7 - Measuring rate of a reaction

What is meant by the ‘rate of reaction’ ?

The changes in the concentration of reactants or products over time

How can the rate of reaction be measured?

• Initial rates method: iodine clock reaction

• Continuous monitoring method: measuring the volume of gas produced in a reaction over time

What is an initial rates method?

Involves measuring the initial rate of reaction for multiple different concentrations to observe how rate of reaction varies 

In the iodine clock reaction, I^- reacts with hydrogen peroxide to form I₂. This reacts with HSO₃^- & forms further iodide ions, until the supply of HSO₃^- is exhausted. At this stage, I₂ forms a blue complex with starch.

You are given solution A (containing iodide ions), solution B (containing hydrogen peroxide) & a starch solution. Describe how you would design a series of experiments to determine the order of reaction with respect to the iodide ions (6 marks)

1. Perform a series of experiments where only the concentration of solution A varies, meaning the volumes & the concentration of B & the starch remain the same

2. Control the temperature using a water bath

3. Add 5cm³ of solution B & 1cm³ of starch to the beaker (add solution A last)

4. Simultaneously add 5cm³ of solution A & start the timer

5. Record the time taken for a blue-black colour to appear

6. Dilute solution A, so the concentration is half of the original

7. Repeat the experiment & record the time taken for a blue-black colour to appear

8. Use 1/time taken for a blue-black colour to appear as the surrogate rate

How are the results interpreted from the initial rates (iodine clock reaction) experiment?

• If the rate doubles/halves when [A] doubles/halves, the reaction is 1st order with respect to A

• If the rate doesn’t change when [A] doubles/halves, the reaction is 0 order with respect to A

• If the rate quadruples when [A] doubles/halves, the reaction is 2nd order with respect to A

What are some issues with the iodine clock reaction?

• Some low I^- concentrations may take too long to react

• Delayed stopwatch reactions

• Concentrations may not be exact due to measuring apparatus

What is a continuous monitoring method?

Involves measuring the change in concentration of a reactant or product over time (or measuring volume of gas released) as the reaction progresses 

A student investigates the following reaction: 2H₂O₂ (l) → 2H₂O (l) + O₂ (g)

The reaction is catalysed by manganese (IV) dioxide.

She wishes to determine the partial order with respect to hydrogen peroxide.

Outline the experimental steps & data analysis she should do to achieve this (6 marks)

1. Use a burette to measure 50cm³ of 0.1moldm^-3 of hydrogen peroxide into a conical flask (allows us to close the opening & collect the gas)

2. Use a high precision balance to measure 1g of manganese (IV) dioxide → significant excess, so concentration is effectively unchanging

3. Prepare the bung with a delivery tube attached to a gas syringe

4. Add the catalyst to the flask, insert the bung & start the stopwatch

5. Record the volume of gas released every 10 seconds until the syringe is full

• n (O₂) produced = n (H₂O₂) reacting ÷ 2

6. Calculate [H₂O₂] remaining for each point in time

7. Plot [H₂O₂] on the y-axis against time on the x-axis

8. Draw tangents at each time point to establish the rate for each concentration

How are the results interpreted from the continuous monitoring method experiment?

Plot a graph of [H₂O₂] on the x-axis & the rate on the y-axis