Knowledge Organiser: Endothermic Reactions
Key Terms
- Endothermic — absorbs energy from surroundings
- ΔH — enthalpy (energy change) — positive for endothermic
- Thermal decomposition — breakdown by heat
- Photosynthesis — key endothermic example
- Reversible reaction — reverse has opposite ΔH sign
Must-Know Facts
- Temperature of surroundings DECREASES
- Products have MORE energy than reactants
- ΔH is POSITIVE
- Products are HIGHER on energy profile
- Examples: photosynthesis, thermal decomposition
- Energy needed breaking bonds > energy released making bonds
Key Equations
- ΔH = energy required breaking bonds − energy released making bonds
- Endothermic: ΔH is positive (energy absorbed from surroundings)
- Reversible reaction: if forward is endothermic, reverse is exothermic by same ΔH
Common Mistakes
- Saying endothermic reactions release heat: Endothermic reactions ABSORB energy from surroundings — the temperature of the surroundings decreases
- Getting products position wrong on energy profile: For endothermic, products are HIGHER than reactants on the y-axis — draw the profile arrow going UP at the end
- Forgetting cooling packs are endothermic: Endothermic reactions feel cold — hand warmers are exothermic; cooling packs (sports injuries) are endothermic
- Saying reversible reactions have no energy change: The reverse reaction has the OPPOSITE energy change — if forward is endothermic (+x kJ), reverse is exothermic (−x kJ)
Memory Aid
- ENDO = IN = energy goes IN from surroundings = temperature goes DOWN
- The "endo" prefix means "inside" or "within". Energy enters the chemical system from the surroundings, cooling them down. Cold packs, photosynthesis, and dissolving ammonium nitrate all pull heat IN.
- ΔH sign rule: ENDO = ENTERS = positive — positive energy change means energy is gained by the system
- Endothermic → ΔH is POSITIVE (the system gains energy, so the change is positive)
- On the energy profile: "ENDothermic ENDS higher — the products are higher up on the diagram than the reactants."
- Exo vs Endo summary: "EXO = EXit (out, hot), ENDO = ENter (in, cold)"