Module 5.1.1- How fast?

• rate of reaction

  the change in concentration of a reactant or product per unit time
• rate of reaction units

  mol dm-3 s-1
• overall order


  the sum of the orders of all reactants in a chemical reaction

  
• rate constant

  k, proportionality constant in the rate equation
• half-life


  time taken for half a reactant to be used up
• Zero order


  changing concentration of reactant has no effect on the rate rate= k
• First order


  rate is directly proportional to the concentration of the reactant term - 5 rate= k [A]
• Second order


  rate depends on its concentration raised to the power of two rate= k [A]^2
• Units of the rate constant (k)

  1) rearrange equation for k

  2) substitute units into expression

  3) cancel common units out

  
• Example: Units of first order reaction


  rate = k[A]rate/ [A] = k (rearrange for k)mol dm^-3 s^-1 / mol dm^-3 (substitute units)s^-1 = units for first order
• Initial rate


  the instantaneous rate determined just after the reaction begins at t=0
• Rate constant from experimental results

  
• Continuous monitoring of rate

  - gas collection- mass loss- colorimeter
• Concentration time graph- Zero order


  Zero order- straight line, negative gradient, reaction rate remains unchanged

  
• Concentration time graph- First order


  First order- downward curve with decreasing gradient, constant half-life

  
• Concentration time graph- Second order


  Second order- steeper (than 1) downward curve with a decreasing gradient

  
• Determining k from a first order reaction (1)

  rate= k [N2O5(g)] rearrange for k

  k= rate/ [N2O5(g)]work out the rate by working out the gradient using a tangent. use conc at the point of the tangent
• Determining k from a first order reaction (2)

  
• Rate concentration graph- zero order


  straight horizontal line

  because rate does not change with changing concentration

  
• Rate concentration graph- first order


  rate is directly proportional to concentration

  
• Rate concentration graph- second order


  upward curve with increasing gradient

  
• Initial rates method


  An experimental technique that can be used to work out the orders of a reaction.
• Initial rates method- Clock reaction


  a reaction in which some visible change takes place in the reaction mixture when a certain point in the reaction is reachedinitial rate is proportional to 1/t
• Iodine clock reaction


  As aqueous iodine is coloured orange-brown, the time from the start of the reaction and the appearance of the iodine colour can be measured. Starch usually added -> blue-black
• How accurate clock reactions


  the accuracy of a clock reaction is dependent in the time measured
• rate determining step


  the slowest step in a reaction mechanism
• reaction mechanism


  A series of steps that, together, make up the overall reaction.
• Explain why this mechanism is consistent with the kinetics data' (question)


  Because the stoichiometry of reactants

  in the slowest step

  (or ‘up to and including the slowest step’, if the RDS isn’t the first step) matches their orders in the rate equation.
• Predicting reaction mechanisms

  The rate equation only includes reacting species involved in the rate-determining step.The orders in the rate equation match the number of species involved in the rate-determining step.
• Hydrolysis of tertiary haloalkanes

  
• As temperature increases, the rate ________ and therefore the rate constant k _______

  increases, increases
• How does temperature affect rate constant

  As temperature increases, particles move faster due to increased KE and collide more frequently

  Increasing temperature shifts the Boltzmann distribution graph to the right

  increase proportion of particles to exceed the activation energy Ea.

  (change in rate is mainly determined Ea)

  Particles must collide in the same orientation
• rate constant (k) vs temperature graph

  exponentially increases

  
• Another form of Arrhenius equation


  ln k= -Ea/RT + ln A

  
• y value of Arrhenius equation

  ln k
• m value of Arrhenius equation

  -Ea/R
• x value of Arrhenius equation

  1/T
• c value of Arrhenius equation

  ln A
• Arrhenius equation graph


  plot ln k against 1/t straight line graph

  gradient = -Ea/R

  intercept = ln A

  
• How can you work out the activation energy from an Arrhenius graph?

  Work out gradient of the line

  gradient= -Ea/R

  Rearrange for Ea