6.4 Heat Capacity and Calorimetry

Heat capacity basics

Heat capacity is the amount of heat needed to raise a substance's temperature by 1°C. Specific heat capacity (c) is per gram; molar heat capacity is per mole. The key equation is q = mcΔT, where q is heat transferred, m is mass, c is specific heat, and ΔT is temperature change. Substances with high c (like water) resist temperature change.

Calorimetry

Calorimetry measures heat flow using a calorimeter, an insulated device that minimizes heat loss to surroundings. In a simple calorimetry experiment, heat lost by one substance equals heat gained by another: q_lost = -q_gained. This assumes no heat escapes the system, letting us calculate unknown specific heats or final temperatures.

Applying q = mcΔT

When two substances at different temperatures mix, they reach a common final temperature where energy is conserved. Set q1 = -q2 and solve using each substance's mass and specific heat. ΔT is always final minus initial; a positive q means heat was absorbed (endothermic for that substance), while negative q means heat was released.

Key terms

Heat capacity

The amount of heat required to raise a substance's temperature by 1°C.

Specific heat capacity (c)

Heat required to raise 1 gram of a substance by 1°C; units are J/(g·°C).

q = mcΔT

Equation for heat transfer: q = mass × specific heat × temperature change (final − initial).

Calorimeter

An insulated device used to measure heat flow during physical or chemical processes.

Conservation of heat in calorimetry

In an isolated system, heat lost by one substance equals heat gained by another: q_lost = -q_gained.

Try an AP-style question

How much heat raises 100. g of water by 5.00 °C? (c = 4.18 J/g·°C)

1. 2,090 J
2. 20.9 J
3. 500 J
4. 418 J