Exothermic and Endothermic Reactions, Calculating Energy Changes

Dr. Walt Volland all rights reserved revised August 20, 2012

The Law of Conservation of energy.

This means the amount of energy that exists today is the the same as the amount of energy that existed thousands and millions of years ago. The energy in use today existed ages ago but was stored in some concentrated form like a high energy bond.

The concentrated regions of energy release energy to regions with lower concentrations. This is what happens when heat energy flows from a hot flame to something cooler. This is the same thing that happens when light energy in the form of photons radiates away from a light source. TYPICALLY chemical reactions involve an energy change. The reaction is examined as a system and the rest of the universe makes up the surroundings.

DEuniverse = DE system +     DEsurroundings

delta Euniverse = delta Esystem + delta Esurroundings

The total energy change for universe adds up to zero and comes from the system and surroundings. This has to happen because of the Law of Conservation of energy. Energy is released when we replace unstable bonds with more stable bonds. This occurs because matter always acts to go to the most stable state possible.


Energy Calculations, where the energy values appear in an equation

An exothermic reaction releases energy. The energy change that accompanies a reaction can be written in the equation.

Here the 213 kcal are a product and appear on the right hand side of the equation. The reaction produces or releases energy so the sign for the 213 kcal is negative, (-213 kcal). The reaction is exothermic.

The amount of energy change is proportional to the mass of material consumed in the reaction. If two moles of methane are burned the heat effect will be doubled. If one mol of methane is burned this way 213 kcal will be lost to the surroundings. When the amount of CH4 is increased to ten mols then the reactants will release 2130 kcal.



An endothermic reaction uses energy as a reactant.

The energy change that accompanies a reaction can be written in the chemical equation. The reaction requires the addition of energy to the reactants to form the bonds in the products. In this reaction 43 kcal are needed to make the reaction occur. The sign for the energy change is +. This is an endothermic reaction. The surroundings must provide energy to make this reaction happen. We are fortunate that the oxygen and nitrogen require energy to force them to react. Otherwise we could have lost our atmosphere in a burst of flame years ago.


What is the energy change when 5 moles of oxygen and 11 moles of nitrogen react?

Use the energy change per mol from the equation

N2(g) + O2(g) + 43 kcal -----> 2 NO(g) .


Step 1. Check the balanced equation mole ratio for moles of N2 to moles of O2.

mole ratio N2 to O2                is 1 mole N2 / 1 mole O2


Step 2. Compare actual numbers of moles available with the ideal mole ratio and see which reactant is limiting. The actual mole ratio is

11 mole N2 / 5 mole O2 but this is greater than the 1 mole N2 / 1 mole O2

The mols of N2 that can react is limited to only "5" because the 11 mols N2 needs 11 mol O2

You can verify the number of mols of N2

moles N2 = [ 5 mols O2] [1 mol N2 / 1 mol O2] = 5 mols N2

Step 3. Determine the kcal change.

kcal = [ 5 moles N2 ][43 kcal /mole N2 ] = 215 kcal

The reactants in this reaction must take energy from the surroundings to form products.


Online Introductory Chemistry

Dr. Walt Volland, revised August 21, 2012
 All rights reserved, copyright 1998-2012