AS Chemistry – Revision Notes
Unit 2 – Foundation Physical And Inorganic Chemistry
1. An enthalpy change is a change in energy at constant pressure (i.e. in normal ‘open container’
conditions where there is no change in pressure during the reaction).
2. A positive enthalpy (DH) signifies an endothermic reaction, whereas a negative enthalpy will be given by an exothermic reaction.
3. The energy released (or absorbed) in a reaction can be calculated by E = mcDT , where c is the specific heat capacity of the reaction mixture (usually taken as water, i.e. 4200 Jkg–1K–1). The enthalpy for the reaction in Jmol–1 can then be calculated by dividing E by the number of moles reacting.
4. A simple calorimeter measures the energy released in a reaction by using it to heat a mass of water – the increase in temperature of the water gives the energy released by the reaction. For combustion of alcohols, the mass of the burner before and after gives the mass of alcohol used, which can be used to calculate the moles used.
5. Standard enthalpy of combustion is the enthalpy change when one mole of a substance in its standard state is completely burned in air or oxygen to produce the products in their standard states, under standard conditions (298K, 101kPa). This is DHo
6. Standard enthalpy of neutralisation is the enthalpy change when one mole of water is produced when an acid neutralises an alkali under standard condition (298K, 101kPa). This is DHo neut.
7. Standard enthalpy of formation is the enthalpy change when one mole of a substance in its standard state is formed from its constituent elements in their standard states under standard conditions (298K, 101kPa). This is DHo
8. Hess’ Law states that the enthalpy change of a reaction depends only upon the initial and final states of the reaction and is independent of the route the reaction takes.
9. Bond dissociation enthalpy is the enthalpy change when one mole of a substance in its gaseous state is split into individual gaseous atoms.
10. The mean bond enthalpy is the average enthalpy required to break or make one mole of a bond (e.g. C–H, C–C etc.). This is taken as an average from many (but not all) molecules containing the bond.
11. Mean bond enthalpies can be used to estimate enthalpy…