Temperature and rate
In this topic you will:
- investigate the effect of temperature on the rate of reaction
- carry out a trial run and preliminary work for the investigation and use the information to improve the investigation method
- select equipment
- plot a graph of results.
Key words
- precipitate
- trial run
- preliminary work
Changing the temperature of reactants
If you change the temperature of the reactants in a reaction, the rate of reaction will change.
If you mix hydrochloric acid with a solution of sodium thiosulfate (a salt), the mixture becomes cloudy. This is because sulfur is produced. Sulfur is insoluble in water, so it forms a precipitate. The precipitate is opaque – so, when you can no longer see through the solution, you know the sulfur has been formed.
The reaction is good for investigating reaction rates, because you can easily time how long it takes for the sulfur precipitate to be formed.
And by changing the temperature of the solution, you can investigate the effect of temperature on reaction rates.
Key Concept
Precipitate – a solid that forms in a solution during a chemical reaction and makes the solution cloudy or opaque.
Think Like a Scientist
The effect of temperature on the rate of reaction
You are going to measure the rate of reaction between sodium thiosulfate and hydrochloric acid at different temperatures.
There are three parts to this: the trial run, the preliminary work and the investigation.
Part 1: The trial run
First you need to carry out a trial run. A trial run means you will carry out a practice experiment to make sure you can do the experiment safely and effectively. You will then be prepared and be ready to overcome any problems you find.
Safety
Make sure the room is well ventilated, because sulfur dioxide gas will be produced. Place any reacted solutions into a container with some solid sodium hydrogencarbonate for your teacher to remove later. The sodium hydrogencarbonate will react with the sulfur dioxide.
You will need:
• safety glasses, test tube with stopper, clamp stand, white card with a cross marked on it, timer or stop clock, sodium thiosulfate solution, dilute hydrochloric acid, large beaker containing solid sodium hydrogencarbonate.
Method
- Place 10 cm³ of sodium thiosulfate solution in a test tube.
- Fix the test tube into the clamp stand and arrange the card with the black cross behind it as in the diagram.
- Add 1 cm³ hydrochloric acid to the test tube and put the stopper in the tube. Start the timer.
- Time how long it takes before you can no longer see the cross.
- When you have finished, place the contents of the test tube in the large beaker containing sodium hydrogencarbonate.
Questions
Show Answer
It helped you test your setup, timing process, and recognize any issues in observation or measurement before starting the main experiment.
Show Answer
You might adjust how you mix the solutions, improve how you observe the cross, or refine the timing method to improve reliability.
Part 2: The preliminary work
The next step is to do some preliminary work to decide which temperatures you will use. To change the temperature of the sodium thiosulfate solution, warm it in a suitable water bath before adding the acid. Make sure there is a stopper in the tube while it is warming.
Discuss in your group how you will do this preliminary work. Here are some of the things you will need to consider:
- How large a change in temperature is needed to give a reaction time difference we can measure?
- Will we start the temperatures at room temperature?
- How big a range of temperatures will we use?
- Will we increase the temperature by 5°C or 10°C or 20°C?
Questions
Show Answer
You will need safety glasses, a water bath, thermometer, test tubes with stoppers, timer, measuring cylinders, and all standard reaction apparatus.
Show Answer
For the preliminary work, you will test a few different temperatures using the same method as the trial run to decide which range gives measurable results.
Show Answer
It helps you choose appropriate temperature intervals and confirm that the setup produces observable changes.
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Use insulation or conduct the reaction quickly after heating to minimize heat loss.
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Plan includes testing at 20°C, 30°C, 40°C, 50°C; recording time to cross disappearance; and organizing results in a table with temperature vs. reaction time.
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You identified a good range of temperatures and refined your technique for better consistency.
Part 3: The investigation
Discuss in your group how you will carry out the final investigation using the information you got from the trial run and the preliminary work.
Questions
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The plan should include steps, equipment list (same as previous stages), and a table with temperature and reaction time columns.
Show Answer
Kept volumes and concentrations constant, used same timing method, and repeated measurements.
Show Answer
Plot temperature on the x-axis and reaction time (or rate) on the y-axis to identify trends.
Looking at typical results
Here is a graph of some typical results for the rate of reaction between marble chips (calcium carbonate) and hydrochloric acid. This was carried out as in the previous topic.
You can see from the graph that you do not get any more of the product (carbon dioxide gas) at the higher temperature. You get the same volume of gas but in less time.
Common Mistake
Students often think a higher temperature produces more product. The graph shows the final volume of gas is the same — the reaction just happens faster at higher temperatures.
Questions
Show Answer
The line that rises more steeply at the beginning shows the faster reaction, because the gas is produced more quickly in a shorter time.
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The line would rise even more steeply than the one at 40°C, but still level off at the same final volume, showing a faster reaction that finishes sooner.
Explaining the effect of temperature
Particles move all the time. When the temperature of the reaction is increased, the particles move faster. They collide more often, and with more energy.
Right: Reaction at 40°C – more frequent and energetic collisions.
The higher the temperature, the faster the rate of reaction.