23.2 A2 Level

Enthalpies of solution and hydration

11 flashcards to master this topic

Definition Flip

Define enthalpy change of hydration (ΔHhyd).

Answer Flip

ΔHhyd is the enthalpy change when one mole of gaseous ions dissolves in water to form an infinitely dilute solution.

Example: Na+(g) + (aq) → Na+(aq) has a negative ΔHhyd because energy is released as ions attract water molecules.
Definition Flip

Define enthalpy change of solution (ΔHsol).

Answer Flip

ΔHsol is the enthalpy change when one mole of a substance dissolves in water to form an infinitely dilute solution.

Example: NaCl(s) + (aq) → Na+(aq) + Cl-(aq) can be endothermic or exothermic depending on the balance of lattice energy and hydration enthalpies.
Key Concept Flip

Draw an energy cycle that relates enthalpy change of solution, lattice energy, and enthalpy change of hydration for NaCl.

Answer Flip

Cycle: NaCl(s) + H₂O(l) -> Na+(aq) + Cl-(aq) (ΔHsol). Alternative route: NaCl(s) -> Na+(g) + Cl-(g) (ΔHlattice) then Na+(g) + H₂O(l) -> Na+(aq) (ΔHhyd(Na+)) AND Cl-(g) + H₂O(l) -> Cl-(aq) (ΔHhyd(Cl-)). ΔHsol = ΔHlattice + ΔHhyd(Na+) + ΔHhyd(Cl-).

Key Concept Flip

How does ionic charge affect the magnitude of the enthalpy change of hydration?

Answer Flip

Higher ionic charge leads to a more negative (larger magnitude) enthalpy change of hydration. This is because more highly charged ions attract water molecules more strongly, releasing more energy.

Key Concept Flip

How does ionic radius affect the magnitude of the enthalpy change of hydration?

Answer Flip

Smaller ionic radius leads to a more negative (larger magnitude) enthalpy change of hydration. Smaller ions have a higher charge density, leading to stronger attraction to water molecules and greater energy release.

Definition Flip

Define entropy (S).

Answer Flip

Entropy (S) is a measure of the number of possible arrangements of particles and their energy in a system. Higher entropy corresponds to greater disorder or randomness.

Key Concept Flip

Predict the sign of the entropy change (ΔS) for the boiling of water.

Answer Flip

ΔS is positive. Boiling increases the disorder as liquid water becomes gaseous water, increasing the number of possible arrangements and energy distributions of the water molecules.

Calculation Flip

Calculate the entropy change for a reaction given standard entropies of reactants and products.

Answer Flip

ΔS ⦵ = ΣS ⦵ (products) – ΣS ⦵ (reactants). Sum the standard entropies of all products and subtract the sum of standard entropies of all reactants, considering stoichiometric coefficients.

Definition Flip

State the Gibbs equation.

Answer Flip

The Gibbs equation is ΔG ⦵ = ΔH ⦵ – TΔS ⦵, where ΔG ⦵ is the Gibbs free energy change, ΔH ⦵ is the enthalpy change, T is the temperature in Kelvin, and ΔS ⦵ is the entropy change.

Key Concept Flip

How does the sign of ΔG predict the feasibility of a reaction?

Answer Flip

If ΔG is negative, the reaction is feasible (spontaneous). If ΔG is positive, the reaction is non-feasible (non-spontaneous). If ΔG is zero, the reaction is at equilibrium.

Key Concept Flip

Predict the effect of increasing temperature on the feasibility of a reaction if ΔH is positive and ΔS is positive.

Answer Flip

Increasing temperature will make the reaction more feasible (more likely to be spontaneous). Since ΔH is positive and ΔS is positive, as T increases, the -TΔS term becomes more negative, eventually making ΔG negative.

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23.1 Lattice energy and Born-Haber cycles 24.1 Electrolysis