Rates of Reaction (Edexcel A Level Chemistry)

• The rate of reaction refers to the change in the amount or concentration of a reactant or product per unit time
  
  • The units for rate of reaction are mol dm^-3 s^-1 
• It can be found by measuring:
  • The mass lost over time
  • The volume of produced over time
  • Colour changes, including by the use of colorimetry
  • pH changes over time
  • Changes in electrical conductivity
• The rate of reaction can be calculated by:

Rate of reaction = 

Rate of Reaction

• A directly proportional relationship between the rate of reaction and concentration of D is observed when the results are plotted on a graph:

Rate of reaction over various concentrations of D

• This leads to a very common rate expression:

Rate ∝ [D]       or       Rate = k[D]

• This rate expression means that if the concentration of D is doubled, then the rate doubles
• Equally, if the concentration of D halves, then the rate halves

Rate Equations

• The following reaction will be used to discuss rate equations:

A (aq) + B (aq) → C (aq) + D (g) 

• The rate equation for this reaction is:

Rate of reaction = k [A]^m [B]ⁿ

• Rate equations can only be determined experimentally and cannot be found from the stoichiometric equations
• In the above rate equation:
  • [A] and [B] are the concentrations of the reactants
  • m and n are orders with respect to each reactant involved in the reaction

• Products and catalysts may feature in rate equations
• Intermediates do not feature in rate equations