Chemistry - Unit 2

• Chemical equilibrium


  A reaction is in equilibrium when the concentrations of reactants and products are constant
• Equilibrium constant (K)


  General expression: K = [products] / [reactants]
• Meaning of equilibrium constant value

  • K less than 1 - equilibrium lies more to the left (reactants) side
  • K approximately 1 - neither reactants nor products are favoured
  • K greater than 1 - equilibrium lies more to the right (products) side
• The numerical value of the equilibrium constant depends on the reaction temperature and is independent of concentration and/or pressure
• For endothermic reactions


  A rise in temperature causes an increase in K and the yield of the product is increased
• For exothermic reactions


  A rise in temperature causes a decrease in K and the yield of the product is decreased
• The presence of a catalyst does not affect the value of the equilibrium constant
• Chemical equilibrium


  The forward reaction (reactants to products) and the reverse reaction (products to reactants) do not stop, but the rates are equal
• Factors that can alter the position of equilibrium

  • Changing the concentrations of reacting substances
  • Changing the pressure of reacting gases
  • Changing the temperature
• A catalyst will speed up the rate of both the forward and the reverse reactions but has no effect on the position of equilibrium
• The equilibrium constant has no units as it represents a ratio
• Homogeneous equilibrium


  All the species are in the same gaseous phase
• Heterogeneous equilibrium


  The species are in more than one phase
• Equilibrium constants are independent of changes of concentrations or partial pressures of species in any given reaction
• Knowing the value of the equilibrium constant, K, allows us to determine the direction in which a reaction will proceed to achieve equilibrium and the ratio of the concentrations of reactants and products when equilibrium is reached
• For endothermic reactions

  Increasing the temperature favours the endothermic reaction and K will increase
• For exothermic reactions


  Increasing the temperature causes a decrease in K and the yield of the product is decreased
• Catalysts speed up the rates of the forward and reverse reactions but have no effect on the equilibrium constant or equilibrium position
• Acids and bases


  Fully dissociate in water to ions. Weak acids and bases only dissociate partially
• Strong acids


  • Nitric, sulfuric and hydrochloric acids
• Strong bases


  • Sodium hydroxide and potassium hydroxide
• Weak acids


  • Ethanoic acid, carbonic acid and sulfurous acid
• Weak bases


  • Ammonia and amines
• pH for strong acids and bases

  pH = -log[H]
• Acids


  • Proton donors
  • Monoprotic or monobasic acids can donate one hydrogen ion per molecule
  • Diprotic or dibasic acids can donate two hydrogen ions
• Degree of ionisation


  Ability of the non-ionised acid molecule to produce hydrogen ions. Determines if an acid is classified as strong or weak
• Ionisation of strong acids

  1. One-way arrow equation

  2. HNO3 → H+ + NO3-

  3. H2SO4 → 2H+ + SO42-
• Strength of an acid is not related to the concentration of the acid
• Weak acids


  Equilibrium mixture of non-ionised acid molecules, hydrogen ions and the conjugate base of the acid
• Ionisation of weak acids

  1. Reversible arrow equation

  2. CH3COOH ⇌ H+ + CH3COO-
• Bases and alkalis


  Substances that react with an acid and accept the proton from the acid are classified as bases. Soluble bases are called alkalis as they produce hydroxide ions in aqueous solution
• Strong bases


  Dissociate completely in aqueous solution, typically group 1 metal hydroxides
• Dissociation of sodium hydroxide


  NaOH → Na+ + OH-
• Weak bases


  Only partially dissociated in aqueous solution, form an equilibrium mixture
• Ionisation of ammonia


  NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)
• Acid dissociation constant, Ka


  Measure of how weak (or dissociated) an acid is. Smaller Ka means weaker acid
• pKa


  • log Ka, helps Ka become whole numbers. Larger pKa means weaker acid
• Equimolar solutions of weak and strong acids/bases have different pH, conductivity and reaction rates, but same stoichiometry
• Soluble salts


  Strong acid + strong base = neutral solution

  Weak acid + strong base = alkaline solution

  Strong acid + weak base = acidic solution
• Indicators


  Weak acids for which the dissociation can be represented as: HIn(aq) + H2O(l) ⇌ H3O+(aq) + In-(aq)

  Colour determined by ratio of [HIn] to [In-]