Chemistry - 7 Energy Changes - 7.3 Reaction Profiles & 7.4 Bond Energy Calculations

Exothermic reaction profile

High bar, curve, low bar

Endothermic reaction profile

Low bar, curve, high bar

Energy change during reaction =

difference between products and reactants

Activation energy

minimum energy needed to get a reaction started

Activation energy =

reactants to peak of curve

Bond breaking: endo- or exo-?

endothermic

Bond making: endo- or exo-?

exothermic

Bonds in exothermic reactions

making bonds > breaking bonds

Bonds in endothermic reactions

making bonds < breaking bonds

Bond energy

the amount of energy that will break a bond between two atoms

To calculate the energy change for a chemical reaction:

• how much energy is needed to break the bonds

• how much energy is released when bonds are formed

If the overall energy change is negative, the reaction is...

...exothermic

To find the overall energy change:

add the energy required to break bonds and the energy released by their formation