Reactivity 1.1.2
Exothermic & Endothermic Reactions
Classifying reactions based on the direction of heat flow.
Exothermic
Heat exits the system ()
- System: Loses chemical potential energy.
- Surroundings: Gain heat energy.
- Observation: Temperature RISES.
- Bonds: Making bonds (Exothermic).
Products are more stable than Reactants
Examples:
Combustion, Neutralization, Respiration.
Endothermic
Heat enters the system ()
- System: Gains chemical potential energy.
- Surroundings: Lose heat energy.
- Observation: Temperature FALLS.
- Bonds: Breaking bonds (Endothermic).
Reactants are more stable than Products
Examples:
Thermal Decomposition, Photosynthesis.
Deep Think Concept
Examiner's Warning: Stability
"Lower Energy = Higher Stability"
Think of a ball on a hill.
- High Potential Energy = Unstable (wants to roll down).
- Low Potential Energy = Stable (at the bottom, won't move).
In an Exothermic reaction, the chemicals lose energy and fall down the energy hill. Therefore, the Products are more stable than the reactant.
Putting it into Practice
Classifying Reactions
Paper 2 StyleClassify the following changes as Exothermic or Endothermic:
- Photosynthesis in a plant leaf.
- Magnesium burning in oxygen with a bright white light.
- Water freezing into ice.
- 1. Endothermic. Photosynthesis requires energy (sunlight) to occur. .
- 2. Exothermic. Combustion reactions release massive amounts of energy (light/heat). .
- 3. Exothermic. Freezing is proper bond formation (intermolecular forces). To go from a chaotic liquid to an ordered solid, particles must lose/release energy to settle down.
Practice: Bond Enthalpy Logic
Explain, in terms of bond breaking and bond making, why the combustion of methane is exothermic.