CHEMISTRY UNIT 8 - ENERGY CHANGES 
Practical
Determination of the amount of energy released by a fuel
Exothermic 
Endothermic 
Energy is released into the surroundings
Eg. Burning fuels & neutralisation reactions (acid & alkali)
More chemical energy in the reactants than the products
Energy is transferred to the surroundings as the reaction proceeds & surroundings get hotter
Less chemical energy in the reactants than products
ΔH > 0
Eg. Thermal decomposition - heating calcium carbonate causes it to decompose into Calcium Oxide & CO2
Energy is absorbed from the surroundings
Exo means "release" & thermic means "heat"
Change in energy shown by ΔH
In Exothermic reaction ΔH < 0
ΔH = Eprod−Ereact
ΔH is less than zero because the energy of the reactants is greater than the energy of the products. Energy is released in the reaction.
ΔH is greater than zero because the energy of the reactants is less than the energy of the products. Energy is absorbed in the reaction.
Activation energy
Bond Making 
Activation energy is the minimum energy required to start a chemical reaction or ignite a combustible vapor, gas or dust cloud.
Represented on an energy level diagram as the difference between the reactants’ energy level and the top of the curve.
For example, burning methane in a Bunsen burner:
methane + oxygen → carbon dioxide + water
The activation energy must be supplied in the form of a flame or a spark to get the methane to ignite.
Bond Breaking 
Energy used to break bonds is greater than the energy released by forming them
Breaking bonds absorbs energy - endothermic
Energy released by forming bonds is greater than energy used to break them.
Forming bonds releases energy - exothermic
Bond Energy Calculations
Reaction Profiles
Endothermic
Exothermic
Diagrams that show the relative energies of the reactants, products in a reaction & how energy changes over the course of the reaction
Can use known bond energies to calculate overall energy change for a reaction.
Energy change = energy in - energy out
Every chemical bond has particular bond energy
Eg. H−H + Cl−Cl → 2 × (H−Cl)
H-H = 436, Cl-Cl = 243, H-Cl = 432
Energy in = 436 + 243 = 679 kJ/mole
Energy out = 2 x 432 = 864 kJ/mole
Energy change = in - out = 679 - 864 = -185 kJ/mole