentrated acid and the same mass of marble. Describe how the gradient and the final volume of gas change.[2]
3. (c)Explain why the final volume of gas is unchanged.[1]

Two gases react when they are mixed, but the reaction is very slow at room temperature.

1. (a)State the two conditions that must be met for a collision between the particles to lead to a reaction (Supplement).[2]
2. (b)Explain why warming the mixture speeds up the reaction (Supplement).[2]

Sodium thiosulfate solution reacts with dilute hydrochloric acid to form a cloudy precipitate of sulfur. A student records the time taken for the precipitate to hide a cross marked under the beaker, repeating the experiment at different temperatures.

1. (a)Explain how the rate of reaction is obtained from the recorded time.[1]
2. (b)Predict, and explain in terms of particles, what happens to the rate as the temperature is increased (Supplement).[3]

In the Contact process, sulfur dioxide reacts with oxygen: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). The forward reaction is exothermic.

1. (a)Name the product of this reaction.[1]
2. (b)Name the catalyst used in the Contact process.[1]
3. (c)Explain why a temperature of about 450 °C is used rather than a much lower temperature, even though a lower temperature would give a higher yield. (Supplement)[2]

In the Haber process, nitrogen and hydrogen react: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction is exothermic.

1. (a)Predict and explain the effect of increasing the pressure on the yield of ammonia.[2]
2. (b)Explain why a catalyst is used even though it does not change the yield.[2]
3. (c)Explain why a temperature of about 450 °C is used rather than a much lower one.[1]

Oxidising agents and reducing agents are used to identify redox reactions.

1. (a)State the colour change when acidified potassium manganate(VII) is reduced.[1]
2. (b)State the colour change when aqueous potassium iodide is oxidised.[1]
3. (c)In the reaction Zn + Cu²⁺ → Zn²⁺ + Cu, identify the oxidising agent and explain your answer in terms of electrons.[2]
4. (d)State the change in the oxidation number of zinc in this reaction.[1]

Chlorine water is added to aqueous potassium bromide: Cl₂ + 2KBr → 2KCl + Br₂.

1. (a)State the oxidation number of bromine before and after the reaction. (Supplement)[1]
2. (b)Identify the species that has been oxidised and the species that has been reduced, explaining your answer in terms of electrons. (Supplement)[2]
3. (c)Identify the oxidising agent in this reaction. (Supplement)[1]
4. (d)State the colour change that shows the product in part (a) has formed.[1]