C5 - Bond Energy Calculations 1

What is bond energy?

The amount of energy needed to break one mole of a specific bond

Is breaking bonds endothermic or exothermic?

Endothermic - it requires energy

Is making bonds endothermic or exothermic?

Exothermic - it releases energy

What is the energy change for an exothermic reaction?

Negative, because energy is released to the surroundings

What is the energy change for an endothermic reaction?

Positive, because energy is absorbed from the surroundings

What is the general formula for calculating overall energy change?

Energy in (breaking bonds) - Energy out (making bonds)

What must you do with bond energies when breaking bonds?

Add them as positive values

What must you do with bond energies when making bonds?

Add them as negative values (energy released)

Why do we need bond energies to calculate energy change?

To compare total energy used to break bonds with energy released when bonds are made

In bond energy calculations, what does a negative final value mean?

The reaction is exothermic (energy released)

In bond energy calculations, what does a positive final value mean?

The reaction is endothermic (energy absorbed)

What two pieces of information are needed to calculate energy change using bond energies?

1) The structure of the molecules (bonds involved), 2) Bond energy values (given in exam)

If a bond is formed twice in a reaction, what must you do with the bond energy?

Multiply the bond energy by 2

If you break two identical bonds, how do you calculate the energy required?

Multiply the bond energy by the number of bonds broken

What is the bond energy of a H-H bond?

436 kJ/mol

What is the bond energy of a Cl-Cl bond?

242 kJ/mol

What is the bond energy of a H-Cl bond?

431 kJ/mol

In the reaction H₂ + Cl₂ → 2HCl, which bonds are broken?

H-H and Cl-Cl bonds

In the reaction H₂ + Cl₂ → 2HCl, which bonds are formed?

Two H-Cl bonds

How much energy is needed to break 1 mole of H-H and Cl-Cl bonds together?

436 + 242 = 678 kJ/mol

How much energy is released when 2 moles of H-Cl bonds are made?

2 × 431 = 862 kJ/mol

What is the overall energy change for the reaction H₂ + Cl₂ → 2HCl?

678 - 862 = -184 kJ/mol

What does the negative sign in the energy change mean?

The reaction is exothermic

What step comes first in a bond energy calculation?

Identify and list all bonds being broken (reactants)

What step comes after listing bonds broken?

Add their bond energies (positive values) to find total energy in

What step comes after listing bonds made?

Add their bond energies (as negative values) to find total energy out

What is the final step in a bond energy calculation?

Subtract total energy out from energy in (energy in - energy out)

What should you always include in your final answer?

The sign (+ or -) and the unit (kJ/mol)

Why is making bonds exothermic?

Because energy is released when atoms form stable bonds

Why is breaking bonds endothermic?

Because energy is required to overcome the forces holding atoms together

What unit is used for bond energies?

kJ/mol (kilojoules per mole)

Why must bond energy values be given in the exam?

Because they are specific to each bond and needed for calculation

What makes a reaction overall exothermic or endothermic?

Whether more energy is released making bonds or absorbed breaking them

What if energy in

energy out in a reaction? = The reaction is neither exothermic nor endothermic (no net energy change)