chem unit 3 revision

• Chemical equilibrium


  A state where the forward and reverse reactions occur at the same rate, resulting in constant concentrations of reactants and products
• Exothermic reaction


  A chemical reaction that releases energy as heat to the surroundings (ΔH = -ve)
• Exothermic reactions


  • 2H2(g) + O2(g) → 2H2O(g) + 572 kJ
  • C6H12O6(s) + 6O2(g) → 6CO(g) + 6H2O(l) ΔH = -2870kJ
• Endothermic reaction


  A chemical reaction that absorbs energy from the surroundings (ΔH = +ve)
• Endothermic reactions


  • H2O(s) → H2O(l) ΔH = 6kJ
  • C2H5OH(l) + 43kJ → C2H5OH(g)
• Enthalpy change (ΔH)


  The energy exchange between a chemical reaction and its surroundings at constant pressure
• Enthalpy change (ΔH) = enthalpy of products - enthalpy of reactants
• Rate of reaction


  The speed at which reactants are used up or products are formed
• Graph of rate of reaction


  • Steepest slope at start when concentrations are maximum
  • Curve steadily gets less steep as reactants are used up
  • Curve levels off at end when all reactants have been used
• Collision theory


  Reacting particles must collide in the correct orientation and with sufficient energy to overcome the activation energy barrier
• Successful collision


  • Correct orientation and sufficient energy to result in a reaction
• Activation energy (Ea)


  The minimum amount of energy molecules must have in order for the reaction to take place
• According to collision theory, a reaction will speed up if the frequency of collisions increases or the proportion of particles with energy greater than the activation energy increases
• Effect of concentration on rate of reaction


  More concentrated reactants have a faster rate of reaction due to more frequent collisions
• Effect of pressure on rate of reaction


  Increasing pressure of reacting gases increases the number of gas molecules in a given volume, leading to more collisions and a faster rate of reaction
• Effect of state of subdivision on rate of reaction


  Smaller size particles provide a larger surface area, increasing the chances of collisions and the rate of reaction
• Maxwell-Boltzmann distribution


  • At a given temperature, particles will not all possess the same amount of energy
  • Increasing temperature shifts the curve to the right and flattens it, increasing the proportion of particles with energy greater than the activation energy
• A 10°C rise in temperature approximately doubles the rate of a reaction
• Catalyst


  A substance that speeds up a chemical reaction by providing an alternative pathway with lower activation energy, without being consumed in the process
• Effect of catalyst on activation energy


  • Catalyst lowers the activation energy, increasing the proportion of particles with sufficient energy to react
• Catalyst does not affect the shape of the Maxwell-Boltzmann distribution
• Rate determining step


  The slowest step in a multi-step reaction, which governs the overall rate
• The rate equation may not contain all the reactants if there is more than one step
• Most chemical reactions are considered irreversible - the products cannot readily be changed back into the original reactants
• Reversible reaction


  A reaction that can go in either direction, where the products can react together to reform the original reactants
• Reversible reactions have low activation energies for both the forward and reverse reactions, and do not go to completion
• Characteristics of equilibrium


  • Requires a closed system
  • Is dynamic - forward and reverse reactions occur at the same rate
  • Concentrations of reactants and products remain constant
• In homogeneous equilibria, all reactants and products are in the same phase. In heterogeneous equilibria, reactants and products are present in more than one physical state
• For heterogeneous equilibria, the concentrations of pure solids and liquids are omitted from the equilibrium constant expression
• Interpreting the equilibrium constant


  • If K > 1 x 10^4, products are favored over reactants
  • If K < 1 x 10^-4, reactants are favored over products
• Types of equilibria


  • Homogeneous
  • Heterogeneous
• Homogeneous equilibria


  • Include all (g), (l), (s), (aq)
• Heterogeneous equilibria


  • Omit (s) and (l), include (g) and (aq)
• Solids and liquids are NOT included in the K expression of heterogeneous reactions. The concentration of a solid does not vary, nor does the concentration of liquid (water) in an aqueous solution.
• Interpreting the equilibrium constant

  If K > 1 x 10^4, products are favored over reactants

  If K < 1 x 10^-4, reactants are favored over products
• The value of K varies only with temperature
• For endothermic reaction


  K increases with increasing temperature
• For exothermic reaction


  K decreases with increasing temperature
• Factors that do not change K: concentration/partial pressure, total pressure, catalyst
• Factors that do change K: temperature