Enthalpy changes for solids and solutions

Enthalpy Changes for Solids and Solutions

I. Basic Definitions

  • Enthalpy (H) represents the total energy of a system.
  • Enthalpy change (∆H) is the difference in enthalpy before and after a chemical reaction.
  • Exothermic reaction releases heat and has a negative enthalpy change.
  • Endothermic reaction absorbs heat and has a positive enthalpy change.

II. Standard Enthalpy Changes

  • Standard enthalpy change of formation (∆Hf) is the enthalpy change when one mole of a substance is formed from its constituent elements.
  • Standard enthalpy change of reaction (∆Hr) is the enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation.
  • Standard enthalpy change of combustion (∆Hc) is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions.

III. Enthalpy Changes in Solutions

  • Solution formation involves three steps: breaking solute-solute bonds (endothermic), breaking solvent-solvent bonds (endothermic), and forming solute-solvent bonds (exothermic).

IV. Calculating Enthalpy Change

  • Enthalpy change can be calculated using Hess’s Law, which states that the total enthalpy change for a reaction is the same, regardless of the route by which the reaction is carried out.
  • Calorimetry can be used to measure the heat change in a chemical reaction. By knowing the mass of the solution and its specific heat capacity, the enthalpy change can be determined.

V. Bond Enthalpies

  • Bond enthalpy is the energy required to break one mole of a particular bond in a given substance in the gas phase. This process is always endothermic.
  • The overall enthalpy change of a reaction can be estimated using average bond enthalpies.

VI. Lattice Enthalpy

  • Lattice enthalpy describes the strength of the forces in an ionic solid. It is the enthalpy change when one mole of an ionic solid is formed from its gaseous ions.
  • It can be determined using Born-Haber cycles, a form of Hess’s Law.

VII. Entropy and Gibbs Free Energy

  • Entropy (S) measures the disorder of a chemical system.
  • Gibbs Free Energy (∆G) combines enthalpy and entropy values to predict whether a reaction is spontaneous or not.