Exothermic and endothermic reactions study guide

Exothermic and Endothermic Reactions

1. Energy Conservation in Chemical Reactions

• Energy is conserved: the total energy before a reaction equals the total energy after. This is the first law of thermodynamics.
• In a chemical reaction, energy can be transferred to or taken from the surroundings. The energy that leaves or enters is called the enthalpy change (ΔH), but GCSE does not require ΔH calculations for this unit.

2. Exothermic Reactions

| Feature | Description | |---------|-------------| | Energy transfer | Energy is released to the surroundings | | Energy of products | Lower than that of reactants by the amount released | | Temperature effect | Surroundings warm up | | Examples | Combustion (e.g. burning wood), oxidation (rusting iron), neutralisation (acid + base), self‑heating cans, hand warmers |

Everyday Uses

• Self‑heating cans: exothermic reaction of iron powder with water releases heat, warming the food.
• Hand warmers: iron powder oxidises slowly, producing heat for a few hours.

3. Endothermic Reactions

| Feature | Description | |---------|-------------| | Energy transfer | Energy is absorbed from the surroundings | | Energy of products | Higher than that of reactants | | Temperature effect | Surroundings cool down | | Examples | Thermal decomposition (e.g. CaCO₃ → CaO + CO₂), citric acid + sodium bicarbonate (baking soda) reaction, sports injury packs (ice packs) |

Everyday Uses

• Sports injury packs: ice packs absorb heat from the body, reducing swelling.
• Baking soda & citric acid: the fizzing reaction is visibly endothermic, cooling the mixture.

4. Distinguishing Exothermic vs. Endothermic

• Temperature change of surroundings: If the surroundings warm, the reaction is exothermic; if they cool, it is endothermic.
• Reaction profile: The overall energy change (ΔE) is shown as a vertical arrow at the end of the curve.

5. Reaction Profiles

A reaction profile is a graph of energy (vertical axis) vs. reaction progress (horizontal axis).

Key Elements

1. Reactants – starting point on the left.
2. Products – ending point on the right.
3. Activation energy (Ea) – the highest point on the curve; the energy barrier that must be overcome.
4. Overall energy change (ΔE) – vertical difference between reactants and products.

Exothermic Profile

• Reactants higher than products.
• ΔE is negative (energy released).

Endothermic Profile

• Reactants lower than products.
• ΔE is positive (energy absorbed).

Drawing a Profile

1. Draw a horizontal line for reactants.
2. Curve up to the activation energy.
3. Curve down to products.
4. Label Ea and ΔE.

6. Bond‑Energy Calculations (HT Only)

6.1 Energy to Break Bonds

• Bond energy is the energy required to break one mole of a specific bond.
• Total energy needed = Σ (bond energy × moles of that bond in reactants).

6.2 Energy Released When Bonds Form

• Total energy released = Σ (bond energy × moles of that bond in products).

6.3 Overall Energy Change

• ΔE = Energy to break bonds – Energy released when bonds form.
• Negative ΔE → exothermic.
• Positive ΔE → endothermic.

Example (Illustrative)

| Bond | Energy (kJ mol⁻¹) | Moles in Reactants | Moles in Products | |------|------------------|--------------------|-------------------| | H–H | 436 | 1 | 0 | | O=O | 498 | 1 | 0 | | H–O | 467 | 0 | 2 |

• Energy to break = 436 kJ + 498 kJ = 934 kJ.
• Energy released = 2 × 467 kJ = 934 kJ.
• ΔE = 934 kJ – 934 kJ = 0 kJ → reaction is neither exothermic nor endothermic (idealised).

7. Practical Work (AT 1, 3, 5, 6)

• Measure temperature change: Use a thermometer or digital temperature probe.
• Variables to test: concentration of acid, type of metal, type of carbonate, reaction time.
• Record data: Initial temperature, final temperature, ΔT, and calculate percentage change.
• Safety: Wear goggles, gloves, and work in a well‑ventilated area.

8. Summary of Key Points

1. Energy conservation: total energy is constant.
2. Exothermic: energy released, surroundings warm, ΔE negative.
3. Endothermic: energy absorbed, surroundings cool, ΔE positive.
4. Reaction profiles: visualise Ea and ΔE.
5. Bond‑energy calculations: compare energy to break bonds vs. energy released when bonds form.
6. Practicals: measure ΔT, vary variables, record data safely.

---

Further Reading

• AQA GCSE Chemistry specification, Unit 6 – Energy changes.
• Practical guidelines for measuring temperature changes in GCSE chemistry.

---

Remember: For GCSE, you do not need to calculate ΔH, but you should understand the concepts of energy transfer, reaction profiles, and bond‑energy reasoning.

Energy changes focus for 830

This undefined is anchored to AQA GCSE Chemistry 8462 Unit 4.5. It separates exothermic reactions, endothermic reactions, reaction profiles, activation energy, bond-energy calculations, chemical cells and fuel cells so students do not collapse nearby ideas into one generic energy answer.

How to answer exam questions

Start by naming the energy-change idea being tested. For reaction profiles, label reactants, products, activation energy and overall energy change. For bond-energy calculations, add the energy needed to break bonds, subtract the energy released when bonds form, keep the sign, and state whether the result is exothermic or endothermic.

Common checks

Check whether the question asks for temperature change, energy transfer, a diagram label, a calculation, a cell voltage pattern, a fuel-cell comparison or an evaluation. Use the exact subtopic wording and avoid drifting into chemical changes, rates, equilibrium or electrolysis unless the question explicitly connects them.

Practice method

After reading the section, write a three-part response: define the key idea, apply it to the named reaction or device, then explain the evidence using the correct GCSE Chemistry term. For calculations, show formula, substitution, calculation, final answer, unit and conclusion.