Energy in chemical reactions
Every chemical reaction involves energy. Breaking bonds absorbs energy, and forming bonds releases energy. Whether a reaction gives off or takes in heat depends on which is larger.
Enthalpy
Enthalpy (H) is the heat content of a system at constant pressure. Chemists measure the change, ΔH (delta H), in kilojoules (kJ).
| Exothermic | Endothermic | |
|---|---|---|
| Heat | Released to surroundings | Absorbed from surroundings |
| ΔH | Negative | Positive |
| Surroundings | Get warmer | Get colder |
| Bonds | Stronger bonds formed than broken | Weaker bonds formed than broken |
| Examples | Combustion, respiration, hand warmers, neutralization | Photosynthesis, cold packs, melting ice, cooking an egg |
Example (exothermic): CH₄ + 2O₂ → CO₂ + 2H₂O ΔH = −890 kJ/mol
Example (endothermic): 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ ΔH = +2803 kJ/mol
Energy diagrams
On a potential energy diagram:
- Exothermic: products are lower than reactants.
- Endothermic: products are higher than reactants.
- Both have a "hill" in between: the activation energy barrier.
Measuring heat: calorimetry
Specific heat capacity (c) is the energy needed to raise 1 g of a substance by 1 °C.
q = mcΔT
- q = heat (J)
- m = mass (g)
- c = specific heat capacity (J/g·°C)
- ΔT = temperature change (°C)
| Substance | c (J/g·°C) |
|---|---|
| Water | 4.18 |
| Aluminum | 0.90 |
| Iron | 0.45 |
| Copper | 0.39 |
Water’s high specific heat means it heats and cools slowly. That’s why coastal cities like Vancouver have milder climates than inland cities.
Example: How much heat raises 200 g of water from 20 °C to 80 °C?
q = 200 × 4.18 × 60 = 50 160 J ≈ 50.2 kJ
A calorimeter measures heat changes by tracking the temperature of surrounding water.
Hess’s law
The total enthalpy change of a reaction is the same no matter what path it takes. You can add equations (and their ΔH values) to find ΔH for a reaction that’s hard to measure directly.
Reaction rates
The reaction rate measures how fast reactants are used up or products form, for example in mol/L per second or mL of gas per minute.
Collision theory
For a reaction to happen, particles must:
- collide
- with enough energy (at least the activation energy)
- in the correct orientation
Factors that affect rate
| Factor | Effect | Why |
|---|---|---|
| Temperature | Higher → faster | Particles move faster, collide more often and with more energy. A 10 °C rise often roughly doubles the rate. |
| Concentration (or pressure for gases) | Higher → faster | More particles, more collisions |
| Surface area | Larger → faster | More particles exposed (powdered sugar burns faster than a sugar cube) |
| Catalyst | Present → faster | Provides a pathway with lower activation energy |
| Nature of reactants | Varies | Some bonds break more easily |
Catalysts
A catalyst speeds up a reaction without being used up.
- Enzymes are biological catalysts. Catalase breaks down hydrogen peroxide in your cells millions of times faster than it would on its own.
- Catalytic converters in cars use platinum, palladium and rhodium to change carbon monoxide and nitrogen oxides into less harmful gases.
- The Haber process uses an iron catalyst to make ammonia.
Everyday applications
- Refrigerators slow food spoilage by lowering the reaction rate.
- Instant cold packs use the endothermic dissolving of ammonium nitrate.
- Fine flour dust in mills can explode because of its huge surface area.
Common mistakes
- Thinking catalysts change ΔH. They lower activation energy only.
- Mixing up sign conventions. Exothermic ΔH is negative.
- Thinking heat and temperature are the same. Heat is energy transferred; temperature measures average particle energy.
Try it yourself
- How much energy is needed to heat 500 g of aluminum from 25 °C to 125 °C?
- Is dissolving ammonium nitrate (it gets cold) exothermic or endothermic? What is the sign of ΔH?
- Explain, using collision theory, why chewing food helps digestion.
- Sketch an energy diagram for an exothermic reaction with and without a catalyst.
(Answers: q = 500 × 0.90 × 100 = 45 000 J = 45 kJ; endothermic, positive; chewing increases surface area, so enzymes collide with more food particles)
Quick recap
- Exothermic reactions release heat (ΔH negative); endothermic reactions absorb it (ΔH positive).
- q = mcΔT calculates heat transferred.
- Collision theory: particles must collide with enough energy and the right orientation.
- Temperature, concentration, surface area and catalysts affect reaction rate.
- Catalysts lower activation energy without being used up.