C5 - Bond Energy Calculations 2

What is the first step in calculating energy change using bond energies?

Identify and list all the bonds being broken in the reactants

What is the second step in bond energy calculations?

Identify and list all the bonds being made in the products

Why do we multiply bond energy values?

To account for the number of times each bond appears in the balanced reaction

In bond energy calculations, what sign do we use for bonds being broken?

Positive, because breaking bonds is endothermic

In bond energy calculations, what sign do we use for bonds being made?

Negative, because making bonds is exothermic

How do you calculate total energy in (bonds broken)?

Add up the energy for all the bonds broken using positive values

How do you calculate total energy out (bonds made)?

Add up the energy for all the bonds made using negative values

How do you calculate overall energy change in a reaction?

Energy in (bonds broken) - Energy out (bonds made)

What does a negative energy change mean?

The reaction is exothermic (releases energy to surroundings)

What does a positive energy change mean?

The reaction is endothermic (absorbs energy from surroundings)

In the reaction 2H₂ + O₂ → 2H₂O, which bonds are broken?

Two H-H bonds and one O=O bond

What is the bond energy of H-H?

436 kJ/mol

What is the bond energy of O=O

(oxygen double bond)? = 495 kJ/mol

How much energy is needed to break 2 H-H bonds and 1 O=O bond?

(2 × 436) + 495 = 872 + 495 = 1,367 kJ

What bond is made in the reaction 2H₂ + O₂ → 2H₂O?

H-O bonds (in water)

How many H-O bonds are formed in 2 H₂O molecules?

4 H-O bonds

What is the bond energy of H-O?

463 kJ/mol

How much energy is released when 4 H-O bonds are made?

4 × 463 = 1,852 kJ

What is the overall energy change for 2H₂ + O₂ → 2H₂O?

1,367 - 1,852 = -485 kJ (exothermic)

In the combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O, which bonds are broken?

4 C-H bonds and 2 O=O bonds

What is the bond energy of C-H?

413 kJ/mol

How much energy is needed to break 4 C-H bonds?

4 × 413 = 1,652 kJ

How much energy is needed to break 2 O=O bonds?

2 × 495 = 990 kJ

Total energy needed to break all bonds in CH₄ + 2O₂

1,652 + 990 = 2,642 kJ

What bonds are made in the combustion of methane?

2 C=O bonds (in CO₂) and 4 H-O bonds (in 2 H₂O)

What is the bond energy of C=O?

799 kJ/mol

How much energy is released from 2 C=O bonds?

2 × 799 = 1,598 kJ

How much energy is released from 4 H-O bonds?

4 × 463 = 1,852 kJ

Total energy released in CH₄ + 2O₂ reaction

1,598 + 1,852 = 3,450 kJ

What is the overall energy change for CH₄ + 2O₂ → CO₂ + 2H₂O?

2,642 - 3,450 = -808 kJ (exothermic)

What does a large negative energy change suggest?

A lot of energy is released - very exothermic

What must you always check in bond energy calculations?

You've counted all bonds in the correct quantities from a balanced equation

What unit is used for energy change in bond energy calculations?

kJ/mol (kilojoules per mole)

What does it mean if total energy in is greater than total energy out?

The reaction is endothermic (absorbs energy)

What does it mean if total energy out is greater than total energy in?

The reaction is exothermic (releases energy)

Why are balanced equations essential for bond energy calculations?

So you know the correct number of each bond to include