Definitions + Equations

Define enthalpy

The measure of the total energy of a thermodynamic system

Includes the internal energy of the system + the product of pressure and volume.

Define Enthalpy Change

The amount of heat released or absorbed during a reaction

What are the Standard Conditions?

Temperature - 298K or 25*C

Pressure - 1.01 kPa or 1 atm

Concentration - 1.00 mol/dm3

(Represented by the symbol θ)

Define Enthalpy change of atomisation, ΔatHθ

This is the enthalpy change when one mole of gaseous atoms forms from an element in its standard state

Define bond dissociation enthalpy

The enthalpy needed to break one mole of the bond to give separated atoms, with everything being in the gaseous state

Define Enthalpy change of lattice formation, ΔLeHθ

This is the enthalpy change when one mole of an ionic compound forms from its constituent gaseous ions

e.g. 2Na+(g) + O2– —> (g) Na2O(s)

Define Enthalpy change of hydration, ΔhydHθ

This is the enthalpy change when one mole of gaseous ions dissolves in enough water to give an infinitely dilute solution.

e.g. Li+(g) + aq —> Li+(aq)

Define Enthalpy change of solution, ΔsolHθ

This is the enthalpy change when one mole of an ionic substance dissolves completely in water to form a solution (understand conditions).

LiCl(s) + aq —> Li+(aq) + Cl–(aq)

What is the equation for determining enthalpy change of solution?

q = mc ΔT

or ΔsolHθ = ΔhydHθ - ΔLeHθ

Define Hess Law

Hess’ Law states that the enthalpy change going from reactants to products is independent of the route taken.

What happens when a solid dissolves in water?

When a salt dissolves in water, the water molecules overcome the electrostatic forces between the ions in the solid and break up the giant ionic lattice structure.

What are the two factors affecting the lattice enthalpy of an ionic crystal?

1. The size of the ion - small ions result in larger attraction between ions

2. The charge of the ion - higher charges result in larger attractions between ions