Physical Chemistry

Energetics and Bond Enthalpies – Revision Notes

Key Definitions and Terminology

• **Exothermic reaction**: A reaction that transfers energy to the surroundings, causing the temperature of the surroundings to increase. The products have less energy than the reactants.
• **Endothermic reaction**: A reaction that takes in energy from the surroundings, causing the temperature of the surroundings to decrease. The products have more energy than the reactants.
• **Activation energy**: The minimum amount of energy that colliding particles must have in order to react.
• **Bond energy (bond enthalpy)**: The amount of energy needed to break one mole of a particular covalent bond in the gaseous state, measured in kJ/mol.
• **Energy change of a reaction (ΔH)**: The overall energy difference between the energy needed to break bonds in the reactants and the energy released when new bonds are formed in the products.
• **Catalyst**: A substance that increases the rate of a reaction by providing an alternative pathway with a lower activation energy, without being chemically changed itself.

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Main Concepts

1. Energy Changes in Reactions

• In every chemical reaction, energy is required to **break bonds** (this is an endothermic process — energy is taken in).
• Energy is **released** when new bonds are **formed** (this is an exothermic process).
• The overall energy change depends on the balance between energy taken in to break bonds and energy released when forming bonds.

2. Exothermic vs Endothermic — The Key Rule

• If **more energy is released forming new bonds** than is needed to break old bonds → **exothermic** (ΔH is **negative**).
• If **more energy is needed to break old bonds** than is released forming new bonds → **endothermic** (ΔH is **positive**).

3. Energy Level Diagrams

• **Exothermic**: Products are drawn **lower** than reactants on the energy axis. The difference in height represents the energy released to the surroundings.
• **Endothermic**: Products are drawn **higher** than reactants. The difference in height represents the energy absorbed from the surroundings.
• Both diagrams include an **activation energy "hump"** between reactants and products, showing the energy barrier that must be overcome for the reaction to proceed.
• A catalyst lowers this hump (provides a lower activation energy pathway) but does **not** change the overall energy change (ΔH).

4. Calculating Energy Changes Using Bond Enthalpies

The formula used is:

ΔH = Σ (bond energies of bonds broken) − Σ (bond energies of bonds formed)

• Step 1: Draw out the full structural formulae of all reactants and products showing every bond.
• Step 2: List every bond broken in the reactants and add up their bond energies.
• Step 3: List every bond formed in the products and add up their bond energies.
• Step 4: Subtract the total energy released (bonds formed) from the total energy taken in (bonds broken).

5. Everyday Examples of Exothermic and Endothermic Reactions

• **Exothermic**: Combustion of fuels (e.g., burning methane), neutralisation reactions, oxidation reactions, self-heating food cans.
• **Endothermic**: Thermal decomposition (e.g., calcium carbonate breaking down), photosynthesis, instant cold packs (ammonium nitrate dissolving in water).

6. Interpreting the Sign of ΔH

• A **negative ΔH** value confirms an **exothermic** reaction.
• A **positive ΔH** value confirms an **endothermic** reaction.
• Bond enthalpy calculations give **approximate** values because the bond energies used are **averages** taken from many different compounds, not exact values for the specific molecules involved.

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Worked Examples

Worked Example 1: Combustion of Hydrogen

Calculate the energy change for:

H₂(g) + Cl₂(g) → 2HCl(g)

Given bond energies: H–H = 436 kJ/mol, Cl–Cl = 242 kJ/mol, H–Cl = 431 kJ/mol

Step 1 — Bonds broken (reactants):

| Bond | Number | Energy |

|------|--------|--------|

| H–H | 1 | 436 |

| Cl–Cl | 1 | 242 |

| Total | | 678 kJ |

Step 2 — Bonds formed (products):

| Bond | Number | Energy |

|------|--------|--------|

| H–Cl | 2 | 2 × 431 = 862 |

| Total | | 862 kJ |

Step 3 — Calculate ΔH:

ΔH = 678 − 862 = −184 kJ/mol

Conclusion: The reaction is exothermic because ΔH is negative (more energy is released forming bonds than is taken in breaking bonds).

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Worked Example 2: Combustion of Methane

Calculate the energy change for:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

Given bond energies: C–H = 413 kJ/mol, O=O = 498 kJ/mol, C=O = 805 kJ/mol, O–H = 463 kJ/mol

Bonds broken (reactants):

• 4 × C–H = 4 × 413 = 1652 kJ
• 2 × O=O = 2 × 498 = 996 kJ
• **Total = 2648 kJ**

Bonds formed (products):

• 2 × C=O = 2 × 805 = 1610 kJ
• 4 × O–H = 4 × 463 = 1852 kJ
• **Total = 3462 kJ**

ΔH = 2648 − 3462 = −814 kJ/mol

The reaction is exothermic. This is consistent with combustion reactions releasing heat energy to the surroundings.

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Worked Example 3: Drawing an Energy Level Diagram (Exothermic)

For the combustion of methane (ΔH = −814 kJ/mol):

```

Energy

↑

| CH₄ + 2O₂ ← Reactants (higher energy level)

| /\

| / \ ← Activation energy (Ea)

| / \

| / \

| \_______ CO₂ + 2H₂O ← Products (lower energy level)

|

| |← ΔH = −814 kJ/mol →|

|______________________________→ Progress of reaction

```

• The downward arrow between reactants and products represents the **energy released**.
• The hump represents the **activation energy** needed to start the reaction (e.g., a spark to ignite methane).

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Exam Technique Tips

Tip 1: Show Every Step in Bond Energy Calculations

Edexcel mark schemes award marks separately for: correctly identifying bonds broken, correctly identifying bonds formed, and the final subtraction. Even if your final answer is wrong, you can still gain 2 out of 3 marks by showing your working clearly. Always write out the equation: ΔH = bonds broken − bonds formed, then substitute values. Never skip to the answer.

Tip 2: Use Precise Language for Exothermic/Endothermic Definitions

Edexcel examiners look for specific phrasing. Always say energy is "transferred to the surroundings" (exothermic) or "taken in from the surroundings" (endothermic). Do not write "the reaction gives out heat" — instead say "the reaction transfers thermal energy to the surroundings, so the temperature of the surroundings increases." When asked to describe an energy level diagram, explicitly state whether products are at a higher or lower energy level than reactants — this is a commonly missed mark.