How fast

• Calculate Ea using gradient


  gradient x 8.314
• State what is meant by the term rate-determining step.


  slowest step
• A proposed mechanism for this reaction takes place in several steps. Suggest two reasons why it is unlikely that this reaction could take place in one step.

  rate equation does not match in overall equation

  Collision unlikely with more than 2 ions
• Why would use of excess HCOOH ensure that the order with respect to HCOOH is effectively zero?


  Concentration of HCOOH would be constant
• What is meant by half-life of a reaction 


  time for concentration to fall to half the original value 
• What is the rate of reaction?

  The change in concentration of reactants or products over time.
• oncentration−time graphs for zero and first order reactants

  A) zero order

  B) first order

  C) time

  D) concentration

  
• Rate−concentration graphs for zero, first, and second order reactants

  A) zero order

  B) first order

  C) rate

  D) concentration

  E) second order

  
• Variation of rate constant k with temperature T

  
• Arrhenius plot with ln k positive

  A) inA

  B) -Ea/R

  C) ink

  D) 1/T

  
• The student uses a large excess of CH3OH. The temperature is kept constant throughout the experiment.

  Explain why the student uses a large excess of methanol in this experiment.

  Zero order with respect to CH3OH
• zero order

  rate is constant throughout
• first order conc vs time

  half life constant
• the chemist proposes a three-step mechanism for the reaction:

  2H2(g) + 2NO(g) → N2(g) + 2H2O(g)

  step 1: 2NO → N2O2 fast

  step 2: H2 + N2O2 → N2O + H2O slow

  Explain why this mechanism is consistent with the rate equation rate = k[H2(g)][NO(g)]2

  Step 1 AND Step 2 together give 2NO + H2
• A proposed mechanism for this reaction takes place in several steps.

  Suggest two reasons why it is unlikely that this reaction could take place in one step.

  rate equation does not match stoichiometry in overall equation

  Collision unlikely with more than 2 species
• Why would use of excess HCOOH ensure that the order with respect to HCOOH is effectively zero

  Concentration of HCOOH would be constant
• what is stoichiometery

  the relative number of atoms and molecules which are involved in a chemical reaction (balancing numbers)
• what is the effect of temperature on the rate constant?

  as the temp increases, the rate increases so the value of K will also increase
• what are the factors affecting K?

  -more particles exceeding Ea when the temp increased, so Boltzmann distribution shifts to right.

  Particles move faster and collide more frequently as temp increases
• describe the procedure used to investigate reaction rates by continuous monitoring and colorimetry

  • prepare standard solutions of iodine
  • zero the colorimeter with water
  • measure the absorbance of standard solutions of iodine
  • plot a calibration curve of absorbance against iodine concentration
  • carry out the reaction and measure the absorbance at time intervals
  • use the calibration curve to measure the concentration of iodine at each reading
  • plot a graph of concentration against time
• how do you calculate half life?

  ln (2/k)
• explain why the order with respect to H2O is effectively zero

  the water is in excess
• how can a gas syringe be used to continuously monitor a reaction?

  attach airtight gas syringe

  
  

  measure gas volume in syringe at regular intervals

  USING STOPWATCH

  
  

  until constant gas volume

  
• how can change in mass be used to continuously monitor a reaction?

  mass of has lost from vessel

  
  

  measure mass at regular intervals

  USING STOPWATCH

  
  

  until constant mass

  
• gradient of curved graph

  draw a tangent

  
  

  calculate gradient

  
• how to find initial rate using tangent?

  draw a tangent at t = 0

  
  

  calculate gradient

  
• zero order

  
  

  concentration-time graph

  if concentration doubled

  
  

  rate DOES NOT change

  
• zero order

  
  

  rate-concentration graph


  if concentration doubled

  
  

  rate DOES NOT change

  
• first order

  
  

  concentration-time graph

  if concentration doubled

  
  

  rate ALSO DOUBLES

  
• first order

  
  

  rate-concentration graph

  rate is directly proportional to the reactant concentration (straight diagonal line)

  
  

  
  

  if concentration doubled

  
  

  rate ALSO DOUBLES

  
• second order

  
  

  concentration-time graph

  if concentration doubled

  
  

  rate QUADRUPLED

  
• second order

  
  

  rate-concentration graph

  Rate is proportional to the reactant concentration squared (curved line getting steeper)

  
  

  if concentration doubled

  
  

  rate QUADRUPLED

  
• units for rate


  mol dm-3 s-1
• predicting rate-determining step

  
  

  example --> original equation:

  2N2O5 --> 4NO2 + O2

  
  

  rate equation:

  rate = k[N2O5]

  e.g. N2O5 --> NO2 + NO3

  
  

  NO3 + N2O5 --> 3NO2 + O2
• Arrhenius plot


  x axis = 1/T (K^-1 units)

  
  

  y axis = In k

  
• what is the exponential factor?

  proportion of molecules with sufficient energy reaction to occur