Lesson video

Hello, I am Mrs. Adcock, and welcome to today's lesson on the collision theory.

We are going to be looking at what is the collision theory and why does changing certain factors impact the rate of reaction.

Today's lesson outcome is I can explain why increasing pressure, surface area, or concentration increases the frequency of collisions between particles and therefore the rate of reaction.

Some of the keywords we will be using in today's lesson include collision theory, activation energy, and rate of reaction.

Here you can see each of those keywords written in a sentence.

It would be a good idea to pause the video now and read through those sentences.

You might like to make some notes so that you can refer back to them later in the lesson if needed.

Today's lesson on the collision theory is split into three main parts.

First of all, we are going to look at collision theory.

Then we are going to have a look at concentration and pressure.

And then finally, we are going to move on to look at surface area and temperature.

Let's get started on the first part of our lesson on collision theory.

For a chemical reaction to take place, particles in the reactants must collide with each other.

We have the reactant particles in our reaction vessel and these need to collide with each other in order to react and form products.

However, if those reactant particles collide with too little energy, then a reaction will not occur and no new products will be formed.

In the animation there, we can see the particles collide, but they do not have sufficient energy to react and so do not form a new product.

We have AB, that's one of our reactants, and we have C, and they do collide, but the particles do not have sufficient energy and therefore no reaction occurs.

For particles to have successful collisions where they react, they must collide with sufficient energy, and this is known as the collision theory.

Just to recap, the collision theory is where particles collide with sufficient energy in order to react.

And the minimum energy that the particles must have in order to react is known as the activation energy.

If we look at the animation here, we can see that the particles have sufficient activation energy when they collide and therefore they are able to react and form a new product.

We have the particles AB, and along chem C, they collide together with sufficient energy and then we form new products.

And we end up with A and BC.

Time for a question.

In order for reactant particles to react, they need to A, have energy lower than the activation energy, B, collide with energy lower than the activation energy, or C, collide with the activation energy.

The correct answer is C.

So well done if you got that question right.

In order for reactant particles to react, they need to not only collide but collide with sufficient energy and that minimum energy that they need is the activation energy.

The rate of a reaction and the rate of reaction is the speed with which a chemical reaction takes place.

And the rate of a reaction depends on the frequency of successful collisions between reactant particles.

So the higher the frequency of successful collisions, the higher the rate of reaction.

And by successful collision, we mean a collision where the particles have got the activation energy and therefore they can react and form a product.

Factors that affect the frequency of successful collisions and therefore affect the rate of reaction are the concentration of reactants in solution, the pressure of reacting gases, the surface area of solid reactants, or the temperature.

So these are all things that can affect the rate of the reaction.

What effect does a higher frequency of successful collisions have on the rate of reaction? A, it decreases the rate of reaction, B, it has no impact on the rate of reaction, C, it increases the rate of reaction.

A higher frequency of successful collisions increases the rate of reaction.

So well done if you chose C.

Time for our first practise task of today's lesson, and you've got four questions that you need to complete here.

Number one is define activation energy.

Number two, what is the collision theory? Three, explain why particles may collide with each other but not react to produce new products.

And question four, can you state two factors that can affect the rate of reaction? Pause the video now.

Have a go at answering these four questions.

Then when you come back, we'll go over the answers.

Let's see how you got on.

Question one was define activation energy.

An activation energy is the minimum energy that the particles must have in order to successfully react.

Two, what is the collision theory? The collision theory states that in order for a chemical reaction to take place, the reactant particles must collide with each other with sufficient energy.

And the particles must have at least the activation energy for a reaction to occur.

So the reactant particles not only need to collide, but they have to collide with the activation energy for a reaction to take place.

You may have used different wording for your answer there, but hopefully you've got those key points in your answer.

Question three, explain why particles may collide with each other but not react to produce new product.

The particles may collide but not react if the particles do not contain the activation energy.

Remember, they must contain the activation energy in order for that collision to be successful and for products to be made.

Question four, state two factors that can affect the rate of reaction.

So you may have chosen the concentration of the reactants if we're using solutions, the pressure if your reactants are gases, the surface area if you've got a solid reactant, or the temperature.

Well done if you remembered two factors that could affect the rate of reaction.

We have looked at the collision theory and how our reactant particles need to collide with the activation energy in order to react and produce new products.

