[Music] hi and welcome back to three science lessons code okay by the end of this video you should be able to describe what's meant by an exothermic or an endothermic reaction you should then be able to draw energy profiles showing exothermic and endothermic reactions and finally you should be able to describe what's meant by the activation energy for reaction let's start by looking at x of them reactions I'm showing you an exothermic reaction here this shows wood burning now the key fact about exothermic reactions is that they transfer energy from the reacting molecules to the surroundings so that means that an exothermic reactions the temperature of the surroundings increases in other words the surroundings get hotter now we've already seen that combustion in other words burning is an exothermic reaction other exothermic reactions include certain oxidation reactions our neutralization it's worth learning these examples as they could come up in your exams and we're going to look at all of these reactions and more detail in later videos now scientists represent the energy changes that take place in reactions by drawing an energy profile diagram I'm showing you here the energy profile found exothermic reaction the first thing to notice is that the products have got less energy than the reactants that's because energy has been transferred from the reaction to the surroundings the difference between the energy of the reactants are the energy of the product tells us the energy that's been released to the surroundings so I'm showing you that here in the exam you could be asked to label an energy profile diagram showing the energy change now exothermic reactions can be very useful and I'm showing you two uses here we find exothermic reactions in hand-warmers and we also find them in self heating cons for example containing food or drink okay we're going to continue now by looking at endothermic reactions endothermic reactions take in energy from their surroundings so in the case of endothermic reactions the temperature of the surroundings decreases in other words they get colder and a good example of an endothermic reaction is thermal decomposition which we look at in a later video I'm showing you here the energy profile for an endothermic reaction as you can see the products have gotten more energy than the reactants and that's because energy has been taken in from the surroundings the difference in energy between the reactants and the product tells us the amount of energy that's been taken in by the reaction and I'm showing you that here now if we look again at the energy profiles for both exothermic and endothermic reactions we can see they've got something in common in both cases the energy rises to a peak and I'm showing you these here this is called the activation energy and we're going to look at that now the first key idea is that reactions can only occur when the particles collide with each other and they have to do that with sufficient energy the minimum amount of energy that particles must have in order to react is called the activation energy now you could be asked to represent the activation energy on an energy profile diagram so let's look at how to do that the activation energy is from the reactants to the peak of the curve here's the activation energy on the exothermic energy profile and this shows the activation energy on an endothermic energy profile remember you'll find plenty more questions on exothermic and endothermic reactions and on reaction energy profiles in my revision workbook and you can get that by clicking on the link above [Music]