Bond Enthalpy - Definition, Energy Change, Solved Examples - Free Printable

Let’s solve this step by step.

We are given a table of average bond enthalpies (in kJ/mol). These values tell us how much energy is needed to break one mole of a specific type of bond in the gas phase.

The problem doesn’t state what we’re calculating — but since it’s common in chemistry homework, and based on typical problems using such tables, I’ll assume you’re being asked to calculate the enthalpy change (ΔH) for a reaction using bond energies.

But wait — there’s no reaction given! That means either:

1. The original problem had a reaction that wasn’t included in your upload, or
2. You’re expected to know which reaction to use with this table.

Since no reaction was provided, let me show you how to use this table with an example reaction so you can apply it to your actual problem.

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Example Reaction:

Let’s say you’re asked to find ΔH for:

CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)

This is a classic substitution reaction.

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Step 1: Draw Lewis structures (mentally or on paper) to see which bonds break and form.

In reactants:
- CH₄ has 4 C–H bonds
- Cl₂ has 1 Cl–Cl bond

In products:
- CH₃Cl has 3 C–H bonds and 1 C–Cl bond
- HCl has 1 H–Cl bond

So, bonds broken:
- 4 C–H (but only 1 is replaced — actually, we break 1 C–H and 1 Cl–Cl)
Wait — better way: count total bonds broken vs formed.

Actually, net change:
Break: 1 C–H and 1 Cl–Cl
Form: 1 C–Cl and 1 H–Cl

Yes — because 3 C–H bonds remain unchanged.

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Step 2: Use bond enthalpy values from the table.

From the table:

C–H = 413 kJ/mol
Cl–Cl = 242 kJ/mol
C–Cl = 328 kJ/mol
H–Cl = 431 kJ/mol

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Step 3: Calculate energy required to break bonds (positive, endothermic)

Energy absorbed (bonds broken):
= 1 × C–H + 1 × Cl–Cl
= 413 + 242 = 655 kJ/mol

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Step 4: Calculate energy released when new bonds form (negative, exothermic)

Energy released (bonds formed):
= 1 × C–Cl + 1 × H–Cl
= 328 + 431 = 759 kJ/mol

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Step 5: Calculate ΔH

ΔH = Energy absorbed – Energy released
OR more accurately:
ΔH = Σ (bond energies of bonds broken) – Σ (bond energies of bonds formed)

So:

ΔH = 655 – 759 = –104 kJ/mol

That means the reaction releases 104 kJ per mole — exothermic.

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✔ So if your actual problem was something like this, you’d follow the same steps.

But since no reaction was given, perhaps you were meant to look up a specific value? Or maybe the question was “What is the bond energy of O=O?” — which from the table under Multiple Bonds is 495 kJ/mol.

Wait — looking again at the table, under “Multiple Bonds”, it lists:

O₂ → 495

That likely refers to the O=O double bond in oxygen gas.

Similarly, N≡N is 941, etc.

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Perhaps the question was simply:
“What is the average bond enthalpy for the O=O bond?”

Then the answer is directly from the table: 495 kJ/mol

Or maybe:
“What is the bond energy of N≡N?” → 941 kJ/mol

Without the exact question, I can’t be sure — but since the table includes “O₂ 495” under multiple bonds, and that’s unusual notation (usually written as O=O), it might be testing whether you recognize that O₂ has a double bond with enthalpy 495 kJ/mol.

Alternatively, maybe the question was to compare single vs multiple bonds — e.g., why is C=C (614) less than twice C–C (2×348=696)? Because double bonds aren’t exactly twice as strong.

But again — without the actual question, I’m guessing.

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However, since you uploaded only the table and said “solve the problem”, and no problem statement is visible, the most reasonable assumption is that you’re being asked to identify a specific bond energy from the table, and the most notable one listed differently is O₂ = 495, which represents the O=O bond.

Alternatively, perhaps the question was:
“What is the bond enthalpy of the nitrogen triple bond?” → N≡N = 941 kJ/mol

But let’s check — in many textbooks, they ask:
“What is the average bond energy for the O=O bond in O₂?” → Answer: 495 kJ/mol

Given that, and since it’s listed uniquely as “O₂ 495” instead of “O=O”, it might be the intended target.

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To be safe, let’s consider another possibility: sometimes questions ask for the bond energy of a bond not explicitly labeled — but here everything is labeled.

Wait — look at the very bottom of Multiple Bonds:
S=S 418 — that’s sulfur-sulfur double bond.

But again — no context.

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I think the best approach is to provide the most commonly tested value from such tables: the O=O bond energy, which is listed as 495 kJ/mol under “O₂”.

So I’ll go with that.

Final Answer:
495