Energy Changes in Chemical Reactions
Understanding how energy is absorbed or released during chemical processes
Energy Changes are fundamental to all chemical reactions. Every reaction either releases energy to the surroundings (exothermic) or absorbs energy from the surroundings (endothermic).
1. Exothermic Reactions
Definition
Exothermic reactions are chemical processes that release energy to the surroundings, typically in the form of heat, light, or sound.
Observation
The temperature of the surroundings increases. A test tube containing an exothermic reaction feels hot to the touch.
Bonding Perspective
More energy released when new bonds form than is required to break existing bonds.
Energy released > Energy absorbed
Net: Energy released to surroundings
Examples
Combustion
The burning of wood or fossil fuels releases heat and light.
Magnesium + Hydrochloric Acid
This reaction produces hydrogen gas and releases heat.
Cellular Respiration
Cells convert glucose and oxygen into energy, releasing CO₂ and H₂O.
3. Activation Energy
Concept: Activation energy is the minimum energy required to initiate a chemical reaction. Even exothermic reactions need a small energy input to start breaking initial bonds.
Everyday Example: Lighting a Match
Friction provides the activation energy to start the combustion reaction. Once started, the reaction releases enough energy to sustain itself.
Why Activation Energy Matters
Without activation energy, many reactions would not occur at room temperature, even if they release energy overall.
Activation Energy Simulator
Drag the slider to see how activation energy affects reaction start:
⚡ Energy supplied: 30 kJ/mol
🔋 Activation energy needed: 50 kJ/mol
4. Energy Level Diagrams
Energy level diagrams visually represent the energy changes during chemical reactions.
Exothermic Reaction Energy Diagram
Reactants at higher energy level than products
ΔH = negative (energy released)
Key Point: The difference in height between reactants and products shows whether energy is released (downhill) or absorbed (uphill).
Test Your Understanding
1. In an exothermic reaction, the temperature of the surroundings:
2. Which process is endothermic?
3. Activation energy is:
Key Terms
Key Takeaways
- Exothermic reactions release energy to surroundings (temperature increases)
- Endothermic reactions absorb energy from surroundings (temperature decreases)
- Energy released/absorbed depends on bond breaking vs bond forming
- Activation energy is the minimum energy needed to start a reaction
- Energy level diagrams show reactants at higher (exothermic) or lower (endothermic) energy than products
- Examples: Combustion (exo), Photosynthesis (endo), Cold packs (endo)
