A2- Enthalpy change of Solution Flashcards
What is enthalpy change of solution?
The enthalpy change when one mole of the solid ionic compound is completely dissolved in water to infinite dilution
What 2 things happen when a substance dissolves?
- ionic bonds break
- new bonds form between solvent and substance
What solvents dissolve ionic compounds?
polar solvents (eg H2O)
What type of reaction is breaking bonds?
endothermic
What type of reaction is making bonds?
exothermic
What has to be true for an ionic substance to be able to dissolve?
the bonds formed are the same strength or greater than those broken
Why do ionic compounds tend to have exothermic enthalpies of solution?
as the exothermic reaction forming bonds is greater than the endothermic reaction breaking them
What 2 enthalpies can be used to calculate enthalpy change of solution?
- lattice dissociation
- hydration
What is enthalpy change of hydration?
the enthalpy change when one mole of gaseous ions is diluted to form one mole of aqueous ions
How would you work out enthalpy of solution?
- assume lattice is broken up to ions (enthalpy of lattice dissociation)
- assume ions are dissolved in water (enthalpy change of hydration)
Draw a solution enthalpy cycle for MgBr2
includes:
- lattice dissociation of MgBr2 to Mg2+ ions and 2Br- ions
- enthalpy of hydration for Mg2+ and 2 x Br-
- enthalpy of solution from MgBr2 to Mg2+ (aq) and 2Br-(aq)
For an exothermic enthalpy of solution, explain the change in solubility as temperature is increased
- exists in eqm, forwards reaction is exothermic
- will shift to decrease temperature, favouring the backwards reaction
- solubility decreases
What factors affect hydration enthalpy?
- size of ion
- charge of ion
What is enthalpy change of solution equal to?
enthalpy of l.d + enthalpy of hydration for all ions
Is enthalpy of solution endo or exothermic? Why?
exothermic, as energy is released forming an attraction between partial charges on water molecule and ions
