RP2 - Measuring enthalpy change Flashcards
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
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
Measure out desired volumes of solutions with volumetric pipettes and transfer to
insulated cup
clamp thermometer into place making sure the thermometer bulb is immersed in solution measure the initial temperatures of the solution or both solutions if 2 are used. Do this
every minute for 2-3 minutes
At minute 3 transfer second reagent 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.
If using a solid reagent then use ‘before and after’ weighing method
stirs mixture (ensures that all of the solution is at the same temperature) Record temperature every minute after addition for several minutes
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.
errors in method
energy 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 energy absorbed by the apparatus.
* reaction or dissolving may be incomplete or slow.
* density of solution is taken to be the same as water.