Enthalpy Change Practical

Enthalpy Change Practical

1. Non-Combustion Reaction

   • Spontaneous reaction at room temperature
     • Salt + Water
       • Enthalpy change of solution
     • Acid + Alkali
       • Enthalpy change of neutralisation
   • Used for both exothermic and endothermic reactions

2. Combustion Reaction

   • Specifically measures the heat given out (exothermic)
   • Method
     • Measure the mass of the burner before and after
       • Thermometer records the temperature of the known mass of water
   • Limitations
     • Very Inaccurate
       • Huge losses of heat
         • Radiation from flame and calorimeter
         • Conduction through copper calorimeter
         • Convection from the flame gases passing by calorimeter
   • Used for the comparison of the heat output of different fuels
     • If same burner and set-up is used

3. Q = mcΔT

   • Calculation
     • For both experiements
       • Calculate the change in temperature
         • Apply mcΔT, using the heat capacity of and mass of water
           • Calculate the number of moles of reactants
             • Alcohol for combustion reaction, use mass = mr x mole
             • Q x 1/moles of burner
               • Put into KiloJoules
             • Solution, use moles = conc x vol

4. Free Energy

   • ΔG = ΔH - TΔS
     • ΔS(system) = S(products) - S(reactants)
       • Entropy
         • Measure of the disorder in a system
     • Use the temperature in Kelvin
       • 273 + T in degress
     • Hess's law cycles can also be applied
   • If ΔG is positive the reaction is not spontaneous at 298K