A1 Choose from the following equations to answer the questions below.
| (a) Ag⁺(aq) + I⁻(aq) → AgI(s) |
| (b) Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) |
| (c) CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l) |
| (d) Cu(s) → Cu²⁺(aq) + 2e⁻ |
| (e) Cu²⁺(aq) + 2e⁻ → Cu(s) |
| (f) Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) |
| (g) Fe(s) + 2H⁺(aq) → Fe²⁺(aq) + H₂(g) |
| (h) H⁺(aq) + OH⁻(aq) → H₂O(l) |
| (i) 2I⁻(aq) + Br₂(aq) → I₂(aq) + 2Br⁻(aq) |
| (j) NH₄⁺(aq) + OH⁻(aq) → H₂O(l) + NH₃(g) |
| (k) 4OH⁻(aq) → O₂(g) + 2H₂O(l) + 4e⁻ |
Each equation can be used once, more than once, or not at all.
Give the letter of an equation which
(a) shows the formation of gas that turns moist red litmus blue,
Equation J makes ammonia gas NH₃. Ammonia is basic and so it turns moist red litmus paper blue.
(b) shows a reaction that forms a white precipitate,
Equation B forms barium sulfate BaSO₄ as a solid. BaSO₄ is an insoluble salt and appears as a white precipitate.
(c) shows only reduction,
Equation E is Cu²⁺ + 2e⁻ → Cu(s). Electrons are only on the left side of the species being reduced. This is a reduction reaction only.
(d) shows the neutralisation of dilute hydrochloric acid by aqueous sodium hydroxide,
Neutralisation of an acid by sodium hydroxide is H⁺ + OH⁻ → H₂O. That exact equation is H, so H shows the neutralisation of dilute hydrochloric acid by aqueous sodium hydroxide.
(e) shows the reaction at an inert positive electrode when copper(II) sulfate is electrolysed.
In electrolysis, the positive electrode (anode) is where oxidation happens.
When using an inert anode (like carbon or platinum) in a copper(II) sulfate solution, the hydroxide ions (OH⁻) from water lose electrons. This produces oxygen gas (O₂) and water (H₂O).
So, equation K (4OH⁻ → O₂ + 2H₂O + 4e⁻) shows what happens at the anode —
oxygen gas is made, and electrons are released.
A2 A power station burns methane, CH₄, which is contaminated by hydrogen sulfide, H₂S.
The equation shows the combustion of methane:
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
The combustion of the hydrogen sulfide forms water and sulfur dioxide.
(a) Construct the equation to show the combustion of hydrogen sulfide.
2H₂S(g) + 3O₂(g) → 2SO₂(g) + 2H₂O(g)
(b) Explain why the burning of the contaminated methane at the power station causes atmospheric problems.
Carbon dioxide is produced, which is a greenhouse gas and causes global warming.
Sulfur dioxide is also formed, which causes acid rain and air pollution
(c) A 1000 dm³ sample of the contaminated methane gas burnt at the power station produces 999 dm³ of carbon dioxide and 1 dm³ of sulfur dioxide. All gas volumes are measured at room temperature and pressure.
(i) What is the volume of methane, at room temperature and pressure, in the 1000 dm³ of the gas burnt?
CH₄ → CO₂
(1 volume : 1 Volume)
Volume of CH₄ = Volume of CO₂ = 999 dm³
(ii) What is the volume of hydrogen sulfide, at room temperature and pressure, in the 1000 dm³ of the gas burnt?
2H₂S → 2SO₂
(2 Moles : 2 Moles)
(1 Moles : 1 Moles)
(1 Volume : 1 Volume)
Volume of H₂S = Volume of SO₂ = 1 dm³
(iii) Calculate the percentage, by volume, of hydrogen sulfide in the contaminated methane. You must show your working.
The volume of a gas changes if the pressure is increased or the temperature is increased.
(i) Describe and explain qualitatively the effect of increasing the pressure on the volume of a gas if the temperature remains constant.
Answer:
When pressure increases, the volume of the gas decreases.
This is because the gas particles are forced closer together when pressure is applied.
At constant temperature, the particles move with the same energy but occupy a smaller space.
(ii) Describe and explain qualitatively the effect of increasing the temperature on the volume of a gas if the pressure remains constant.
Answer:
When temperature increases, the volume of the gas increases.
This is because heating gives the gas particles more kinetic energy, so they move faster and push outwards with greater force.
At constant pressure, the gas expands to keep the pressure the same.