enthalpy change of solution

what is enthalpy change of solution

• The standard enthalpy change of solution (Δ_solH) is the enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form an infinitely dilute solution

what does charge of enthalpy change of solution depend on?

enthalpy change of solution can be positive or negative and depends on:

• the energy required to break up a lattice -positive enthalpy change

• the energy released as the gaseous ions are hydrated- negative enthalpy change

what is enthalpy change of hydration?

one mole of aqueous ions are formed from 1 mole of gaseous ions under standard conditions

what does enthalpy change of hydration measure?

• 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

entropy of enthalpy of lattice dissociation

• When an ionic compound dissolves in water, there are two changes that need to take place

  • The lattice structure needs to be broken down and the ions must become hydrated

• The breaking down of the lattice structure is endothermic (equivalent to the reverse lattice energy) it also results in an increased number of moles of particles present, therefore entropy will increase

entropy change during hydration of ions

• The hydration of the ions produces solvent cages and results in water molecules becoming more ordered as they arrange themselves around the cations and anions 

• This increasing in order decreases the entropy of the water 

• Therefore to explain why some ionic compounds are soluble and some are insoluble we must consider both entropy and enthalpy changes involved 

what does solubility depend on

• The entropy of the system, ΔS_{system. Total entropy must be positive}

• The enthalpy change of solution, Δ_solH

• The temperature, in K, of the water, T

trends in hydration enthalpy

• Ionic radius: Enthalpy change becomes less exothermic as Ionic size increases. Charge density decreases so the forces of attraction to water molecules are weaker .

• Ionic charge: Ionic radius decreases as the charge increases. As the charge increases, enthalpy of hydration becomes more negative/exothermic