To estimate an energy change from bond energies, add up the energy needed to break every bond in the reactants, add up the energy released by making every bond in the products, and subtract: broken minus made. The result is in kJ/mol.
This lesson builds on interpreting an energy profile and is the calculation part of energy changes and bonds.
Why “broken minus made”?
Breaking bonds absorbs energy, so the broken total is the energy put in. Making bonds releases energy, so the made total is the energy given out. If more is given out than put in, the reaction is exothermic overall and the answer is negative.
The bond energy values are supplied in the question. The numbers in this lesson are rounded illustrative values for practice, so use the table in your own question, not these.
How do you work through it?
- Balance the equation and write out the structures so that every bond can be seen. The equation balance reasoning trainer helps with counting atoms.
- List the bonds broken, with how many of each, using the coefficients.
- Total the energy to break them.
- List the bonds made and total the energy released.
- Subtract: broken − made.
- Interpret the sign, then add kJ/mol.
Worked example
The bond energies are supplied values for practice, in kJ/mol: C–H 413, O=O 498, C=O 805, O–H 464.
Estimate the energy change for CH₄ + 2O₂ → CO₂ + 2H₂O.
Step 1, bonds broken: one CH₄ has 4 C–H bonds. Two O₂ molecules have 2 O=O bonds.
Step 2, energy to break: 4 × 413 = 1652. 2 × 498 = 996. Total = 1652 + 996 = 2648 kJ/mol.
Step 3, bonds made: one CO₂ has 2 C=O bonds. Two H₂O molecules have 2 × 2 = 4 O–H bonds.
Step 4, energy released: 2 × 805 = 1610. 4 × 464 = 1856. Total = 1610 + 1856 = 3466 kJ/mol.
Step 5, subtract: 2648 − 3466 = −818 kJ/mol.
Step 6, interpret: the sign is negative, so the reaction is exothermic.
Check: more energy is released in making bonds (3466) than is needed to break them (2648), which agrees with a negative answer. Recalculating, 1652 + 996 = 2648 and 1610 + 1856 = 3466, so the subtraction gives −818 again.
The mistakes to watch for
There are two frequent slips.
Mistaken answer 1: using 1 × 498 for oxygen, giving broken = 1652 + 498 = 2150 and ΔH = 2150 − 3466 = −1316 kJ/mol.
The student ignored the coefficient 2 in front of O₂.
Mistaken answer 2: made − broken = 3466 − 2648 = +818 kJ/mol.
The student reversed the subtraction, which flips the sign and gives the wrong label for the reaction.
The corrections are simple habits. Multiply every bond count by the coefficient, and write “broken − made” as a heading above your working before you start. Then check the sign against the idea that more energy released than needed means negative.
Check yourself
Use these supplied values for practice: H–H 436, Cl–Cl 242, H–Cl 431, N≡N 945, N–H 391.
1. Estimate the energy change for H₂ + Cl₂ → 2HCl.
Show answer
Broken: 436 + 242 = 678. Made: 2 × 431 = 862. Energy change = 678 − 862 = −184 kJ/mol, exothermic.
2. Estimate the energy change for N₂ + 3H₂ → 2NH₃. (Each NH₃ has 3 N–H bonds.)
Show answer
Broken: 945 + 3 × 436 = 945 + 1308 = 2253. Made: 2 × 3 × 391 = 6 × 391 = 2346. Energy change = 2253 − 2346 = −93 kJ/mol, exothermic.
3. A student gets +184 kJ/mol for question 1. What went wrong?
Show answer
They subtracted the other way round (made − broken). The correct order is broken − made, which gives −184 kJ/mol.
Where this leads next
Next, separate bond breaking from bond making so that the two totals stay clean. Then try the full practice set. The mole and equation-ratio tutor also shows how coefficients carry through amounts.
Students who can do every step on a good day still slip under pressure. A teacher in online one-to-one Chemistry tuition can watch your layout and help you build a working order that protects the sign.