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?°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

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