1. Learning Objective Draw and calculate energy change To be able to:
Key Words:
Exothermic, endothermic, energy

2. Energy Change Calculations
Sunday, 19 May 2019
Energy Change Calculations
Date and title in books. You need a pen, pencil & ruler.
Starter Activity – 5 minutes
Define:
An exothermic reaction
An endothermic reaction

3. Calculate bond enthalpies
(F)
Target 9-1
Identify if a reaction is exothermic or endothermic using the reaction profile
(F/H)
Target 4-9
Draw reaction profiles for exothermic and endothermic reactions
(H)
Target 5-9
Calculate bond enthalpies
Extension: Application of Knowledge
Target 8/9
Explain where the energy for the bond enthalpies comes from
Learning Outcomes

4. Activation Energy
Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
The minimum amount of energy that particles must have to react is called the activation energy (more next lesson!)

5. Exothermic Energy Graphs
“Energy is released to the surroundings because more heat energy is released making bonds in the products than is needed to break bonds in the reactants”

6. Endothermic Energy Graphs
“Energy is taken in from the surroundings because less heat energy is released making bonds in the products than is needed to break bonds in the reactants”

7. Bond Energy Values
It is possible to calculate the energy change of a reaction using bond energy values.
The bond energy value tells us how much energy is needed to break a bond.

8. Bond Energies During a chemical reaction:
energy must be supplied to break bonds in the reactants
energy is released when bonds in the products are formed.
The energy needed to break bonds and the energy released when bonds are formed can be calculated from bond energies.

9. Worked Example
This is how to calculate the energy change when steam is made from hydrogen and oxygen: 2H2(g) + O2(g) → 2H2O(g)
The bond energy values will be given in the exam. The units of bond strength are kJ, (which stands for kilojoules).
Here are some examples of bond strengths:
H-H is 436 kJ, O=O is 498 kJ, H-O is 464 kJ
Here are the steps needed to calculate the energy change:
1.Draw out the bonds in each of the reactants and products.
2.Calculate the energy absorbed when all of the reactant bonds are broken.
3.Calculate the energy released when all of the new bonds in the products are made.
4.Subtract the total for the new bonds from the total for the old bonds. A negative final answer means that the reaction is exothermic.

11. Making and breaking bonds
In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds.
In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.