Rate equations

• Define rate of reaction

  The change in the concentration of reactants or products per unit time
• how can the rate by found ?

  • measuring the decrease in the concentration of reactants over time
  • measuring the increase in the concentration of product over time.
• what are the units for rate of reaction ?

  mol dm-3 s-1
• Define concentration

  The number of particles of solute in a particular volume of solution
• two values are proportional if they have a consistent relationship
• What is the proportionality constant used for ?


  To quantify a proportional relationship
• State the meaning of the term order or reaction

  The power to which the concentration of a substance is raised to in the rate equation
• Why cant a catalyse have an order of 0 ?

  catalysts increase the rate of reaction. If it is zero order it means it has no effect on the rate of the reaction.
• X + 2Y —> 2Z

  rate = $[W]^2[Y]$

  what is the catalyst in this reaction?

  The catalyst is W as it appears in the rate equation, therefore has an affect on the rate of reaction but does not appear in the balanced equation for the reaction.
• what are the units for this third order reaction

  $K =$$rate/[A]^2[B]$


  $K =$$(moldm^-3 s^-1) / (moldm^-3)^2(moldm^-3)$

  K = $S^-1 / (moldm^-3)^2$

  K = $Mol^-2dm^6s^-1$
• define the average rate of reaction

  the total change in the amount of reactants or products over a certain period of time
• state what is meant by the term instantaneous rate of reaction

  the rate of reaction at a particular point in time
• state the meaning of the term elementary steps

  basic steps that cannot be broken down any further
• what are the three elementary steps in this equation:

  Cl2 + CHCl3 —> HCl + CCl4 


  step 1: Cl2 —> Cl + Cl (decomposition)

  Step 2: Cl + CHCl3 —> HCl + CCl3 (substitution)

  step 3: CCl3 + Cl —> CCl4 (synthesis)
• state what is meant by the rate determining step?

  the slowest step in the reaction
• what are the intermediates in this reaction mechanisms and explain your answer.

  Cl2 + CHCl3 —> HCl + CCl4

  step 1: Cl2 —> Cl + Cl

  step 2: Cl + CHCl3 —> HCl + CCl3

  step 3: CCl3 + Cl —> CCl4

  Cl and CCl3 - they are produced during one step of the reaction and then used up in another step
• What is the overall equation for these steps ?

  state the intermediates

  step 1 : A + A —> B + C

  step 2 : D + B —> E

  step 3 : F + E —> G

  Intermediates - B and E

  overall equation - 2A + D + F —> C + G
• Any reactants in steps that come after the rate determine step must be zero order.
• In this reaction what is the rate determine step:

  $Rate =$$k[H2O2]^2$

  Step 1: H2O2 —> OH + OH

  step 2: H2O2 + OH —> H2O + HO2

  step 3: HO2 + OH —> H2O + O2

  step 2 - the rate equation shows that H2O2 is 2nd order therefore must appear twice before or during the rate determine step.
• What affects the rate constant ?

  Temperature

  catalyst
• state what the Arrhenius equation lets us calculate:

  1. how much factors such as temperature and catalyst affect the rate of reaction
  2. a reactions activation energy
• What does this expression let us calculate?

  $e ^-Ea/ RT$


  The proportion of particles with energy greater than or equal to Ea
• What are experimental methods that track changes in the amount of reactants or products over time called ?

  Continuous monitoring methods
• What is the average rate of reaction

  The total change in the amount of reactants and products over a certain period of time
• what is the instantaneous rate of reaction

  the rate of reaction at a particular point in Time
• What are elementary steps ?

  Basic steps in a reaction that can’t be broken down any further (each step can occur at different rates)
• What are the three types of elementary steps

  Decomposition - one molecule breaks up into two

  substitution - two particles collide and form two completely different particles

  synthesis - two particles collide and join together as a single particle
• What are intermediates

  Substances that are produced in one step of a multiple step reaction and used up in another step
• How do we write a equation for a multi step Reaction?

  Write down the reactants that are involved in the reaction before and during the slowest step
• What is an elementary reaction

  Is an overall reaction which involves only one elementary step
• what does ln(7x) expand to ?

  ln(7) + ln(x)
• Why do we rearrange the Arrhenius equation into the form y=mx+c ?

  -to clarify the relationship between T and K

  -so we can plot a graph showing the relationship between T and K
• what is the answer for activation energy usually between ?

  40kJmol-1 - 400kJmol-1
• The equation and rate law for the reaction of substance P with substance Q are given below. 2P + Q → R + S

  rate = k[P]2[H+]

  Under which one of the following conditions, all at the same temperature, would the rate of

  reaction be slowest?

  [P] / mol dm−3 pH

  A 0.1 0

  B 1 2

  C 3 3

  D 10 4

  Answer = C

  A = $(0.1)^2(10^-0) =$$0.01K$

  B = $(1)^2(10^-2)$= 0.01k

  C = $(3)^2(10^-3) =$$0.009K$

  D = $(10^2)(10^-4) =$$0.01K$