Now, we are going to move on to have a look at how changing the concentration or pressure affects the rate of reaction.

The rate of reaction can be affected by changing the concentration of the reactants.

When we increase the concentration of a solution, there are more reactant particles per unit volume and these particles are therefore more likely to collide.

Let's have a look at these animations to help us understand.

We've got a low concentration of reactants in solution here.

And here, we've got a high concentration of reactants in solution.

So we have the same unit volume, but in the high concentration, we have got more reactant particles in that volume.

And therefore the particles are more likely to collide.

Increasing the concentration therefore increases the rate of reaction as there are more frequent successful collisions.

We can see in the graph here, we've got as the concentration is increasing, the rate of reaction is also increasing.

And the graph shows that concentration is directly proportional to rate of reaction.

That means if we double the concentration, then the rate of reaction will also double.

They are directly proportional, the concentration and the rate of reaction.

How can we increase the rate of a reaction? A, increase the concentration of reactants in solution, B, decrease the concentration of reactants in solution, C, increase the volume of reactant used, or D, increase the volume of product made.

And we are trying to increase the rate of reaction, so choose any answers that you think are correct.

The correct answer is to increase the rate of reaction, we need to A, increase the concentration of reactants in solution.

If we increase the concentration, then there will be more particles per unit volume and therefore this increases the frequency of successful collisions.

Decreasing the concentration of reactants in solution would lead to a decrease in the rate of reaction.

C, increasing the volume of reactant used, that may lead to an increase in the volume of product made.

And increasing the volume of the product that we make has no impact on the rate of reaction.

Experimental data shows the effect of changing the concentration of a reactant on the rate of reaction.

And the higher the concentration of the reactant, the steeper the gradient and higher the rate of reaction.

We have a chart here that shows the volume of gas produced in centimetres cubed.

And along the X-axis, we have got the time in seconds.

We have got two sets of data shown on this graph.

We have a pink line which is data for a reactant with a high concentration, and the green line is data from a reaction where the reactant had a low concentration.

The curve for the reactant with a high concentration has a steeper gradient, and therefore when we had a high concentration, we had a higher rate of reaction.

For both the high and the low concentrations, the final volume of gas produced is the same.

And this is because the concentration does not affect the overall volume of gas that is produced.

Concentration affects the rate of reaction but does not affect the final volume of gas that is produced.

The rate of reaction can also be affected by changing the pressure of reacting gases.

So we've looked how we can change the concentration of solutions.

We can also change the pressure of reacting gases to impact the rate of reaction.

When we increase the pressure of reacting gases, there are more reacting particles per unit volume, and therefore the particles are closer together and are more likely to collide.

Here, we can see reacting gases under low pressure.

And in this animation, we can see the reacting gases under high pressure.

And hopefully you can notice that when we've increased the pressure and we have a high pressure, we've got more reactant particles per unit volume, and therefore they are more likely to collide.

Increasing the pressure increases the rate of reaction as there are more frequent, successful collisions.

We can see in the graph here that this looks very similar to when we had the rate of reaction and concentration, but in this time, we've got rate of reaction and pressure.

And the graph shows that pressure like concentration is directly proportional to rate of reaction.

So if we double the pressure, then the rate of reaction will also double.

And why is this? Because we will have more particles per unit volume and therefore we are increasing the frequency of successful collisions.

Time for a true or false question.

Increasing the pressure of reacting gases decreases the rate of reaction.

Is that statement true or false? That statement is false.

Can you explain why this statement is false? At higher pressures, the particles in reacting gases are closer together with a higher frequency of successful collisions.

Therefore increasing the pressure increases the rate of reaction.

So this statement is false because the statement says that increasing the pressure decreases the rate of reaction.

Hopefully you got that question correct and you were able to explain in detail why that statement was false.

It's time for our second practise task of today's lesson, and for this task, you need to explain how decreasing the concentration of reactance affects the rate of reaction.

Be careful because we have looked at how increasing affects the rate of reaction.

So can you think about what will happen if we decrease the concentration of the reactants? So just apply the same principles that we've learned.

Pause the video now.

Have a go at answering this question in detail and then come back when you're ready to go over the answer.

Explain how decreasing the concentration of reactants affects the rate of reaction.

Decreasing the concentration of a reactant solution means there will be less reactant particles in the same volume.

This leads to a decrease in the frequency of successful collisions, causing the rate of reaction to decrease.

