Understanding Heat Transfer and Capacity Concepts

What is heat (q)?

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

When does heat stop flowing?

When the system and surroundings are at the same temperature (thermal equilibrium).

How does heat flow at the molecular level?

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

What happens when a pure substance is heated?

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

What is a diathermic boundary?

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

What is an adiabatic boundary?

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

What does q > 0 mean?

Heat is absorbed by the system (endothermic process).

What does q < 0 mean?

Heat is released by the system (exothermic process).

What does a flat section on a heating curve represent?

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

What does a rising section on a heating curve represent?

A temperature increase where kinetic energy increases.

Why is q_vap > q_fus?

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

What is the formula for heat absorbed in warming?

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

What is specific heat capacity (c)?

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

What is molar heat capacity (C̄)?

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

What is heat capacity?

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

What is the formula for average heat capacity?

C = q / ΔT.

What is q_fus?

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

What is q_vap?

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

What affects heat capacity of a substance?

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

What's the difference between C_P and C_V?

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

What are the units for heat capacity?

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