Exothermic and Endothermic Reactions

Today we learned about endothermic and exothermic reactions.

Exothermic reactions are reactions that give off energy.
Endothermic reactions absorb energy. 

Today we also discovered the joy of energy diagrams. These show the energy of the reactants and the energy of the products.

 The reaction is exothermic because, the energy of products is less than energy of reactants. This means energy was released.

The x axis represents the time needed to undergo the reaction and the y-axis represents potential energy in kJ.

The diagram is very straightforward.

The initial flat line represents the energy of the reactants. As you can see, the line transforms into a hill. The distance between the initial flat line and the top of the hill is called the activation energy.This is the amount of energy needed to carry out the reaction.  The level of the hill (on the y axis) is called the activated complex. It is the highest amount of energy that the reactants can reach in the reaction process. The curve then drops off and then evens out. The distance between the energy of the reactants line and the energy of the products is known as delta H. Delta H is the change in enthalpy,(heat contained in the system).

This is found using the equation
Delta H = energy of products - energy of the reactants.
A negative value of H  signifies an exothermic reaction because you end up with less energy than you started with.

A positive value of H signifies and endothermic reaction because you end up with less energy than you started with.

Once H is found, you use it in chemical equations like this

Mg + Cl2 ------> MgCl2 + 347 kJ

Notice that the 347 is on the side of the reactants. This means that the reaction is exothermic because:

Energy on one side = energy on the other
So the MgCl2 needs the 347 kJ to make up the energy so it equals the reactants.
This means that the reaction released energy.