Understanding Heat Transfer and Capacity Concepts

heat (q)

Energy that flows from hot to cold when there's a temperature difference between a system and its surroundings.

thermal equilibrium

When the system and surroundings are at the same temperature.

molecular heat flow

Fast-moving (hot) molecules transfer energy to slower (cold) ones through collisions.

heating a pure substance

Either the temperature increases (kinetic energy ↑) or a phase change occurs (potential energy ↑).

diathermic boundary

A boundary that allows heat to pass between system and surroundings (e.g. metal).

adiabatic boundary

A boundary that does not allow heat to pass (e.g. thermos).

q > 0

Heat is absorbed by the system (endothermic process).

q < 0

Heat is released by the system (exothermic process).

flat section on a heating curve

A phase change, where temperature stays the same and potential energy increases.

rising section on a heating curve

A temperature increase where kinetic energy increases.

q_vap > q_fus

It takes more energy to turn a liquid into gas than to melt a solid.

formula for heat absorbed in warming

q = C ΔT or q = mcΔT or q = nC̄ΔT (for pure substances).

specific heat capacity (c)

The heat required to raise 1 g of a substance by 1 K (J/g·K).

molar heat capacity (C̄)

The heat required to raise 1 mol of a substance by 1 K (J/mol·K).

heat capacity

The amount of heat needed to increase temperature by 1°C or 1 K.

average heat capacity formula

C = q / ΔT.

q_fus

The amount of heat needed to melt a substance at constant temperature.

q_vap

The amount of heat needed to boil a substance at constant temperature.

factors affecting heat capacity

Its structure — more complex molecules absorb more energy and have higher heat capacity.

C_P vs C_V

C_P is heat capacity at constant pressure; C_V is at constant volume.

units for heat capacity

Usually J/K for objects, J/g·K for specific heat, and J/mol·K for molar heat.