2 When substances dissolve in water, an energy change usually occurs. Enthalpy of solution is defined as the enthalpy change when 1 mole of a substance completely dissolves (kJ mol-1)
3 Experiment50cm3 of H2OThermometerBalance2g NaOH or 4g NH4NO3
4 Measurements Taken Volume (mass) of water Mass of substance dissolves Temperature of water at startTemperature of water at end
5 Specimen Results1.03g of NaOH was dissolved in 50g (0.05kg) of water. The temperature of water rose by 5oC.Calculate the enthalpy of solution of NaOH.
6 1. Calculate energy change EH = c x m x ΔT= 4.18x0.05x5= kJ2. Calculate the number of moles of NaOH1 mole NaOH=40gso 1.03g=1.03/40= mol3. Calculate enthalpy of solution0.026 moles: 1.045kJ1 mole : 40.2kJ
7 Therefore the Enthalpy of solution= -40.2kJ Note that HSOLN can be a +ve or –ve value.You can work out which it is from the question.