Chemistry: Atoms to AP Chemistry · Module 8 of 15
Thermochemistry & Reaction Rates
How energy moves during reactions, and what speeds a reaction up or slows it down.
Energy in Reactions
- Exothermic reactions release energy (usually as heat) to the surroundings, often feeling warm; endothermic reactions absorb energy from the surroundings, often feeling cold.
- Energy changes are often described using enthalpy (ΔH): negative ΔH means exothermic, positive ΔH means endothermic.
A chemical hand warmer gets hot when activated. Is this reaction exothermic or endothermic?
Solution. Exothermic — it releases heat energy to its surroundings (your hands).
Calorimetry & Specific Heat
- Specific heat is the energy needed to raise 1 gram of a substance by 1°C; water has an unusually high specific heat (4.18 J/g°C).
- q = mcΔT calculates heat energy (q) from mass (m), specific heat (c), and temperature change (ΔT).
How much heat is needed to raise the temperature of 100 g of water from 20°C to 40°C (c = 4.18 J/g°C)?
Solution. q = mcΔT = (100)(4.18)(40 − 20) = (100)(4.18)(20) = 8360 J.
Reaction Rates & Factors Affecting Them
- Reaction rate measures how fast reactants turn into products; higher temperature, higher concentration, more surface area, and a catalyst all increase rate.
- Higher temperature increases rate because particles move faster and collide more often and with more energy.
Why does food spoil more slowly in a refrigerator than at room temperature?
Solution. Lower temperature slows down the reaction rates of the chemical processes — from bacteria and enzymes — that cause spoilage.
Catalysts & Activation Energy
- Activation energy is the minimum energy needed for a reaction to occur — the "energy hill" reactants must climb.
- A catalyst speeds up a reaction by providing an alternative pathway with lower activation energy, without being consumed or permanently changed itself.
Enzymes are biological catalysts. How do they speed up reactions in the body?
Solution. Enzymes lower the activation energy needed for a reaction, letting it proceed faster at normal body temperature, without being used up themselves.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What type of reaction releases energy to its surroundings?
2. Using q = mcΔT, how much heat is released when 50 g of water cools from 30°C to 10°C (c = 4.18 J/g°C)?