Full chapter
Chemical Energetics
Explain temperature changes and the energy cost of rearranging particles.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Follow the energy transfer
A temperature change is evidence about the surroundings.
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A solid dissolves in water and the beaker feels cold to touch. What happened to energy?
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The dissolving took in energy from the water and your hand
The process absorbed energy from its surroundings, so the surroundings cooled. That is an endothermic change.
Define the system as the reacting substances and the surroundings as what exchanges energy with them, such as the water, container and air. An exothermic process transfers energy to its surroundings, often as heat; the measured mixture temperature usually rises. An endothermic process takes energy from the surroundings, so their temperature usually falls.
| Process | Observation | Energy direction |
|---|---|---|
| Aqueous sodium hydroxide reacts with hydrochloric acid | Temperature rises | Reaction releases energy to the surroundings |
| Ammonium nitrate dissolves in water | Temperature falls | Dissolving process takes energy from the surroundings |
NaOH(aq) + HCl(aq) -> NaCl(aq) + H2O(l) is a neutralisation. Dissolving ammonium nitrate is a physical process with an energy change; an energy transfer does not by itself prove that a new substance has formed.
Enthalpy change, ΔH, records the energy change of the reacting system at constant pressure. For an exothermic reaction products have lower enthalpy than reactants, so ΔH is negative. For an endothermic reaction products have higher enthalpy, so ΔH is positive. The sign describes the system, not the thermometer.
Check your understanding
A reaction raises the temperature of the surrounding solution from 22 degrees C to 29 degrees C. Is it exothermic or endothermic?
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Exothermic, because energy moved from the reacting chemicals to the solution
The surroundings (the water and thermometer) got warmer, so they gained energy from the reaction. The reacting system lost energy, so delta H is negative.
Investigate an energy change
Make the measurement large enough to interpret fairly.
Measure the initial temperatures, mix the specified quantities in an insulated cup, stir consistently and record the highest or lowest temperature reached. A lid reduces energy transfer to the air; the probe should measure the mixture rather than rest on the cup. Use the same volumes, concentrations and apparatus when comparing repeats.
Worked exampleInterpret a cooling experiment
Water starts at 24 degrees C. After a salt dissolves, the lowest temperature is 18 degrees C, before slowly returning towards 24.
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- The initial fall is evidence that the dissolving process absorbed energy.
- The change in the measured temperature is 18 - 24 = -6 degrees C.
- The later warming occurs as the cooler solution gains energy from the room.
Dissolving is endothermic. The later return towards room temperature does not change that classification.
A smaller observed temperature change may result from heat exchange with the room, a different amount of reacting substance or a different amount of water being warmed. It need not mean a different reaction type. Report measurements and limitations rather than claiming that temperature change alone is an energy value in joules.
Check your understanding
How would heat loss to the room affect the measured temperature rise in an exothermic experiment?
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The measured rise would be smaller than the true rise.
Some of the energy released escapes to the room instead of warming the solution, so the thermometer under-reads. The reaction itself is still exothermic.
Read an energy profile
The barrier and the overall change are different quantities.
An energy profile plots the energy of the reacting system against reaction progress, not time. Reactants start on one level, the path rises to an energy barrier, and products finish on another. Activation energy, Ea, is the minimum energy barrier that must be overcome for a successful reaction. Read it from the reactant level to the peak; read ΔH from reactant level to product level.
An exothermic profile
The reactants lie above the products; the activation barrier rises from reactants to the peak, while the enthalpy change points down towards products.
For an endothermic profile, draw products above reactants, so ΔH is positive. There is still an activation barrier above the reactant level. A catalyst provides a lower-barrier route but leaves reactant and product levels unchanged, so it changes the rate rather than ΔH.
An endothermic profile
Products finish above reactants. Activation energy still runs from the reactant level to the peak; the enthalpy increase is smaller than the full barrier.
Watch out for this
A student says: "On an energy profile, Ea is a double-headed arrow between the reactant level and the peak." What is wrong with this?
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Draw Ea and delta H as arrows with one head. The Ea arrow starts at the reactant level and points up to the peak. The delta H arrow starts at the reactant level and points to the product level: down for an exothermic reaction, up for an endothermic one. A double-headed arrow does not show the direction of the change, and mark schemes give no mark for it. Label both axes: energy, and progress of reaction.
Worked exampleConstruct a profile from energy levels
On a relative energy scale in kJ mol-1, reactants are at 80, products at 30 and the uncatalysed peak at 140. Sketch and label the profile.
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- Label the vertical axis energy / kJ mol-1 and the horizontal axis reaction progress. Draw the reactant level at 80 and the lower product level at 30.
- Join them by a curve that rises to 140 before falling to the product level. Draw an upward activation-energy arrow from the reactant level to the peak: Ea = 140 - 80 = 60 kJ mol-1.
