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Enthalpy Changes in Solution

Enthalpy Changes in Solution. Ionic Compounds. 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

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Enthalpy Changes in Solution

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  1. Enthalpy Changes in Solution Ionic Compounds

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

  3. 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). • HLE - Lattice Enthalpy • Hhyd - Hydration Enthalpy Enthalpy Changes in Solution

  4. Lattice Enthalpy • Definition • The enthalpy change that takes place when1 mole of a solid ionic lattice forms from its gaseous ions. Na+(g) + Cl-(g)  Na+Cl-(s) HLE = -776 kJ mol-1 • HLE is defined exothermically. • The more closely ions pack together in the solid lattice the more exothermic is HLE. The smaller and more highly charged the ions (the greater their charge density) the closer they pack. Enthalpy Changes in Solution

  5. 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) Hhyd = - 406 kJ mol-1 • The smaller and more highly charged the ions (the greater their charge density), the more exothermic is Hhyd. Enthalpy Changes in Solution

  6. 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 Solution

  7. Enthalpy of Solution Click to complete the diagram. • Enthalpy Diagram Na+(g) + Cl-(g) Hhyd = -771 kJ mol-1 -HLE = +776 kJ mol-1 Na+(aq) + Cl-(aq) Hsol = +5 kJ mol-1 NaCl(s) + aq Hsol = -HLE + (Hhyd(cation) + Hhyd(anion)) Hsol = - -776 + -771 = + 5 kJ mol-1 Enthalpy Changes in Solution

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