Thermochemistry

Introduction to Thermochemistry

Thermochemistry is the branch of chemistry that studies the energy changes, particularly heat, that occur during chemical reactions. Understanding these energy changes is essential for predicting reaction behavior and for applications in fields such as biochemistry, engineering, and environmental science.

• Energy: The capacity to do work or to transfer heat.

• Work: The energy used to cause an object to move against a force.

• Heat (q): The energy used to cause the temperature of an object to rise.

Energy and Its Units

Definitions and Units

Energy is a fundamental concept in thermochemistry, and it is measured in specific units.

• Joule (J): The SI unit of energy. Defined as .

• Calorie (cal): A non-SI unit of energy, commonly used in chemistry and biology. .

Example: If a reaction releases 100 cal of energy, this is equivalent to J.

Law of Conservation of Energy

First Law of Thermodynamics

The First Law of Thermodynamics states that energy cannot be created or destroyed in a chemical reaction, but it can be converted from one form to another. The total energy of the universe remains constant.

• Mathematical Expression:

• In a chemical reaction: Energy lost by the system = Energy gained by the surroundings

Heat and Temperature Changes

Heat Transfer and Temperature

Heat (q) is the form of energy transferred between objects due to a temperature difference. When two objects at different temperatures come into contact, heat flows from the hotter to the cooler object until thermal equilibrium is reached.

• The amount of heat required to change the temperature of a substance depends on:

  • Magnitude of temperature change ()

  • Quantity of substance (mass or moles)

  • Chemical nature of the substance

Heat Capacity

Definitions and Calculations

Heat capacity is a measure of the amount of heat required to change the temperature of a substance by a certain amount.

• Heat Capacity (C): Amount of energy required to raise the temperature of a substance by 1°C. Units: J/°C.

• Specific Heat Capacity (s): Amount of energy required to raise the temperature of 1 gram of a substance by 1°C. Units: J/g·°C.

• Molar Heat Capacity (MHC): Amount of energy required to raise the temperature of 1 mole of a substance by 1°C. Units: J/mol·°C.

Key Equations:

• For a given mass:

• For a given number of moles:

• General:

Example: Calculating the specific heat of lead using heat transfer between lead and water.

Types of Chemical Reactions by Energy Change

Exothermic and Endothermic Reactions

Chemical reactions can be classified based on whether they release or absorb energy.

• Exothermic Reaction: Releases energy to the surroundings. is negative. Example: Combustion reactions.

• Endothermic Reaction: Absorbs energy from the surroundings. is positive. Example: Photosynthesis.

General Equations:

• Exothermic: Reactants Products + Energy

• Endothermic: Reactants + Energy Products

Calorimetry

Measuring Heats of Reaction

Calorimetry is the technique used to measure the heat change in chemical reactions. A calorimeter is the apparatus used for this purpose.

• Heat lost by the reaction = Heat gained by the calorimeter and water.

• Key equations:

Bomb Calorimeter: A specialized calorimeter for measuring the heat of combustion reactions at constant volume.

Enthalpy and Heats of Reaction

Internal Energy and Enthalpy

Internal Energy (U): The total energy contained within a system, including chemical bonds, intermolecular forces, and kinetic energy.

Enthalpy (H): The heat absorbed or released in a chemical reaction at constant pressure. The change in enthalpy () is a key quantity in thermochemistry.

Examples:

• Phase changes: ()

• Chemical reactions:

Standard Enthalpy and Standard State

Standard Enthalpy Change ()

The standard enthalpy change refers to the enthalpy change when reactants and products are in their standard states (most stable form at 25°C, 1 atm, 1 M concentration).

• Symbol:

• Standard state conditions: 25°C, 1 atm, 1 M concentration

Hess's Law

Indirect Determination of Enthalpy Changes

Hess's Law states that if a reaction is carried out in a series of steps, the overall enthalpy change is the sum of the enthalpy changes for the individual steps. This allows calculation of enthalpy changes for reactions that are difficult to measure directly.

• Mathematical Expression:

Example: Calculating the enthalpy of formation of a compound using known enthalpy changes of related reactions.

Standard Enthalpy of Formation

Definition and Application

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

• By definition, the standard enthalpy of formation of an element in its standard state is zero.

• Example: (C, graphite) = 0; (O, gas) = 0

General Reaction:

• For ethanol:

Tabulated Standard Enthalpies of Formation

Standard enthalpies of formation for various substances are experimentally measured and tabulated for use in calculations.

Calculating Enthalpy Changes: Direct Method

Using Standard Enthalpies of Formation

The standard enthalpy of reaction () can be calculated using the standard enthalpies of formation of the reactants and products.

• General Equation:

• Where and are the stoichiometric coefficients from the balanced chemical equation.

Example: For the combustion of ethane:

• kJ/mol