5.1.1Monitoring and Controlling Chemical Reactions
Measuring Rates — OCR Gateway GCSE Combined Science A J250 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 2 of 7
Part 1 of 5What is the rate of a reaction?
Fast and slow reactions
- Fast and slow reactions Reactions happen at very different speeds.
- Rusting is slow, whereas explosions are extremely fast.
- The rate tells us how quickly reactants are used up or how quickly products are formed.
Calculating rate
- Calculating rate Rate = amount of reactant used or product formed ÷ time taken.
- Typical units are g/s for a mass, or cm3/s for a volume of gas. Time is normally measured in seconds.
Why measure a product?
- Why measure a product?
- It is usually easier to measure a product appearing, such as a gas, than a reactant disappearing, so most practicals follow the product.
- Several readings are taken during the reaction so that a graph can be plotted.
Part 2 of 5Three methods for measuring rate
| Method | What is measured | Example reaction | Limitation |
|---|---|---|---|
| Loss in mass | Mass of a flask on a balance falls as gas escapes | Calcium carbonate with hydrochloric acid (CO2 escapes) | Unsuitable for hydrogen, because the mass lost is too small to measure |
| Gas volume | Gas collected in a gas syringe or an upturned measuring cylinder over water | Magnesium with hydrochloric acid (H2 collected) | Gas must not dissolve in water if collected over water |
| Precipitate and cross | Time for a cross under the flask to disappear | Sodium thiosulfate with hydrochloric acid, forming sulfur | Subjective; gives only one result per experiment, so no rate graph |
Part 3 of 5Measuring loss in mass
1
Place a conical flask containing the acid on a balance and add a loose plug of cotton wool to the neck. This lets gas escape but stops spray being lost.
2
Add the solid, such as calcium carbonate chips, and start the stopwatch at once.
3
Record the mass at regular intervals, for example every 10 seconds.
4
Plot the mass against time. The line falls steeply at first and then levels off when the reaction has finished.
5
The mass lost is a measure of the carbon dioxide made, so the steepest part of the graph shows the fastest rate.
Part 4 of 5Measuring gas volume and the disappearing cross
Other methods
Collecting gas
- Collecting gas A gas syringe, or a measuring cylinder filled with water and inverted in a trough, collects the gas through a delivery tube.
- Read the volume every few seconds and plot volume against time.
- The curve shows amount of product against time.
Disappearing cross
- Disappearing cross Sodium thiosulfate and hydrochloric acid are mixed in a flask placed over a cross.
- Sulfur forms as a cloudy yellow precipitate.
- Stop the clock when the cross can no longer be seen. A shorter time means a faster reaction.
- Different people may judge the end point differently, so the result is subjective.
Fair test
- Fair test When investigating one factor such as temperature, keep all control variables the same: volumes, concentrations, mass and surface area of the solid.
Part 5 of 5Investigating factors that change the rate (IGCSE)
1Surface area
- Surface area React different sizes of calcium carbonate chips with the same acid and measure the gas collected in a fixed time.
2Concentration
- Concentration Dilute the sodium thiosulfate with water in different amounts, add acid and time the cross to disappear.
3Temperature
- Temperature Warm the acid in a water bath to a set temperature, add magnesium and time how long it takes to dissolve completely.
4Catalyst
- Catalyst Collect the oxygen from hydrogen peroxide with and without manganese(IV) oxide and compare the volumes.
Key terms
| Term | Meaning |
|---|---|
| Rate of reaction | How quickly reactants are used up or products are formed, measured as change in amount per unit time. |
| Precipitate | A solid that forms when two solutions react and which makes the mixture cloudy. |
| Gas syringe | A graduated syringe used to collect and measure the volume of a gas produced. |
| Control variable | A condition kept the same during an investigation so that the test is fair. |
| Subjective | Depending on the judgement of the person observing, so that different people may get different results. |
Avoid these mistakes
1
Using the loss in mass method for hydrogen. Its mass is so small that the change cannot be measured accurately.
2
Forgetting the cotton wool plug, which stops liquid being thrown out of the flask along with the gas.
3
Trying to draw a rate graph from the disappearing cross method. Only one time is obtained per experiment.
4
Writing the unit of rate as just 's' or 'g'. The unit must show the amount and the time, such as g/s or cm3/s.
5
Leaving out control variables when describing a method to test one factor.
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