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.
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 understandingA reaction raises the surrounding solution temperature from 22 to 29 degrees C. Is it exothermic or endothermic?Think it through, then reveal the answer
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 example
Interpret 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.
- 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 understandingHow would heat loss to the room affect the observed rise in an exothermic experiment?Think it through, then reveal the answer
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.
Worked example
Construct 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.
- 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.
Check your understandingAn exothermic reaction has a large activation energy. Is that contradictory?Think it through, then reveal the answer
Breaking bonds costs energy; making bonds releases it
The net change depends on both parts of the rearrangement.
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 example
Explain hydrogen combustion qualitatively
Why can hydrogen reacting with oxygen release energy even though bonds must first break?
- 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 understandingA student says "an endothermic reaction forms no bonds". Correct the statement.Think it through, then reveal the answer
Quick revision
Revisit the essentials, then return to an explanation when you need it.
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 and references
Learning outcomes and sources
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
- 2026 Pure Chemistry 6092
Official topic 9, pages 19. Original explanations mapped to the stated outcomes; 2026 and 2027 topic content agrees.
- 2027 Pure Chemistry K324
Official topic 9, pages 19. Original explanations mapped to the stated outcomes; 2026 and 2027 topic content agrees.
- 2026 Combined Chemistry 5086 / 5088
Official topic 9, pages 33. Original explanations mapped to the stated outcomes; 2026 and 2027 topic content agrees.
- 2027 Combined Chemistry K326 / K328
Official topic 9, pages 33. Original explanations mapped to the stated outcomes; 2026 and 2027 topic content agrees.
- Grail: 6092 Chemistry Complete Notes, Version 1
Background consultation: Chapter 16, energy profiles, pp. 68-70. Teaching additions and examples are original; syllabus scope and chemistry independently checked.