Lesson video

Hello, my name is Mrs. Collins and I'm going to be taking you through your learning today.

This lesson forms part of the unit, "Energy Changes in Reactions", and it's called, "Reaction Profiles".

So let's get started.

So during this lesson, we are going to learn how to draw, label and interpret reaction profiles for exothermic and endothermic reactions and understand the changes these cause in the surroundings.

Here are the keywords for today's lesson.

Exothermic, endothermic, activation energy and reaction profile.

Now, some of those words we've used already during the topic, but others will be new to you.

So pause the video here and read through the definitions and make any notes you feel you need to.

Today's lesson is divided into two parts.

We're going to talk about the energy changes in a reaction first of all and then we're going to draw and interpret some reaction profiles.

Let's get started on part one, energy changes in a reaction.

So you'll remember that exothermic chemical reactions are ones in which energy is released to the surroundings from the reaction, and this causes an increase in temperature.

And a good example of this is a displacement reaction.

So here we've got an image of a displacement reaction taking place.

So we've got a coil of copper wire placed into a silver nitrate solution and we can see that the copper is displacing the silver from the solution and the silver's forming on the surface of the copper.

So an endothermic chemical reaction absorbs energy from the surroundings and it causes a decrease in temperature.

So thermal decomposition is a good example, and here we've got copper carbonate decomposing to copper oxide.

So the green copper carbonate decomposing to the black copper oxide.

Here is a question based on your previous learning.

Combustion is an endothermic reaction.

Is that statement true or false? And use the statements below to justify your answer.

So pause the video here and I'll see you when you're finished.

Welcome back.

Hopefully you recognise that that statement is false and the reason it's false is because combustion is an exothermic reaction.

It transfers energy to the surroundings, making it exothermic.

So well done if you got that correct.

So during a chemical reaction, both exothermic and endothermic processes are taking place.

To give an example, we've got carbon reacting with oxygen to form carbon dioxide.

Now, what needs to happen first, is we need to break the bonds between the oxygen atoms and between the carbon atoms, and in order to do this, energy is absorbed.

So that's an endothermic process.

Energy is being absorbed.

To form the dioxide at the end, we need to make new bonds between the carbon and the oxygen, and during this process, energy is released to form these bonds.

So that's an exothermic process.

So bond breaking is endothermic and bond making is exothermic.

So the energy needed to break the bonds in the reactants and start the reaction is called the activation energy.

So activation energy is needed to start a reaction.

It's the minimum amount of energy needed for a reaction to take place and different amounts of activation energy are needed for different reactions.

So if we think about striking a match, that has quite a low activation energy, not much energy is needed to start that reaction taking place.

But rusting is a very slow process, so it's got a high activation energy.

It takes a lot of energy to start that reaction.

Here we've got a question based on that learning.

So which of these reactions will have the higher activation energy? So there are two reactions there which have got a high activation energy, higher than the others.

So pause the video here, read through those statements and decide which reactions it is.

Welcome back.

So hopefully you've recognised that the chemical weathering of rock will have a higher activation energy and so will photosynthesis.

And this is because rocks take a long time to weather and plants require time to grow.

So those two processes are slower and require high activation energies.

As both exothermic and endothermic processes are taking place in a chemical reaction, the difference between those two processes is the overall energy change of the reaction.

So we're looking at the difference between bond making and bond breaking.

So neutralisation of strong acids and bases remember, is an overall exothermic reaction.

So more energy is released forming the products than was absorbed by the reactants.

This means the products contain less energy than the reactants, because more energy has been released.

Electrolysis on the other hand, is an endothermic reaction remember, as more energy is absorbed by the reactants than is released forming the products.

This means the products contain more energy than the reactants as less energy is released.

Let's have a go at task A.

In the following reactions, A equals energy absorbed by reactants and B equals energy released by products and C is the overall energy change.

Select the correct relationship between A and B.

So what I'd like you to do is pause the video and have a go at filling in the table.

