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

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

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

     

  • calorimeter can be made up of a polystyrene drinking cup, a vacuum flask or metal can

 

1.5-Chemical-Energetics-Calorimeter

 

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

 

 

 

  • A fixed volume of one reagent is added to the calorimeter and the initial temperature taken with a thermometer
  • An excess amount of the second reagent is added and the solution is stirred continuously
  • The maximum temperature is recorded and the temperature rise calculated

 

 

 

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

 

5.1.4-Simple-Calorimeter_1A 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

     

 

Exam Tip

For both types of calorimetry experiment you should be able to give an outline of the experiment and be able to process experimental data.

转载自savemyexams

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