Predicting solubility from the enthalpy and entropy of solution A2 Unit 4.4 How far? Entropy p48-49

Dissolving and enthalpy changes Warning: Hess’s law is back, please remain calm!

Dissolving and enthalpy changes There are three key enthalpy changes involved when a substance dissolves: Enthalpy change of solution ∆Hsol Lattice enthalpy ∆Hlatt Enthalpy change of hydration ∆Hhyd

Lattice energy Explain it: Standard lattice enthalpy (∆Hθlatt)is the energy change when 1 mole of a solid ionic compound is formed from gaseous ions, under standard conditions How is lattice dissociation enthalpy related? What would be the difference between the two? Explain it: when the ionic radii increases (cation and anion), the lattice energy decreases the bigger the charge on the ion, the higher the lattice enthalpy

Enthalpy of hydration Explain it: The energy released when 1 mole of gaseous ions dissolve in water under standard condition to produce a 1.0 mol dm-3 solution. Mg2+(g) → Mg2+(aq) Always -ve Explain it: The smaller and more highly charged the ion, the more negative the hydration enthalpy What interactions take place between the ions and water molecule?

Enthalpy of solution The enthalpy change when 1 mole of a solute is dissolved in a sufficient volume of solvent that no further enthalpy change occurs on dilution. NaCl(s) → Na+(aq) + Cl-(aq) This can’t be measured directly – you need an ENTHALPY CYCLE Difference between the energy needed to separate the ions from the lattice and the energy given out when the ions are hydrated

Practical 4.5 Draw a Hess cycle linking enthalpy of solution, the hydration enthalpies and the lattice enthalpy for calcium chloride.

Questions State two assumptions made during this experiment State the main source of error during the experiment Explain why hydration enthalpies are negative values State and explain the relationship between enthalpy of solution values and the solubility of a salt Apart from lattice and hydration enthalpies what other factor is important in determining the solubility of a salt in water?

Apply it Pg 49 Q1 & Q2

Predicting solubility We can use ∆G = ∆H - TΔS to predict whether an ionic compound will dissolve under standard conditions e.g. does silver bromide dissolve in water at 298K? ΔSsys = + 47.7 J mol-1 K-1 , and ∆H = + 84.4 kJ mol-1 and does sodium bromide dissolve in water at 298K? ΔSsys = + 55.0 J mol-1 K-1 , and ∆H = -0.6 kJ mol-1

Learning objectives define the term enthalpy of hydration of an ion and use it and lattice energy to calculate the enthalpy of solution of an ionic compound understand the factors that affect the values of enthalpy of hydration and the lattice energy of an ionic compound use entropy and enthalpy of solution values to predict the solubility of ionic compounds

Jan 2010 Q25