Now, that probably took a little bit of thinking to try and work out what the different symbols meant, but let's go through those answers.

So first of all, the first one is endothermic.

So we need to think about is the energy absorbed by the reactants the same as the energy released by the products or is it greater than or less than? So for endothermic reactions, A, the energy absorbed by the reactants is greater than the energy released by the products, whereas for an exothermic reaction, the energy absorbed by the reactants is less than the energy released by the products.

And again, we've got an endothermic and an exothermic reaction.

So well done if you've got that correct.

For question two, we're gonna read through the statements that these four students have made.

So some students are discussing the energy changes that occur in exothermic and endothermic reactions.

We need to read through them, decide which ones are correct and correct those that are incorrect.

So make some corrections.

So read through each of those four statements.

Decide first of all, are they correct and if they're wrong, you need to amend them.

So pause the video here and I'll see you when you're finished.

Welcome back.

So let's go through those answers then.

So first of all, hopefully you've recognised that actually Alex had the correct answer.

So all reactants need activation energy before they can react.

Jun's statement is slightly incorrect, so it's actually less energy is absorbed by the reactants, not more.

Izzy's answer is also incorrect, because we need to say less energy again.

And Aisha's answer is incorrect, because it's energy that's released, not activation energy.

So well done if you got that correct.

Now we're going to move on to part two and we're going to use that knowledge we gleaned in part one to draw and interpret reaction profiles.

So reaction profiles are diagrams which show the energy changes during an exothermic and endothermic reaction.

They can also be called energy level diagrams. So it depends which examination board you're working with.

So the Y-axis represents energy and it has no scale to it.

They have horizontal lines which represent the reactants and the products and they have a loop showing the energy change during the reaction.

They also have vertical arrows which represent the activation energy and the overall energy change.

So here we have the activation energy and the overall energy change that's happened.

So the activation energy always goes to the top of the loop, but the energy change is the difference between the two horizontal lines, and we're going to have a look at some examples so that you can understand that in a little bit more depth.

So if we wanted to draw a reaction profile for an exothermic reaction, it would look like this.

So we'd have that vertical energy line to start off with, with no scale on it.

We would have the energy level for the reactants and then the energy level for the products.

And notice that the products are lower than the reactants.

So the products contain less energy than the reactants.

And remember that's because energy has transferred to the surroundings.

We then put the loop in to show that the reaction has taken place.

We have the activation energy.

So the activation energy is the part where the loop goes higher than the reactants.

So we need to put in energy into the reaction for the reaction to start, and the energy change here is from the reactant down to the product.

So it's a negative value.

So there's a negative energy change as energy is released to the surroundings.

Now, you might want to pause the video here and just sketch that reaction profile.

So the key features of an exothermic reaction profile are that the energy of the product is lower than the reactants.

The overall energy change is always negative as energy is released to the surroundings.

And energy in the reactants is converted into energy in the products and the energy which is released to the surroundings.

And look at the link there to your physics learning.

So we've got chemical energy stores and thermal energy stores.

So we've got a question here based on the learning so far.

So select the key features of a reaction profile of an exothermic reaction.

So pause the video here and I'll see you when you're finished.

Let's go through the answers.

So hopefully you've recognised that the overall energy change is negative and the reactants contain more energy than the products.

So well done if you got that correct.

Now let's look at an endothermic reaction profile.

So again, we have the vertical line representing energy with no values on it.

The reactants this time contain less energy than the products, because energy is transferring from the surroundings into the products.

We can show this happening with our loop.

And again, remember we've got this increase of activation energy.

So the loop always goes higher than the products and then comes down to the level of the products.

This time, the energy change is positive, so there's more energy in the products than there were in the reactants.

So the positive energy change as the energy is absorbed from the surroundings.

And again, you may wish to pause the video at this stage and sketch that out.

So the key features of an endothermic reaction profile are that the energy of the products is higher than the energy of the reactants, that the overall energy change is always positive as energy is absorbed from the surroundings and the energy in the reactants and energy in the surroundings is converted into the energy of the products.

