Enthalpy of Solution - Key Terms (Edexcel International A Level Chemistry)

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Enthalpy of Solution - Key Terms

Enthalpy of solution

  • The standard enthalpy change of solution (ΔsolH) is the enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form an infinitely dilute solution
  • The symbol (aq) is used to show that the solid is dissolved in sufficient water
    • For example, the enthalpy changes of solution for potassium chloride are described by the following equations:

KCl (s) + aq → KCl (aq)

OR

KCl (s) + aq → K(aq) + Cl(aq)

  • ΔsolHcan be exothermic (negative) or endothermic (positive)

Enthalpy of hydration

  • The standard enthalpy change of hydration hydH) is the enthalpy change when 1 mole of a specified gaseous ion dissolves in sufficient water to form an infinitely dilute solution
    • For example, the enthalpy change of hydration for magnesium ions is described by the following equation:

Mg2+(g) + aq → Mg2+(aq)

    • Hydration enthalpies are the measure of the energy that is released when there is an attraction formed between the ions and water molecules
    • Hydration enthalpies are exothermic

  • When an ionic solid dissolves in water, positive and negative ions are formed
  • Water is a polar molecule with a δ- oxygen (O) atom and δ+ hydrogen (H) atoms which will form ion-dipole attractions with the ions present in the solution
  • The oxygen atom in water will be attracted to the positive ions and the hydrogen atoms will be attracted to the negative ions

Chemical Energetics - Ion-Dipole Bonds, downloadable AS & A Level Chemistry revision notes

The polar water molecules will form ion-dipole bonds with the ions in solution causing the ions to become hydrated 

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Stewart

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