1.8.3 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

   Answer

   Step 1:

• • q = m x c x ΔT
  • m (of water) = 500 g
  • c (of water) = 4.18 J g^{-1 o}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

• 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:

ΔH = 

• 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, e.g. combustion

Answer

Step 1: Calculate the energy released by the organic liquid

• • Q = mcΔT
  • Q = 100 x 4.18 x (75 - 20)
  • Q = 22990 J
  • Q = 22.99 kJ

Step 2: Calculate the number of moles of the organic liquid

• • Number of moles = 0.0259 moles (to 3s.f.)

Step 3: Calculate the enthalpy change of combustion

• • Δ_cH^θ -887 kJ mol^-1 (to 3s.f.)
    • Remember, combustion is an exothermic process and will, therefore, be a negative enthalpy change value