Representing endothermic and exothermic processes using energy diagrams
About this video
Using a hypothetical reaction A + B -> C in a beaker, the video shows how feeling the beaker tells you whether a reaction is exothermic (warm) or endothermic (cool). It then uses energy profiles to calculate delta H as the energy of the products minus the energy of the reactants: -50 kJ/mol for the exothermic case and +50 kJ/mol for the endothermic case.
You'll be able to
- decide whether a reaction is exothermic or endothermic from how the container feels.
- read an energy profile and calculate delta H as products minus reactants.
- recognise exothermic and endothermic energy profiles from the relative heights of reactants and products.
Key words
- energy profile
- A graph of potential energy (kJ/mol) against reaction progress, showing reactants on the left and products on the right.
- exothermic
- A reaction where heat flows from the system to the surroundings; products are lower in energy and delta H is negative.
- endothermic
- A reaction where heat flows from the surroundings to the system; products are higher in energy and delta H is positive.
- delta H
- The change in enthalpy, found here as energy of products minus energy of reactants.
Transcript
- [Instructor] Let's say we run an experiment to determine if a reaction is endo or exothermic. For our hypothetical reaction, A reaction would B to form C. And let's say this reaction takes place in aqueous solution in a beaker. We can define our system as the reactants and products that make up our chemical reaction, and everything else is part of the surroundings, for example, the water and also the beaker the reaction is taking place in.
Let's say we run the reaction and we put our hand on the beaker and we feel that the beaker is warm. If the beaker is warm since the beaker is part of the surroundings, energy must've been transferred from the system to the surrounding. So heat flowed from the system to the surroundings.
And that's an example of an exothermic reaction. So Delta H is negative. We can determine the amount of energy that flowed from the system to the surroundings by looking at the energy profile for our hypothetical reaction. In an energy profile, potential energy is in the y-axis in kilojoules per mole and reaction to progress is on the x-axis.
So as we move to the right on the x-axis, the reaction is occurring. Our reactants, which are A and B have a certain amount of potential energy. So that's right here on our energy profile. So that part represents the energy of our reactants. Our reactants react together to form our product which is C and that's at the very end.
So over here, this line represents the energy the potential energy of our products. Notice how the potential energy of our reactants is higher than the potential energy of the product. So if we were to find the change in energy, that would be the final minus the initial.
So the energy of the products minus the energy of the reactance for this energy profile, the energy of the products is about 50 kilojoules per mole, and the potential energy of our reactance is at 100. So this would be 50 minus 100 which is equal to negative 50 kilojoules per mole.
So on our energy profile, we could show Delta E which would be this difference right here. So that represents Delta E and the change in energy Delta E is also equal to the change in the enthalpy Delta H for this reaction. So we know that the change in enthalpy is equal to negative 50 kilojoules per moles.
Let's go ahead and plug that in over here on the left. So by feeling the outside of the beaker, we knew that the reaction was exothermic, but the energy profile allowed us to figure out how much energy was transferred from the system to the surroundings. So for an energy profile, when the energy of the reactants is higher than the energy of the products, this is the energy profile for an exothermic reaction.
Let's say we ran a similar reaction where A plus B turned into C, but this time when we felt the beaker, the beaker felt cool to the touch. If that's the case, it's because energy was being transferred from the surroundings to the system. And since these surroundings was losing energy, that's why the beaker felt cool.
So heat flows from the surroundings to the system, and this occurs in an endothermic reaction and the change in entropy, Delta H is positive for an endothermic process. When we look at the energy profile for an endothermic reaction. The energy of the reactants.
So go ahead and write reactants in here. The energy of the reactants is lower than the energy of the products. So this time, if we find Delta E that would be the energy of our products minus the energy of our reactants. And the energy of our products is about 100 kilojoules per mole and the energy of our reactants is about 50.
So let's say it's 100 minus 50, which would be positive 50 kilojoules per mole. So on our diagram if we represent Delta E, that would be this difference here on the energy profile. And once again, Delta E is equal to the change in the enthalpy Delta H for the reaction.
So Delta H for this hypothetical reaction is positive 50 kilojoules per mole. Since Delta H is positive, we know that energy was transferred from the surroundings to the system. And that's the reason why the products have a higher potential energy than the reactants in our energy diagram.
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