Thermochemistry

Introduction

Thermochemistry is the study of energy changes, particularly heat, that occur during chemical reactions and changes of state. It is a fundamental topic in general chemistry, providing insight into how energy is transferred and conserved in chemical processes.

Energy in Chemistry

Definition and Types of Energy

• Energy: The potential or capacity to move matter. It exists in various forms and is essential for all physical and chemical processes.

• Units of Energy:

  • calorie (cal)

  • kilocalorie (kcal)

  • joule (J)

  • kilojoule (kJ)

• Conversion Factor:

• Law of Conservation of Energy: Energy is neither created nor destroyed, but can be converted from one form to another.

• Potential Energy: Stored energy due to position or composition.

• Kinetic Energy: Energy of motion.

Energy Unit Conversion Factors

Energy can be measured in different units. The following table summarizes common conversion factors:

Energy Transfer

System and Surroundings

• System: The part of the universe being studied (e.g., the contents of a reaction vessel).

• Surroundings: Everything outside the system.

• Energy transfers between the system and surroundings until thermal equilibrium is reached (no temperature difference).

Examples

• Ice cube melting: The ice (system) absorbs energy (gets warmer), and the surroundings lose energy (get cooler).

• Water freezing: The water (system) loses energy (gets cooler), and the surroundings (e.g., freezer) absorb energy (get warmer).

Heat of Reaction

Definition of Heat ()

• Heat (): Energy that flows into or out of a system due to a temperature difference between the system and surroundings.

Exothermic and Endothermic Reactions

• Exothermic Reaction: Releases heat to the surroundings. The reaction vessel warms. Example: kJ (heat given off)

• Endothermic Reaction: Absorbs heat from the surroundings. The reaction vessel cools. Example: kJ (heat taken up)

Enthalpy ()

Definition and Properties

• Enthalpy (): The heat of a reaction at constant pressure.

• Meaning of Heats of Reaction:

  • If the reaction equation is doubled, is doubled.

  • If the reaction equation is reversed, the sign of is reversed.

Examples

• kJ (heat given off)

• kJ (double equation, double )

• kJ (reverse equation, reverse sign)

Stoichiometry of

• Enthalpy change is given per mole of reaction as written.

• Example: kJ

• To calculate heat evolved for a given mass:

  • Convert mass to moles.

  • Multiply by per mole.

Example Calculation

• Calculate the heat evolved when 7.33 g of burns:

Calorimetry

Measuring Heat Changes

• Calorimetry: The measurement of heat changes in chemical reactions.

• Specific Heat (): The amount of heat required to raise the temperature of 1 g of a substance by 1°C at constant pressure.

• Heat Capacity (): The amount of heat required to raise the temperature of an object by 1°C.

Formulas

• For specific heat: where:

  • = heat (J)

  • = specific heat (J/g°C)

  • = mass (g)

  • (°C)

• For heat capacity: where:

  • = heat (kJ)

  • = heat capacity (kJ/°C)

  • (°C)

When to Use Each Formula

• Use if the problem mentions specific heat or specific heat capacity.

• Use if the problem mentions heat capacity of the calorimeter.

Example Calculations

• Calculate the heat needed to raise 19.0 g of from 11°C to 88°C (specific heat of water = 4.184 J/g°C):

• Calculate the heat needed to raise 104 g of Cu from 17°C to 34°C (specific heat of copper = 0.384 J/g°C):

Summary Table: Energy Conversion Factors

Key Concepts

• Energy is conserved in all chemical processes.

• Heat and enthalpy are central to understanding energy changes in reactions.

• Calorimetry allows quantitative measurement of heat changes.

• Unit conversions are essential for comparing energy values.

Additional info: These notes cover the foundational concepts of thermochemistry, including energy types, unit conversions, system/surroundings, heat of reaction, enthalpy, and calorimetry, as presented in a typical General Chemistry curriculum.