Thermochemistry: Heat in Reactions
Why some reactions burn hot while others get cold. Endothermic vs exothermic, ΔH sign conventions, specific heat calculations, calorimetry, and phase-change energies.
Energy and chemistry are inseparable
Every chemical reaction involves an energy change. Burning gasoline releases tremendous heat. Dissolving ammonium nitrate cools your hand (basis of instant cold packs). Photosynthesis stores sunlight in chemical bonds.
Thermochemistry studies these energy flows in reactions and physical changes.
Endothermic vs exothermic
- Exothermic — system RELEASES heat to surroundings. Surroundings warm up. ΔH is NEGATIVE.
- Endothermic — system ABSORBS heat from surroundings. Surroundings cool down. ΔH is POSITIVE.
The sign convention is from the system's perspective. When the system loses energy (gives heat away), its ΔH is negative.
Memory aid: EXothermic = EXits (heat exits the system).
Examples:
- Exothermic: combustion (burning), neutralization, condensing, freezing
- Endothermic: melting, evaporating, photosynthesis, dissolving NH₄NO₃
Specific heat — how much energy per gram per degree
Specific heat (c) is the energy required to raise the temperature of 1 gram of a substance by 1 °C. The formula:
q = m × c × ΔT
where q = heat (J), m = mass (g), c = specific heat (J/(g·°C)), ΔT = temperature change (°C).
Approximate specific heats (J/(g·°C)):
- Water — 4.18 (very high; moderates climate)
- Ice — 2.09
- Ethanol — 2.44
- Aluminum — 0.897
- Iron — 0.449
- Copper — 0.385
Metals have LOW specific heats — they warm up quickly. Water's HIGH specific heat is why oceans buffer climate and why pots take a while to boil.
Worked example
How much heat is needed to warm 100 g of water from 20 °C to 80 °C?
q = m × c × ΔT = 100 × 4.18 × (80 − 20) = 100 × 4.18 × 60 = 25,080 J (or ~25 kJ).
Sign check: heat is ABSORBED (water warms), so q is positive.
Calorimetry — measuring heat experimentally
A calorimeter is an insulated container where you can measure heat exchanged between two substances. In a typical experiment, a hot metal is dropped into water, and heat flows from metal to water until they reach the same temperature.
The KEY principle: in an insulated system, heat lost by the hot object = heat gained by the cold object.
−q_metal = q_water or m_metal × c_metal × ΔT_metal = −m_water × c_water × ΔT_water
This lets you find the specific heat of an unknown metal by knowing only masses and temperatures.
Phase changes — energy without temperature change
During a phase change (melting, vaporizing, condensing, freezing), the temperature stays CONSTANT even as heat is added or removed. All the energy goes into breaking or forming intermolecular forces, not into kinetic energy.
Two key quantities:
- Heat of fusion (H_f) — energy to melt 1 g of solid at melting point. Water: 334 J/g.
- Heat of vaporization (H_v) — energy to vaporize 1 g of liquid at boiling point. Water: 2,260 J/g.
Vaporization requires MUCH more energy than melting because all intermolecular forces must be broken (vs partial loosening during melting). This is why steam burns are far worse than boiling-water burns — when steam condenses on skin, it releases 2,260 J/g BEFORE the resulting water even begins to cool.
Formula for phase change heat:
q = m × H_f (or m × H_v)
Reaction energy diagrams
An energy diagram plots energy on the y-axis vs reaction progress on the x-axis. Standard features:
- Reactants level — energy of starting materials.
- Activation energy (Ea) — the energy "hump" that must be climbed for the reaction to proceed. Measured from reactants up to the peak.
- Transition state — the peak of the energy diagram, an unstable intermediate.
- Products level — energy of products. If lower than reactants → exothermic. Higher → endothermic.
- ΔH — energy difference between products and reactants.
A catalyst LOWERS the activation energy (provides an alternative pathway), making the reaction faster but NOT changing ΔH.
Heat always flows hot → cold
This is the second law of thermodynamics in everyday form. Heat spontaneously flows from a HOTTER region to a COOLER one until both reach the same temperature (thermal equilibrium). To pump heat the other way (refrigerator), you must do WORK.