2When 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)
3Experiment50cm3 of H2OThermometerBalance2g NaOH or 4g NH4NO3
4Measurements Taken Volume (mass) of water Mass of substance dissolves Temperature of water at startTemperature of water at end
5Specimen 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.
61. 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
7Therefore 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.