Well done if you were able to identify that decreasing the concentration decreases the rate of reaction, and you were able to explain why.

We have looked at the collision theory and how the concentration and pressure can affect the rate of reaction.

Now, we are going to move on to see if we can explain how changing the surface area and temperature will affect the rate of reaction.

The rate of reaction can also be affected by changing the surface area of solid reactants.

To increase the surface area of a solid, we can break it into smaller pieces.

Here in the image, we can see we have metal strips and they have a smaller surface area.

Then if we break them up into smaller pieces such as a metal powder, and then the metal powder has a larger surface area.

So if we want to increase the surface area, we need to break something up into smaller pieces.

If we break a solid into smaller pieces, the volume stays the same.

We still have the same mass of our solid, but because we have broken it into smaller pieces, the surface area increases.

So the volume stays the same, but the surface area increases.

Let's look at an example.

We have a large cube here that is two centimetres by two centimetres.

The surface area of one side will be two times two which will be four centimetres squared.

So the total surface area of all six sides will be four which is the surface area of one side times by six.

So we get the surface of all six sides is 24 centimetres squared.

If we have a look at the volume, the volume for one cube will be two times two times two.

So we have got the width times the length times the depth, so two times two times two, and this is eight centimetres cubed.

We can now think about the surface area to the volume ratio.

The surface area was 24 and the volume of our cube is eight, so our surface area to volume ratio is 24 to eight, and we can put this into its simplest form and identify the surface area to volume ratio as three to one.

So what happens if we now break this large cube into smaller pieces? Here, we can see we have broken our larger cube into eight smaller cubes with sides of one centimetre.

If we work out our surface area now, well, the surface area of one side will be one times one which will be one centimetre squared.

But this time, we have got eight cubes each with six sides.

So that means we have got 48 sides in total.

So if we do one times 48, our total surface area for all the 48 sides will be 48 centimetres squared.

We can work out the volume of one cube and one cube will have a volume of one times one times one which is one centimetre cubed.

However, we have now got eight cubes, so our volume will be one times eight which will be eight centimetres cubed.

You should notice that even though we have broken our cube into smaller pieces, the volume is still the same.

So when we use the large cube, our volume was eight centimetres cubed, and when we have now broken it into eight smaller cubes, the volume is still eight centimetres cubed.

So the volume has not changed, but the surface area has changed.

Let's have a look at the surface area to volume ratio.

This has now become 48 to eight, and we can simplify that to six to one.

So we can see that breaking a large cube into smaller cubes or smaller pieces increases the surface area, but the volume remains the same.

Increasing the surface area to volume ratio increases the rate of reaction.

When the surface area of a solid reactant increases, we have got more particles that are exposed on the surface that are able to react.

This means more collisions can occur per second which means we have more frequent successful collisions and this results in a higher rate of reaction.

Which of the following will react with hydrochloric acid at a higher rate? Is it A, 10 grammes of calcium carbonate powder, B, 10 grammes of calcium carbonate granules, or C, 10 grammes of calcium carbonate chips? The correct answer is A, 10 grammes of calcium carbonate powder.

Now, we have the same mass for each of these different options.

The only thing therefore that has changed is the surface area, and powder will have a larger surface area than the granules or the chips.

As we now know, the larger the surface area, the higher the rate of the reaction.

Well done if you chose A.

We have had a look at how changing the concentration and pressure and the surface area can affect the rate of reaction.

Now, we're going to have a look at temperature, and when we increase the temperature, particles move faster resulting in more frequent collisions.

The particles collide with greater energy, increasing the likelihood of successful collisions.

So when those particles move faster, not only are they more likely to collide with each other, so we get more frequent collisions, but also they've got more energy.

So more of those collisions will be successful collisions because more of those collisions are going to be ones where the particles have got energy higher or equal to the activation energy.

Here, we can see particles moving at a low temperature.

And in this animation, we can see the particles moving at a high temperature.

And hopefully you can spot those particles moving at a high temperature are moving faster and they are colliding more frequently.

Increasing the temperature therefore increases the rate of reaction as there are more frequent successful collisions.

Experimental data shows us the effect of changing temperature on the rate of reaction.

Here, we can see the volume of gas produced in centimetres cubed and the time in seconds.

And we've got two sets of data on our chart.

The blue line represents a reaction that took place at a higher temperature, and the green line represents a reaction that took place at a lower temperature.

