Calculating the enthalpy change

Enthalpy change is the  difference between the energy taken to break the bonds and energy released by making the bonds. These bond values are available in chemical data books and can be used to calculate the overall energy change.

e.g: Enthalpy change for the burning of methane

CH₄  + 2O₂ --->  CO₂  +2 H₂O

Bond energies should be given as

C– H   =  99 KJ/mol
O=O   = 119 KJ/mol
C=O   = 192 KJ/mol
O– H  = 111 KJ/mol

By looking at the structures of the molecules involving in the reaction for breaking bonds

• 4 C-H bonds
• 2 O=O bonds has to be broken

Total energy for breaking the bonds 

99*4 +119*2 = 

396+238=

634 KJ/mol

For making bonds from the reaction

• 2 C=O bonds
• 4 O– H bonds has to be made

Total energy for making the bonds

192* 2+ 111*4= 

384 +444= 

828  KJ/mol

Enthalpy change= energy needed to break the bonds- energy taken to make the bonds

         ΔH°        =634-828

                        = - 194  KJ/mol

If the enthalpy change come as a negative value it is an exothermic reaction. If it was positive it is endothermic. According to this example this reaction is exothermic.

To refer the standard bond energy values click this link

Next about hydrogen as a fuel

            

Energy Profile Diagrams

When there is an energy change, the energy of the products become different from the energy that was present in the reactants.

If the reaction was an exothermic one;  the energy is given out by the overall reaction. Therefore the products will have a lesser energy than the reactants.This can be represented in a energy profile as follows

Energy profile diagram for exothermic reaction

• Activation energy is the energy barrier for the reactants to become products.In an energy profile it can be represented by an arrow from the reactants to the peak 

• Enthalpy change is from reactants to the products.

• Energy profile can also be represented with real energy values and the names of the reactants and products

Endothermic reactions will have a different energy profile than the exothermic one because in endothermic reactions overall energy change is absorption of heat. The products will have a higher energy than the reactants.

Energy profile for endothermic reaction

Next about calculating the enthalpy change

Energy from Chemicals

Chemical reactions are always associated with energy changes. Due to some chemical reactions the container gets warm up where the chemical reaction occurred where as some chemical reactions will cool the container. Based on the above observations chemical reactions can be catergorized in to two main types.

1. Exothermic reactions

'exo' means out. If a chemical reaction gives out heat it is called 'exothermic'
e.g : burning of coal
      reaction between sodium hydroxide and hydrochloric acid

2. Endothermic reactions

'Endo' means taking in heat. when energy or heat is absorbed from surrounding due to a chemical reaction it will cool the container.

e.g: dissolving ammonium chloride in water
      photosynthesis

Chemical reactions happen when bonds between the reactants are broken and new bonds are made to form products. breaking the bonds requires energy. Which means
     Breaking bonds is Endothermic

Making bonds releases energy. which means
     Making bonds is exothermic

for any chemical reaction the energy change means the difference between the energy needed to break the bonds and energy given by making the bonds. which is also referred by the term
Enthalpy change

   Next about Energy Profile Diagrams