Topic 6 · page 15 of 15

Which bond breaks first?

A lower bond enthalpy means a weaker bond that needs less energy to break, so reactions that start by breaking it tend to go faster at room temperature. This is a guide, not a guarantee.

9 min read6.11

Read first: Limits of bond enthalpy calculations, and finding a mean bond enthalpy

Weak bonds break first 6.11

Every reaction has to break at least one bond in the reactants. At room temperature the particles do not have much energy to share, so the question is how easy it is to break the first bond.

A lower bond enthalpy means a weaker bond and less energy needed to break it.
A higher bond enthalpy means a stronger bond and more energy needed.
In a molecule with more than one kind of bond, the weakest bond tends to break first.
A reaction that starts with a weak bond usually goes more rapidly at room temperature, because more of the colliding molecules have enough energy to break it.

Bar chart of bond enthalpies on one scale in kilojoules per mole: the nitrogen N-N triple bond 945, oxygen O=O 498, chlorine Cl-Cl 243, fluorine F-F 158. The nitrogen bar is about six times as long as the fluorine bar. — On one scale, the N≡N bond is about six times as strong as the F–F bond.

Examples 6.11

Nitrogen and fluorine. The N≡N triple bond (945 kJ mol⁻¹) is very strong, so a lot of energy is needed to break it. Nitrogen is so unreactive that it is the main gas in air, mixed with oxygen but not reacting with it at room temperature. The F–F bond (158 kJ mol⁻¹) is very weak and breaks easily, so fluorine reacts readily with many substances.

Nitrogen and oxygen. Nitrogen and oxygen sit together in air without reacting. The reaction $\ce{N2(g) + O2(g) -> 2NO(g)}$ is endothermic (ΔH = +181 kJ mol⁻¹) and begins with the strong N≡N bond. It only happens where energy is plentiful, such as lightning or a hot car engine.

Bonds in one molecule. In a halogenoalkane the carbon–halogen bond is the one that breaks in many reactions. The mean bond enthalpies fall down the group:

Bond	Mean bond enthalpy / kJ mol⁻¹
C–Cl	346
C–Br	290
C–I	228

The C–I bond is the weakest, so iodoalkanes are the most reactive and chloroalkanes the least. You will meet this again in halogenoalkane reactions.

Worked exampleUsing the data in an explanation

Explain why iodoethane reacts faster than chloroethane with aqueous sodium hydroxide at room temperature, using bond enthalpies.

The reaction involves breaking the carbon–halogen bond.
C–I has a lower bond enthalpy (228 kJ mol⁻¹) than C–Cl (346 kJ mol⁻¹), so the C–I bond is weaker.
Less energy is needed to break it, so it breaks more easily and the reaction is faster at room temperature.

Check yourself

Suggest why nitrogen is unreactive at room temperature but fluorine is reactive. Use bond enthalpies.
In ethane, CH₃–CH₃, state which bond is the weakest, using C–C 347 and C–H 413 kJ mol⁻¹, and what this suggests about which bond breaks first.
Explain why nitrogen and oxygen in the air do not react with each other at room temperature.
The mean bond enthalpy of C–Br is 290 kJ mol⁻¹ and of C–Cl is 346 kJ mol⁻¹. Which of bromoethane and chloroethane is likely to react faster with aqueous alkali at room temperature, and why?
A student writes "iodine is less reactive than chlorine, so bond enthalpy cannot be used to explain reactivity." Comment on what the student has noticed and on the conclusion.

Answers cover these until you have tried the questions

N≡N has a very high bond enthalpy (945 kJ mol⁻¹), so a very large amount of energy is needed to break it, and few collisions at room temperature have enough. F–F has a low bond enthalpy (158 kJ mol⁻¹), so it is easily broken and fluorine reacts readily.
The C–C bond (347) is weaker than C–H (413). The weakest bond tends to break first, so the C–C bond is the more likely to break first, as an indication.
The reaction needs the N≡N bond (945 kJ mol⁻¹) to break. That requires a lot of energy and the reaction is endothermic, so at room temperature the molecules do not have enough energy and the gases coexist without reacting.
Bromoethane. The C–Br bond (290) is weaker than C–Cl (346), so less energy is needed to break it, and the reaction is faster.
The student has noticed that the weaker bond in I₂ does not make iodine more reactive. That does not make bond enthalpy useless: it is an indication and other factors matter as well. The conclusion is too strong. Bond enthalpy is a useful guide when comparing similar reactions, such as the carbon–halogen bonds in halogenoalkanes.