Rate Equations

• zero order rate graph

  A) zero

  
• first order rate graph

  A)

  
• second order rate graph

  A)

  
• the Arrhenius Equation

  A) RT

  B) arrhenius constant

  C) activation energy

  
• rearranged arrhenius equation

  A) RT

  B) (lnA - lnk)

  
• determining the rate from a straight line graph:

  • calculate gradient
  • gradient = change in y/change in x
• determining the rate from a curve:

  • draw a tangent to the curve
  • calculate gradient
• orders of reaction - the power to which a concentration is raised. tells us how the concentration affects the rate
• zero order - has no effect on the rate
• first order - the change in concentration has a proportional effect on the rate of reaction
• second order - change in concentration has a squared proportional effect on the rate
• the rate constant K changes with temperature
• • if temperature increases, k increases
  • the larger the value of k, the faster the rate of reaction
• rate units - mol dm-3 s-1
• iodine clock experiment:

  • add sodium thiosulfate and starch
  • sodium thiosulfate reacts with the iodine produced
  • once Na₂S₂O₃ runs out, iodine reacts with starch
  • this gives the blue-black colour

  
• the rate determining step is the slowest step in a multi-step reaction
• • reactants that appear in the rate equation affect the rate of reaction
  • only these reactants appear in the rate determining step
• 
  

  • as the activation energy decreases, the rate constant increases
  • more particles have enough energy to collide and react
• units for the Arrhenius Constant: s-1
• Explain why doubling the temperature has a greater effect on the rate of the reaction than doubling [reactant].

  • a reaction occurs when molecules have energy greater than or equal to the activation energy
  • doubling T causes a lot more molecules to have this energy
  • doubling [reactant] doubles the number of molecules with the same energy, so they might not have sufficient energy to collide and react
• in acid-catalysed reactions, small portions of the reaction mixture are added to an excess of sodium hydrogen carbonate to:

  • neutralise all the acid
  • stops the reaction
• the order of reaction with respect to the reactant you aren't investigating can be ignored because:

  • the concentration is constant
  • doesn't affect the rate of reaction
  • concentration is much larger
• • For a 1st order overall reaction the unit of k is s -1
  • For a 2nd order overall reaction the unit of k is mol-1 dm3 s-1
  • For a 3rd order overall reaction the unit of k is mol-2 dm6 s-1
• justify the statement that adding more water to the equilibrium mixture will lower the amount of A

  • All concentrations decrease
  • equilibrium moves to the side with more moles to oppose the decrease in concentration
• explain how the graph shows that the order with respect to H+ is zero

  • rate/gradient is constant as [H+] decreases so concentration of H+ has no effect on the rate
• how to use a series of experiments to determine the order of reaction with respect to A

  • measure known volumes of some reagents and measure known amount of X into two separate beakers, add A, B, C last
  • start timer when both substances are mixed together
  • record the time taken for the blue colour to appear/when the cross disappears
  • same concentrations and volumes of B and C, same volume/amount of X
  • same temperature - use a water bath
  • repeat with different concentrations of A
  • calculate rate by doing 1/time taken
  • plot graph of 1/T against concentration of A
  • interpret order of reaction from the shape of the graph
• deduce the order of reaction from the graph

  • second order
  • [C6H5CHO]2 is directly proportional to rate