exam questions mistakes

• = G

  • there is no linear relationship as you increase the temperature the rate of reaction gets dramatically faster

  
• Explain qualitatively why doubling the temperature has a much greater effect on the rate of the reaction than doubling the concentration of E

  • reaction only occurs when energy of the molecules is greater than the activation energy
  • doubling the temp causes many more molecules to have this greater amount of energy
  • while doubling the conc of E only doubles the number with this energy
• Suggest why the 1.0 cm3 portions of the reaction mixture are added to an excess of NaHCO3 solution.

  • The sodium hydrogencarbonate solution neutralises the acid
  • in order to stop the reaction

  
• Suggest why the order of this reaction with respect to propanone can be ignored in this experiment

  • The concentration of propanone is much larger than iodine
  • the concentration of propanone is constant
• Explain how the graph shows that the reaction is zero-order with respect to iodine in the reaction between propanone and iodine.

  • the graph is a straight line (with a constant gradient)
  • so rate does not change as concentration changes

  
• Define the term overall order of reaction

  • sum of powers/ individual orders by which the concentration are raised to in the rate equation
• Explain why the use of a large excess of H2O2 and I– means that the rate of reaction at a fixed temperature depends only on the concentration of H+ (aq)

  • so H2O2 and I- concentrations are constant
  • if they are constant that wont have an effect on rate of reaction
• Samples of the reaction mixture are removed at timed intervals and titrated with alkali to determine the concentration of H+ (aq).

  State and explain what must be done to each sample before it is titrated with alkali.

  • need to stop the reaction
  • by dilution/ cooling
• to find rate of reaction from a graph find its gradient (change in y/ change in x)
• working out Ea from a graph

  1. change in y/ change in x
  2. Gradient = -Ea / R
  3. -Ea = gradient x 8.31

  
• Suggest why initial rates of reaction are used to determine these orders rather than rates of reaction at other times during the experiments.

  • (At time zero/start) the concentrations are known
• State how the initial rate is obtained from a graph of the concentration of the product against time.

  • Calculate gradient of tangent/curve
  • at t=0 or at start of graph/curve
• describe and explain the shape of the graph:

  • as temp increases rate of reaction (1/t) increases
  • at an exponential rate
  • because a significantly greater proportion of particles have energy greater than activation energy
  • more frequent successful collisions