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

This question is about enthalpy changes.

Theoretical values for enthalpies of lattice dissociation can be calculated using a perfect ionic model.

state the meaning of the term perfect ionic model (1)

(ions are) point charges

OR

(ions are) perfect spheres

OR

No covalent character (1)

Explain why the second electron affinity of oxygen has a positive value (1)

O^- repels the electron (being added) (1)

Give a reason why data books do not contain a value for the enthalpy of solution of sodium oxide (1)

It reacts with water (1)

Explain why the enthalpy of hydration of fluoride ions is more negative than the enthalpy of hydration of chloride ions (2)

fluoride ions are smaller (1)

stronger attraction (of fluoride ion) to δ+ on H/ electron deficient H (in water) (1)

Explain why the second ionisation energy of calcium is greater than the first ionisation energy of calcium. (1)

electron removed closer to nucleus (1)

Table 3 shows lattice enthalpies based on a perfect ionic model and lattice enthalpies from Born–Haber cycles for three metal chlorides.

Discuss the values in table 3

in you answer you should

• compare the three values based on a perfect ionic model

• compare the values based on a perfect ionic model to the values from a Born-Haber cycle for each compound (6)

Level 3 5-6 marks All stages are covered and each stage is generally correct and virtually complete

stage 1

1a Value for CaCl₂ is larger/Values for KCl and AgCl are similar/ Values for CaCl₂

1b Ca²⁺ has a larger charge/ is a smaller ion / Ag⁺ and K⁺ have smaller charge or larger ions

1c CaCl₂ has stronger ionic bonds or stronger attraction between + and - ions (Ca²⁺ and Cl^- )

stage 2

2a CaCl₂ has similar values (between the perfect ionic model and Born-Haber cycle)

2b KCl has similar values (between the perfect ionic model and Born-Haber cycle)

2c CaCl₂ and KCl have (almost) perfect ionic bonding or + ions are point charges/(perfectly) spherical

stage 3

3a AgCl has larger difference in values (between the perfect ionic model and Born-Haber cycle)

3b AgCl contains some covalent character

3c Ag⁺ more polarising/distorts electron cloud more

Another student obtained a value of +15kJmol^-1 using the same method.

Suggest the main reason for the difference between this experimental value for the enthalpy of solution and the correct value of +26 kJ mol^-1 (1)

heat gain (from the surroundings)/ incomplete dissolving (1)