RP7: Measuring the rate of reaction by an initial rate method and by a continuous monitoring method

Describe how the initial rate method is used to measure the rate of a reaction. (5 marks)

1. Mix the reactants and start timing immediately.

2. Follow a property that changes as the reaction proceeds, and record it at regular intervals.

3. Plot concentration against time and draw a tangent at time zero.

4. The gradient of that tangent is the initial rate.

5. Repeat with a different concentration of one reactant, keeping all other variables constant.

Describe the iodine clock reaction and explain how it gives an initial rate. (5 marks)

1. Mix the reactants with a small, fixed amount of sodium thiosulfate and starch indicator.

2. The thiosulfate removes the iodine as it forms, so no colour appears at first.

3. Once all the thiosulfate has been used up, the iodine reacts with the starch and the solution turns blue-black suddenly.

4. Time how long this takes; the same small amount of iodine has formed each time.

5. The initial rate is proportional to 1 ÷ time, so 1/t can be used as a measure of it.

Explain why the iodine clock gives a good measure of the initial rate. (2 marks)

• Only a very small fraction of the reactants has been used when the colour appears.

• The concentrations are therefore still close to their starting values.

Describe two continuous monitoring methods for following a reaction. (4 marks)

• Loss of mass: carry out a gas-producing reaction on a balance and record the mass at regular intervals.

• This cannot be used for a toxic or flammable gas released into the room.

• Gas collection: fit a gas syringe and record the volume collected at regular intervals.

• The apparatus must be airtight and the syringe large enough, or the plunger may be forced out.

State how to tell from a continuous monitoring graph that the reaction has finished. (2 marks)

• The readings stop changing.

• The graph levels off into a horizontal plateau.

State three variables that must be controlled in a rate experiment. (3 marks)

• Temperature, since the rate constant depends on it.

• The total volume of the mixture, so the concentrations are as intended.

• The concentrations of every reactant other than the one being varied.

Explain how colorimetry can be used to follow a reaction. (3 marks)

1. It is used when a reactant or product is coloured.

2. Take samples at intervals and measure the absorbance with a colorimeter.

3. Convert absorbance to concentration using a calibration curve made from solutions of known concentration.

Write the two equations involved in the iodine clock reaction. (2 marks)

• Main reaction, slow: H₂O₂ + 2I⁻ + 2H⁺ → I₂ + 2H₂O

• Removal of iodine, fast: I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻

State the three assumptions made in a clock reaction. (3 marks)

• The concentrations of the reactants barely change before the end point, so the rate is effectively the initial rate.

• The temperature stays constant throughout each run.

• The end point is reached when the same fixed amount of product has formed every time.

Explain why the total volume must be kept constant when the volume of one reactant is varied. (2 marks)

• Water is added to make up the difference, so the total volume is the same in every run.

• Otherwise the concentrations of all the other reactants would change too, and the effect of the one being varied could not be isolated.

Explain why a pipette is rinsed with the solution it will deliver. (2 marks)

• Any water left in the pipette would dilute the solution.

• The volume delivered would then contain fewer moles than intended.

Describe how gas volume or mass loss data is converted into a reactant concentration. (3 marks)

1. Convert the gas volume to moles using pV = nRT, or convert the mass lost to moles using moles = mass ÷ Mr.

2. Use the molar ratio in the equation to find the moles of reactant used up.

3. Subtract from the initial moles and divide by the volume of the solution to give the concentration remaining.