Chemistry C7: Organic Chemistry

Describe what happens to energy when bonds are broken and when bonds are made.

• Bond breaking is endothermic (takes in energy).

• Bond making is exothermic (releases energy).

• This is because energy is needed to overcome the forces holding atoms together, but energy is released when new bonds form.

Explain how you know a reaction is exothermic in terms of bond energies.

• More energy is released making new bonds than is taken in to break old bonds.

• Therefore, the overall energy change is negative (energy is released to the surroundings).

• Examples: combustion, respiration.

Hydrogen reacts with chlorine to form hydrogen chloride:

H₂ + Cl₂ → 2HCl

Bond energies (kJ/mol):

• H–H = 436

• Cl–Cl = 243

• H–Cl = 432

Calculate the overall energy change for this reaction. State whether the reaction is exothermic or endothermic.

Energy in (bonds broken):

• 1 × H–H = 436

• 1 × Cl–Cl = 243

• Total = 436 + 243 = 679 kJ/mol

Energy out (bonds made):

• 2 × H–Cl = 2 × 432 = 864 kJ/mol

Overall energy change:

• 679 − 864 = −185 kJ/mol

Exothermic (negative value means energy is released).

Nitrogen and oxygen react to form nitrogen monoxide:

N₂ + O₂ → 2NO

Bond energies (kJ/mol):

• N≡N = 941

• O=O = 498

• N–O = 630

Calculate the overall energy change for this reaction. State whether the reaction is exothermic or endothermic.

Energy in (bonds broken):

• 1 × N≡N = 941

• 1 × O=O = 498

• Total = 941 + 498 = 1439 kJ/mol

Energy out (bonds made):

• 2 × N–O = 2 × 630 = 1260 kJ/mol

Overall energy change:

• 1439 − 1260 = +179 kJ/mol

Endothermic (positive value means energy is absorbed).

Hydrogen reacts with oxygen to form water:

2H₂ + O₂ → 2H₂O

How many O–H bonds are made in this reaction? Explain your answer.

• There are 2 water molecules (2H₂O).

• Each water molecule is H–O–H, which has 2 O–H bonds.

• 2 × 2 = 4 O–H bonds made.

Common mistake to avoid: Don't write "2 × O–H". That would mean only 2 bonds, but you actually have 4.

Explain how this graph shows the reaction is endothermic.

Products are at a higher energy level than reactants, so energy is absorbed (ΔH is positive).