Measuring the rate of reaction
In this topic you will:
- learn how the rate of reaction can change
- measure the rate of reaction
- use graphs to discuss and measure the rate of reaction
- explain why the rate of reaction changes.
Key words
- anomalous result
- collecting a gas over water
- collisions
- gradient
- rate of reaction
Rate of reaction
When magnesium ribbon is added to dilute sulfuric acid, you can tell a reaction is taking place because bubbles of hydrogen gas are given off. When you carried out this reaction in the laboratory, you may have noticed that at the start of the reaction a lot of bubbles were given off quickly. As the reaction came to an end, fewer bubbles were produced. Eventually, no more bubbles were produced. This shows that the reaction started quickly, then slowed, and eventually stopped.
How could you measure how quick the reaction was? This is called the rate of reaction.
The rate of reaction can be measured by working out how much of one of the products has been made in a given time, or how much of a reactant has been used up in a given time.
In the magnesium ribbon experiment, it is difficult to measure how quickly the reactants are used up or how quickly the magnesium sulfate is formed. The easiest way to measure the rate is to measure how quickly the hydrogen gas is produced. You can do this by measuring the volume of gas produced in a particular period of time.
To collect the gas, you can attach a syringe to the top of a flask so that no hydrogen can escape, as shown in the diagram. You can use the scale on the syringe to measure the volume of gas produced at different times during the reaction.
Example: Measuring Rate Using a Gas Syringe
1. Add a known mass of magnesium ribbon to a conical flask containing dilute sulfuric acid.
2. Immediately seal the flask with a bung connected to a gas syringe.
3. Start a timer and record the volume of hydrogen gas every 10 seconds.
4. Continue until no more gas is produced.
5. Use your data to plot a graph of volume of gas vs time to analyze the rate of reaction.
Questions
Show Answer
Word equation: magnesium + sulfuric acid → magnesium sulfate + hydrogen
Symbol equation: Mg + H₂SO₄ → MgSO₄ + H₂
How can you tell from the table that the reaction has finished at 270 seconds?
| Time in s | Total volume of hydrogen gas produced in cm³ | Time in s | Total volume of hydrogen gas produced in cm³ | Time in s | Total volume of hydrogen gas produced in cm³ |
|---|---|---|---|---|---|
| 0 | 0 | 120 | 40 | 240 | 64 |
| 30 | 10 | 150 | 48 | 270 | 66 |
| 60 | 20 | 180 | 54 | 300 | 66 |
| 90 | 26 | 210 | 60 | 330 | 66 |
Show Answer
The volume of gas remains constant at 66 cm³ from 270 seconds onward. This shows that no more hydrogen is being produced and the reaction has finished.
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Too much gas could build pressure and cause the syringe or apparatus to pop off. You could use a larger syringe or a gas collection trough to safely measure volumes over 100 cm³.
Which of the results is anomalous? Explain how you identified the anomalous result, and say what you would expect the result to be.
Show Answer
The value at 90 seconds (26 cm³) is lower than expected based on the graph’s trend. It should be closer to 30–35 cm³. This could be due to experimental error or a temporary blockage in the syringe.
When you measure the rate of reaction, you find that the rate of reaction changes as the reaction proceeds. Another reaction you could look at is the one between calcium carbonate and dilute hydrochloric acid.
As carbon dioxide gas is lost from the flask, the mass of the flask decreases. If you measure the mass every 30 seconds, you find that the mass decreases quickly at first but, as the reaction continues, the mass decreases more and more slowly.
Using the graph
A graph of the results from investigations into the rate of reaction can be used to measure the rate of reaction at any given time. The slope or gradient of the line tells you how quickly the reaction is taking place.
The steeper the slope, the faster the reaction.
This graph shows the results of an investigation into the rate of reaction between calcium carbonate and hydrochloric acid.
The line is steepest at the start of the reaction. This is when the reaction is fastest. As the slope of the line becomes less steep, the reaction is becoming slower. When the line levels out, it shows that no more carbon dioxide is being lost. This means that the reaction has ended.
This graph shows the rate of reaction between copper carbonate and hydrochloric acid. You can use a graph like this to measure and compare the rates of reaction at different times during the reaction.
Example: Interpreting a Rate Graph
Suppose a graph of gas produced over time shows a steep slope between 0–60 seconds, a gentler slope from 60–150 seconds, and a flat line after 150 seconds.
This means:
- The reaction is fastest in the first 60 seconds (steepest gradient).
- The reaction slows down as the reactants are used up (slope decreases).
