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

\[q=mc\Delta T\]