4.2 - Measuring Enthalpy Changes

Question 1

Use of enthalpy change calculation:

Answer 1

Question 2

determining the temperature change

Answer 2

1. Plot the data - record temperature against time
2. Establish baseline - draw a line of best fit for the initial temperature readings before the reaction commences
3. Post reaction baseline - after peak reaction temperature is reached, draw another line of best fit for the cooling period
4. Extrapolation - extend both lines to intersect at the reaction’s start time
5. True temperature change - difference between the two extrapolated lines of best fit at the start of the reaction - giving the accurate temperature change

Question 3

Solution calorimetry

Answer 3

1. Add a measured volume of one reactant to a polystyrene cup calorimeter and record initial temperature
2. Add a measured volume of a second reactant and quickly seal the container with a lid
3. Carefully stir the mixture with the thermometer and record temperatures at regular intervals over time as heat is released or abandoned

Note - polystyrene cup is a better insulator, lightweight, waterproof and low SHC compared to glass which reduces heat loss to the surroundings and minimise the heat absorbed by the calorimeter itself

Question 4

Common inaccuracies in enthalpy change

Answer 4

• heat loss - heat escaping to the surroundings meaning less water is absorbed and leads to a lower measured enthalpy change
• incomplete combustion - when fuel isn’t completely burned
• fuel evaporation - if some fuel evaporates before burning - reduces the amount of heat generated and decreases the enthalpy change value

Question 5

Combustion calorimetry process

Answer 5

1. Weigh a fuel sample and burn it underneath a calorimeter containing a known mass of water
2. As the fuel combusts, heat is transferred to the water and a thermometer measures the temperature change
3. Temperature change along with the mass and heat capacity of water are used to calculate the heat energy

Question 6

How do you measure enthalpy changes with calorimetry? (combustion and solution)

Answer 6

measure the energy transferred in a reaction as heat using an apparatus (calorimeter) which determines enthalpy changes
* combustion calorimtry - used to find the enthalpy change when a fuel is burned
* solution calorimetry - reactions occurring in aqueous solutions like enthalpy change of neutralisation