Rate Equations

• rate of reaction?

  change in amount of reactant/product over time
• gradient unit?

  mol dm−³s−¹
• order of reaction?

  denotes how concentration of reactant affects rate of reaction
• overall order of reaction?

  sum of individual order of reactants
• first order?


  if concentration doubles, rate doubles
• zero order?

  if concentration changes but rate stays the same
• second order?

  if concentration doubles, rate quadruples
• bigger the value of k, faster the reaction
• initial rates method?

  sample of reaction mixture is taken and analysed after a certain observation is made (colour change) - happens after a short period of time. process is repeated using different concentrations of a reactant whilst keeping the others constant to observe the impact it has on the time taken for the observation to occur. initial rate is calculated to determine the order of reaction with respect to different reactants
• continuous monitoring?


  reaction is monitored over time by recording the amount of reactant/product you have at certain time intervals. this allows you to work out how the rate changes over time rather than just getting the initial rate
• rate of reaction?


  refers to the charge in the amount or concentration of a reactant OR product per unit time
• for catalyst to appear in the rate equation:

  • must have a measurable and quantifiable effect on the rate of reaction
  • must be homogeneous
  • if a chemical appears in the rate equation but is not a reactant, then it is a catalyst
• order of a reactant?

  shows how concentration of a reactant affects the rate of reaction. the power to which the concentration of reactant is raised in the rate equation
• when order of reaction of reactant is 0?

  no affect on rate of reaction therefore not included in rate equation at all
• when order of reaction of reactant is 1?

  rate is directly proportional to concentration of reactant
• when order of reaction of reactant is 2?


  rate is directly proportional to square of concentration of reactant
• overall order of reaction?

  sum of the powers of the reactants in a rate equation
• half-life?


  time taken for concentration of a limiting reactant to become half of its initial value
• zero order in a concentration vs time graph?

  concentration of reactant is inversely proportional to time - concentration of reactant decreases with increasing time - graph is a straight line
• first order in a concentration vs time graph?

  concentration of reactant decreases with time - graph is a curve going downwards and eventually plateaus
• second order in a concentration vs time graph?

  concentration of reactant decreases more steeply with time, compared to in a first-order reaction - graph is a steeper curve going downwards
• zero order in a rate vs time graph?

  reaction rate does not depend on the concentration of reactant. rate of reaction remains constant - graph is a horizontal line
• first order in a rate vs time graph?

  reaction rate is directly proportional to the concentration of a reactant. if concentration doubles, rate also doubles - graph is a straight line
• second order in a rate vs time graph?

  reaction rate is directly proportional to square of concentration of a reactant. if concentration doubles, rate increases by 4 - graph is a curved line
• k?


  remains constant if concentration of reactants is the only factor which is changed
• rate equation?

  shows how each of the reactants in a reaction effects the rate of reaction and includes rate constant, k
• when does k change?


  if temperature changes or catalyst is used
• at higher temperatures, a greater portion of molecules have energy greater than the activation energy
• Arrhenius equation?

  k = Ae^−Ea/RT
• ln k = ln A - Ea/RT
• what does increasing the temperature do?

  gives a greater value of ln k since rate of reaction depends on rate constant and increase in k means an increase in rate of reaction
• what does increasing activation energy means? 


  means proportion of molecules which possess at least the activation energy is less, this means rate of reaction and value of k decreases
• initial rates method?


  method is used to gather experimental data, to determine the order with respect to the reactants in the reaction. involves setting up a series of experiments:

  • for each experiment, concentration of reactants is altered - the rest remains the same
  • when results are collected, they are used to determine the order with respect to each reactant
  • for each experiment, concentration - time graph is drawn and initial rate is calculated by drawing a tangent and working out the gradient
• what happens in a zero-order concentration-time graph?


  concentration of reactant is inversely proportional to time. concentration decreases with increasing time - graph is a straight line going down
• what happens in a first-order concentration-time graph?


  concentration of reactant decreases with time - graph curves downwards and eventually plateaus
• what happens in a second-order concentration-time graph?


  concentration of reactant decreases more with increasing time compared to first-order reaction - graph is a steeper curve going down
• concentration-time graph of a zero-order reaction:

  • rate of reaction is not changing - gradient is a constant value
  • order with respect to reactant is 0, a change in concentration will have no effect on the rate of reaction
  • rate = k
  • rate of reaction is gradient of graph so rate constant will also be gradient of graph
• zero-order from initial rate:

  • rate does not depend on the concentration of reactant
  • rate of reaction remains constant throughout reaction
  • graph is a horizontal line
• first-order from initial rate:

  • rate is directly proportional to concentration of a reactant
  • rate increases as concentration of reactant increases, vice versa
  • graph is a straight line
• second-order from initial rate:

  • rate is directly proportional to the square of a concentration of a reactant
  • rate of reaction increases more as concentration of reactant increases, vice versa
  • graph is a curved line