Reaction Rates (II)

• Continuous monitoring involves collecting experimental data throughout the course of a reaction to plot a concentration-time graph.
• Two of the most common way to collect data for concentration-time graphs are:

  • measuring the volume of gas evolved over time
  • measuring the mass of reactants lost over time
• To find the rate of reaction at any given point, a tangent is drawn and the gradient is determined. The gradient gives the rate of reaction.
• How do you find the rate of reaction from a graph?

  Draw a tangent. The gradient of the tangent gives the rate of reaction.
• You can also plot a concentration-time graph through colorimetry.
• Q: Describe in detail an experiment to measure the rate of reaction of hydrochloric acid with limestone chips.

  -Use a measuring cylinder to measure 50cm3 of 0.100 mol dm-3 hydrochloric acid.

  -Add acid to excess limestone chips, insert bung as rapidly as possible.

  -Use gas syringe to measure and record volume of gas collected at set intervals of time.
• Q: Briefly describe a method to measure the rate of reaction for the equation : Br2 + HCO2H ----> 2HBr + CO2.


  • Use colorimeter to measure and record transmittance of solution at set intervals of time using a stopwatch.
  • Use calibration curve to determine concentration of bromine.
  • Plot graph of Bromine concentration against time.
  • Find the gradient of a tangent to graph at time t to find the rate of reaction at time t.
• The effect that a concentration of a reactant has on the rate of reaction is called the order with respect to a reactant.
• In a zero-order reaction, the concentration of the reactant is inversely proportional to time. This means that the reactant concentration decreases as time increases. The graph is a straight line going down.
• If the concentration of a chemical is directly proportional to the rate of the reaction, the chemical is first order.
• If the rate is directly proportional to the square of the concentration of a chemical, then the chemical is second order.
• If changing the concentration of a reactant has no effect on the rate of the reaction, then it is zero order.
• On a zero-order graph, the gradient of the line is the rate of reaction.
• In a first-order reaction, the concentration of the reactant decreases with time. The graph is a curve going downwards and eventually plateaus.
• In a second-order reaction, the concentration of the reactant decreases more steeply with time. The concentration of a reactant decreases more with increasing time compared to a first-order reaction. The graph is a steeper curve going downwards.
• In a zero-order reaction, the rate does not depend on the concentration of the reactant. The rate of the reaction therefore remains constant throughout the reaction. The graph is a horizontal line.
• In a first-order reaction, the rate is directly proportional to the concentration of a reactant. The graph is a straight, upwards line.
• In a second-order reaction, the rate is directly proportional to the square of concentration of a reactant. The graph is an upwards curved line.
• The rate constant, k, is the constant that links the rate of reaction with the concentrations of the reactants raised to the powers of their orders in the rate equation.
• The rate equation shows the relationship between the rate of a reaction and the concentrations of the reactants raised to the powers of their orders.
• If the temperature of a reaction is increased, then the rate constant k increases and hence the rate of reaction also increases.
• Order is the order with respect to a reactant is the power to which the concentration of the reactant is raised in the rate equation.
• What does k represent?


  The rate constant.
• To find the overall order of a reaction, add up all the individual orders of reactants.
• The initial rate can be found by:

  1. Drawing a concentration-time graph.
  2. Adding a tangent at t=0
  3. Calculating the gradient of the tangent
• Half life of a reactant is the time taken for the concentration of the reactant to reduce by half its original value.
• For a zero-order reaction the successive half-lives decrease with time. This means that it would take less time for the concentration of a reactant to halve as the reaction progresses.
• The half-life of a first order reaction remains constant throughout the reaction. The amount of time required for the concentration of the reactant to halve will be the same during the entire reaction.
• For a second-order reaction, the half-life increases with time. This means that as the reaction takes place, it takes more time for the concentration of reactants to halve.
• The half-life of a first order reaction independent of the concentration and is therefore constant. You can prove this by:

  1. Measuring two or more half lives
  2. Showing they are constant