Chap 6 (enthalpy changes)

Exothermic reaction

Chemical reactions that transfer heat energy to the surroundings. The value of 🔼H is negative

Endothermic reaction

Chemical reactions that absorb heat energy from the surroundings. The value of 🔼H is positive

Enthalpy change, 🔼H

The heat energy transferred during a chemical reaction

Reaction pathway diagram

shows the relative enthalpies of the reactants (on the left) and the products (on the right) and the enthalpy change as an arrow. It may also include the activation energy

Activation energy, Ea

The minimum energy that colliding particles must possess to break bonds to start a chemical reaction

Standard conditions

A pressure of 101 kPa and temperature of 298K, shown by ø

Standard enthalpy change of reaction, 🔼Hr

The enthalpy change when the amounts of reactants shown in the stoichiometric equation react to give products under standard conditions

Standard enthalpy change of formation, 🔼Hf

The enthalpy change when one mole of a compound is formed from its elements under standard conditions

Standard enthalpy change of combustion, 🔼Hc

The enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions

Standard enthalpy change of neutralisation, 🔼Hneut

The enthalpy change when one mole of water is formed by the reaction of an acid with an alkali under standard conditions

Specific heat capacity, c

the energy needed to raise the temperature of 1g of a substance by 1°C (by 1K)

Hess's Law

The enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions and states of reactants and products are the same for each route.

Exact bond energy

the energy needed to break a specific covalent bond in a named molecule in the gaseous state. E.g. the O-H bond to water. Also called the bond dissociation energy or bond enthalpy

Average bond energy

the average energy needed to break a specific covalent bond averaged from a variety of molecules in the gaseous state.