Concentration and rate
In this topic you will:
- investigate the effect of concentration on the rate of reaction
- plan an investigation
- explain the effect of changes in concentration on the rate of reaction, using particle theory
Key words
- concentration
- dilute
In this unit you have investigated different variables and how they affect the rate of reaction. You have also used different methods in your investigations. Now you will investigate the effect of concentration on the rate of reaction.
Think Like a Scientist
Investigating the effect of concentration on the rate of reaction
In this investigation you will change the concentration of the acid used. Instead of measuring the volume of carbon dioxide produced every 30 seconds, you will use a slightly different method of measuring the rate of reaction.
You will time how long it takes to collect 25 cm³ of carbon dioxide in the measuring cylinder.
Part 1: Planning the investigation
Discuss in your group how you will carry out this investigation.
- What mass of marble chips will you use?
- Which factors will you keep the same?
Some of the other things you could consider are:
- What volume of acid will you use?
- Which concentrations of acid will you use?
- What are the risks in this investigation?
- What safety precautions should you take?
- What equipment will you need?
- You will need items that are not shown in the diagram.
- Do you need to do a trial run or some preliminary work?
- If so, what do you want to find out?
- Do you need to repeat any of your readings?
Question
Show Answer
Group discussions help identify variables to control, safety steps, and clarify steps in the procedure to improve reliability and accuracy.
Part 2: Carrying out the investigation
Safety
Make sure you follow the safety precautions in your plan.
You will need:
• safety glasses, top pan balance, timer, measuring cylinders (to collect the gas and to measure the volume of acid used), beakers and a marking pen to label the different concentrations of acid, conical flask fitted with a bung and delivery tube, large container for holding the water when collecting the gas over water, marble chips, dilute hydrochloric acid, water.
Method
- Make up your concentrations of acid. Use the table below to help you. Make 50 cm³ each time by mixing acid and water as shown:
| Solution | Acid in cm³ | Water in cm³ | Concentration |
|---|---|---|---|
| A | 10 | 40 | least concentrated |
| B | 20 | 30 | |
| C | 30 | 20 | |
| D | 40 | 10 | |
| E | 50 | 0 | most concentrated |
- Set up the apparatus as shown in the previous diagram.
- Weigh out the marble chips and place them in the flask.
- Add the acid and time how long it takes to collect 25 cm³ of carbon dioxide. Record the acid used and the timing.
- Repeat with different concentrations of acid.
Questions
Show Answer
You controlled variables like mass of chips, volume of acid, and used consistent timing methods.
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You repeated readings, used precise measurements, and averaged data where appropriate.
Show Answer
Create a table with columns for concentration, time to collect 25 cm³, and any calculated rate values.
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Plot acid concentration on the x-axis and time or rate on the y-axis to show the trend.
Show Answer
The most concentrated acid (Solution E) gave the fastest reaction time.
Show Answer
As acid concentration increased, the rate of reaction increased, shown by shorter collection times.
Typical results
The graph below shows some typical results for the rate of reaction between marble chips and dilute hydrochloric acid. These results have been obtained from an experiment in which the volume of carbon dioxide has been measured every 10 seconds. Not like the experiment you have just carried out.
You can see from the graph that you do not get any more product (carbon dioxide gas) at the higher concentration. You get the same volume of gas, but more quickly.
Questions
Show Answer
The faster reaction is shown by the line that rises more steeply at the start, because the gas is produced more quickly in less time.
Show Answer
The line would rise more slowly and take longer to level off. It would still reach the same final volume, but over a longer time period.
Explaining the effect of concentration
Once again, we can use particle theory to help explain these results.
The higher the concentration of hydrochloric acid, the more hydrochloric acid particles there are in a given amount of space. This means that there will be more frequent collisions between hydrochloric acid particles and calcium carbonate particles.
Right: Reaction in acid that is twice as concentrated. More particles increase the chance of collisions.
Common Mistake
It’s a common mistake to think that a higher concentration always produces more gas. In reality, the total amount of gas stays the same — but it forms more quickly due to more frequent collisions.