Equilibria & Acid & Bases

• Reversible reaction


  A reaction which can be made to go in either direction depending on the conditions
• Dynamic equilibrium


  When the rate of the forward reaction and backward reaction is the same; there is no further change in the concentrations of reactants and products
• Position of equilibrium


  The proportion of products to reactants in an equilibrium mixture
• If a system at equilibrium is subjected to a change then the position of equilibrium will shift to minimise that change
• If you change the conditions in a way which changes the relative rates of the forward and backward reactions, you change the position of equilibrium, i.e. the proportion of products to reactants in the equilibrium mixture
• Factors that influence the position of equilibrium


  • Concentration
  • Pressure
  • Temperature
• A catalyst decreases the time it takes to reach equilibrium but does not alter the position of equilibrium
• Le Chatelier's principle


  If the concentration of a reactant is increased, the position of equilibrium moves to the right and more products are formed
• Equilibrium reaction


  • 2CrO4(2-)(aq) + 2H+(aq) ⇌ Cr2O7(2-)(aq) + H2O(l)
• Adding hydrochloric acid


  Position of equilibrium moves to the right, more H+ ions added so equilibrium shifts to decrease the concentration of the H+ ions
• Adding sodium hydroxide


  Position of equilibrium shifts to the left, concentration of H+ ions decreases
• Increasing pressure


  Position of equilibrium moves to whichever side of the equation has fewer gas molecules
• Equilibrium reaction


  • 2NO2(g) ⇌ N2O4(g)
• Increasing temperature


  Position of equilibrium moves in the endothermic direction
• Equilibrium reaction


  • N2(g) + 3H2(g) ⇌ 2NH3(g)
• Kc


  Equilibrium constant in terms of concentration
• Solids are never included in the expression for Kc
• The value of Kc is not affected by pressure or concentration, it is only affected by temperature
• Equilibrium reaction


  • CH3CO2H(aq) + C2H5OH(aq) ⇌ CH3CO2C2H5(aq) + H2O(l)
• Acid


  A proton (H+) donor
• Base


  A proton (H+) acceptor
• Strong acid


  Fully dissociated (or ionised) in aqueous solution
• Strong acid


  • HCl(aq) → H+(aq) + Cl-(aq)
• Weak acid


  Only partially dissociated in aqueous solution
• Weak acid


  • CH3COOH(aq) ⇌ CH3COO-(aq) + H+(aq)
• Concentrated acid


  Consists of a large quantity of acid and a small quantity of water
• Dilute acid


  Contains a large quantity of water
• pH


  A measure of the concentration of aqueous hydrogen ions, H+(aq)
• pH


  pH = -log[H+]
• pH calculation


  • pH = -log(3.9 x 10-6) = 5.4
• Aqueous hydrogen ion concentration calculation


  • [H+] = 10-pH = 10-2.2 = 6.3 x 10-3 mol dm-3
• Standard solution


  A solution whose concentration is accurately known
• Preparing a standard solution

  1. Calculate mass of solid required

  2. Accurately weigh solid into weighing bottle

  3. Transfer solid into beaker and dissolve

  4. Pour solution into volumetric flask

  5. Add water to graduation mark

  6. Mix thoroughly
• Titration


  Used to calculate the exact concentrations of acid or base solutions
• Performing a titration


  1. Pour acid into burette

  2. Use pipette to add measured volume of base into conical flask

  3. Add indicator

  4. Run acid from burette into flask, stop at endpoint

  5. Read final burette volume, calculate titre

  6. Repeat until consistent readings
• Pipette


  Measures a set volume of solution
• Burette


  Measures different volumes and allows adding solution drop by drop