Bond Energies and Energy Changes Flashcards

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What is bond energy?

Bond energy is the energy needed to break one mole of a particular chemical bond.

What are the usual units for bond energies?

Bond energies are usually given in kilojoules per mole, written as kJ/mol.

When a chemical bond is broken, is energy taken in or given out?

Energy is taken in when a bond is broken, so bond breaking is endothermic.

When a new chemical bond is made, is energy taken in or given out?

Energy is given out when a bond is made, so bond making is exothermic.

In terms of bond energies, when is a reaction exothermic overall?

A reaction is exothermic if more energy is released making bonds than is taken in breaking bonds.

In terms of bond energies, when is a reaction endothermic overall?

A reaction is endothermic if more energy is taken in breaking bonds than is released making bonds.

What is the general formula for energy change in a reaction using bond energies?

Energy change = total energy in (bonds broken) − total energy out (bonds made).

How do you use bond energies to find the total energy in for a reaction?

List all the bonds broken, multiply each by its bond energy, then add these values to get the total energy in.

How do you use bond energies to find the total energy out for a reaction?

List all the bonds made, multiply each by its bond energy, then add these values to get the total energy out.

What sign does the energy change have for exothermic and endothermic reactions?

Exothermic reactions have a negative energy change; endothermic reactions have a positive energy change.