Consider three reactions with different values of E_a and ΔE:

Reaction 1. E_a = 20 kJ>mol; ΔE = -60 kJ/mol

Reaction 2. E_a = 10 kJ>mol; ΔE = -20 kJ/mol

Reaction 3. E_a = 40 kJ>mol; ΔE = +15 kJ/mol

(c) Which reaction is the most endothermic, and which is the most exothermic?

Video transcript

Hello. In this problem we are asked to determine which of the following reactions is the most exotic thermic and which is the most endo thermic. We are given three reactions reaction A. B and C. Were given their activation energy and their change in internal energy. So let's begin by looking at a couple of reaction profile diagrams where we have energy on the Y axis and reaction progress on the X axis. So when we begin a reaction we start with the energy of our reactant. We have some energy barrier. Peak is the transition state. And then we have our products. So this energy of our reactant products and the transition state. Can we begin with react ints you have some energy barrier and then we have the energy of our products. So reactant product energy and energy at the transition state. The difference between the energy of our transition state and the energy of our reactant is the activation energy and the difference between the energy of our products and that other reactant is a change in internal energy. The change in internal energy is equal to the difference between energy of our products and that of our reactant. Looking at the profile diagram on the left, our products are lower in energy than our reactant. So that means that the change in internal energy is going to be negative and sign when our products are lower energy than our reactive means the system lost energy. This will be Exxon thermic and for the diagram on the right the change in internal energy will be positive. Our products are higher energy than our reactant, which means the products gained energy. This is endo thermic as we can tell by looking at these two reaction profile diagrams. The activation energy is independent of the change in internal internal energy, activation energy depends on the chemical transformations taking place. So we are breaking bronze and our reactors to form new bronze in our products. So when trying to determine which is most exotic thermic and which is most endo thermic, we're just paying attention to then the change in internal energy. Since um whether reaction is XR thermic or endo thermic is independent of the activation energy. So if we go back up above, then the most exotic emmick will be the one that has the larger negative change in internal energy. And the one that is the most endo thermic is going to have more positive change in internal energy. So, if we look at our answers that were provided, the first one says reaction C. Is the most eggs are thermic because it has the largest negative value for the change in internal energy which we said is true. While reaction B. Is the most endo thermic because it has the largest positive value for the change in internal energy. Which is also true. So answer A. Is the correct answer. Let's look at the other possible answers to determine why they are not correct. So next one B says reaction B is the most eggs are thermic because it has the largest activation energy, the whether it's exo thermic or endo thermic, it's independent of the activation energy. So this is incorrect. See says Reaction A. Is the most eggs are thermic because it has the smallest activation energy. Again, whether reaction is exit thermic or endo thermic is independent of the activation energy, so that is incorrect. The last one says, we cannot determine with just the activation energy and the change in internal energy. Which is not true. We can determine whether reaction is endo thermic or exo thermic by looking at the change in internal energy. Thanks for watching. Hope this help.