And again, the link there to your physics knowledge, chemical energy stores and thermal energy stores.

So here's a question based on that learning.

Which reaction profile is or are correctly drawn? Use the key to help.

So R stands for reactants, P for products.

The red line is the activation energy and the blue lines are the overall energy changes.

So look carefully at those three different diagrams. Which one or ones are correct? Pause the video now and answer the question.

Welcome back.

So the correct answer is B.

Well done if got that correct.

So a catalyst, remember, is a substance added to a reaction to speed it up.

It lowers the activation energy needed for a reaction to take place and it does this by providing an alternative pathway.

Now remember, a catalyst is not used up in the chemical reaction.

So let's have a look at the effect of a catalyst on an exothermic reaction profile.

So remember, we've got the vertical line showing the energy, we've got the reactants and the products.

Now remember, in an exothermic reaction, the products contain less energy than the reactants, because energy has transferred to the surroundings.

We put in the loop line and we can see that the energy change is negative.

And here is the original activation energy, so from that reactant line up.

Now, what a catalyst does is provide an alternative pathway with a lower activation energy.

So you can see here that the activation energy is lower than the original and this speeds up the chemical reaction.

So a catalyst speed up reactions by providing an alternative pathway for the reaction requiring a lower activation energy.

So let's see what happens with an endothermic reaction profile.

So remember, an endothermic reaction profile, we've got the energy line as before.

This time, the reactants have less energy than the products and this is because energy is transferred from the surroundings into the products, remember? We place it in the loop and here we've got the energy change and there we've got the original activation energy.

So that's from the reactants all the way to the top of the loop.

So the catalyst provides an alternative pathway with a lower activation energy and we can see the drop in activation energy here.

So catalysts speed up reactions by providing an alternative pathway for the reaction, requiring a lower activation energy, and that works in exothermic and endothermic reactions.

So let's have a go at task B.

The reaction between magnesium and hydrochloric acid is exothermic.

Draw a reaction profile for this reaction.

So we know it's an exothermic reaction, so think about what the reaction profile might look like.

Pause the video here and I'll see you when you're finished.

Let's go through the answer.

So here's what your reaction profile should look like.

You need to make sure that the products have a lower line than the reactants, that you've shown the activation energy and you've shown the energy change.

Make sure that blue arrow there, the arrow head is on the right end, because it's a negative energy change.

So well done if you've got that correct.

So let's have a go at question two.

This time, we're going to explain the key features of an endothermic reaction.

We're gonna think about the position of the reactants and the products, the activation energy, the direction and sign of the overall energy change and definition and effect of a catalyst.

So what you might want to do here, is sketch the reaction.

So what you might want to do here, is sketch the reaction profile first and then explain it.

So pause the video here and I'll see you when you're finished.

So let's go through the answers.

So in terms of the position and the reactants and the products, because it's endothermic, the products are higher than the reactants and this is because they have more energy due to the energy being absorbed from the surroundings.

For B, the activation energy is the minimum energy required to start a reaction.

On the reaction profile, it starts at the reactant and stretches to the top of the energy transfer loop.

Then for C, the direction and sign of the overall energy change.

So the overall energy change spans from the reactants to the products and is always positive for an endothermic reaction.

And then a catalyst is a substance that speeds up a chemical reaction without being used up.

It works by providing an alternative pathway for the reaction, requiring a lower activation energy.

So well done if you got that correct.

Here is a summary of today's lesson.

Exothermic chemical reactions transfer energy from the reactants to the surroundings, causing an increase in temperature.

Endothermic chemical reactions transfer energy from the surroundings to the products, causing a decrease in temperature.

Activation energy is the minimum amount of energy transferred to the reactants that allows a chemical reaction to start.

And a reaction profile shows the progress of a reaction and compares the amount of energy the reactants have to the products.

Thank you very much for joining me for today's lesson.