A match head holds immense energy. It will never ignite without friction.
Strike a match on the box and it bursts into flame. The reaction releases heat and light. Yet the same match can sit in a drawer for fifty years and never ignite. Even a reaction that releases energy must first spend energy to break the bonds of its reactants. This page compares the energy that breaking bonds absorbs with the energy that forming bonds releases. Energy profiles show the result.
Core Claims
- BENDO MEXO. Breaking bonds always absorbs energy (endothermic, +). Forming bonds always releases energy (exothermic, −). Breaking bonds never releases energy by itself.
- Releases or absorbs. If the new bonds release more energy than the old bonds cost, the reaction releases energy (ΔH < 0, exothermic). If the old bonds cost more, it absorbs energy (ΔH > 0, endothermic).
- Activation energy (Ea). The minimum energy colliding molecules need to reach the transition state and start rearranging their bonds.
- Catalysts. A catalyst speeds up a reaction by giving it a path with a lower Ea. It does not change the reactant energy, the product energy, or ΔH.
Energy profile
Retrieval Checklist
- State whether breaking bonds absorbs or releases energy.
- Say why a reaction releases or absorbs energy, using the bonds that break and form.
- Identify the reactants, products, Ea, and ΔH on a reaction profile.
- Explain what a catalyst does to Ea and to ΔH.
Breaking bonds takes energy. Forming bonds releases energy.
A common misconception says bonds release energy when they snap. In chemistry the opposite is true.
A chemical bond forms because two atoms lower their potential energy by sharing electrons. The negative electrons attract both positive nuclei. The bonded molecule rests in a stable well of potential energy.
Breaking bonds: always endothermic (+ΔH)
To pull two bonded atoms apart, you pull opposite charges away from each other against their attraction. That always takes energy. You must do work to lift atoms out of the well. Breaking bonds never releases energy.
Forming bonds: always exothermic (−ΔH)
When separate atoms approach, opposite charges attract. As the atoms settle into the well, potential energy becomes thermal energy and light. Forming bonds always releases energy to the surroundings.
BENDO: Breaking is ENDOthermic (energy goes in, +ΔH).
MEXO: Making is EXOthermic (energy comes out, −ΔH).
The net energy change of a reaction (ΔHrxn) comes from both processes together.
Thinking energy comes out when a bond breaks. Breaking a bond always costs energy. A reaction releases energy only when the new bonds release more than the old bonds cost.
Reaction profiles: climbing the energy hill.
Chemists plot potential energy along the reaction coordinate to track energy changes. This idealized path follows the atoms from the reactants to the products.
Every reaction profile has three features.
- Activation energy (Ea). The energy difference between the reactants and the peak. Colliding molecules need at least this much kinetic energy to stretch their bonds. With less, they bounce apart unchanged.
- Transition state (activated complex, ‡). The peak of the curve. Old bonds are partly broken and new bonds are partly formed. It has the highest potential energy and the lowest stability on the path.
- Reaction enthalpy (ΔH). The difference in potential energy between the products and the reactants: ΔH = Eproducts − Ereactants. In an exothermic reaction the products sit lower (ΔH < 0). In an endothermic reaction they sit higher (ΔH > 0).
Reading the activation energy as the energy released. Ea is the climb from the reactants to the peak. ΔH is the gap between the reactants and the products. A reaction can release a lot of energy and still have a large Ea, like a match.
Reaction profile and transition state.
Pick a reaction and drag the slider through it. Switch the catalyst on to see how the barrier changes.
The barriers without a catalyst are simplified. Each equals the energy of the first bond to break: H–H at 436 kJ/mol, or N≡N at 945 kJ/mol. Measured barriers can be lower.
Why does a reaction that releases energy still need a push to start?
The explanation appears after you predict.
- Energy profile without a catalyst
- Energy profile with a catalyst
- Hydrogen atom
- Oxygen atom
- Nitrogen atom
- Load hydrogen burning with the catalyst off, then press Record. Look for: a barrier of +436 kJ/mol and an energy change of −484 kJ/mol, with the marker at the peak.
- Load the same reaction with the catalyst on, then press Record. Look for: the dashed green peak is lower, at +125 kJ/mol, and the energy change is still −484.
- Load ammonia with the catalyst off and press Record, then load it with the catalyst on and press Record. Look for: the barrier falls from +945 to +180 kJ/mol while the energy change stays −92.
- Load water splitting, then press Record. Look for: the products sit higher than the reactants, so the energy change is +484 and the barrier is +920 kJ/mol.
Use your evidence
Calculating ΔH from bond energies.
EnrichmentAdd up the bonds: ΔH = ∑(broken) − ∑(formed)
Extra depth. The rest of the lesson works without it.