- Draw the enthalpy-change arrow from reactants down to products: delta H = 30 - 80 = -50 kJ mol-1. For a catalysed route, lower only the peak.
An exothermic profile: products are lower, delta H is negative, and Ea is measured up from the reactants, not from zero. The energy zero is arbitrary; differences determine the labels.
Try it step by step
Work it out in steps: Enthalpy change, then Activation energy, then Exothermic or endothermic?
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Problem 1.
Enthalpy change: delta H = products - reactants = 40 - 90 = -50 kJ mol-1.
Activation energy: Ea = peak - reactants = 150 - 90 = 60 kJ mol-1. Measure it up from the reactants, not from zero.
Exothermic or endothermic?: The products are lower than the reactants and delta H is negative, so the reaction is exothermic.
Problem 2.
Enthalpy change: delta H = 85 - 20 = +65 kJ mol-1.
Activation energy: Ea = 130 - 20 = 110 kJ mol-1.
Exothermic or endothermic?: The products are higher than the reactants, so the reaction is endothermic.
Problem 3.
Enthalpy change: delta H = 35 - 110 = -75 kJ mol-1.
Activation energy: Ea = 160 - 110 = 50 kJ mol-1.
Exothermic or endothermic?: The products are lower and delta H is negative, so the reaction is exothermic.
Check your understanding
An exothermic reaction has a large activation energy. Is that a contradiction?
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No. Ea is the barrier at the start, while delta H compares the products with the reactants.
A reaction can need energy to get started (a high Ea) and still release more energy overall. Burning fuel needs a spark but is strongly exothermic.
Check your understanding
In an energy profile for the endothermic reaction CaCO3(s) -> CaO(s) + CO2(g), how are the arrows for Ea and delta H drawn?
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Both start at the reactant level: Ea up to the peak, delta H up to the products.
Use single-headed arrows. Ea goes from CaCO3 up to the top of the curve. delta H goes from CaCO3 up to CaO + CO2, which sit higher, so delta H is positive.
Breaking bonds costs energy; making bonds releases it
The net change depends on both parts of the rearrangement.
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A molecule of hydrogen, H-H, is split into two hydrogen atoms. Does breaking this bond take in energy or give it out?
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It takes in energy to pull the atoms apart
Atoms in a bond attract the shared electrons. Pulling them apart works against that attraction, so energy must be supplied.
Breaking a covalent bond separates atoms that attract one another, so energy must be supplied: it is endothermic. Forming a bond releases energy as atoms become more strongly held together: it is exothermic. A chemical reaction usually does both; never explain combustion by saying that breaking fuel bonds releases energy.
- Break reactant bonds
Energy is absorbed.
- Form product bonds
Energy is released.
- Compare the amounts
More released than absorbed gives an exothermic reaction; more absorbed than released gives an endothermic reaction.
Worked exampleExplain hydrogen combustion qualitatively
Why can hydrogen reacting with oxygen release energy even though bonds must first break?
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- H-H and O=O bonds must be broken, requiring energy.
- O-H bonds form in water, releasing energy.
- The energy released in making product bonds exceeds that needed to break reactant bonds.
The net change is exothermic. This explanation compares totals, not just whether a bond is strong.
Check your understanding
A student says, "An endothermic reaction forms no bonds." Which correction is right?
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Bonds do form, but less energy is released making them than is taken in breaking bonds.
Every reaction breaks reactant bonds (taking in energy) and makes product bonds (releasing energy). Endothermic means the energy taken in is greater than the energy released.
Quick revision
Revisit the essentials, then return to an explanation when you need it.
| Term | Definition to write |
|---|---|
| Exothermic reaction | A reaction that transfers energy, usually as heat, to the surroundings, so the temperature of the surroundings rises. |
| Endothermic reaction | A reaction that takes in energy, usually as heat, from the surroundings, so the temperature of the surroundings falls. |
| Term | Definition to write |
|---|---|
| Enthalpy change, ΔH | The heat energy change of a reaction. ΔH is negative for an exothermic reaction and positive for an endothermic reaction. |
| Activation energy, Ea | The minimum energy that colliding particles must have for a reaction to occur. |
Exothermic: energy out to surroundings. Endothermic: energy in from surroundings. Use the initial change and account for later heat exchange.
Pure: ΔH compares product and reactant levels; Ea is the barrier. Breaking bonds absorbs energy; making bonds releases it. Compare both totals.
Scope
Learning outcomes
9. Chemical Energetics (6092 / K324). Use the outcome map to find the explanation for a particular syllabus requirement.
See the learning outcome map
9(a) Explain enthalpy-change sign
- Exothermic negative dH
- Endothermic positive dH
9(b) Represent energy profiles
- Reactant/product levels
- Activation energy
- Reaction enthalpy change
9(c) Explain bond-breaking/making energy
- Breaking endothermic
- Making exothermic
9(d) Explain overall enthalpy qualitatively
- Compare breaking and making covalent bonds