Topic 5.2: Calculation of enthalpy changes

5.2.1 Calculate the heat energy change when the temperature of a pure substance is changed

The heat energy change or enthalpy change is dependent on three factors:

1. The temperature change
2. The mass that changes temperature
3. The specific heat capacity of the mass

The final product is measure in kJ (energy).

Temperature change will simply be a difference in temperature (Celsius or Kelvin)

Mass is in Kg (In aqueous solutions, it will be the mass of water which is the same as it's volume in dm^3)

C is the specific heat capacity and in an aqueous solution this will be 4.18 kJ kg^-1 K^-1

The final equation is:

To change the final equation to enthalpy change, you need the equation to divide by the number of moles of the limiting reagent.

5.2.2 Design suitable experimental procedures for measuring the heat energy changes of reactions

The enthalpy changes of reaction in solution can be calculated by carrying out the reaction in an insulated system, for example, a polystyrene cup. The heat released or absorbed by the reaction can be measured from the temperature change of the water.

This additional statement is extremely important:

Errors due to heat loss to the surroundings can be minimized by plotting the temperature rise against time and then extrapolating the graph back to estimate the temperature rise for an instantaneous reaction.

5.2.3 Calculate the enthalpy change for a reaction using experimental data on temperature changes, quantities of reactants and mass of water

Using this equation, you can plot all the information and retrieve the enthalpy change.

Enthalpy change = [(Mass of water x 4.18 kJ kg^-1 K^-1 x temperature changes) = heat change]  / [(quantites of reactants / Molar mass of reactants) = moles]


5.2.4 Evaluate the results of experiments to determine enthalpy changes

This is similar to the syllabus statement 5.2.3 except you will be given data instead. Use the formula as a guideline to find what's important in the table.

Finding the moles would most likely be in the data booklet. Double check results with data booklet as well