4. Mixtures & Solutions

Why can’t we use ideal mixing (Ĥmix = Σxᵢ·Ĥᵢ) for HCl and water?

• It only works for gases and similar liquids.

• For dissimilar compounds, mixing breaks old intermolecular forces and forms new ones, so there is an extra enthalpy change (the heat of mixing) that is hard to predict.

Define the heat of solution

What are the two reference conditions for heat of solution data?

• (1) Pure solute and pure solvent at 25 °C.

• (2) Pure solvent and infinitely dilute solution.

• Both are per mole of solute, and the problem decides which to use.

What do you do when calculations are at temperatures different to the reference conditions (25 °C)