Calculation of Enthalpy Changes
Calculate the heat energy change when the temperature of a pure substance is changed
Specific Heat Capacity – Symbol c – The amount of heat energy required to raise the temperature of 1.00g of the substance by 1.00°C or 1.00K
The specific heat capacity for some common substances is shown below
The equation to calculate the heat energy change is
q = heat energy change – J
m = mass of the substance – g
c = specific heat capacity of the substance – J °C-1 g-1 or J K-1 g-1
ΔT = change in temperature – °C or K
For the equation to work, all these figures must relate to the one substance
Design suitable experimental procedures for measuring the heat energy changes of reactions
Here, you need to know two types of experiments. One involves a reaction in aqueous solution and the other involves combustion reactions.
A simple Calorimetry is the most appropriate method here. In this type of procedure, the heat energy released is measured
The heat transfers to the can of water holding the reactants; however this is an incomplete transfer. The calculations of enthalpy change are based on the temperature change of the water and the mass of the fuel.
Reactions in Aqueous Solutions
These reactions can be done in a simple calorimeter, such as a Styrofoam cup. The volume of water must be accurately known for accurate calculations.
There will always be error in this measurement as heat is lost to the surroundings, as well as the can
Combustion Reactions
For these reactions, we measure the amount of heat produced by the combustion of one mole of the fuel. Note that this is not necessarily the same as the enthalpy of combustion. The heat of combustion for the reaction below is actually 5464 kJ mol-1, however the enthalpy is twice this because there are two moles of octane involved.
When the fuels are used in the form of complex mixtures, the heat of combustion is expressed as kJ g-1 or MJ dm-3
Some examples of the heat of combustion for certain compounds are given below
Calculate the enthalpy change for a reaction using experimental data on temperature changes, quantities of reactants and mass of water
In thermochemical equations, the value for ΔH is always given for the equation exactly as it is written. Therefore, if you double the equation, then ΔH is also doubled. Also, if the reaction is reversed [swapping the products and the reactants], then the sign changes.
Compare the equations below
For example:
Calculate the amount of energy released when 500cm3 of methane gas at STP reacts with excess air according to the equation
Therefore, the energy released in this combustion reaction is 19.9 kJ
Evaluate the results of experiments to determine enthalpy changes
Basically what you need to do here is take the results from an experiment, put them into the equation, and do the appropriate calculations.
Once you have done this, you also need to be able to consider the problems or errors with the results. These should include:
- In combustion reactions, heat is lost to the surroundings
- There is a great deal of error in this method
o Does not allow for the heat energy absorbed by the can or polystyrene cup
- Therefore, the calculated heat energy change is inaccurate