Exothermic Reactions
Key idea: Exothermic reactions: definition, ΔH is negative, and how to interpret energy profile diagrams and activation energy without common traps.
Before you start: Describe exothermic and endothermic enthalpy changes, including the sign of ΔH
By the end, you can
- Represent exothermic reactions using energy profile diagrams
Exothermic questions test whether you can state the direction of heat transfer, use the sign of Δ H, and interpret an energy profile without confusing enthalpy change with activation energy.
What this page is testing:
- Can you define exothermic using system/surroundings language (not just temperature)?
- Can you link energy profile shape to sign of Δ H confidently?
- Can you separate enthalpy ideas from rate/catalyst ideas in one clear sentence?
1. Definition
A. Exothermic Reaction
An exothermic reaction is a reaction that releases heat energy to the surroundings.
B. Enthalpy Change (Δ H)
For an exothermic reaction, the enthalpy change is negative:
2. Key Ideas
- Exothermic means the surroundings warm up because heat is released from the reacting chemicals.
- Products have lower energy than reactants.
- Δ H is negative.
- Most reactions still require activation energy to start (even exothermic ones).
Exothermic reactions release heat to the surroundings, so Δ H is negative.
3. Detailed Explanations
- Exothermic reactions release heat to the surroundings, so the surroundings often warm up.
- Products are at lower energy than reactants, so Δ H is negative.
- On an energy profile: label reactants, products, activation energy (hump), and Δ H (downward arrow).
- A catalyst lowers activation energy but does not change Δ H.
A. System vs Surroundings
- System: the reacting chemicals.
- Surroundings: the solution/container/air.
If the system releases heat, the surroundings get hotter (exothermic).
B. Energy Profile Diagram (Exothermic)
In an exothermic energy profile:
- reactants start at a higher energy level,
- products end at a lower energy level,
- Δ H is downward (negative),
- there is usually an activation energy “hump”.
A catalyst lowers activation energy but does not change Δ H.
C. Examples You Can Write Safely
| Example | Why exothermic? |
|---|---|
| Combustion (burning in oxygen) | Releases heat/light |
| Neutralisation (acid + alkali) | Releases heat (temperature rises) |
| Many oxidations | Often release heat |
4. Common Mistakes
- Writing “temperature increases” as the definition. Definition must mention heat released to surroundings.
- Getting the sign wrong: exothermic is Δ H negative.
- Thinking exothermic means “no activation energy”. Exothermic reactions can still need energy to start.
- Forgetting units for Δ H (commonly kJ mol⁻¹).
- Confusing “heat released” with “rate of reaction”. Fast/slow is a different topic.
5. Exam Tips
If asked to sketch/label: label reactants, products, activation energy, and ΔH (downward arrow).
- In optional molar-enthalpy calculations, say “released” or use a negative sign consistently.
- Use the word surroundings (container/solution) in explanations.
A. Phrase-level exam wording reminders
- “Heat is released from the system to the surroundings, so Δ H is negative.”
- “Products are at lower energy than reactants on the energy profile.”
- “A catalyst lowers activation energy only; it does not change Δ H.”
- “Temperature rise is evidence consistent with exothermic change, assuming limited heat loss.”
6. Worked Examples
Example 1: Decide the Sign of ΔHCore
Is Δ H positive or negative for an exothermic reaction? Explain using “products vs reactants energy”.
Show Answer
Δ H is negative because products have lower energy than reactants, so energy is released to the surroundings.
Example 2: Calculate ΔH From Energy LevelsCore
Energy of reactants = 250 kJ and energy of products = 180 kJ. Calculate Δ H and state whether the reaction is exothermic or endothermic.
Show Answer
Δ H is negative, so the reaction is exothermic.
Example 3: Use ΔH to Find Energy ReleasedOptional
Out-of-syllabus enrichment:
The combustion of carbon has Δ H = -394kJ mol⁻¹ (for the reaction as written). How much heat is released when 0.25 mol of carbon burns completely?
Show Answer
Heat released = 0.25 × 394 = 98.5kJ (released).
Example 4: Read an Energy ProfileCore
On an energy profile diagram, the products are lower than the reactants. What does this tell you about Δ H and the reaction type?
Show Answer
Δ H is negative and the reaction is exothermic (heat released to surroundings).
7. Mind Stretchers
Mind stretcher 1: Catalyst ConfusionExtension
A student says: “A catalyst makes a reaction more exothermic because it lowers activation energy.” Identify the mistake.
Show Answer
Lowering activation energy changes the rate, not the energy difference between reactants and products. A catalyst does not change Δ H.
Mind stretcher 2: Temperature Observation TrapExtension
A student measures no temperature rise in a reaction and concludes “it is not exothermic”. Give one valid reason their conclusion could be wrong.
Show Answer
Heat could be lost to the surroundings (poor insulation), or the temperature rise could be too small to detect. Exothermic means heat is released; the observed temperature change depends on heat loss/gain.
8. Quiz
First sketch one exothermic energy profile from memory. Then use the shared objective-selected practice for K324 / 6092.
K324 / 6092 Practice