O-LevelChemistryRates of Reaction5070/22 · May/June 2018 · Q810 Marks

A scientist investigates the thermal decomposition of calcium carbonate in a closed system. A dynamic equilibrium mixture is established. CaCO3(s) ⇌ CaO(s) + CO2(g)

(a)What is meant by the term dynamic equilibrium?[2]
(b)The pressure of the equilibrium mixture is decreased. The temperature of the closed system is kept constant. Predict and explain what will happen to the composition of the equilibrium mixture.[2]
(c)The temperature of the equilibrium mixture is increased. The pressure within the closed system is kept constant. The position of equilibrium shifts to the right hand side. What conclusion can be made about the enthalpy change of the reaction?[1]
(d)Calcium carbonate reacts with dilute nitric acid to form three compounds, X, Y and Z. • X is a salt. • Y is a colourless gas. • Z is a colourless liquid.
(d)(i)Name Y and describe a test for this gas. name test observation[2]
(d)(ii)Name Z and describe a chemical test for this liquid. name chemical test observation[2]
(d)(iii)Construct the equation for the reaction between calcium carbonate and nitric acid.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[2] 8(a) a reversible reaction / backward AND forward reaction (1) the concentrations of reactants AND products do not change / amounts of reactants AND products constant/moles of reactants AND products constant (1) OR rate of forward reaction = rate of backward reaction (2)
[2] 8(b) amount of carbon dioxide increases / moles of carbon dioxide increases / mass of carbon dioxide increases (1) no gas on left but gas on right (1)
[1] 8(c) endothermic / the enthalpy change is positive (1)
[2] 8(d)(i) carbon dioxide (1) limewater turns milky (1)
[2] 8(d)(ii) water (1) copper(II) sulfate goes from white to blue / cobalt chloride goes from blue to pink (1)
[1] 8(d)(iii) CaCO3 + 2HNO3 → Ca(NO3)2 + CO2 + H2O (1)

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