3.4.2 Calorimetry

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 can

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:

Equation for calculating energy transferred in a calorimeter

Worked Example

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

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