6.1.6Chemical Change: Rate and Extent
Required Practical: Investigating the Effect of Concentration on Rate of R — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 7 of 16
Part 1 of 5Method A: disappearing cross
1
Draw a black cross on a piece of paper and place a conical flask on top of it.
2
Measure 10 cm3 of sodium thiosulfate solution of a known concentration with a measuring cylinder and pour it into the flask; add water if you are diluting it, so the total volume is always the same.
3
Measure 10 cm3 of dilute hydrochloric acid in a second cylinder.
4
Add the acid to the flask, swirl once and start the stopwatch immediately.
5
Look down through the solution and stop the timer when the cross can no longer be seen because sulfur has made the liquid cloudy.
6
Pour the contents into a sink with plenty of water, rinse the flask, and repeat with a different concentration of thiosulfate.
7
Repeat each concentration at least twice and calculate a mean time.
Part 2 of 5Variables
| Type | Variable | How it is handled |
|---|---|---|
| Independent | Concentration of sodium thiosulfate | Changed in steps, for example by mixing with different volumes of water |
| Dependent | Time for the cross to disappear | Measured with a stopwatch |
| Control | Volume and concentration of acid, total volume, temperature, same flask and cross | Kept the same every time so the test is fair |
Part 3 of 5Method B: collecting a gas
Set-up
- Set-up React magnesium ribbon of the same length with hydrochloric acid of different concentrations in a flask connected to a gas syringe.
- Hydrogen is given offMg + 2HCl → MgCl2 + H2.
Measurements
- Measurements Record the volume of gas every 10 seconds, or the time to reach a fixed volume such as 40 cm3.
- Start the stopwatch the moment the acid touches the metal.
Part 4 of 5Processing and concluding
1Calculate rate
- Calculate rate For the cross method use rate = 1 ÷ time (s-1); for gas use volume ÷ time (cm3/s).
2Plot a graph
- Plot a graph Plot concentration on the x-axis and rate on the y-axis, and draw a line or curve of best fit.
3Describe the trend
- Describe the trend A higher concentration gives a shorter time and a higher rate.
4Explain
- Explain More particles per unit volume cause more frequent successful collisions.
Part 5 of 5Safety and accuracy
Safety
- Safety Wear eye protection throughout.
- Sulfur dioxide can be released in the thiosulfate reaction and may trigger asthma, so work in a well-ventilated area and do not sniff the flask.
- Mop up spills of acid straight away.
Improving reliability
- Improving reliability Repeating and averaging reduces the effect of random error, and any anomalous result should be spotted and ignored.
- Different people judging the cross vanishing gives personal error, so use the same observer or a light sensor.
Key terms
| Term | Meaning |
|---|---|
| Independent variable | The factor you deliberately change in an investigation. |
| Dependent variable | The factor you measure to see the effect of the change. |
| Control variable | A factor kept constant so that the test is fair. |
| Precipitate | An insoluble solid formed when two solutions react. |
| Anomalous result | A result that does not fit the pattern and is likely to be caused by a mistake. |
| Mean | The sum of repeated readings divided by the number of readings. |
Avoid these mistakes
1
Changing the total volume as well as the concentration, which makes the test unfair.
2
Plotting time instead of rate and then saying 'the line goes up' when the reaction is actually getting faster as time falls.
3
Starting the stopwatch before the reactants are mixed or forgetting to start it at the same moment each run.
4
Including an anomalous result in the mean instead of identifying and excluding it.
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