Chemical Energetics 1: All chemical reactions involve an energy change. The transfer of energy, usually heat, into or out of the reaction mixture’. For example: When petrol burns heat is given out. When ammonium nitrate dissolves in water heat is taken in.
Chemical Energetics 2: Instead of writing ‘energy change’ all the time chemists use the symbols: H Pronounced ‘delta aitch’. =greek letter ‘delta’ meaning change. H=heat. So, H means ‘heat change’.
These are a more detailed version of the energy level diagrams previously looked at.
They show the endothermic bond breaking and the exothermic bond making stages of a reaction.
Reaction profiles can be used to do numerical calculations on the energy changes that happen in chemical reactions. The next slide shows the energy profile for the reaction between hydrogen and chlorine to form hydrogen chloride.
The energy change for a reaction can be calculated using ‘bond energies’.
‘ Bond energy’ is the amount of energy required to break a bond. This value is always endothermic , ie. has a positive value. When a bond is formed the energy given out is exactly equal to, but of opposite sign, to the energy required to break the bond. Bond making is always an exothermic process . .
Consider the reaction between hydrogen and chlorine:
H 2 + Cl 2 2HCl to see the reaction profile. In this reaction one H-H bond and one Cl-Cl bond is broken and two H-Cl bonds are formed. The H-H bond energy is 436kJ/mol The Cl-Cl bond energy is 242kJ/mol So, the energy needed to break these bonds is 436 + 242 = 678kJ The H-Cl bond energy is 431kJ So the energy given out when these bonds are formed is 2 x -431 = -862kJ Overall change = 678 – 862 = -184kJ, an exothermic reaction. Next slide shows the reaction profile with these values added.