Combustion as an Exothermic Reaction

Introduction

We burn fuels for one reason: to get energy out of them. A gas hob heats a pan, a car engine turns wheels and a power station makes electricity, all by combustion. But where does that energy actually come from? It is not stored in a single "energy bond" in the fuel. It comes from the difference between the energy needed to break the bonds in the fuel and oxygen and the energy released when new bonds form in carbon dioxide and water.

Core explanation

Breaking bonds takes energy; making bonds releases it. Atoms in a molecule are held together by covalent bonds. To pull two bonded atoms apart you must put energy in, so bond breaking is endothermic . When atoms come together to form a bond, energy is given out, so bond making is exothermic .

The overall balance. In any reaction, old bonds break and new bonds form. The overall energy change is:

energy change = energy taken in to break bonds − energy released when bonds form

If more energy is released by bond making than is absorbed by bond breaking, the reaction is exothermic and the energy change (ΔH) is negative. Combustion reactions are strongly exothermic because the products, CO₂ and H₂O, contain very strong C=O and O–H bonds.

Average bond energies (kJ/mol):

Bond Bond energy --- --- C–H 413 O=O 498 C=O (in CO₂) 805 O–H 464

Methane as an example. For CH₄ + 2O₂ → CO₂ + 2H₂O:

- Bonds broken: 4 C–H and 2 O=O = 4 × 413 + 2 × 498 = 1652 + 996 = 2648 kJ - Bonds formed: 2 C=O and 4 O–H = 2 × 805 + 4 × 464 = 1610 + 1856 = 3466 kJ - Energy change = 2648 − 3466 = −818 kJ per mole of methane

The negative sign shows that about 818 kJ is released for every mole (16 g) of methane burnt. The measured value is about −890 kJ/mol; the difference arises because the table uses averages over many molecules and because the measured value is for liquid water.

Activation energy. If combustion releases so much energy, why does a gas tap not burst into flame the moment it is opened? The reactant bonds must start to break before new bonds can form. This needs an initial input of energy, the activation energy , supplied by a spark or a flame. Once the reaction starts, the energy it releases provides the activation energy for neighbouring molecules, so burning continues on its own.

Formulae

ΔH = Σ(bond energies of bonds broken) − Σ(bond energies of bonds formed). A negative ΔH means exothermic; a positive ΔH means endothermic.

Step-by-step reasoning

To estimate the energy change of a combustion reaction:

1. Write the balanced equation. 2. Draw or list every bond in the reactants and count them. 3. Add up the energy needed to break all reactant bonds. 4. Count and add up the energy released forming all product bonds. 5. Subtract: bonds broken minus bonds formed. A negative answer means exothermic.

Visual explanation

On an energy profile diagram, draw the reactants (CH₄ + 2O₂) on a high level at the left and the products (CO₂ + 2H₂O) on a much lower level at the right. The curve between them first rises in a hump — the activation energy — then falls steeply. The vertical drop from reactants to products is ΔH, pointing downwards because energy leaves to the surroundings.

Real-world analogy

Think of a boulder resting in a small dip at the top of a tall hill. You must push it up over the lip of the dip first (activation energy), but once over, it rolls all the way down, releasing far more energy than your push. The height of the whole hill is like the energy released by combustion.

Real-world example

A natural-gas boiler relies on this energy balance. An electric spark supplies the activation energy to light the burner. After that, each mole of methane releases roughly 890 kJ, which heats water flowing through a heat exchanger for radiators and taps.

Why?

Why are the products of combustion so low in energy? Carbon and hydrogen both form very strong bonds with oxygen. The C=O bonds in carbon dioxide and the O–H bonds in water are among the strongest common covalent bonds, so forming them releases a large amount of energy, more than enough to repay the cost of breaking C–H and O=O bonds.

Common misconception

"Energy is released when bonds in the fuel break." Breaking any bond always requires energy. The energy from combustion comes from forming new, stronger bonds in the products, not from breaking bonds in the fuel.

Worked example

Question: Using the table, estimate the energy change when one mole of hydrogen burns: 2H₂ + O₂ → 2H₂O. The H–H bond energy is 436 kJ/mol.

Reasoning: Broken: 2 H–H + 1 O=O = 2 × 436 + 498 = 1370 kJ. Formed: 4 O–H = 4 × 464 = 1856 kJ. Change for the equation as written = 1370 − 1856 = −486 kJ, which is for 2 moles of H₂.

Answer: About −243 kJ per mole of hydrogen; the reaction is exothermic.

Quick check

1. Is bond breaking endothermic or exothermic? Answer: Endothermic, because energy must be supplied to separate bonded atoms.

Exam focus

Show every step in bond-energy calculations: list bonds, multiply by the number of each, add, then subtract "broken minus formed". Always include the sign and the unit kJ/mol. Be able to label reactants, products, activation energy and ΔH on an energy profile.

Advanced insight

Bond-energy calculations give only estimates because a given bond's strength varies slightly between molecules and the values assume everything is gaseous. Accurate values come from calorimetry and Hess's law. Real flames also proceed through many short-lived fragments, called radicals, in a chain of fast steps rather than one single collision.

Summary

Combustion is exothermic because the energy released when strong C=O and O–H bonds form in the products is greater than the energy needed to break C–H and O=O bonds in the reactants. The energy change equals bonds broken minus bonds formed and is negative. A spark or flame supplies the activation energy needed to start the reaction.

Practice questions

1. State the formula used to calculate an energy change from bond energies. Answer: Energy change = total energy of bonds broken − total energy of bonds formed. 2. Why is the energy change for combustion negative? Answer: More energy is released forming the product bonds than is absorbed breaking the reactant bonds, so energy is transferred to the surroundings. 3. What is the purpose of the spark in a gas cooker? Answer: It supplies the activation energy needed to start the combustion reaction. 4. On an energy profile for combustion, are the products above or below the reactants? Answer: Below, because energy has been released to the surroundings. 5. How many C–H bonds are broken when one molecule of ethane, C₂H₆, burns completely? Answer: Six.