A-LevelChemistryGroup 29701/22 · Feb/Mar 2020 · Q119 Marks

Group 2 metals form alkaline solutions in water.

(a)(i)Write the equation for the reaction of calcium oxide with water.[1]
(a)(ii)Identify the ion that causes an aqueous solution to be alkaline.[1]
(b)The table shows the melting points of some Group 2 metal oxides. compound melting point/°C MgO 2825 CaO 2613 Sro 2531 BaO 1923 Explain the trend in the melting points of the oxides down Group 2.[2]
(c)Oxygen reacts readily with some metals, but each Group 2 metal requires strong heating to start the reaction with oxygen. Suggest why strong heating is required to start these reactions.[1]
(d)Beryllium oxide reacts with hydrochloric acid to form molecules of BeCl2. Deduce the bond angle in BeCl2.[1]
(e)(i)Unlike the other oxides of Group 2 metals, beryllium oxide is amphoteric. Give the meaning of the term amphoteric.[1]
(e)(ii)Beryllium oxide and aluminium oxide have similar chemical properties. The Be(OH)42- anion is a product of the reaction between beryllium oxide and excess concentrated OH-(aq). Construct an equation for this reaction.[1]
(f)(i)Magnesium oxide reacts reversibly with chlorine according to the following equation. 2MgO(s) + 2Cl2(g) ⇌ 2MgCl2(s) + O2(g) Under certain conditions, a dynamic equilibrium is established. State two features of a reaction that is in dynamic equilibrium. 1 2[2]
(f)(ii)The equilibrium constant, Kp, is given by the following expression. Kp = Po₂ / (pCl₂)₂ At 1.00 × 10^5 Pa and 500 K, 70% of the initial amount of Cl2(g) has reacted. Calculate Kp and state its units. Kp = units =[3]
(g)(i)Magnesium peroxide, MgO2, is made in the following reaction. MgO(s) + H2O2(l) → MgO2(s) + H2O(l) ∆H = -96 kJ mol-1 compound enthalpy change of formation, ∆Hf / kJ mol-1 MgO(s) -602 H2O2(l) -188 H2O(l) -286 The peroxide ion is O22-. Deduce the average oxidation number of oxygen in the peroxide ion.[1]
(g)(ii)Define the term enthalpy change of formation.[2]
(g)(iii)Use the data given to calculate the enthalpy change of formation of MgO2(s). ∆Hf MgO2(s) =[2]
(g)(iv)Magnesium peroxide decomposes slowly to form magnesium oxide and oxygen. MgO2(s) → MgO(s) + ½O2(g) Use your answer to (g)(iii) and the data in the table to calculate the enthalpy change of this reaction. If you were unable to obtain an answer to (g)(iii), use the value ∆Hf = −550 kJ mol−¹. This is not the correct answer. enthalpy change of reaction =[1]

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2020). The mark or mark type is shown in brackets.

[1] CaO + H2O → Ca(OH)2
[1] OH-/hydroxide
[2] M1 (decreasing melting point down the group because) lower forces of attraction / weaker bonds (between cations and anions / oxide / O2-)
[2] M2 larger cations and constant charge OR decreasing charge density of cation (down group)
[1] high(er) activation energy / heating overcomes activation energy
[1] 180(°)
[1] reacts with / behaves as both acid and base
[1] BeO + 2OH- + H2O → Be(OH)42-
[2] M1 equal rates of forward and backward reactions
[2] M2 closed system OR macroscopic properties unchanging
[3] M1 Cl2: initial x, equilibrium 0.3x mol, mol fraction 6/13. O2: initial 0, equilibrium 0.35x mol, mol fraction 7/13
[3] M2 Kp = (100000×7/13) / (100000×6/13)² = 2.53×10-5
[3] M3 Pa-1
[1] -1
[2] M1 (enthalpy / energy change) when one mole of a compound / substance is formed
[2] M2 from its elements in their standard states
[2] -(-602 + −188) + (∆Ηf [MgO2] + −286) = -96
[2] ΔΗ[MgO2] = −600 (kJ mol-1)
[1] -(-600) – (+602) = −2 (kJ mol-1)