Thermochemistry (intro)
The idea
Every chemical or physical change comes with an energy transaction. Reactions that release heat to the surroundings are exothermic (burning fuel, hand warmers); reactions that absorb heat are endothermic (cold packs, melting ice). Thermochemistry is the accounting system for these flows, and its basic measuring trick is simple: let the heat flow into or out of something whose warming you can track — usually water — and read the temperature change.
The central tool is q = mcΔT: heat transferred equals mass times specific heat capacity times temperature change. Specific heat c is the joules needed to raise 1 g of a substance by 1 °C, and water's value, 4.18 J/(g·°C), is unusually large — water is a heat sponge, which is why coastal climates are mild and why water makes a good coolant. A positive q means the substance absorbed energy; negative means it released energy.
Keep heat and temperature distinct — conflating them is the classic error. Temperature measures how vigorously particles are moving on average; heat is energy in transit between objects at different temperatures. A bathtub at 40 °C holds vastly more thermal energy than a teaspoon of water at 90 °C, despite the lower temperature, because energy scales with the amount of matter as well.
Worked example
A kettle heats 250.0 g of water from 22.0 °C to 98.0 °C. How much heat does the water absorb? The specific heat of water is 4.18 J/(g·°C).
- Find the temperature change first: ΔT = 98.0 − 22.0 = 76.0 °C. Only the change matters to the formula, not the individual readings.
- Set up q = mcΔT with everything in matching units: q = 250.0 g × 4.18 J/(g·°C) × 76.0 °C.
- Compute in stages to stay honest: 250.0 × 4.18 = 1045 J/°C — the energy to lift this particular mass of water by one degree — and then 1045 × 76.0 = 79,420 J.
- Convert to friendlier units and fix the sign: 79,420 J ≈ 79.4 kJ, and q is positive because the water absorbed energy from the kettle element (endothermic from the water's point of view).
- Sanity-check the scale: tens of kilojoules to nearly boil a large mug of water matches everyday experience — an electric kettle delivering about 2000 J each second would need roughly 40 seconds.
Answer. The water absorbs about 79,400 J, or 79.4 kJ, of heat.
Check your understanding
- How would you explain the difference between heat and temperature using two containers of water of very different sizes?
- Why does water's unusually high specific heat matter for climates, oceans, and engine coolants?
- In a calorimetry experiment, why does measuring the water's q tell you the reaction's q, and why do the signs flip?
- What happens to the heat required if you double the mass of water but halve the temperature change, and why?
Build the foundations first
Thermochemistry (intro) builds on these concepts. If any feel shaky, start there.