9.1 Thermochemistry and Heats of Reaction

Thermochemistry is all about heat in chemical reactions. It's crucial for understanding how energy flows during combustion and other processes. We'll look at enthalpy, a key measure of heat content, and how it changes in different reactions.

We'll also explore ways to calculate and measure heat changes. This includes using Hess's law, standard enthalpies, and calorimetry. Understanding these concepts helps predict and control energy transfer in chemical reactions.

Enthalpy in Thermochemistry

Definition and Significance of Enthalpy

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• Enthalpy is a state function that represents the total heat content of a system at constant pressure, symbolized by H and has units of energy (joules (J) or kilojoules (kJ))
• The change in enthalpy (ΔH) during a chemical reaction or physical process equals the heat absorbed or released by the system at constant pressure
  • A positive ΔH indicates an endothermic process, while a negative ΔH indicates an exothermic process
• Enthalpy is a crucial concept in thermochemistry, the study of heat changes associated with chemical reactions and physical transformations
  • Thermochemistry deals with the energy transfer between a system and its surroundings

Standard Enthalpy of Formation and Enthalpy Changes

• The standard enthalpy of formation (ΔH°f) is the enthalpy change that occurs when one mole of a compound is formed from its constituent elements in their standard states at a specified temperature (usually 298 K) and 1 atm pressure
• Enthalpy changes can be used to:
  • Predict the direction and extent of chemical reactions
  • Calculate the heat absorbed or released during various processes (phase changes, dissociation, and combustion)

Hess's Law and Enthalpy Calculations

Hess's Law

• Hess's law states that the total enthalpy change for a reaction is independent of the pathway taken from reactants to products
  • The overall enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps of the reaction
• Hess's law allows for the calculation of the enthalpy change of a reaction by combining the enthalpy changes of other reactions, as long as the sum of the reactions equals the desired reaction

Calculating Enthalpy Changes using Standard Enthalpies of Formation

• The standard enthalpy of formation (ΔH°f) can be used to calculate the enthalpy change of a reaction using the following equation:
  • ΔH°rxn = ∑(n × ΔH°f(products)) - ∑(n × ΔH°f(reactants)), where n is the stoichiometric coefficient of each species
  • By convention, the standard enthalpy of formation for an element in its standard state is zero

Other Methods for Calculating Enthalpy Changes

• Enthalpy changes for reactions can also be calculated using bond dissociation energies
  • Bond dissociation energies represent the energy required to break a specific bond in a molecule
  • The enthalpy change of a reaction equals the sum of the bond dissociation energies of the bonds broken minus the sum of the bond dissociation energies of the bonds formed
• Enthalpy changes for reactions involving ions in aqueous solution can be calculated using:
  • The standard enthalpies of formation of the ions
  • The enthalpy of hydration (the enthalpy change associated with the dissolution of a gaseous ion in water to form an aqueous ion)

Calorimetry and Heats of Reaction

Calorimetry Concepts

• Calorimetry is an experimental technique used to measure the heat absorbed or released during a chemical reaction or physical process
  • It involves the use of a calorimeter, an insulated device that minimizes heat exchange with the surroundings
• The heat capacity (C) of a substance is the amount of heat required to raise the temperature of the substance by one degree Celsius or Kelvin
  • The specific heat capacity (c) is the heat capacity per unit mass of the substance

Constant-Pressure Calorimetry

• In a constant-pressure calorimeter, the heat absorbed or released by the system (q) is equal to the product of the mass of the substance (m), its specific heat capacity (c), and the change in temperature (ΔT):
  • $q = m × c × ΔT$

Bomb Calorimetry

• Bomb calorimetry is a specific type of constant-volume calorimetry used to measure the enthalpy of combustion for a reaction
  • The sample is placed in a sealed "bomb" and ignited in the presence of excess oxygen
  • The heat released by the combustion reaction is absorbed by the calorimeter and its contents, allowing for the calculation of the enthalpy change

Applications of Calorimetry

• Calorimetry can be used to determine the enthalpy changes associated with various processes:
  • Neutralization reactions
  • Dissolution of solids
  • Phase changes
• These measurements provide valuable information about the thermodynamics of the system and can be used to predict the behavior of similar reactions or processes

Endothermic vs Exothermic Reactions

Endothermic Reactions

• Endothermic reactions are chemical reactions or physical processes that absorb heat from their surroundings
  • In an endothermic reaction, the enthalpy of the system increases, and the change in enthalpy (ΔH) is positive
• Examples of endothermic reactions:
  • Photosynthesis
  • Dissolution of ammonium nitrate in water
  • Decomposition of calcium carbonate (CaCO3) to form calcium oxide (CaO) and carbon dioxide (CO2)

Exothermic Reactions

• Exothermic reactions are chemical reactions or physical processes that release heat to their surroundings
  • In an exothermic reaction, the enthalpy of the system decreases, and the change in enthalpy (ΔH) is negative
• Examples of exothermic reactions:
  • Combustion
  • Formation of water from hydrogen and oxygen
  • Neutralization of an acid with a base

Thermodynamic Favorability and Potential Energy Diagrams

• The sign and magnitude of the enthalpy change can provide information about the thermodynamic favorability of a reaction
  • Exothermic reactions are generally thermodynamically favored and tend to occur spontaneously
  • Endothermic reactions typically require an input of energy to proceed
• The enthalpy change of a reaction can be represented graphically using a potential energy diagram, which plots the enthalpy of the system as a function of the reaction coordinate
  • Endothermic reactions have a positive slope
  • Exothermic reactions have a negative slope

Effect on Temperature

• The heat absorbed or released during a reaction can affect the temperature of the system and its surroundings
  • In an endothermic reaction, the temperature of the system may decrease as heat is absorbed
  • In an exothermic reaction, the temperature of the system may increase as heat is released