A level chemistry AQA - Energetics

Enthalpy definition

A thermodynamic property of a system linked to internal energy

Energy of reactants vs products in exothermic reactions

Energy of reactants is higher than energy of products

Energy of reactants vs products in endothermic reactions

Energy of products is higher than energy of reactants

Activation energy definition

The minimum energy required to break bonds in reactants before new bonds can form in the products (minimum energy required for reaction to start)

Standard enthalpy pressure

100kPa

Mean bond enthalpy definition

The measure of the energy required to break one mole of a specific type of a covalent bond in a gaseous molecule in kJmol^-1

Bond making

exothermic

Bond breaking

endothermic

Effect of bond length on bond enthalpy

The shorter the covalent bond, the shorter the bon and the higher the bond enthalpy

Hess Law

The enthalpy change for a chemical reaction is independent of the route taken and depends only on the initial and final states

Infinite separation definition

All bonds between ions must be broken to calculate enthalpy of lattice dissociation so the ions must be infinitely seperated.

Drawing Born-Haber cycles - direction of exothermic arrow

Down

Drawing Born-Haber cycles - direction of endothermic arrow

Up

Factors affecting lattice enthalpy values

Ion size, ion charge

How does ion size affect lattice enthalpy values?

Smaller ions are more closely packed together in the lattice and so are more attracted to each other

How does ion charge affect lattice enthalpy values?

Ions with a higher charge are attracted more strongly to each other

Assumptions of the Perfect Ionic Model

1. Assumes that bonding in the compound is 100% ionic and that the compound does not have any covalent character

2. The ions are regarded as point chargers and are not distorted