hello and welcome this is AQA GCSE chemistry the unit is called energy changes and this along with the other four whole unit videos that I've published on here is good for revision for paper 1 this video is for Combined science the triple science video is available on the channel remember time stamps in the description below but for now let's get going in with exothermic and endothermic reactions exothermic and endothermic reactions reactions can transfer energy to the surroundings imagine we have some reactants with 100 Jews of energy in their bonds we produce some products which have 70 Jew of energy in their bonds 30 Jew of energy have been transferred to the surroundings this is an exothermic reaction for exothermic reactions these transfer energy to the surroundings so imagine we have a reaction going on in the flask if it was an exothermic reaction we would detect a temperature rise as shown by the thermometer there examples of exothermic reaction include combustion reactions which is the same thing as burning many oxidation reactions are exothermic in fact combustion is a oxidation reaction and neutralization reactions are exothermic for example acid with Alkali or acid with base these types of reactions are used in self heating cans and hand warmers for example in terms of our self-heating can here's a small diagram to show the idea of that we got some self-heating soup there's a little button at the bottom of the can if we look inside we can see that when we press the button we Pierce a little membrane which causes two chemicals to mix and causes an exothermic reaction which Heats our soup endothermic reactions we would detect a temperature drop that's because these take in energy from the surroundings examples include th decomposition and a very specific example the reaction of citric acid with sodium hydrogen carbonate these type of reactions are often used in sports injury packs for example to reduce or stop swelling of an injury activation energy reactions happen when reacting particles collide with enough energy here is a flask with a reaction going on and I've just shown two of the reacting particles in that flask if they don't react with enough energy there will not be a chemical reaction however in the second example here they have reacted with enough energy so there is a chemical reaction between those particles the minimum energy needed for particles to react is called the activation energy reaction profiles here's a reaction profile showing the energy on the y-axis and the progress of the reaction on the x-axis for our reactant we can see the energy level there however the energy in the products is a bit lower and we can show the activation energy on this reaction as well the activation energy is the difference between the reactant line and the peak of that hump there so there's our activation energy we also need to know what's called the overall energy change and that's shown by the reactant line and the products line you should recognize a reaction profile and you should be able to label those two parts on a reaction profile graph for exothermic reactions I've just repeated the diagram again there but this time just to show that energy is transferred to the surroundings in an exothermic reaction and that's what the reaction profile would look like for an exothermic reaction for an endothermic reaction we have the product's energy line is higher than the energy of the reactants and in this case the energy has been taken in from the surroundings also worth labeling the parts here so the activation energy X is shown by that line there and the overall energy change is shown by y between the reactants and the products there the energy change of reactants this is for the higher tier only the energy change of reactions let's take an example of the reaction of hydrogen with oxygen to form water this is a balanced formula equation for that reaction we can see that there's two molecules of hydrogen that react with one molecule of oxygen to produce two molecules of water to understand what's going on here we need to remember that energy is supplied to break the bonds in the molecules of hydrogen and oxygen and energy is released when making bonds to produce the water molecules the overall energy change is the energy supplied to breake bonds minus the energy released when making bonds so let's take a look at what's called a display formula for this reaction we have a hydrogen molecule there as shown by H with a single line between the two and in the reaction you can see we actually have two of those so we put a two in front of the formula there we react that with one molecule of oxygen there's a double bond between the atoms in oxygen and that produces two molecules of water we can see all the various different bonds that are involved in this reaction here is the energy in the bonds in KJ per mole so for the bond between the hydrogen atoms that's 436 KJ per mole for the double bond between the oxygen atoms it's 498 and for the bond between hydrogen and oxygen in the water it's 464 so we can see this 436 Jews between the hydrogen atoms there's two of those so we put a two in front there there's 49 8 Jew in the double bond between the oxygen atoms and for each of our o Bonds in the water there's 464 K per mole we can use this to calculate the overall energy change so breaking Bonds in the reactants that's the energy supplied well we have 2 * 436 + 498 that gives us 1370 and in terms of making Bonds in the product that's the energy release we have 2 * 464 in each water molecule but remember there's two molecules of water in the balanced reaction so that gives us a total of 1856 KJ per mole to calculate the overall energy change it's the energy supplied minus energy released that gives us the overall energy change of 1370 - 1856 which is -486 K per mole this actually tells us that this is an exothermic reaction because more energy is released when making bonds then absorbed to break bonds let's take a look at one more example this is the reaction of methane with chlorine and again we're going to show the display formula and there it is and the bond energies in KJ per mole are shown by this table if you want to give this one a go please pause here and give it a try but if not we'll go through the answers in a moment so in terms of breaking Bonds in the reactants that's the energy supplied we have Bonds in CH4 and there's four CH bonds there as shown by the CH in the table and there's our four bonds so that gives us 4 * 413 which is 1652 we then have one cl to CL Bond as shown in the table there and shown in the reaction there so that gives us 243 and that's a total of 1895 KJ per mole in terms of making Bonds in the products that's the released we have three C to H bonds as we can see there 1 2 3 that gives us 1 2 3 9 then we have one C to CL Bond as shown there that gives us 327 and we have one H to CL bond which gives us that product there and that's 432 so if we add those together we get 1 1998 K per mole and again energy supplied minus energy released gives usus 103 KJ per mole again this is exothermic because more energy is released when making bonds than absorbed to break bonds so in summary for exothermic reactions when the overall energy change is negative for example - 103 K per mole the reaction is exothermic and here is the reaction profile for an exothermic reaction if we have an endothermic reaction when the overall energy change is positive for example 178 KJ per mole the reaction is endothermic and here's our reaction profile for an endothermic reaction so that's what you need for the energy change of reactions for Combined science do go over the key points of that again if you need to but for now that's it thank you for watching and I'll see you say