3 - Effect of temperature on rate of a reaction

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

Why does the rate of reaction increase with increasing temperature?

• As temperature increases, the kinetic energy of the molecules also increases, meaning more molecules have energy greater than or equal to the activation energy, so can react

• As the kinetic energy of the molecules increases, their velocities increase, resulting in more frequent & successful collisions

How would you investigate how rate of this reaction varies with temperature?

This reaction produces a cloudy sulfur precipitate:

• time how long it takes for a cross underneath a test tube to disappear, using a stopwatch

• repeat this at 5 different temperatures & compare times taken

What are some issues with the continuous monitoring method?

• The disappearing cross is judged by the human eye, therefore can be unreliable

• There may be a delayed reaction time with the stopwatch

• At low temperatures, the reaction may be too slow

• There may not be enough precipitate for the cross to disappear

How could the data be analysed for this practical?

• For a graph of the amount of sulfur against time, it would be a straight line initially, as the reaction has only just begun

• Therefore, the gradient of this straight section = rate of reaction

• Compare the rate of reaction for each temperature (for further analysis, plot a graph of rate of reaction against temperature)

What is a control variable?

A variable that is kept constant throughout the experiment

In an experiment investigating the effect of temperature on rate of reaction, what are the control variables?

• Concentrations/volumes of the reactants & products

• Use the same test tubes & cross

• Use the same person to decide when the cross has disappeared

How can a substance be kept at a constant temperature?

Using a thermostatically controlled water bath

What is the method for this practical?

1. Add 10cm of 1.00 moldm^-3 HCl to the ‘acid’ tube & place this tube into the hole of the plastic container, without the cross under it

2. Use a measuring cylinder to add 10cm of 0.05 moldm^-3 of sodium thiosulfate solution to the second tube. Place this tube into the hole of the plastic container with the cross under it & carefully place a thermometer in this tube. Leave both test tubes for 3 minutes to equilibrate

3. Record the start temperature & then add 1cm of HCl to the thiosulfate solution & start timing

4. Look down through the tube from above & record the time for the cross to disappear

5. Record the final temperature of the reaction mixture, then pour the cloudy contents of the tube into a sodium carbonate solution (prevents any toxic SO₂ gas being produced)

6. Add water from a kettle to the plastic container (no hotter than 55°C). Add cold water if necessary

7. Measure another 10cm of 0.05moldm^-3 sodium thiosulfate into a clean test tube & insert into the correct hole in the plastic container. Leave the tubes to equilibrate to 3 minutes

8. Repeat to obtain results for at least 5 different temperatures

What are some safety precautions to be aware about?

• To minimise the escape of toxic SO₂ gas, use a lid (two holes should be made using a hot, wide cork borer)

  • could also perform the experiment in a fume cupboard or well-ventilated room

• Wear gloves & safety goggles, as HCl can cause skin burns & eye damage (irritant)