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


Organic Chemistry

  • Aromatic Compounds

    • Compunds that contain one or more benzene rings

  • Aliphatic Compounds

    • Compounds that contain no benzene rings

  • Functional Group

    • A modified responsible for the characteristic chemical reactions of a molecule

  • Saturated

    • A hydrogen containing the maximum number of hydrogens possible, with no double or triple bonds

  • Homologous series

    • A series of compounds in which all members have the same general molecular formula