3.2.1Physical Chemistry
Measuring Rates — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 12 parts Lesson 9 of 19
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 12What 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.
Section 2 of 12Three 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 |
Section 3 of 12Measuring 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.
Section 4 of 12Measuring 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.
Section 5 of 12Investigating 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.
Section 6 of 12Methods of Measuring Rate
Rate is the change in quantity of a reactant or product divided by the time taken.
Gas volume
- Gas volume Collect the gas in an inverted measuring cylinder or gas syringe and record the volume at set time intervals.
Mass loss
- Mass loss Place the flask on a balance and record the decrease in mass as a gas escapes.
Disappearing cross
- Disappearing cross Look down through the flask at a cross on paper and time how long it takes to disappear.
Colour change
- Colour change Time a colour change, or a fixed amount of product, to compare the speed of different reactions.
| Method | Measured quantity | Rate expression |
|---|---|---|
| Gas collection | Volume of gas | Volume ÷ time |
| Mass loss | Decrease in mass | Mass lost ÷ time |
| Cross disappearing | Time taken | 1 ÷ time |
Section 7 of 12Experiments to Compare Rates
Each factor is investigated by changing one variable while keeping the others fixed.
1
Surface area: use calcium carbonate chips of different sizes with acid, and measure the gas volume in a fixed time.
2
Concentration: mix different volumes of sodium thiosulfate with water and time the disappearance of a cross.
3
Temperature: warm the acid in a water bath to set temperatures, add magnesium, and time until it fully dissolves.
4
Catalyst: add manganese(IV) oxide to hydrogen peroxide, and compare the gas volume with a run without the catalyst.
Section 8 of 12Interpreting Rate Graphs
Interpreting Rate Graphs
Steepness
- Steepness A steeper initial gradient on a volume-time graph means a faster rate at the start.
Gradient
- Gradient The gradient of the tangent at time zero gives the initial rate of reaction.
Levelling off
- Levelling off A curve that flattens shows that a reactant has been used up, so the reaction has stopped.
Units
- Units Rate has units such as cm³/s or g/s, depending on the quantity measured.
Section 9 of 12Worked Example
Question
- In an experiment, 48 cm³ of gas is collected in 40 s.
- Calculate the mean rate of reaction in cm³/s.
- The same reaction is then run with a catalyst, and 48 cm³ is collected in 20 s.
- Calculate the mean rate for the second run and state the effect of the catalyst.
- Mean rate 1 = volume ÷ time = 48 cm³ ÷ 40 s = 1.2 cm³/s.
- Mean rate 2 = 48 cm³ ÷ 20 s = 2.4 cm³/s.
- The rate has doubled, so the catalyst increased the rate of reaction.
Answer
- Without catalyst 1.2 cm³/s;
- with catalyst 2.4 cm³/s.
- The catalyst doubled the rate.
Section 10 of 12Exam Checklist & Tips
3.2.1 – Key Points for the Exam
- Rate of reaction The change in quantity of reactant or product per unit time.
- Mean rate Total change divided by the total time taken for the reaction.
- Initial rate The rate at the start of a reaction, found from the gradient of a curve at time zero.
- Gas syringe A calibrated syringe used to measure the volume of gas produced in a reaction.
Common Mistakes
- Dividing time by quantity instead of quantity by time when calculating the rate.
- Failing to state units for the rate, such as cm³/s or g/s.
- Changing more than one variable at a time, which makes the experiment unfair.
- Measuring the cross disappearance time and treating it directly as a rate, when a rate is proportional to 1 ÷ time.
Section 11 of 12Key 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. |
Section 12 of 12Exam technique
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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