OCR A Level Chemistry

Revision Notes

3.4.2 Calorimetry

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Calorimetry & Specific Heat Capacity

Measuring enthalpy changes

  • Calorimetry is the measurement enthalpy changes in chemical reactions
  • A simple calorimeter can be made from a polystyrene drinking cup, a vacuum flask or metal canChemical Energetics Calorimeter, downloadable AS & A Level Chemistry revision notes

A polystyrene cup can act as a calorimeter to find enthalpy changes in a chemical reaction

  • The energy needed to increase the temperature of 1 g of a substance by 1 oC is called the specific heat capacity (c ) of the liquid
  • The specific heat capacity of water is 4.18 J g-1 K-1
  • The energy transferred as heat can be calculated by:Chemical Energetics Equation for Calculating Energy Transferred in Calorimeter, downloadable AS & A Level Chemistry revision notes

Equation for calculating energy transferred in a calorimeter

Worked example

Specific heat capacity calculations

In a calorimetry experiment 2.50 g of methane is burnt in excess oxygen.

30% of the energy released during the combustion is absorbed by 500 g of water, the temperature of which rises from 25 °C to 68 °C.

The specific heat capacity of water is 4.18 J g-1 K−1

What is the total energy released per gram of methane burnt?

Answer

Step 1

  • q = m x c x ΔT
  • m (of water) = 500 g
  • c (of water) = 4.18 J g-1 °C-1
  • ΔT (of water) = 68 oC - 25 oC = 43 oC

Step 2: 

  • q = 500 x 4.18 x 43 = 89 870 J

Step 3: 

  • This is only 30% of the total energy released by methane
  • Total energy x 0.3 = 89 870 J
  • Total energy = 299 567 J

Step 4:  

  • This is released by 2.50 g of methane
  • Energy released by 1.00 g of methane = 299 567 ÷ 2.50 = 120 000 J g-1 (to 3 s.f.) or 120 kJ g-1

Exam Tip

Aqueous solutions of acid, alkalis and salts are assumed to be largely water so you can just use the m and c values of water when calculating the energy transferred.

To calculate any changes in enthalpy per mole of a reactant or product the following relationship can be used:

ΔHq over n space or space fraction numerator m space cross times space c space cross times straight capital delta T over denominator n end fraction

When there is a rise in temperature, the value for ΔH becomes negative suggesting that the reaction is exothermic

  • This means that your value should be negative for an exothermic reaction, e.g. combustion

When the temperature falls, the value for ΔH becomes positive suggesting that the reaction is endothermic

  • This means that your value should be positive for an endothermic reaction

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