A-LevelChemistryChemical periodicity9701/41 · Oct/Nov 2023 · Q47 Marks

Cobalt(II) nitrate, Co(NO3)2, is a reddish-brown crystalline solid. It dissolves in water to form a solution containing [Co(H2O)6]2+ complex ions.

(a)(i)Complete Table 4.1 giving the formula of the cobalt-containing species that is formed in each of the three reactions described. Table 4.1 reaction reagent added to [Co(H2O)6]2+(aq) cobalt-containing species formed 1 NaOH(aq) 2 an excess of NH3(aq) 3 an excess of conc. HCl(aq)[2]
(a)(ii)Describe the colour change seen in reaction 3. original colour of [Co(H2O)6]2+(aq) final colour after addition of an excess of conc. HCl(aq)[1]
(b)Calcium nitrate, Ca(NO3)2, is a white crystalline solid. When heated, it starts to decompose at approximately 500°C.
(b)(i)Write an equation for the decomposition of Ca(NO3)2.[1]
(b)(ii)Suggest temperatures at which Mg(NO3)2 and Ba(NO3)2 start to decompose. Explain your answer. temperature at which Mg(NO3)2 starts to decompose temperature at which Ba(NO3)2 starts to decompose explanation[3]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2023). The mark or mark type is shown in brackets.

• Co(OH)2 OR Co(OH)2(H2O)4
• [Co(NH3)6]2+ OR [Co(NH3)6]3+
• [CoCl4]2-
any two [1] all three [2]
[1] pink to blue
[1] 2Ca(NO3)2 → 2CaO + 4NO2 + O2 OR Ca(NO3)2 → CaO + 2NO2 + 1/2O2
[1] M1: Mg(NO3)2 below 480 °C, Ba(NO3)2 above 520 °C BOTH u / c
[1] M2: ionic radii of M2+ increases down the group OR radii of Ba2+ is greater (than Mg2+) OR charge density of M2+ decreases down the group OR charge density of Ba2+ is smaller (than Mg2+) ORA u/c
[1] M3: (larger cations) polarise / distort anion / NO3¯ less OR (larger ions) weaken N-O / N=O (bond) less ORA

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