Endothermic and Exothermic Reactions

In this video we are going to talk about

endothermic

and

exothermic

reactions

, so in a delta h

endothermic

reaction, the enthalpy change is positive, the system absorbs thermal energy. Now, in an

exothermic

reaction, it is the other way around. In an exothermic reaction, the enthalpy change is. Negative and thermal energy is released from the systems into the environment. Now let's draw a typical potential energy diagram for an endothermic reaction, so it will look like this, here we have the reactants, the products at the top, we have the transition state also known as the activated complex delta h the enthalpy of the reaction is the difference between the potential energy of the products and the reactants, so because the products have more energy than the reactants, the delta h is positive and that is why this reaction is considered an endothermic reaction, consider this then notice that the energy of the products is less than that of the reactants, therefore energy was released as the potential energy of the system decreased, so delta h the enthalpy 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 here we have the reactants the products and the intermediate then in the first step it is an endothermic reaction or an exothermic reaction now the first step involves the reactants changing to intermediates and now we notice that it is going up in the potential energy diagram, so the first step is an endothermic step delta h is positive.

Now what about the second step as we move from intermediates to products? So notice that it's going down the potential energy diagram, so the The second step is an exothermic exothermic step. I think I said it wrong. So what about the general reaction that goes from reactants to products? We are going down the potential energy diagram so that the overall reaction remains exothermic. Now you know how to analyze a potential. energy diagram and you can tell which steps are endothermic and which are exothermic so now let's talk about phase changes so when ice melts into liquid water we have solid h2o which becomes liquid h2o is this a process endothermic or an exothermic process? making a solid become a liquid, this is called melting now, to melt ice you need to add heat to it, as ice needs to absorb heat, delta h is positive and therefore this is an endothermic process.

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 remove heat? Vaporization The process of converting a liquid into a gas is always an endothermic process. delta h is positive, heat must be added to vaporize liquid water into gas. Now, what about going from gas to liquid? This is called condensation and is an exothermic process. Heat must be released for a gas to condense. in a liquid now to understand a condensation process let's say that if the temperature of the room is 25 degrees Celsius and there are water molecules in the air let's say that it is a humid environment and let's say that this is a table and you pour cold water you pour water cold in this glass and let's say that there are some ice cubes floating, so let's say that the temperature of this glass of cold water is approximately zero degrees Celsius.

Heat will flow from the surroundings into the system. The system is the glass water code. The environment is the air and the water molecules in the air, so the environment is at a higher temperature than cold water. Heat flows from hot to cold, so heat will flow into the glass of cold water, so eventually the water will evaporate. the temperature will increase as it gains energy and because the cold glass of water is absorbing thermal energy it is endothermic to the water and now to the surroundings it is exothermic because the surroundings are releasing energy now the water molecules that are at 25 degrees Celsius are The water molecules in the air are going to transfer thermal energy to the glass water and as they lose that thermal energy they will condense into a liquid, so every time a gas becomes a liquid it is a exothermic process. are released, so these gaseous water molecules will release their thermal energy into the cold water and liquefy.

You can see these water droplets forming on the side of the container outside of it and so on. That's condensation. You see it happen all the time. Every time you place a glass of cold water in a humid environment, you will see how water condenses on the outer surface of this glass. What happens is that the gaseous water that is in the air is condensing into a liquid and as a result it releases heat, thus heating the water that is at a cold temperature and that is condensation and as you can see it is an exothermic process, so Every time you go from a solid to a liquid to a gas it is endothermic, you have to put heat to achieve it, go from a solid to a gas directly, this is called sublimation and it is an endothermic process.

Now, the other way around, going from a gas to a liquid, which is condensation, or from a liquid to a solid, which is known as freezing. To do this, you must remove heat from the system, therefore this is an exothermic process. Thermal energy must be released when going from a gas to a solid. This is called deposition. This is also an exothermic process. Another thing you need to understand is breaking and forming a bond every time a bond is broken it requires energy which is an endothermic process you must put energy to break a bond every time a bond is formed this is an exothermic process energy is released so Let's say if you have a solid in In a solid, the atoms are close together and that's why they have strong bonds, but when you heat a solid, the atoms separate and as they separate, they become more fluid and the solid becomes or it melts into a liquid and as you continue to heat a liquid, eventually the particles will separate and become a gas, so as you can see, every time you add heat, you are breaking bonds, atoms are moving apart, so that this is an endothermic process, now the other way around if you were. to remove heat, if you take away heat from a liquid, the atoms will lose their thermal energy and get closer and closer and form a bond and become a solid, so every time To break a bond where the atoms are separated , you need to add thermal energy to it, that is an endothermic process and to bring the atoms together you want to remove the heat and that will cause the atoms to come together and form a solid, so let's say if you have a chlorine molecule and it breaks into two chlorine radicals, is this an endothermic or exothermic process?

Since we are breaking the chlorine bond, this will be an endothermic process, so the enthalpy of the reaction is positive now if we have two radicals and if they come together to form a bond, it is an exothermic process. Every time a bond forms, thermal energy is released. Now there are some examples of

reactions

that are very exothermic. An example is the combustion reaction of, say, a hydrocarbon. If you burn methane in air, to produce co2 and water, most of the combustion reactions are highly exothermic, releasing a lot of thermal energy. Sometimes you can dissolve salts in water and they will also release a lot of heat energy, for example, calcium chloride, let's say if you have a solid piece of calcium chloride. and let's say if you take a glass stern rod and there are some water molecules in a Glaston rod, once you come into contact with this anhydrous or dry form of calcium chloride, it will release a lot of heat, in some cases it can cause a small quantity. of water that is in the glass rod of the sternum to vaporize into steam, the dissolution of calcium chloride releases a lot of thermal energy, making it an exothermic reaction.

Another example is sodium hydroxide, when sodium hydroxide dissolves in water it also releases a lot of heat energy if you pour sodium hydroxide into a cup of water and if you have a thermometer inside you will see that the temperature increases so many of These dissolution reactions are highly exothermic, but there are some that are endothermic, you just have to find them, but that's the way it is. it's because of this video thanks for watching