DF1

Energy Changes

• Exothermic reactions give out energy and have a negative enthalpy change

• Endothermic reactions take in energy and have a positive enthalpy change

Enthalpy Change

The enthalpy, $$H$$, of an individual substance cannot be measured. However, the enthalpy change can. $\Delta H = H_{\textup{products}}-H_{\textup{reactants}}$

Standard Conditions

• 298K (25$$^{\circ}$$C)

• 1 atm

• 1 moldm-3

• The standard state is the physical state of a substance under standard conditions

Standard Enthalpy Change for a Reaction

• The enthalpy change when molar quantities of reactants as stated in the equation react together under standard condtions

• $$\Delta_rH^{\ominus}_{298}$$

Standard Enthalpy Change of Combustion

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

• $$\Delta_cH^{\ominus}_{298}$$

Standard Enthalpy Change of Formation

• Then enthalpy change when one mole of a compound is formed from its element under standard states

• $$\Delta_fH^{\ominus}_{298}$$

Standard Enthalpy Change of Neutralisation

• The enthalpy change when one mole of hydrogen ions react with one mole of hydroxide ions to form one mole of water under standard conditions

• $$\Delta_{neut}H^{\ominus}_{298}$$

Measuring Enthalpy changes

• Use of a bomb calorimeter

• Measuring energy transferred to or from water surrounding a reaction vessel

$q=mc\Delta T$

DF2

Enthalpy Cycles

• Often difficult to measure $$\Delta H$$

• Uses Hess’ Law:

• The enthalpy change for any chemical reaction is independent of the intermediate stages, so long as the initial and final conditions are the same

An example enthalpy cycle