Exothermic and Endothermic Reactions, Calculating Energy Changes

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 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 system and surroundings adds up to zero. 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.

Example:

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 moles of N2 that can react is limited to only "5" because the 11 moles N2 needs 11 mole O2 You can verify the number of moles of N2 moles N2 = [ 5 moles O2] [1 mole N2 / 1 mole O2] = 5 moles 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.