AQA A level Chem 1.08 Thermodynamics

Magnesium chloride dissolves in water. Give an equation, including state symbols, to represent the process that occurs when the enthalpy of solution of magnesium chloride is measured. (1)

MgCl₂ → Mg²⁺ (aq) + 2Cl^- (aq)

The enthalpy of hydration of Ca²⁺(g) is –1650 kJ mol^–1 Suggest why this value is less exothermic than that of Mg²⁺(g) (2)

Ca²⁺ (ion) bigger/lower charge to size than Mg²⁺ (1)

weaker attraction to (O^δ- in) water (1)

This question is about thermodynamics. Consider the reaction shown.

2 Al₂O₃(s) + 3 C(s) → 4 Al(s) + 3 CO₂(g)

The table below shows some thermodynamic data.

Explain why the standard entropy value for carbon dioxide is greater than that for carbon (1)

CO₂ is more disordered (than solid) (1)

Table 2 shows the theoretical lattice enthalpy, based on a perfect ionic model, and an experimental value for the enthalpy of lattice formation of silver chloride.

State why there is a difference between the theoretical and experimental values (1)

Has covalent character or partial covalent bonding (as well as ionic bonding) (1)

Calculate a value for the enthalpy of lattice formation for magnesium oxide. (3)

_∆fH = _∆aH (Mg) + ½ _∆BDH (O₂) + _{∆1st IE}H (Mg) + _{∆2nd IE}H (Mg) + _{∆1st EA}H (O) + _{∆2nd EA}H(O) + _∆LEH (MgO) (1)

- 602 = 150 + (½ × 496) + 736 +1450 – 142 + 844 + _∆LEH (MgO) (1)

∆LEH (MgO) = -3888 / -3890 (kJ mol^–1)

A calculation of the enthalpy of lattice formation of silver iodide based on a perfect ionic model gives a smaller numerical value than the value calculated in part (b)

Explain this difference. (2)

AgI contains covalent character (1)

Forces/bonds (holding the lattice together) are stronger (1)

Anhydrous magnesium chloride, MgCl₂, can absorb water to form the hydrated salt MgCl₂.4H₂O

MgCl₂(s) + 4H₂O(l) ⟶ MgCl₂.4H₂O(s)

Suggest one reason why the enthalpy change for this reaction cannot be determined directly by calorimetry. (1)

not possible to prevent some dissolving