Exothermic and endothermic reactions
Cambridge IGCSE Chemistry (0620) · Unit 5: Chemical energetics · 12 flashcards
Exothermic and endothermic reactions is topic 5.1 in the Cambridge IGCSE Chemistry (0620) syllabus , positioned in Unit 5 — Chemical energetics , alongside Bond energies. In one line: An exothermic reaction transfers thermal energy to the surroundings, leading to an increase in the temperature of the surroundings.
This topic is examined in Paper 1 (multiple-choice) and Papers 3/4 (theory), plus Paper 5 or Paper 6 (practical / alternative to practical).
The deck below contains 12 flashcards — 6 definitions and 5 key concepts — covering the precise wording mark schemes reward. Use the 6 definition cards to lock down command-word answers (define, state), then move on to the concept and application cards to handle explain, describe and compare questions.
An exothermic reaction
An exothermic reaction transfers thermal energy to the surroundings, leading to an increase in the temperature of the surroundings.
What the Cambridge 0620 syllabus says
Official 2026-2028 specThese are the exact learning objectives Cambridge sets for this topic. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks.
- State State that an exothermic reaction transfers thermal energy to the surroundings leading to an increase in the temperature of the surroundings
- State State that an endothermic reaction takes in thermal energy from the surroundings leading to a decrease in the temperature of the surroundings
- Interpret Interpret reaction pathway diagrams showing exothermic and endothermic reactions
- State State that the transfer of thermal energy during a reaction is called the enthalpy change, ΔΗ, of the reaction. ΔΗ is negative for exothermic reactions and positive for endothermic reactions Supplement
- Define Define activation energy, Ea, as the minimum energy that colliding particles must have to react Supplement
- Draw Draw and label reaction pathway diagrams for exothermic and endothermic reactions using information provided, to include: (a) reactants (b) products (c) enthalpy change of the reaction, ΔΗ (d) activation energy, Ea Supplement
- State State that bond breaking is an endothermic process and bond making is an exothermic process and explain the enthalpy change of a reaction in terms of bond breaking and bond making Supplement
- Calculate Calculate the enthalpy change of a reaction using bond energies Supplement
What is an exothermic reaction?
An exothermic reaction transfers thermal energy to the surroundings, leading to an increase in the temperature of the surroundings.
What is an endothermic reaction?
An endothermic reaction takes in thermal energy from the surroundings, leading to a decrease in the temperature of the surroundings.
What does a reaction pathway diagram show for an exothermic reaction?
The reactants have higher energy than the products. The diagram shows a decrease in energy from reactants to products. The energy released is given out to the surroundings.
What does a reaction pathway diagram show for an endothermic reaction?
The reactants have lower energy than the products. The diagram shows an increase in energy from reactants to products. The energy is absorbed from the surroundings.
What is enthalpy change (ΔH)?
Enthalpy change (ΔH) is the transfer of thermal energy during a reaction. It's negative for exothermic reactions and positive for endothermic reactions.
What is the sign of ΔH for an exothermic reaction?
ΔH is negative for exothermic reactions because energy is released to the surroundings.
What is the sign of ΔH for an endothermic reaction?
ΔH is positive for endothermic reactions because energy is absorbed from the surroundings.
Define activation energy (Ea).
Activation energy (Ea) is the minimum energy that colliding particles must have to react. It is the energy required to break the initial bonds for a reaction to occur.
Is bond breaking an endothermic or exothermic process?
Bond breaking is an endothermic process. Energy is required to overcome the forces holding the atoms together in the bond. This energy is absorbed.
Is bond making an endothermic or exothermic process?
Bond making is an exothermic process. Energy is released when new bonds are formed as the atoms are attracted to each other. This energy is released.
How do you explain enthalpy change in terms of bond breaking and bond making?
ΔH = Energy required for bond breaking - Energy released during bond making. If more energy is released than required, the reaction is exothermic (negative ΔH). If more energy is required than released, the reaction is endothermic (positive ΔH).
Calculate the enthalpy change for H₂(g) + Cl₂(g) → 2HCl(g), given bond energies: H-H = 436 kJ/mol, Cl-Cl = 242 kJ/mol, H-Cl = 431 kJ/mol.
ΔH = [1(H-H) + 1(Cl-Cl)] - [2(H-Cl)] = [436 + 242] - [2(431)] = 678 - 862 = -184 kJ/mol. Therefore, the reaction is exothermic.
Key Questions: Exothermic and endothermic reactions
What is an exothermic reaction?
An exothermic reaction transfers thermal energy to the surroundings, leading to an increase in the temperature of the surroundings.
What is an endothermic reaction?
An endothermic reaction takes in thermal energy from the surroundings, leading to a decrease in the temperature of the surroundings.
What is enthalpy change (ΔH)?
Enthalpy change (ΔH) is the transfer of thermal energy during a reaction. It's negative for exothermic reactions and positive for endothermic reactions.
What is the sign of ΔH for an exothermic reaction?
ΔH is negative for exothermic reactions because energy is released to the surroundings.
What is the sign of ΔH for an endothermic reaction?
ΔH is positive for endothermic reactions because energy is absorbed from the surroundings.
More topics in Unit 5 — Chemical energetics
Exothermic and endothermic reactions sits alongside these Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 0620 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Key terms covered in this Exothermic and endothermic reactions deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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