What are the key learning points about rates of reaction?
The rate of reaction can be determined by measuring the rate of gas production or The amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). loss in a chemical reaction.
A rate of reaction is determined by how often successful When two objects meet and interact, eg two particles moving towards each other will collide. of particles take place, which are collisions that possess the The activation energy is the minimum energy required for a reaction to occur. (Higher tier only)
The rate of reaction is affected by changing The amount of a substance that has been dissolved in a certain amount of solution. Measured in mol/dm3 (moles per decimetre cubed)., temperature or by using a A substance that speeds up a chemical reaction without being used up or chemically changed. It does this by providing an alternative reaction pathway with a lower activation energy.. (Higher tier only)
How to measure rates of reaction
The rate of reaction can be found by measuring the amount of A chemical which is made in a chemical reaction. Products are written on the right of a chemical equation, after the arrow (→). formed, or the amount of The chemical present at the start of a reaction. Reactants appear on the left of a chemical equation, before the arrow →. used up, over a certain period of time.
The mass of a solid product is measured in grams (g), while the volume of a gaseous product is measured in cm3.
Rate is most often calculated using the equation: rate = \(\frac {1}{time}\) where the time is the time for the reaction to reach a certain point or the time for the reaction to be completed.
The units of rate calculated in this way are s-1.
What experiments are used to measure the rate of reaction?
The experiment you carry out to measure the rate of reaction depends on the nature of the product being measured.
How to measure the loss of mass from a reaction mixture
The loss in mass of a substance is measured with a balance.
How to measure the amount of gas produced
In reactions that produce a gas, the volume of a gas can be measured with a gas syringe (or an upside down measuring cylinder).
What are typical rate of reaction experiments?
Metals reacting with dilute acid
We can use a gas syringe to measure the reaction of metals with dilute acid.
When zinc reacts with hydrochloric acid it produces zinc chloride and hydrogen gas.
We can measure the rate of the reaction by measuring how fast the reaction produces hydrogen. This requires a conical flask and gas syringe.
Zn + HCl → ZnCl2 + H2
What happens when calcium carbonate reacts with hydrochloric acid?
Calcium carbonate with dilute acid
Similarly, when calcium carbonate reacts with dilute hydrochloric acid, it produces carbon dioxide gas.
We can measure the rate of the reaction by measuring how fast the reaction produces carbon dioxide.
This requires a conical flask as well as a measuring cylinder filled with water and inverted in a trough of water.
CaCO3+ HCl → CaCl2 + CO2 + H2O
How to analyse rates of reaction data
In a typical rates experiment, the mass or volume of A chemical which is made in a chemical reaction. Products are written on the right of a chemical equation, after the arrow (→). is measured at regular time intervals.
The results are usually recorded in a table.
For example:
| Time / min | Volume of gas produced / cm3 |
|---|---|
| 0 | 0 |
| 1 | 34 |
| 2 | 42 |
| 3 | 48 |
| 4 | 50 |
| 5 | 50 |
These results show that the reaction had finished by four minutes.
No more gas was produced after that point.
However, the rate decreased during the reaction as the volume of gas produced each minute decreased as the time increased.
Graphs
The rate of reaction can be analysed by plotting a graph of amount of product against time.
The graph below shows this for two reactions.
Compared to the slow reaction, the graph line for the faster reaction:
has a steeper In a graph, the gradient is the steepness of the line. The greater the gradient, the greater the rate of change. at the start,
becomes horizontal sooner (gas stops being produced sooner so the reaction finishes sooner) meaning that the rate of reaction is greater.
Question
The graph below shows the volume of gas produced by a reaction over time.
At what time did the reaction finish and what was the total volume of gas produced?
Answer
The reaction finished at 6 minutes, as the graph line becomes horizontal at this point.
This indicates that no more gas is being produced, so the reaction has stopped.
The total volume of gas produced is 60 cm3.
What other factors affect the rate of reaction?
If the temperature of a reaction is increased, then the rate of the reaction will also increase.
If the concentration of a reactant in a reaction is increased, then the rate of the reaction will also increase.
What is collision theory? (Higher tier only)
For a chemical reaction to occur, the reactant particles must When two objects meet and interact, eg two particles moving towards each other will collide. with each other.
Image caption, 1. Two pairs of particles move towards each other
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However, a collision with too little energy will not produce a reaction.
The colliding particles must have enough energy for the collision to be successful or effective in producing a reaction.
The minimum amount of energy for a collision to be successful is called the activation energy.
The rate of a reaction depends on the rate of successful collisions between reactant particles.
The more successful collisions there are, the faster the rate of reaction.
Image caption, 1. Two pairs of particles move towards each other
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How does temperature affect the rate of reaction?
The rate of a chemical reaction can be changed by altering the temperature.
If the temperature is increased:
- the reactant particles move more quickly,
- they have more energy,
- there are more successful collisions per unit time, and
- the rate of reaction increases.
Compared to a reaction at a low temperature, the graph line for the same reaction but at a higher temperature:
- has a steeper In a graph, the gradient is the steepness of the line. The greater the gradient, the greater the rate of change. at the start,
- becomes horizontal sooner.
This shows that the rate of reaction was greater at the higher temperature.
Question
Which line A, B or C represents the reaction that is carried out at the lowest temperature?
Answer
The answer is C.
The reaction that is carried out at the lowest temperature will be the slowest, this is shown by the line that is the least steep and takes longest to reach the end volume.
