A list of chemical and physical processes, A to H, is shown.
A combustion
B diffusion
C melting
D neutralisation
E photosynthesis
F reversible reaction
G roasting
H thermal decomposition
Answer the following questions about processes A to H.
Each letter may be used once, more than once or not at all.
(a) State which of the processes A to H happens when an acid reacts with an alkali.
(b) State which of the processes A to H reaches a position of equilibrium.
(c) State which of the processes A to H involves particles changing from fixed positions to being mobile, but still touching.
(d) State which two of the processes A to H are physical changes.
(e) State which of the processes A to H is caused by gas particles colliding with each other.
▶️ Answer/Explanation
(a) D (neutralisation)
Neutralisation is the process that occurs when an acid reacts with an alkali to form a salt and water.
(b) F (reversible reaction)
Reversible reactions can reach a position of equilibrium where the forward and reverse reactions occur at the same rate.
(c) C (melting)
Melting involves particles changing from fixed positions in a solid to being mobile (but still touching) in a liquid state.
(d) B (diffusion) and C (melting)
These are physical changes because they don’t involve the formation of new substances. Diffusion is the movement of particles, and melting is a change of state.
(e) B (diffusion)
Diffusion is caused by gas particles colliding with each other and moving from areas of high concentration to low concentration.
This question is about compounds of sulfur.
(a) Sulfuric acid, H2SO4, is manufactured using the Contact process. This manufacture involves four stages.
stage 1 – Molten sulfur burns in air to produce sulfur dioxide.
stage 2 – Sulfur dioxide reacts with oxygen to form sulfur trioxide.
stage 3 – Sulfur trioxide combines with concentrated sulfuric acid to form oleum, H2S2O7.
stage 4 – Oleum reacts to form concentrated sulfuric acid.
(i) Write a chemical equation for the reaction occurring in stage 1.
(ii) State the essential conditions that are necessary for stage 2. Write an equation for the chemical reaction that occurs.
(iii) Write a chemical equation for the reaction occurring in stage 3.
(iv) Name the substance that reacts with oleum in stage 4.
(b) Hydrogen sulfide has the formula H2S.
(i) Complete the dot-and-cross diagram to show the electron arrangement in a molecule of hydrogen sulfide. Show outer shell electrons only.
(ii) Balance the chemical equation for the reaction of hydrogen sulfide with sulfur dioxide shown.
H2S + SO2 → S + H2O
▶️ Answer/Explanation
4(a)(i) \(\text{S} + \text{O}_2 \rightarrow \text{SO}_2\)
Molten sulfur reacts with oxygen to form sulfur dioxide, a key step in the Contact process.
4(a)(ii) Temperature: 450°C (1)
Pressure: 1–2 atmospheres (1)
Catalyst: Vanadium(V) oxide (1)
Equation: \(2\text{SO}_2 + \text{O}_2 \rightleftharpoons 2\text{SO}_3\) (1)
These conditions optimize the yield of sulfur trioxide while maintaining reaction efficiency.
4(a)(iii) \(\text{SO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{H}_2\text{S}_2\text{O}_7\)
Sulfur trioxide reacts with concentrated sulfuric acid to produce oleum.
4(a)(iv) Water
Oleum reacts with water to form concentrated sulfuric acid.
4(b)(i)
Two bonding pairs (one dot and one cross each) between S and H (1). Two lone pairs on sulfur (1).
4(b)(ii) \(2\text{H}_2\text{S} + \text{SO}_2 \rightarrow 3\text{S} + 2\text{H}_2\text{O}\)
Balanced equation ensures conservation of mass and charge.