Heat

The One Thing

Calorimetry applies conservation of energy to heat exchange: in an isolated system, heat lost by a hotter body equals heat gained by a colder body until thermal equilibrium is reached. Latent heat concerns energy absorbed or released during a change of state without a change in temperature.

Reactions, Processes and Experiments

Key Terms

• Heat: Energy transferred from a body at higher temperature to a body at lower temperature because of the temperature difference.
• Calorimetry: The measurement of the quantity of heat exchanged during heating, cooling, or a change of state.
• Thermal equilibrium: The condition in which two bodies reach the same temperature and there is no further net transfer of heat.
• Specific heat capacity: The heat required to raise the temperature of 1 kilogram of a substance by 1 degree Celsius or 1 kelvin.
• Heat capacity: The heat required to raise the temperature of an entire body by 1 degree Celsius or 1 kelvin.
• Calorimeter: An insulated container used to measure heat exchanged in a calorimetry experiment.
• Water equivalent: The mass of water that has the same heat capacity as a calorimeter and its contents.
• Principle of calorimetry: In an isolated system, heat lost by the hotter body equals heat gained by the colder body.
• Latent heat: The heat absorbed or released during a change of state without a change in temperature.
• Specific latent heat: The heat required to change the state of 1 kilogram of a substance at constant temperature.
• Specific latent heat of fusion: The heat required to convert 1 kilogram of a solid into liquid at its melting point without changing temperature.
• Specific latent heat of vaporisation: The heat required to convert 1 kilogram of a liquid into vapour at its boiling point without changing temperature.
• Melting: The change of state from solid to liquid at a fixed temperature called the melting point.
• Freezing: The change of state from liquid to solid, releasing latent heat.
• Boiling: The rapid change of a liquid into vapour throughout the liquid at its boiling point.
• Condensation: The change of state from vapour to liquid, releasing latent heat.
• Cooling curve: A graph showing how the temperature of a substance changes as it loses heat; a horizontal section indicates a change of state.

Easily Confused

• Heat and temperature: Heat is energy transferred because of a temperature difference; temperature indicates the thermal state of a body.
• Specific heat capacity and heat capacity: Specific heat capacity refers to 1 kilogram of a substance, whereas heat capacity refers to the entire body.
• Heat capacity and water equivalent: Heat capacity is the heat required to raise an entire body by 1 °C or 1 K; water equivalent is the mass of water with the same heat capacity as a calorimeter and its contents.
• Specific latent heat and specific heat capacity: Specific latent heat concerns a change of state at constant temperature; specific heat capacity concerns a temperature change without a change of state.
• Melting and boiling: Melting changes a solid into a liquid at the melting point; boiling changes a liquid into vapour throughout the liquid at the boiling point.
• Melting and freezing: Melting absorbs latent heat; freezing releases latent heat.
• Boiling and condensation: Boiling absorbs latent heat; condensation releases latent heat.
• Temperature change in Celsius and kelvin: Temperature differences in degrees Celsius and kelvin have the same numerical value: a change of 1 °C equals a change of 1 K.
• Heat lost and heat gained: In calorimetry, heat lost by the hotter body equals heat gained by the colder body only when heat exchange with the surroundings is negligible or properly included.

What Gets Asked

• Calculations involving temperature change: Use $Q=mc\Delta T$, identifying $Q$ in joules, $m$ in kilograms, $c$ as specific heat capacity, and $\Delta T$ as the temperature change. A common mark-losing error is confusing specific heat capacity with heat capacity.
• Calculations involving the heat capacity of a body: Use $C=Q/\Delta T$ or $C=mc$. The distinction between the entire body and 1 kilogram of substance must be stated correctly.
• Mixing problems: Apply $m1c1(T1-Tf)=m2c2(Tf-T2)$ when no heat is lost. The final temperature, $Tf$, must be used consistently for both bodies.
• Calorimeter corrections: Include the calorimeter’s heat gain when the calorimeter absorbs heat. Omitting this term gives an incomplete heat-balance equation.
• Latent-heat calculations: Use $Q=mL$ or $L=Q/m$. The defining feature is that the temperature remains constant during the change of state.
• Cooling curves and state changes: Identify a horizontal section as a change of state, and distinguish melting, freezing, boiling, and condensation according to the direction of the state change and whether latent heat is absorbed or released.