Endothermic reactions are central to thermochemistry in IB Chemistry Topic 5. Understanding them helps you analyze energy changes, interpret enthalpy diagrams, and predict how temperature affects chemical processes. Students often confuse endothermic and exothermic reactions, but once you understand the energy flow, the differences become clear and intuitive.
What Is an Endothermic Reaction?
An endothermic reaction is a chemical reaction that absorbs heat from its surroundings.
Because energy flows into the system:
- Products have higher enthalpy than reactants
- Temperature of the surroundings decreases
- The enthalpy change (ΔH) is positive
In simple terms:
Endothermic reactions take in heat.
How Endothermic Reactions Work
Chemical bonds must be broken before new bonds can form.
Breaking bonds always requires energy, while forming bonds releases energy.
A reaction is endothermic when:
- Energy required to break bonds > energy released when new bonds form
This energy deficit is absorbed from the surroundings, making the environment cooler.
Examples of Endothermic Reactions
1. Thermal decomposition
Many compounds break down only when heat is supplied.
Example: calcium carbonate decomposing into calcium oxide and carbon dioxide.
2. Dissolving certain ionic compounds
Some salts—like ammonium nitrate—absorb heat when they dissolve.
3. Photosynthesis
Plants absorb light energy to convert CO₂ and H₂O into glucose.
4. Evaporation and boiling
These physical processes require energy input.
5. Endothermic reactions in cold packs
Instant cold packs rely on an endothermic dissolution process.
All these examples involve a net intake of energy.
Enthalpy Profile Diagram for an Endothermic Reaction
An enthalpy profile diagram makes energy flow easy to visualize.
Key features:
- Products are higher in enthalpy than reactants
- Energy arrow points upward
- ΔH is positive
This diagram shows that energy must be added in order for the reaction to proceed.
Endothermic vs Exothermic Reactions
Endothermic:
- Absorb heat
- Surroundings cool down
- ΔH is positive
- Products higher in energy
Exothermic:
- Release heat
- Surroundings warm up
- ΔH is negative
- Products lower in energy
A good way to remember:
Endo = energy in.
Factors Affecting Endothermic Reactions
1. Temperature
Higher temperatures favor endothermic reactions because heat acts like a reactant.
Le Chatelier’s Principle predicts that adding heat pushes equilibrium toward products.
2. Reaction rate
Endothermic reactions often proceed more slowly because energy input is needed to break bonds.
3. Nature of reactants
Strong, stable bonds require more energy to break, increasing endothermic behavior.
Why Endothermic Reactions Feel Cold
When an endothermic reaction occurs:
- The system absorbs heat
- The surroundings lose heat
- You feel the temperature drop
This is why cold packs get cold instantly—energy is drawn from your skin.
Real-World Applications
Endothermic processes appear in:
- Cold packs for injuries
- Industrial absorption reactions
- Refrigeration cycles
- Metal extraction
- Thermal decomposition manufacturing
Understanding these reactions helps explain many practical technologies.
Common IB Misunderstandings
“Endothermic reactions always increase temperature.”
False—they lower the temperature of surroundings.
“Endothermic means non-spontaneous.”
Not necessarily. Spontaneity depends on Gibbs free energy (ΔG), not ΔH alone.
“All decomposition reactions are endothermic.”
Most are, but not always—some can be exothermic.
“Heat is released during endothermic reactions.”
Opposite—heat is absorbed.
FAQs
Why is ΔH positive for endothermic reactions?
Because the system gains energy, increasing enthalpy.
Can an endothermic reaction become spontaneous?
Yes—if entropy (ΔS) increases enough to make ΔG negative.
How can I tell if a reaction is endothermic in the lab?
Measure temperature change; a decrease indicates heat absorption.
Conclusion
Endothermic reactions absorb heat from their surroundings, resulting in a positive enthalpy change and cooler temperatures. They occur when more energy is required to break bonds than is released when new bonds form. Understanding endothermic reactions helps students explain thermal behavior, interpret energy diagrams, and connect thermochemistry to real-world processes.
