Energy Changes In Chemical Reactions
K324 and 6092 energy changes hub: enthalpy, energy profiles, and qualitative bond-breaking and bond-making explanations.
Learning goals
- describe the meaning of enthalpy change in terms of exothermic (∆H negative) and endothermic (∆H positive) reactions
- represent energy changes by energy profile diagrams, including reaction enthalpy changes and activation energies (see also 10(c), 10(d))
- describe bond breaking as an endothermic process and bond making as an exothermic process
- explain qualitatively overall enthalpy changes in terms of the energy changes associated with the breaking and making of covalent bonds.
All chemical reactions involve energy changes: energy is absorbed to break bonds and released when new bonds form. This topic helps you interpret exothermic vs endothermic reactions, use Δ H correctly, and explain overall enthalpy changes.
Jump to: Prerequisites · Quick Reference · Core knowledge · Sub-topics · Quiz
The four core Chemical Energetics outcomes are to describe exothermic and endothermic enthalpy changes and their Δ H signs, represent them with energy profile diagrams, and explain overall enthalpy change qualitatively using bond breaking and bond making.
Prerequisites
- Writing and balancing chemical equations
- Basic covalent bonding ideas
- Distinguishing the reacting system from the surroundings
- Reading an initial and final temperature with units
Follow the two-lesson core path: Introduction to Energy Changes, then Bond Breaking and Bond Forming. Use the focused exothermic or endothermic pages only if you want extra practice with one profile type.
Quick Reference
| Item | Quick rule / reminder |
|---|---|
| Enthalpy change | Δ H = energy of products-energy of reactants (kJ mol⁻¹) |
| Exothermic | heat released to surroundings; Δ H < 0 |
| Endothermic | heat absorbed from surroundings; Δ H > 0 |
| Bond changes | bond breaking absorbs energy; bond making releases energy |
| Energy profile diagram | exothermic: products lower; endothermic: products higher |
| Activation energy (Eₐ) | energy needed to start the reaction; label from reactants → peak |
| Catalyst effect | lowers Eₐ (different pathway); does not change Δ H |
| Temperature clue | exothermic: temperature often rises; endothermic: temperature often falls |
Core knowledge to remember
- Exothermic reaction: releases heat to the surroundings (Δ H < 0).
- Endothermic reaction: absorbs heat from the surroundings (Δ H > 0).
- Enthalpy change (Δ H): heat energy change per mole of reaction.
- Surroundings: everything outside the reacting chemicals (e.g. water/air).
- System: the reacting chemicals themselves.
- Bond breaking: requires energy (endothermic step).
- Bond forming: releases energy (exothermic step).
- Activation energy (Eₐ): minimum energy needed for a reaction to occur.
Sub-topics
Core Chemical Energetics
Introduction to Energy Changes
Classify energy changes and interpret ΔH, activation energy and profiles.
Bond Breaking and Bond Forming
Explain overall energy changes by comparing bond breaking with bond making.
Focused profile practice
These pages revisit one profile type at a time. They are useful when a sign, temperature or activation-energy misconception still needs attention; they are not extra core content to complete before moving on.
Exothermic Profile Practice
Practise negative ΔH, lower-energy products and temperature-rise evidence.
Endothermic Profile Practice
Practise positive ΔH, higher-energy products and temperature-fall evidence.
Common mistakes
- Sign of Δ H: exothermic is negative; endothermic is positive.
- System vs surroundings: temperature rise means the surroundings gained heat.
- Bond changes: breaking bonds absorbs energy; making bonds releases energy. Compare the two effects to explain the overall enthalpy change.
- Δ H vs Eₐ: Eₐ is the height to the peak; Δ H compares reactants with products.
- Catalyst diagrams: a catalyst lowers Eₐ but does not change Δ H.
Quiz
Check your understanding, continue to guided practice, practise with more questions, then try the structured questions.