This calculation goes past what this course assesses. It turns the bond picture into a number. Breaking bonds absorbs energy (+) and forming bonds releases energy (−). To find the reaction enthalpy (ΔH), count every bond broken and every bond formed in the balanced equation.
- ∑ Dbroken is the energy absorbed to break the reactant bonds (+).
- ∑ Dformed is the energy released when the product bonds form (−).
How do the broken and formed bonds add up?
- Bonds broken: they absorb energy
- Bonds formed: they release energy
Bond energies (kJ/mol)
| Bond | Energy (kJ/mol) | Bond | Energy (kJ/mol) | Multiple bond | Energy (kJ/mol) |
|---|---|---|---|---|---|
| H − H | 436 | C − H | 413 | C = C | 614 |
| C − C | 348 | C − O | 358 | C ≡ C | 839 |
| O − H | 463 | N − H | 391 | O = O | 498 |
| Cl − Cl | 242 | H − Cl | 431 | N ≡ N | 945 |
Propane burns completely: C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g)
Average bond energies (kJ/mol):
• C−C = 348
• C−H = 413
• O=O = 498
• C=O (in CO2) = 799
• O−H = 463
Calculate ΔHrxn in kJ/mol. Show the bonds broken in the reactants and the bonds formed in the products. [2 marks]
- Bonds broken (1 mark): You total the broken bonds as +6,490 kJ/mol.
- Bonds formed and net (1 mark): You total 8,498 kJ/mol formed, so ΔH = −2,008 kJ/mol.
Why the nitrogen triple bond feeds the world.
Earth’s atmosphere is 78% nitrogen gas (N2). Plants need nitrogen to build amino acids and nucleic acids. Yet crops cannot take N2 from the air. The two nitrogen atoms share a triple bond with a bond energy of 945 kJ/mol. It is one of the strongest covalent bonds in chemistry.
In the early 20th century, Fritz Haber and Carl Bosch developed an iron catalyst for making ammonia. The metal surface binds N2 and weakens the triple bond in stages. The catalyst lowers the activation energy (Ea), so industry can make ammonia at scale. Today, synthetic ammonia supplies the nitrogen for over half the protein in human diets.
Thinking a catalyst changes how much energy the reaction releases. It only lowers the barrier. Making ammonia releases the same 92 kJ per mole of reaction with or without the iron.
Fill the blanks from memory.
Type each blank from memory, then press Check. Press Show answers after your first check if you are stuck.
1. Breaking chemical bonds always energy.
2. In an exothermic reaction, the potential energy of the products is than that of the reactants.
3. The minimum energy colliding particles need to reach the transition state is the .
4. A catalyst speeds up a reaction by the activation energy.
5. A catalyst leaves the reaction enthalpy (ΔH) .
A biology textbook states: “When ATP is hydrolyzed, energy is released when the terminal high-energy phosphate bond is broken.”
Critique this statement as a chemist. Explain why breaking a bond cannot release energy by itself. Then say where the energy released in ATP hydrolysis comes from.
Use BENDO MEXO. Count both the bond that breaks and the bonds that form. Write your answer first, then compare it with the model answer.
The statement is chemically wrong. Breaking the phosphate bond in ATP always absorbs energy. BENDO: breaking is endothermic. Pulling atoms out of an electrostatic well takes work.
The net release comes when new bonds form with surrounding water molecules. MEXO: making bonds is exothermic. The new bonds release more energy than the broken bond cost. The difference gives a net negative reaction enthalpy (ΔH < 0).
Write your answer first. Then grade yourself.
Give yourself 1 mark for each idea you wrote down.
Propane burns completely and releases energy: C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g)
(a) Say whether this reaction is exothermic or endothermic. Describe its energy profile: where the reactants and products sit, and where the activation energy (Ea) is. [2 marks]
(b) A platinum catalyst is added. Explain how it changes the profile and what it leaves unchanged. [1 mark]
- Direction (1 mark): You say the reaction is exothermic, because the products sit lower in energy than the reactants.
- Profile (1 mark): You say the curve climbs to a peak, then drops to the lower products.
- Catalyst (1 mark): You say a catalyst lowers the peak and leaves the products and ΔH unchanged.
The Saturn V upper stages and the Space Launch System burn liquid hydrogen with oxygen: 2 H2 + O2 → 2 H2O. Hydrogen is light, with a molar mass of only 2 g/mol. The reaction releases 484 kJ for every two moles of water formed. That much energy for so little mass pushes the exhaust out at over 4,500 meters per second.
Adaptive Competency Practice: HS-PS1-4
Test your mastery of chemical reaction energetics one question at a time. The adaptive engine models bond breaking, bond forming, reaction coordinate profiles, and enthalpy calculations. Practice until you achieve verified competency.