in this video we're going to talk about endothermic and exothermic reactions so in an endothermic reaction Delta H the enthalpy change is positive heat energy is absorbed by the system now in an exothermic reaction it's the reverse in an exothermic reac action the change in enthropy is negative and heat energy is released from the systems into the surrounding now let's draw a typical potential energy diagram for an endothermic reaction so it's going to look something like this so here we have the reactants the products on top we have the transition state also known as the activated complex Delta H the eny of the reaction is the difference between the potential energy of the products and the reactants so because the products has more energy than the reactants Delta H is positive and so this reaction is considered to be an endothermic reaction consider this one so notice that the energy of the products is lower than that of the reactants therefore energy was released since the potential energy of the system decreased so Delta H the eny of this reaction is negative so this represents an exothermic reaction consider this particular potential energy diagram so this diagram has multiple steps here we have the first transition state this is the second one here we have the reactants the products and the intermediate so in the first step is it an endothermic reaction or an exothermic reaction now the first step involves the reactants changing into intermediates and notice notice that it's going up the potential energy diagram so the first step is an endothermic step Delta H is positive now what about the Second Step as we go from intermediates to products so notice that it's going down the potential energy diagram so the second step is an exothermic step exothermic I believe I said that wrong so now what about the reaction action overall going from reactants to products we're going down the potential energy diagram so the overall reaction is still exothermic so now you know how to analyze a potential energy diagram and you can tell which steps are endothermic and which ones are exothermic so now let's talk about phase changes so when ice melts into liquid water so we have solid H2O converted into liquid H2O is this an endothermic process or an exothermic process so anytime you have a solid go into a liquid this is called Melon now in order to melt ice you need to add heat to it so because the ice needs to absorb heat Delta H is positive and so therefore this is an endothermic process now what about going from a liquid to a gas is it endothermic or exothermic so think of liquid water if you want to vaporize it into steam do you need to add heat or do you need to remove heat vaporization the process of converting a liquid into a gas is always an endothermic process Delta H is positive you have to add heat to it to vaporize liquid water into a gas now what about going from a gas to a liquid this is called condensation and this is an exothermic process heat has to be released in order for a gas to condense into a liquid now to understand the process of condensation let's say if the temperature of the room is 25° C and there's water molecules in the air let's say it's a humid environment and let's say you let's say this is a table and you pour a cold you pour cold water in this glass and let's say there's some ice cubes floating so let's say the temperature of this cold glass of water is about 0 degrees C heat is going to flow from the surroundings into the system the system is the cold glass water the surroundings is the air and the water molecules that is in the air so the surroundings is at a higher temperature than the cold water heat flows from hot to cold so heat is going to flow into the cold glass of water So eventually the water is going to the temperature is going to go up as it gains energy and because the cold glass of water is absorbing heat energy it's endothermic for the water now for the surroundings it's exothermic because the surroundings are releasing energy now the water molecules which are 25° C that's the water molecules in the air they're going to transfer the heat energy into the glass water and so as they lose that heat energy they will condense into a liquid so whenever you have a gas converting into a liquid it's an exothermic process heat energy is released so these gases water molecules they're going to release their heat energy to the cold water and they're going to liquefy they're going to you're going to see these droplets of water forming at the side of the container outside of it and so that's condensation you see it happening all the time anytime you place a cold cup of water in a humanid environment you're going to see water condense on the outside surface of this glass what's happening in is the gaseous water that's in the air is condensing into a liquid and as a result it's releasing heat does warming up the water that's at a cold temperature and so that's condensation and as you can see it's an exothermic process so anytime you go from a solid to a liquid to a gas it's endothermic you have to put heat to get that going so to go from a solid to a gas directly this is called sublimation and that is an endothermic process now the reverse going from a gas to a liquid which is condensation or from a liquid to a solid which is known as freezing in order to do this you have to remove heat from the system so therefore this is an exothermic process heat energy has to be released by the way going from a gas to a solid this is called deposition this too is an exothermic process another thing you need to understand is breaking and forming a bond whenever you break a bond that requires energy that's an endothermic process you got to put in energy to break a bond anytime a bond forms this is an exothermic process energy is released so let's say if you have a solid in a solid the atoms are very close together and so they have strong bonds but when you heat a solid the atoms they move apart and so as they move apart they become more fluid and so the solid converts or melt into a liquid and as you continue to heat a liquid eventually the particles will move apart and they're going to turn into a gas so as you can see whenever you add heat you're breaking bonds the atoms are moving apart and so that's an endothermic process now the reverse if you were to remove heat if you take out heat from a liquid the atoms will they will lose their thermal energy and they will get closer and closer together and they're going to form a bond and they're going to turn into a solid so anytime you break a bond where you move atoms apart you need to add heat energy to it that's an endothermic process and to bring atoms together you want to remove heat and that's going to cause the atoms to come together form in a solid so let's say if you have a chlorine molecule and it breaks into two chlorine radicals is this an endothermic process or an exothermic process since we're breaking the chlorine chlorine bond this is going to be an endothermic process so the enthropy of reaction is positive now if we have two radicals and if they come together to form a bond that's an exothermic process whenever a bond forms heat energy is released now there are some examples of reactions that are very exothermic one example is the combustion reaction of let's say a hydrocarbon if you burn methane in air it's going to produce CO2 water most combustion reactions are highly exothermic they release a lot of heat energy sometimes you can dissolve salts in water and they too will release a lot of heat energy for example calcium chloride let's say if you have a solid chunk of calcium chloride and let's say if you take a glass Stern rod and there's some water molecules on a glass turn rod once you come in contact with this enhydris or dry form of calcium chloride it's going to release a lot of heat in some cases it can cause the small amount of water that's on its Glide to vaporize into steam the dissolution of calcium chloride releases a lot of heat energy so it's an exothermic reaction another example is sodium hydroxide when sodium hydroxide dissolves in water it too releases a lot of heat energy if you pour sodium hydroxide into a a cup of water and if you have a thermometer inside you'll see the temperature go up and so a lot of these dissolution reactions are highly exothermic but there's some that are endothermic you just got to find them but that is it for this video thanks for watching