Define enthalpy of hydration.

The enthalpy change when one mole of gaseous ions is dissolved in water to form one mole of aqueous ions.

Is enthalpy of hydration endothermic or exothermic?

Always exothermic, because ion–dipole attractions form between ions and water.

Write the equation for the enthalpy of solution of NaCl.

NaCl(s) + aq → Na⁺(aq) + Cl⁻(aq)

State the expression for ΔHsol in terms of lattice dissociation and hydration enthalpies.

ΔHsol = ΔH(lattice dissociation) + ΣΔH(hydration)

How is ΔHsol written using lattice formation?

ΔHsol = −ΔH(lattice formation) + ΣΔH(hydration)

In the cycle for CaCl₂, how many times is the hydration enthalpy of Cl⁻ used?

Twice, as there are two chloride ions per formula unit.

What attractions form when a cation is hydrated?

Ion–dipole attractions between the cation and the δ− oxygen atoms of water.

Why is the hydration of Mg²⁺ more exothermic than that of Na⁺?

Mg²⁺ is smaller and more highly charged, so it has a higher charge density and attracts water more strongly.

Why is the hydration of F⁻ more exothermic than that of Cl⁻?

F⁻ is smaller, so it has a higher charge density and attracts water more strongly.

What happens to the sign of the lattice enthalpy when it is used in the solution cycle?

Use the lattice dissociation value, which is positive; reverse the sign of a lattice formation value.

Enthalpy of solution and hydrationAQA A-Level Chemistry: Flashcards

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Define enthalpy of solution.: The enthalpy change when one mole of an ionic solid dissolves in water to form a dilute solution.
Define enthalpy of hydration.: The enthalpy change when one mole of gaseous ions is dissolved in water to form one mole of aqueous ions.
Is enthalpy of hydration endothermic or exothermic?: Always exothermic, because ion–dipole attractions form between ions and water.
Write the equation for the hydration of Cl⁻.: Cl⁻(g) + aq → Cl⁻(aq)
Write the equation for the enthalpy of solution of NaCl.: NaCl(s) + aq → Na⁺(aq) + Cl⁻(aq)
State the expression for ΔHsol in terms of lattice dissociation and hydration enthalpies.: ΔHsol = ΔH(lattice dissociation) + ΣΔH(hydration)
How is ΔHsol written using lattice formation?: ΔHsol = −ΔH(lattice formation) + ΣΔH(hydration)
Which law is used for the enthalpy of solution cycle?: Hess's law.
In the cycle for CaCl₂, how many times is the hydration enthalpy of Cl⁻ used?: Twice, as there are two chloride ions per formula unit.
What attractions form when a cation is hydrated?: Ion–dipole attractions between the cation and the δ− oxygen atoms of water.
Why is the hydration of Mg²⁺ more exothermic than that of Na⁺?: Mg²⁺ is smaller and more highly charged, so it has a higher charge density and attracts water more strongly.
Why is the hydration of F⁻ more exothermic than that of Cl⁻?: F⁻ is smaller, so it has a higher charge density and attracts water more strongly.
What happens to the sign of the lattice enthalpy when it is used in the solution cycle?: Use the lattice dissociation value, which is positive; reverse the sign of a lattice formation value.

Exam questions on Enthalpy of solution and hydration

A student is studying what happens when the ionic solid sodium chloride dissolves in water. The process can be considered in two stages: the ionic lattice is first broken up into gaseous ions, and the gaseous ions are then surrounded by water molecules.
Define the term enthalpy of hydration.2 marks
The enthalpy of solution of sodium chloride is found using an enthalpy cycle. Data (kJ mol⁻¹): enthalpy of lattice formation of NaCl = −787; enthalpy of hydration of Na⁺(g) = −406; enthalpy of hydration of Cl⁻(g) = −364.
Calculate the enthalpy change when 5.85 g of sodium chloride (Mr = 58.5) dissolves in water. State whether the energy is released or absorbed.2 marks
Magnesium chloride dissolves in water. Data (kJ mol⁻¹): enthalpy of lattice dissociation of MgCl₂ = +2526; enthalpy of hydration of Mg²⁺(g) = −1920; enthalpy of hydration of Cl⁻(g) = −364. The enthalpy of hydration of Na⁺(g) is −406 kJ mol⁻¹.
Calculate the enthalpy of solution of magnesium chloride.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).