Measuring Enthalpy Changes

Measuring Enthalpy Changes

The type of apparatus used in the laboratory is shown below:

Measuring Enthalpy Changes

The following measurements are taken:

  1. Mass of cold water (g)
  2. Temperature rise of the water (K)
  3. The loss of mass of the fuel (g)

We know that it takes 4.2J of energy to raise the temperature of 1g of water by 1K. This is called the specific heat capacity of water, c, and has a value of 4.2Jg-1K-1.

Hence, energy transferred can be calculated using:

Energy transfer = mcΔT (joules)

If one mole of the fuel has a mass of M grams, then:

Enthalpy transfer = m x 4.2 x T x M/y

where y is mass loss of fuel.

The apparatus required is different to that used in measurements of combustion, since we are not burning a fuel.

In these experiments you need to:

  1. Use a polystyrene cup to act as an insulating vessel.
  2. Add known amounts of reactants and stir
  3. Measure temperature change using at thermometer that reads to at least 0.2oC accuracy.
  4. Calculate energy transfer using: Energy transfer = mcΔT (joules)