# How can I draw a simple energy profile for an exothermic reaction in which 100 kJ mol-1 is evolved, and which has an activation energy of 50 kJmol-1?

Feb 26, 2015

You can start with a generic potential energy diagram for an exothermic reaction. A reaction is defined as exothermic if you put in less energy to break the bonds of the reactants - the is the activation energy - than it is released when the products are formed.

So, the activation energy is the minimum amount of energy required for a reaction to take place. In your case, you need at least $\text{50 kJ/mol}$ of energy to get the reaction going.

The heat evolved during the reaction, $\Delta H$, will be negative because the products are at a lower energy level than the reactants. In your case, the heat released will be

$\Delta H = \text{- 100 kJ/mol}$

Here's what an energy diagram for such a reaction could look like I've assumed the energy level of the reactants to be $\text{200 kJ}$. The activation energy will make the energy of the reaction peak at $\text{250 kJ}$; the energy level of the products will be lower than that of the products by $\text{100 kJ}$, the heat evolved during the reaction.

In an exothermic reaction, the products will always be lower in energy than the reactants; in this case, the products are at $\text{100 kJ}$, $\text{100 kJ}$ lower than what the energy of the reactants was.