How does concentration affect the rate of reaction? (Higher tier only)
The rate of a chemical reaction can be changed by altering the concentration of a reactant in solution.
If the concentration is increased:
There are more reactant particles.
They collide more often.
There are more successful collisions per unit time; and
The rate of reaction increases.
Compared to a reaction with a reactant at a low concentration (if a solution), the graph line for the same reaction but at a higher concentration:
- has a steeper In a graph, the gradient is the steepness of the line. The greater the gradient, the greater the rate of change. at the start,
- becomes horizontal sooner.
This shows that the rate of reaction was greater at the higher concentration.
Question
Which line A, B or C represents the reaction that is carried out at the highest concentration?
Answer
The correct answer is A.
The reaction that is carried out at the highest concentration will be the fastest, this is shown by the line that is the steepest and takes the shortest time to reach the end volume.
What are catalysts? (Higher tier only)
The rate of a reaction can be increased by adding a suitable catalyst.
A catalyst is a substance which increases the rate of a chemical reaction but it is not used up (remains chemically unchanged at the end).
Only a very small amount of catalyst is needed to increase the rate of reaction between large amounts of reactants.
A catalyst is specific to a particular reaction:
- different catalysts catalyse different reactions,
- not all reactions have suitable catalysts.
WATCH: Four ways to increase the rate of a reaction
Chemical reactions are all about collisions between particles, and a rate of reaction depends on how frequently particles collide and with how much energy.
There are four methods of increasing the rate of reaction.
My mate Professor Seller is going to talk me through them.
First up – concentration.
What we're going to do is we're going to set up these three reactions.
This one's going to be the high‑concentration one.
You can see there's more stuff in it.
We're going to put medium one, and then finally we're going to have a low‑concentration one down at the other side.
And this is going to be like a race.
We're going to start them off, and when we get to the end of the race, the solution's going to turn blue.
Are you ready. Yeah. Steady. Go.
And now let's just mix them up.
The reaction taking place in the beaker finishes with the sudden release of iodine, which interacts with the starch that is already present to turn the solution blue almost instantly.
We have the reaction going, and what we're doing is we're waiting for those racers to get to the end.
Firm favourite is high concentration.
Also in the running is medium concentration.
Bringing up the rear is low concentration.
Woah, there went the first.
As expected, the firm favourite, high concentration, comes storming through the finish line.
Now what about this one.
Kind of wondering whether that one's going to go.
Oh, there went the second one.
The runners are coming in exactly in the order that we're expecting.
And that one, he's been out of training or something.
That's not— Oh! There it went.
Of the three solutions that were added, it was the solution with the highest concentration that resulted in the quickest reaction.
Because the reactant particles are more crowded, collisions take place more frequently.
So that was concentration. Now on to temperature.
When the temperature is increased, the particles in a solution move more quickly.
This causes an increase in the frequency of collisions and the energy with which they hit each other.
We're going to see how temperature affects the rate of reaction.
We're going to do that by using a glowstick, which reacts when we break it.
Now let's see what would happen if you were to cool it down.
So as you can see, the reaction gives off light.
This isn't giving off light so the reaction appears to have slowed down.
If we cool it down and the reaction slows down, what happens if we heat it up.
Well let's give it a go.
A bit of friction to heat it up.
And as we warm up the solution, what happened to the reaction.
Well it's started to give off light again, even more light than that.
And because it's now got warmer, the reaction has sped up.
So we can say that increasing temperature speeds up a reaction.
And decreasing temperature slows down a reaction.
So stick it in your freezer if you want to keep it for tomorrow to have more raving.
Right, I'm off.
Next up are catalysts.
They work by speeding up a reaction and they do this by increasing the number of successful collisions between particles.
Back to Professor Seller and his great experiments.
Here we have hydrogen peroxide, and I'm going to add a little bit of a solid catalyst.
This is manganese dioxide, tiny bit.
Can you see that the tiny little flecks of manganese dioxide are actually causing the reaction.
They're causing the hydrogen peroxide to decompose to oxygen and water.
So they're reacting and remaining unchanged now.
Absolutely. Now it's interesting that on this side we've got the same hydrogen peroxide but without the catalyst.
And actually it decomposes very, very slowly.
Even if you leave it in the fridge, eventually it will go off.
Let's not mess around.
Let's give it a real load of catalyst to see what happens.
Do I have to step back for this.
Well you'll see.
Go.
It's actually gotten so hot that it's boiling, and you'd see a plume of water vapour which accompanies the oxygen as it comes out.
The catalyst is causing the breakdown of hydrogen peroxide into water and oxygen at a phenomenal rate, but the catalyst has not changed at all throughout this reaction.
The catalyst is still there.
We could pour this all off, we could filter it away, and we would collect all of that black stuff — that's our catalyst.
And finally – the size of particles.
How does that affect the rate of reaction.
Let's burn this lump of sugar.
Well it burns a bit.
As it burns, the sugar is turned into carbon dioxide and water.
What about if we decrease the particle size using something like icing sugar.
Using a smaller particle size increases the surface area.
We've used the same amount of sugar as is in this cube, and we've put it into this tube.
Now let's see what happens when we try and burn it this time.
There was a lot more reacting going on and a lot more heat.
I could even feel it coming off.
So by breaking down the sugar into a powder, its surface area increased.
More of the sugar has been exposed to the oxygen in the atmosphere, so collisions can take place more frequently.
Decreasing the size of the particle increases the rate of reaction, and that's because we have increased the surface area.