1. Chemical Reaction Energy
All chemical reactions involve an energy change.
The transfer of energy, usually heat, into or out of the reaction mixture.
For example:
When petroleum burns, heat is given out.
When ammonium nitrate dissolves in water, heat is taken in.

2. Instead of writing ‘enthalpy change’ all the time, chemists use the symbols:
Pronounced ‘delta H’.
Δ = greek letter ‘delta’ meaning change.
H = enthalpy (see below)
So, ΔH means ‘change in enthalpy’.
(Enthalpy is the heat content of a system; assuming pressure is constant)

3. Temperature Changes 1
Some reactions cause the temperature of the reaction mixture to increase.
This type of reaction is called exothermic.
Heat energy is given out by the reaction so the reaction feels hot.

4. Temperature Changes 2
Some reactions cause the temperature of the reaction mixture to decrease.
This type of reaction is called endothermic.
Heat energy is taken in by the reaction so the reaction feels cold.

5. Representing Energy Changes:
The energy changes in a chemical reaction can be conveniently represented using
energy level diagrams
Energy level diagrams make it easier to decide whether a reaction is exothermic or endothermic
See next slide for some examples.

6. Energy Level Diagrams (copy!)
This energy level diagram shows a reaction where ΔH is positive (an endothermic reaction).
This energy level diagram shows a reaction where ΔH is negative (an exothermic reaction).
Reactants
Products
ΔH negative, exothermic.
energy
Reactants
Products
ΔH positive, endothermic.
energy
Time
Time

7. Making and Breaking Bonds:
Breaking chemical bonds is always endothermic.
Making chemical bonds is always exothermic.
All chemical reactions involve bond ‘rearrangements’. Bonds are broken and new ones are formed.
Chemical reactions are in two stages:
1. Breaking bonds, an endothermic process.
2. Making new bonds, an exothermic process.

8. Calculating Energy Changes:
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, so ΔH 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; so ΔH has a negative value.

9. An Example Calculation:
Consider the reaction between hydrogen and chlorine:
H2 + Cl2 → 2HCl
In this reaction one H-H bond and one Cl-Cl bond is broken and two H-Cl bonds are formed.

10. H2 + Cl2 → 2HCl Reaction Profile for: energy H, H, Cl, Cl (Atoms)
Energy taken in to break bonds.
H, H, Cl, Cl (Atoms)
Energy given out when bonds are made.
H-H, Cl-Cl
energy
Reactants
Overall enthalpy change, ΔH = ???
Need to know the specific energy IN vs energy OUT to figure out

11. An Example Calculation:
Consider the reaction between hydrogen and chlorine:
H2 + Cl2 → 2HCl
In this reaction one H-H bond and one Cl-Cl bond is broken and two H-Cl bonds are formed.
Reference information:
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 = 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.

12. H2 + Cl2 → 2HCl Reaction Profile for: energy H, H, Cl, Cl (Atoms)
Energy taken in to break bonds.
H, H, Cl, Cl (Atoms)
678kJ
0 kJ
energy
H-H, Cl-Cl
Reactants

13. H2 + Cl2 → 2HCl Reaction Profile for: energy H, H, Cl, Cl (Atoms)
Energy taken in to break bonds.
H, H, Cl, Cl (Atoms)
678kJ
0 kJ
Energy given out when bonds are made -862kJ
energy
H-H, Cl-Cl
Reactants
H-Cl, H-Cl
Products

14. H2 + Cl2 → 2HCl Reaction Profile for: energy H, H, Cl, Cl (Atoms)
Energy taken in to break bonds.
H, H, Cl, Cl (Atoms)
678kJ
0 kJ
- 184 kJ
Energy given out when bonds are made -862kJ
energy
H-H, Cl-Cl
Reactants
H-Cl, H-Cl
Products
Overall energy change, ΔH
-184kJ

15. Summary Exothermic rxns get hot, they give out heat.
Chemical reactions happen in two stages:
1. Breaking bonds, an endothermic process.
2. Making new bonds, an exothermic process.
Exothermic rxns get hot, they give out heat.
Endothermic rxns get cold, they take in heat.
(think about that for a second)
The energy changes in a chemical reaction can be represented using energy level diagrams and reaction profile diagrams.