- The reaction ends after 150 seconds (flat line – no more product).
You can use these changes in gradient to compare rates of reaction between different experiments or substances.
The graph below shows how to find out the average volume of carbon dioxide gas given off per second in the period between 10 seconds and 30 seconds.
To do this you should draw a line at 10 seconds on the horizontal axis up to where it meets the line of the graph, labelled A. Then draw another line at 30 seconds up to the point where it meets the graph, labelled B.
Draw line C between A and B as shown.
The distance of C represents the time taken, in this case 20 seconds.
The line D shows you how much carbon dioxide was produced in this time, in this case 20 cm³.
If 20 cm³ of carbon dioxide is produced in 20 seconds, then on average the rate of reaction over this period of time is:
20 cm³ in 20 seconds = 1 cm³ per second
Example: Finding Average Rate from a Graph
Imagine a graph shows that:
- At 15 seconds, the volume of gas is 12 cm³.
- At 35 seconds, the volume of gas is 36 cm³.
The change in volume is:
36 cm³ − 12 cm³ = 24 cm³
The time interval is:
35 s − 15 s = 20 seconds
So, the average rate is:
24 cm³ ÷ 20 s = 1.2 cm³/s
This tells us that, on average, 1.2 cm³ of gas was produced every second during that time interval.
Questions
Show Answer
Copper carbonate + hydrochloric acid → copper chloride + carbon dioxide + water
Show Answer
The average rate between 60 and 80 seconds is slower than between 10 and 30 seconds, as the reaction rate typically decreases over time as reactants are used up.
Think Like a Scientist
Investigation: Measuring the Rate of Reaction
Objective: To measure how quickly calcium carbonate reacts with hydrochloric acid by tracking the volume of carbon dioxide gas produced.
Background:
You can measure the rate of this reaction in different ways: by measuring the mass of carbon dioxide lost (using a balance), or by collecting the carbon dioxide and measuring the volume with a gas syringe or using a water trough and inverted measuring cylinder. This is known as collecting a gas over water.
You will need:
You can work out what you will need when you decide which method you will use to measure the rate of reaction.
Instructions:
Step 1: Assemble your equipment and place the calcium carbonate in the conical flask.
Step 2: Add the hydrochloric acid, start the timer, and measure the initial mass or volume.
Step 3: After 30 seconds, take a new measurement. Continue measuring every 30 seconds until you get three consecutive values that remain the same.
Show Answer
Conical flask, calcium carbonate, hydrochloric acid, stopwatch, measuring cylinder or balance, delivery tube, trough of water, inverted measuring cylinder or gas syringe, clamp stand, safety goggles.
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- Wear goggles to protect from acid splashes.
- Handle glassware carefully to avoid breakage.
- Ensure good ventilation to avoid CO₂ buildup.
- Clean any spills immediately.
- Dispose of chemicals properly.
Show Answer
Example:
Time (s) | Volume of CO₂ (cm³)
---------- | ---------------------
0 | 0
30 | 12
60 | 24
90 | 30
120 | 32
Show Answer
The graph should show a steep curve at first (fast reaction), then flatten as the reaction slows and stops. This is because reactants are used up.
Show Answer
Possible issues include gas leaks, timing delays, or inaccurate volume readings. These could cause underestimation of gas volume or inconsistent data.
Show Answer
Repeat the experiment and average results. Use precise equipment, ensure airtight seals, and start timing immediately after acid is added.
Why does the rate of reaction change?
We can use ideas about particle theory to answer this question.
For a chemical reaction to take place, the particles of the reactants involved have to collide with one another with enough energy to react together.
At the start of the reaction there are a lot of particles that have not reacted. Collisions happen frequently. This means that a lot of carbon dioxide is produced in the first 30-second period.
As the particles react, the number of particles that have not reacted gets lower and lower. The chance of two unreacted particles colliding with each other decreases. This means that less carbon dioxide is formed in the later 30-second periods. This means that the rate of reaction is slower.
Eventually, all the particles have reacted. There are no more collisions that result in the production of carbon dioxide gas. The reaction has finished.
Particle Theory and Reaction Rate
According to particle theory:
- Reactant particles must collide with enough energy to cause a reaction.
- At the start of the reaction, there are many reactant particles, so collisions are frequent and the rate is fast.
- As the reaction proceeds, fewer reactant particles remain, so collisions are less frequent and the rate slows down.
- When all the reactants are used up, the reaction stops as no more collisions can occur.
This explains why the rate of reaction is highest at the beginning and decreases over time.