4.3.5Unit 4. Rates, Equilibria and Further Organic Chemistry
Enthalpy of Solution – Calculations — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 27 of 56 Unit quiz
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Section 1 of 5Building the Cycle
The cycle links the solid, the gaseous ions and the aqueous ions.
1
Write the solid ionic compound at the top left and the aqueous ions at the top right, joined by an arrow labelled ΔsolH.
2
Put the separated gaseous ions at the bottom, joined to the solid by the lattice energy arrow pointing toward the solid.
3
Join the gaseous ions to the aqueous ions by the hydration enthalpy arrows, one for each type of ion.
4
Apply Hess law: the direct route equals the route via gaseous ions, so reverse the lattice energy arrow when going from solid to ions.
Section 2 of 5The Equation
Reading the cycle in the correct direction gives one equation.
Core equation
- Core equation ΔsolH = −ΔLE + ΔhydH(cation) + ΔhydH(anion), where ΔLE is the lattice energy defined for formation of the solid.
Reverse the lattice sign
- Reverse the lattice sign Because the lattice breaks apart on dissolving, the lattice energy enters the sum with its sign changed.
Count the ions
- Count the ions If the formula has two anions, such as MgCl₂, the anion hydration enthalpy must be multiplied by two.
Check the units
- Check the units Enthalpy values are given in kJ mol⁻¹, and the answer keeps the same unit and a sign.
| Compound | Lattice energy / kJ mol⁻¹ | Hydration enthalpy of ions / kJ mol⁻¹ | ΔsolH / kJ mol⁻¹ |
|---|---|---|---|
| NaCl | −787 | Na⁺ −406; Cl⁻ −364 | +17 |
| MgCl₂ | −2526 | Mg²⁺ −1920; Cl⁻ −364 each | −122 |
Section 3 of 5Interpreting the Answer
Interpreting the Answer
Positive result
- Positive result The compound absorbs heat on dissolving, so a thermometer in the solution shows a small fall in temperature.
Negative result
- Negative result The compound releases heat, so the temperature of the solution rises as the compound dissolves.
Solubility link
- Solubility link A slightly endothermic value does not stop dissolving because the increase in entropy can still make it favourable.
Rearranging
- Rearranging The same cycle can be rearranged to find an unknown lattice energy or hydration enthalpy when the other values are known.
Section 4 of 5Worked Example
Question Calculate the enthalpy of solution of magnesium chloride given a lattice energy of −2526 kJ mol⁻¹, ΔhydH(Mg²⁺) = −1920 kJ mol⁻¹ and ΔhydH(Cl⁻) = −364 kJ mol⁻¹.
- Write the equationΔsolH = −ΔLE + ΔhydH(Mg²⁺) + 2 × ΔhydH(Cl⁻).
- SubstituteΔsolH = +2526 + (−1920) + 2 × (−364).
- Evaluate2526 − 1920 − 728 = −122 kJ mol⁻¹.
Answer ΔsolH(MgCl₂) = −122 kJ mol⁻¹, so dissolving is exothermic.
Section 5 of 5Exam Checklist & Tips
4.3.5 – Key Points for the Exam
- Hess law Total enthalpy change is independent of the route taken.
- Enthalpy cycle Diagram linking enthalpy changes to apply Hess law.
- Endothermic Process that absorbs heat, giving a positive enthalpy change.
- Exothermic Process that releases heat, giving a negative enthalpy change.
Common Mistakes
- Using the lattice energy with the wrong sign when going from solid to ions.
- Forgetting to double the chloride hydration enthalpy for MgCl₂.
- Adding the values but writing an answer without a sign or unit.
- Concluding a positive solution enthalpy means the compound is insoluble.
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