5.4 a, d, e

5.4 Introduction

Y12 Chem

​What we will cover

• ​describe and analyse the processes involved in the dissolution of ionic compounds in water

• ​solubility and solubility rules

• ​solubility product

• ​Ksp and Q calculations

​Dissolution of Ionic Compounds

• ​separating of positive and negative ions from a solid ionic compound, forming hydrated ions when dissolved in water = dissociation

  • ​ionic bonds in lattice are broken

  • ​hydrogen bonds in water are broken

  • ​ion-dipole attractions form between ions and polar water molecules

​Dissolution of Ionic Compounds

• ​To answer questions, this is the process of dissolution:

  • Separation of the solvent particles: endothermic process as energy is absorbed to overcome and break intermolecular forces between molecules (mostly talking about water) - H bonds, dipole-dipole, and dispersion break apart.

  • Dissociation of ions: also endothermic as energy is absorbed to overcome and break apart forces present in ionic compound

  • Hydration of the ions: dissociated ions form intermolecular forces with the separated water molecules, releasing energy in process (exothermic process)

  • Dissociation will occur when the energy absorbed exceeds the ionic compounds lattice energy

  • Can be an exo or endothermic process, depending on energy released or absorbed overall

​Solubility

• ​soluble or insoluble

  • soluble if ​the energy required to separate the ions from the lattice is greater than the energy released when the ions are hydrated

• ​maximum amount of solute that can be dissolved in a given quantity of a solvent

• Slightly soluble - aka sparingly soluble

​Solubility Rules

• ​Revision from Year 11

• ​Reality is that ionic substances that are classified as insoluble, can still dissolve to some extent

• ​Solubility depends on the difference in the energy required to separate the ions from the lattice, and the energy released when the ions are hydrated

Multiple Choice

Considering the following precipitation reaction:

Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)

What is the correct complete ionic equation?

• Pb²⁺ + (NO₃)₂^- + 2K⁺ + 2I^- → PbI₂(s) + 2K⁺ + 2NO₃^-

• Pb²⁺ + 2NO₃^- +2K⁺ + I^- → PbI₂(s) + 2K⁺ + NO₃^-

• Pb²⁺ + 2NO₃^- + 2K⁺ + 2I^- → PbI₂(s) + 2K⁺ + 2NO₃^-

• Pb²⁺ + 2NO^3- + 2K⁺ + 2I^- → Pb²⁺ + 2I^- + 2K⁺ + 2NO₃^-

Multiple Choice

Which of the following is insoluble in water?

• Ammonium Hydroxide

• Barium Hydroxide

• Potassium Hydroxide

• Zinc Hydroxide

​Solubility Equilibrium

• ​different compounds have different solubilities

• because no ionic compound is completely insoluble, it may only dissolve a very tiny amount - at this point, the ionic solid and the ionic salt enter in a dynamic equilibrium

• the equilibrium constant for this type of system is called the solubility product, and is written as Ksp - using the salts and not the solid in the equation

​Solubility Product

• ​A large Ksp indicates high solubility - because there are large amounts of the ions at equilibrium

• A small Ksp indicates low solubility

• For example below, silver sulfide has the smallest solubility, and lead fluoride has the highest

​Calculating Solubility Product

​Calculating Solubility

Calculating Solubility Quotient (Q)

• ​Used when a dissociation reaction is not at equilibrium - same as regular reactions!

• When Q < Ksp - system is not saturated - more ions can dissolve

• When Q > Ksp - system is saturated - no more ions can dissolve

• This can help us determine whether there will be a precipitate or not

Open Ended

Open Ended

Open Ended

Open Ended

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Common Ion
Toxicity in foods
Pracs!