QUESTION 25 (5 marks)
2+(aq),
An equilibrium is formed between two differently coloured cobalt species, Co(H2O)6
2‒(aq), which is blue. The equation for this equilibrium is shown.
which is pink, and CoCl4
2+(aq) + 4Cl–(aq) ⇌ CoCl4
2–(aq) + 6H2O(l)
Co(H2O)6
a) Apply Le Châtelier’s principle to predict the visible effect of adding AgNO3 to an
2+ and CoCl4
2– ions.
aqueous blue-coloured solution containing Co(H2O)6 Explain your reasoning.
[3 marks]
b) When a sample of the equilibrium mixture is put into hot water, the mixture turns more blue. Determine whether the forward reaction of the equation is exothermic or endothermic. Explain your reasoning.
[2 marks]
QCAA guide · typeset solution
QCAA sample response and mark allocation
25a) | Adding AgNO3 produces Ag+ ions, which react with Cl− ions to form insoluble AgCl, therefore decreasing the concentration of Cl− ions. Equilibrium shifts to reactants (left) to counteract the decrease by increasing the concentration of Cl− ions. The blue solution will become lighter (pinker). | • correctly identifies that Cl− decreases [1 mark] • identifies that equilibrium shifts to left (reactants) to counteract the change [1 mark] • identifies that the blue solution becomes lighter [1 mark] | Allow FT error for equilibrium shift and change in solution from Cl− increases. Acceptable responses may be: - solution becomes pinker - solution decreases in intensity - or other responses consistent with a reasonable understanding. Do not penalise students who mention the formation of a white precipitate, i.e. AgCl. |
25b) | Adding heat shifts equilibrium towards the endothermic direction and produces CoCl4 2− ions, which are blue. Therefore, the forward reaction is endothermic. | • identifies that the forward reaction has been favoured [1 mark] • determines that the forward reaction is endothermic [1 mark] |
QCAA sample response and marking criteria reproduced from the official guide.
Compare your working with the guide above.