Rate Experiments (DP IB Chemistry: SL)

• During a reaction, the reactants are used up and changed into the products
• This means that as the reaction proceeds, the concentration of the reactants is decreasing and the concentration of the products is increasing
• Because of this, the rate of the reaction is not the same throughout the reaction but changes
• The rate of reaction during the reaction can be calculated from a concentration-time graph.
• The isomerisation of cyclopropane to propene will be taken as an example:

Isomerisation of cyclopropane

• The concentrations of reactant (cyclopropane) and product (propene) over time can be experimentally obtained

Concentrations of Cyclopropane & Propene Table

• When taking the measurements, the temperature should be kept the same at all times as a change in temperature will change the rate of reaction
• A concentration-time graph for the concentration of propene as well as cyclopropane can be obtained from the above results
  • As an example, the concentration-time graph for propene is shown below:

The graph shows that the concentration of propene increases with time

Calculating the rate at the start of a reaction

• At the start of the reaction, the concentration-time curve looks almost linear
• The rate at this point can therefore be found by treating the curve as a linear line and by using:

• Using the graph, the average rate of the reaction over the first 5 minutes for propene is:

Line a shows the average rate over the first five minutes whereas line b shows the actual initial rate found by drawing a tangent at the start of the curve. The calculated rates are very similar for both methods

= 0.0009 mol dm^-3 s^-1

Calculating the rate as the reaction proceeds

• The curve becomes shallower with time which means that the rate decreases with time
• The rate of reaction can be calculated by taking short time intervals

= 0.0005 mol dm^-3 s^-1

• The smaller the time intervals, the more accurate the reaction rate value is
• Even more accurate is to find the rate of reaction at different concentrations of reactant or product at particular time points
• This can be done by drawing tangents at several points on the graph
  • As an example, the rates of reaction at different concentrations of cyclopropane are calculated by drawing the appropriate tangents:

The rate of reaction at 3 different concentrations of cyclopropane is calculated by drawing tangents at those points in the graph

Rate-concentration graph

• The calculated rates can then be summarised in a table to show how the rate of reaction changes with changing concentration of the reactants or products

Change in Rate with Decreasing Concentration of Cyclopropane Table

• This data can then be used to plot a rate-concentration graph
• The graph shows that the rate is directly proportional to the concentration of cyclopropane
  • If you double the concentration of cyclopropane the rate of reaction will double too

The graph shows a directly proportional correlation between the concentration of cyclopropane and the rate of reaction