Thermochemistry and Enthalpy

Introduction to Energy in Chemistry

Energy plays a central role in chemical reactions and physical changes. In chemistry, energy can be stored in chemical bonds or associated with the random motion of molecules.

• Potential Energy: Energy stored in the structural units of a compound, such as chemical bonds, position/arrangement of atoms, and attractive forces.

• Kinetic Energy: Energy associated with the random motion of molecules.

Heat Flow

Heat flow refers to the thermal energy transferred between two bodies at different temperatures.

• Heat (q): The transfer of thermal energy due to temperature difference.

Thermochemistry

Thermochemistry is the study of heat flow in chemical reactions. It focuses on how energy changes during chemical processes.

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

• Surroundings: Everything outside the system.

Types of Systems

Exothermic vs. Endothermic Reactions

• Exothermic: Releases heat to surroundings (system loses energy).

• Endothermic: Absorbs heat from surroundings (system gains energy).

Enthalpy (H)

Enthalpy is a measure of heat flow in or out of a system at constant pressure.

• ΔH (Change in Enthalpy): The heat absorbed or released during a reaction at constant pressure.

• q vs. ΔH: For reactions at constant pressure, q (heat) equals ΔH.

Thermochemical Equations Using ΔH

Thermochemical equations show the enthalpy change associated with chemical reactions.

• Example: ;

Calculating Heat and Enthalpy Changes

• q = m c ΔT: Heat (q) is calculated as mass × specific heat × temperature change.

• Specific Heat (c): Amount of heat needed to raise 1 g of a substance by 1°C. For water, .

Calorimetry

Calorimetry is a lab technique used to measure heat flow between a system and its surroundings.

• Calorimeter: Device used to measure heat changes.

• Types: Open air ("coffee cup"), closed, isolated.

• Formula:

Bond Energy and ΔH

Bond energy is the energy required to break a chemical bond. It is used to estimate the enthalpy change of reactions.

• Breaking bonds: Requires energy (endothermic).

• Forming bonds: Releases energy (exothermic).

Bond Dissociation Energies Table

Additional info: Table values inferred from standard bond energy tables.

Methods for Finding ΔH

• Bond Energy: Use bond dissociation energies to estimate ΔH.

• Calorimetry: Measure heat flow directly.

• Hess's Law: Use enthalpy changes of related reactions to find ΔH for a target reaction.

Hess's Law

If a reaction is carried out in a series of steps, the overall ΔH is the sum of the ΔH values for each step.

• Formula:

• Whatever you do to the reaction (reverse, multiply), do the same to ΔH.

Formation Reactions

Standard enthalpy of formation () is the enthalpy change when 1 mole of a substance is formed from its elements in their standard states.

• Example:

• Formula:

Thermodynamics: Internal Energy and Work

• Internal Energy (ΔE): The total energy of a system.

• Formula:

• Work (w): For gases,

• 1 L·atm = 101.3 J

Example Calculation

• Given: 30.0 g Mg reacts with excess HCl at 1.00 atm and 25°C.

• a) Calculate work in kJ:

• b) If kJ/mol, solve for using

Ionic Solids Dissolving in Water

When ionic solids dissolve, energy changes occur due to breaking ionic bonds and forming new interactions with water molecules.

• Heat of Solution: The enthalpy change when a solute dissolves in a solvent.

• ΔH dilution: Enthalpy change associated with diluting a solution.

Assessment and Study Tips

• Be familiar with periodic table, equation sheet, and unit conversions (1 L·atm = 101.3 J).

• Understand how to find ΔH using bond energies, calorimetry, and Hess's law.

• Know the difference between q and ΔH.

• Watch signs (positive/negative) and follow units.

Key Equations