1.7 - equilibria

• Acid


  Proton donors. These species release hydrogen ions in solution.
• Base


  Proton acceptors. These species release hydroxide ions in solution.
• Dynamic equilibrium


  Reached when the rate of the forward reaction of a reversible reaction equals the rate of the backward reaction. The concentrations of the reactants and products remain constant.
• If the concentration of a reactant increases


  More products will be formed to re-establish the equilibrium
• If pressure is increased

  The position of equilibrium shifts towards the side with the fewest number of molecules
• If pressure is decreased


  The position of equilibrium shifts towards the side with the greatest number of molecules to oppose this change
• If the temperature of a system in equilibrium is increased


  There will be an increase in the relative amount of products for an endothermic reaction and a decrease for an exothermic reaction
• Kc


  A value that relates the concentrations of products and reactants present at equilibrium in a reversible reaction at a specific temperature. The equilibrium constant that is equal to the concentration of products raised to their stoichiometric coefficients divided by the concentration of reactants raised to the power of their stoichiometric coefficients.
• Le Chatelier's principle


  If a reaction at equilibrium is subjected to a change in concentration, temperature or pressure, the position of equilibrium will move to counteract the change.
• pH


  A value that represents the acidity or alkalinity of a solution. Acidic solutions have a pH of less than 7 while alkali solutions have a pH of greater than 7. Neutral solutions have a pH of 7.
• pH = -log[H+]


  [H+] = 10-pH
• Reversible reaction


  A reaction in which the products from the reaction can react together to form the original reactants. The direction of reversible reactions can be changed by changing the conditions.
• Strong acid


  An acid which dissociates/ionises almost completely in water. This means nearly all the H+ ions will be released. E.g. HCl.
• Strong base


  A base which dissociates/ionises almost completely in water. E.g. NaOH.
• Titration


  An experimental technique used to determine the concentration of an unknown solution by using a second solution with a known concentration.
• Weak acid


  Acids which only dissociate/ionise very slightly in water so that only a small number of H+ ions are released. E.g. Ethanoic acid.
• Weak base


  A base which only slightly dissociates/ionises in water. E.g. NH3
• 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
• Dynamic equilibrium


  If a system at equilibrium is subjected to a change then the position of equilibrium will shift to minimise that change
• Factors influencing 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 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, only 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