And when the temperature is higher, the gradient is steeper as the rate of reaction increases.

So hopefully you can notice that the reaction that took place at a higher temperature, the curve has a steeper line, and this is because this reaction is taking place at a higher rate.

The temperature does not affect the overall volume of gas that is produced.

So for both the high and the low temperatures, we can see that the volume of gas produced overall is the same.

What the temperature does affect is the rate at which we produce this gas.

So the higher temperature produces that overall volume of gas in a shorter time than the lower temperature.

Time for another question.

Which statement or statements correctly describe particles kept in an ice bath compared to at room temperature? A, more particles will have the activation energy, B, the particles will move faster, C, the particles will collide less frequently, or D, there will be less particles in the same volume.

Remember, you are describing the particles kept in an ice bath compared to those particles that are kept at room temperature.

The correct answer is C.

This was the only statement that is correct.

The particles that are kept in an ice bath will collide less frequently because they will have less energy than those particles that are kept at room temperature.

Let's have a look at the other answers.

A, more particles will have the activation energy is incorrect because less particles will have the activation energy because the particles are kept as a lower temperature.

B, particles will move faster, well, no, those particles will be moving slower at a lower temperature.

And D, there will be less particles in the same volume.

Remember that temperature does not affect the number of particles that are in a volume.

This looks like if we had changed the concentration and if we had decreased the concentration, then there would be less reactant particles in the same volume.

That was a tricky question.

So well done if you got that correct and chose answer C.

Time for our final practise task of today's lesson, and this task is split into two parts.

First of all, a student reacts calcium carbonate chips with hydrochloric acid.

Describe one change the student could make to the calcium carbonate to increase the rate of reaction.

And use the collision theory that we learned at the beginning of the lesson to explain why the rate will increase.

If you pause the video now and have a go at answering this question and then we will move on to question two.

Hopefully you've answered question one.

Question two, sketch a line on the graph to show the expected result if the same experiment was repeated but at a lower temperature.

On the graph there, we've got the volume of gas that has been collected in centimetres cubed as the reaction progresses.

And we've got the time recorded there in seconds.

Pause the video now and add your line onto the graph.

And then when you come back, we'll go over the answers to question one and two.

Question one, let's see how you got on.

So describe one change the student could make to the calcium carbonate chips to increase the rate of reaction.

And you needed to include the collision theory in your answer.

So your answer may include the student could increase the surface area of the calcium carbonate by using calcium carbonate powder, or you may have mentioned they could use calcium carbonate granules because both of those will have a larger surface area than the calcium carbonate chips.

Now, we're going to use the collision theory to explain why the rate will increase.

The larger surface area means more particles are exposed resulting in an increased frequency of successful collisions.

Well done if you got that question correct.

If you were thrown a bit by that part that told you to use the collision theory, if you see a question like this, you need to remember that the collision theory states that the particles need to collide with the activation energy in order to react.

So if we are trying to increase the rate of a reaction, we need to increase the frequency of those successful collisions.

Question two, you needed to sketch a line on the graph to show the expected result.

If we repeated the same experiment at a lower temperature, your gradient should be shallower as the rate of reaction will be lower at a lower temperature.

So hopefully your curve looks similar to this one here where you have a shallower gradient, but overall, you still produce the same volume of gas.

The final volume of gas collected will be the same for both temperatures, but for the lower temperature, you should have shown that that final volume of gas took a longer time to be collected.

We have reached the end of today's lesson on collision theory.

Let's just summarise some of the key points that we've covered in today's lesson.

Particles of reactants in a chemical reaction can react together if they collide with sufficient energy.

And we know that the minimum energy that these particles must have in order to successfully react is known as the activation energy.

Increasing the surface area of a solid lets more reacting particles in a solution collide with its particles each second and therefore increasing the surface area increases the rate of reaction.

Increasing the concentration of a reacting solution introduces more reactant particles in the same volume, so they collide more frequently and increase the rate of reaction.

Increasing the temperature of reactants increases the speed of particles so they collide more frequently.

And again, this will increase the rate of reaction.

And increasing the pressure of reacting gases pushes reactant particles closer together so they collide more frequently and this will increase the rate of reaction.

Well done for all your hard work throughout the lesson.

I've enjoyed the lesson and I hope you have too, and I hope you're able to join me for another lesson soon.