Thermochemistry

Introduction

Thermochemistry is the study of the energy and heat associated with chemical reactions and physical transformations. 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. In chemistry, energy is crucial for driving reactions and changes in state.

• Units of Energy: Common units include calories (cal), kilocalories (kcal), joules (J), and kilojoules (kJ).

• Conversion Factor:

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

• Kinetic Energy: Energy of motion, proportional to mass and the square of velocity.

Law of Conservation of Energy

• Law of Conservation of Energy: Energy cannot be created or destroyed, only converted from one form to another.

Energy Unit Conversion Factors

The following table summarizes common energy unit conversions:

Energy Transfer

System and Surroundings

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

• Surroundings: Everything outside the system.

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

• Example (Ice Melting): The ice (system) absorbs energy (gets warmer), surroundings lose energy (get cooler).

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

Heat of Reaction

Definition of Heat (q)

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

Exothermic and Endothermic Reactions

• Exothermic Reaction: Releases heat to the surroundings (). The reaction vessel warms up.

• Endothermic Reaction: Absorbs heat from the surroundings (). The reaction vessel cools down.

Examples:

•    (exothermic)

•    (endothermic)

Enthalpy (ΔH)

Definition and Properties

• Enthalpy (ΔH): The heat content of a system at constant pressure. For chemical reactions, is the heat of reaction at constant pressure.

• Mathematically: (at constant pressure)

Manipulating Enthalpy Changes

• If a reaction is multiplied by a factor, is multiplied by the same factor.

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

Examples:

•   

• Double the equation:   

• Reverse the equation:   

Stoichiometry of ΔH

• Enthalpy changes are typically reported per mole of reaction as written.

• To calculate the heat evolved or absorbed for a given amount of reactant, use stoichiometry:

Example:

• Given:   

• Calculate heat for 7.33 g CH4:

Calorimetry

Measuring Heat Changes

• Calorimetry: The measurement of heat flow in a chemical or physical process.

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

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

Calorimetry Equations

• When using specific heat:

• When using heat capacity:

• Where:

  • = heat (J or kJ)

  • = specific heat (J/g·°C)

  • = mass (g)

  • = heat capacity (J/°C or kJ/°C)

  • (°C)

Example: Calculate the heat needed to raise 19.0 g of H2O from 11°C to 88°C. (Specific heat of water = 4.184 J/g·°C)

Hess's Law

Definition and Application

• Hess's Law: The enthalpy change for a reaction is the same, regardless of the number of steps or pathway taken, as long as the initial and final conditions are the same.

• Allows calculation of for a reaction by combining known enthalpy changes of other reactions.

Example:

• Given reactions:

  •   

  •   

• Target reaction:

• Combine and manipulate the given reactions to obtain the target reaction and sum their values accordingly.

Standard Enthalpies of Formation (ΔH°f)

Definition

• Standard Enthalpy of Formation (): The enthalpy change when one mole of a compound is formed from its elements in their standard states at 1 atm and 25°C.

• By convention, for all elements in their most stable form (including H2, O2, N2, etc.) is zero.

Calculating Reaction Enthalpy from Formation Enthalpies

• The standard enthalpy change for a reaction can be calculated using:

Example:

• Given:

• Given values (kJ/mol): CH4(g) = -74.9, Cl2(g) = 0, CCl4(l) = -139, HCl(g) = -92.3

• Calculation:

Important Notes

• Always check the physical states (solid, liquid, gas, aqueous) when using values.

• Use tabulated values for standard enthalpies of formation, typically found in appendices of chemistry textbooks.

Additional info: Some values and examples were inferred or expanded for clarity and completeness based on standard General Chemistry curriculum.