General method • washes the equipment (cup and pipettes etc) with the solutions to be used • dry the cup after washing • put polystyrene cup in a beaker for insulation and support • clamp thermometer into place making sure the thermometer bulb is immersed in liquid • measure the initial temperatures of the solution or both solutions if 2 are used • transfers reagents to cup. If a solid reagent is used then add the solution to the cup first and then add the solid weighed out on a balance. • stirs mixture • Measures final temperature One type of experiment is one in which substances are mixed in an insulated container and the temperature rise measured. This could be a solid dissolving or reacting in a solution or it could be two solutions reacting together Errors in this method • heat transfer from surroundings (usually loss) • approximation in specific heat capacity of solution. The method assumes all solutions have the heat capacity of water. • neglecting the specific heat capacity of the calorimeter- we ignore any heat absorbed by the apparatus. • reaction or dissolving may be incomplete or slow. • Density of solution is taken to be the same as water. C. If the reaction is slow then the exact temperature rise can be difficult to obtain as cooling occurs simultaneously with the reaction To counteract this we take readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together. We also take the temperature of the reactants for a few minutes before they are added together to get a better average temperature. If the two reactants are solutions then the temperature of both solutions need to be measured before addition and an average temperature is used. • Heat losses from calorimeter • Incomplete combustion of fuel • Incomplete transfer of heat • Evaporation of fuel after weighing • Heat capacity of calorimeter not included • Measurements not carried out under standard conditions as H2O is gas, not liquid, in this experiment
3.2.1 Enthalpy changes
(h) the techniques and procedures used to determine enthalpy changes directly and indirectly.
3 Enthalpy determination • Measurement of temperature Determination of the enthalpy change of neutralisation 3.2.1(e), 3.2.1(h)