Enthalpy Changes in Solution Ionic Compounds

Enthalpy Changes in SolutionSlide 2 of 7 Enthalpy of Solution Definition –The enthalpy change that takes place when 1 mole of a solute dissolves in a solvent to form an ‘infinitely’ dilute solution. NaCl(s) + aq  NaCl(aq)  H sol = +5 kJ mol -1  H sol - Enthalpy of Solution.  H sol can be exothermic or endothermic.  H sol can be measured experimentally.

Enthalpy Changes in SolutionSlide 3 of 7 Enthalpy of Solution The sum of two imaginary steps –reverse of the Lattice Enthalpy (as we define it). –the Hydration Enthalpy (of the cations and anions).  H LE - Lattice Enthalpy  H hyd - Hydration Enthalpy

Enthalpy Changes in SolutionSlide 4 of 7 Lattice Enthalpy Definition –The enthalpy change that takes place when 1 mole of a solid ionic lattice forms from its gaseous ions. Na + (g) + Cl - (g)  Na + Cl - (s)  H LE = -776 kJ mol -1  H LE is defined exothermically. The more closely ions pack together in the solid lattice the more exothermic is  H LE. The smaller and more highly charged the ions (the greater their charge density) the closer they pack.

Enthalpy Changes in SolutionSlide 5 of 7 Enthalpy of Hydration Definition –the enthalpy change that takes place when 1 mole of gaseous ions become hydrated (surrounded by water molecules). Na + (g) + aq  Na + (aq)  H hyd = kJ mol -1 The smaller and more highly charged the ions (the greater their charge density), the more exothermic is  H hyd.

Enthalpy Changes in SolutionSlide 6 of 7 Enthalpy of Solution Calculation –the sum of two imaginary steps  H  sol = -  H  LE + (  H  hyd (cation) +  H  hyd (anion) ) standard conditions of temperature and pressure (298K, 1atm.) are implied above. –Thermochemical definitions often refer to Standard Molar Enthalpy Changes (changes measured under standard conditions). an enthalpy diagram can be drawn to represent the calculation. –This is illustrated on the next slide.

Enthalpy Changes in SolutionSlide 7 of 7 Enthalpy of Solution Enthalpy Diagram NaCl(s) + aq Na + (g) + Cl - (g) -  H LE = +776 kJ mol -1 Na + (aq) + Cl - (aq)  H hyd = -771 kJ mol -1  H sol = +5 kJ mol -1 Click to complete the diagram.  H sol = -  H LE + (  H hyd (cation) +  H hyd (anion) )  H sol = = + 5 kJ mol -1