UPCAT CHEM - THERMOCHEMISTRY

Specific Heat Capacity (C)

The amount of heat energy needed to raise the temperature of 1 gram of a substance by 1°C. High C means stubborn to change; low C means sensitive.

Heat of Fusion (ΔHfus)

The heat required to completely change 1 gram of a substance from solid to liquid (or vice versa) at a constant temperature.

Heat of Vaporization (ΔHvap)

The heat required to completely change 1 gram of a substance from liquid to gas (or vice versa) at a constant temperature.

Formula for Temperature Changes

Q = mCΔT (Used when a substance gets hotter or colder but stays in the exact same phase).

Formula for Phase Changes

Q = mΔH (Used when a substance is actively melting, freezing, boiling, or condensing at a constant temperature).

When to use Q = mΔH

Use this when the problem mentions a phase change (melting, boiling) and the temperature stays CONSTANT.

When to use Q = mCΔT

Use this when the problem explicitly gives TWO temperatures (initial and final) and the substance stays in the same phase.

Plateau on a Heating Curve

Represents a phase change where temperature stays constant and Q = mΔH is used.

Slope on a Heating Curve

Represents a temperature change within a single phase where Q = mCΔT is used.

Unit distinction for Phase Change constant (ΔH)

Given in J/g or cal/g (no temperature unit because temperature does not change).

Unit distinction for Specific Heat (C)

Given in J/g°C or cal/g°C (includes a temperature unit because temperature changes).

Open System

A system that can freely exchange BOTH energy (heat) and matter (mass) with its surroundings.

Closed System

A system that can exchange energy (heat) with its surroundings, but NOT matter (e.g., a tightly sealed uninsulated flask).

Isolated System

A system that cannot exchange either energy or matter with its surroundings (e.g., a perfectly insulated thermos or calorimeter).

Sign of Q for Endothermic Processes

Positive (+Q) because the system is absorbing heat from the surroundings to loosen or break bonds (e.g., melting, boiling).

Sign of Q for Exothermic Processes

Negative (-Q) because the system is releasing or exuding heat into the surroundings to form stronger bonds (e.g., freezing, condensing).

Why ΔHvap is larger than ΔHfus

Vaporization requires completely breaking all intermolecular forces to launch liquid molecules into a gas state, while melting only requires loosening them.

Specific Heat Capacity of Liquid Water

4.184 J/g°C or 1.00 cal/g°C (UPCAT Shortcut: If calories are used, C = 1, making manual math super fast).