EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记：3.1.2 Calorimetry

• Calorimetry is a technique used to measure changes in enthalpy of chemical reactions
• There are two types of calorimetry experiments you need to know:
  • Enthalpy changes of reactions in solution
  • Enthalpy changes of combustion

   

Reactions in solution

• The principle of these calorimetry experiments is to carry out the reaction with an excess of one reagent and measure the temperature change over the course of a few minutes
• This technique is suitable for reaction such as dissolving, displacement and neutralisation
• For the purposes of the calculations, some assumptions are made about the experiment:
  • That the specific heat capacity of the solution is the same as pure water, i.e. 4.18 J g^-1 K^-1
  • That the density of the solution is the same as pure water, i.e. 1 g cm^-3
  • The specific heat capacity of the container is ignored
  • The reaction is complete
  • There are negligible heat losses

   

• A calorimeter can be made up of 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

Enthalpy of combustion experiments

• The principle here is to use the heat released by a combustion reaction to increase the heat content of water
• A typical simple calorimeter is used to measure the temperature changes to the water

A simple combustion calorimeter

• A fixed volume of water is measured and transferred to the copper can
• The spirit burner containing, for example, an alcohol, is weighed on a 2 d.p. balance
• The initial temperature of the water is measured and recorded
• The alcohol is ignited and the water is stirred continuously
• When the temperature has risen by about 20 degrees the flame is extinguished and the spirit burner is re-weighed
• The final temperature of the water is recorded

Sources of error

• Not all the heat produced by the combustion reaction is transferred to the water
  • Some heat is lost to the surroundings
  • Some heat is absorbed by the calorimeter

   

• To minimise the heat losses the copper calorimeter should not be placed too far above the flame and a lid placed over the calorimeter
• Shielding can be used to reduce draughts
• In this experiment the main sources of error are
  • Heat losses
  • Incomplete combustion