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  • Chemistry, DF Revision Notes


    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

    \[q=mc\Delta T\]


    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