enthalpy of solution

(noun)

The heat association with dissolving a particular solute in a particular solvent.

Related Terms

Examples of enthalpy of solution in the following topics:

  • Heat of Solution

    • Heat of solution refers to the change in enthalpy when a solute is dissolved into a solvent.
    • The heat of solution, also referred to the enthalpy of solution or enthalpy of dissolution, is the enthalpy change associated with the dissolution of a solute in a solvent at constant pressure, resulting in infinite dilution.
    • The heat of solution, like all enthalpy changes, is expressed in kJ/mol for a reaction taking place at standard conditions (298.15 K and 1 bar).
    • The heat of solution can be regarded as the sum of the enthalpy changes of three intermediate steps:
    • The value of the overall heat of solution, $\Delta H^\circ_{sol}$, is the sum of these individual steps.

  • Solutions and Heats of Hydration

    • The heat (enthalpy) of solution (Hsolution) is the sum of the lattice and hydration energies ( Hsolution = Hhydration + Hlattice energy).
    • Hsolution is just one of the factors determining solution formation, but it is typically the major consideration in solution formation because of the role that enthalpy plays in most thermodynamic considerations.
    • A hot solution results when the heat of hydration is much greater than the lattice energy of the solute.
    • The enthalpy diagram showing exothermic solution formation.
    • Notice that Hsolution is lower in energy than the starting solute/solvent enthalpies.

  • Calorimetry

    • Dividing the energy change by how many grams (or moles) of A were present gives its enthalpy change of reaction.
    • A constant-pressure calorimeter measures the change in enthalpy of a reaction occurring in solution during which the atmospheric pressure remains constant.
    • The inner cup holds a known amount of a solute, usually water, that absorbs the heat from the reaction.
    • where Cp is the specific heat at constant pressure, ΔH is the enthalpy of the solution, ΔT is the change in temperature, W is the mass of the solute, and M is the molecular mass of the solute.
    • Constant-pressure calorimetry is used in determining the changes in enthalpy occurring in solution.

  • Constant-Pressure Calorimetry

    • A constant-pressure calorimeter measures the change in enthalpy of a reaction at constant pressure.
    • A constant-pressure calorimeter measures the change in enthalpy of a reaction occurring in a liquid solution.
    • The heat transferred to/from the solution in order for the reaction to occur is equal to the change in enthalpy ($\Delta H = q_P$), and a constant-pressure calorimeter thus measures this heat of reaction.
    • To determine the standard enthalpy of the reaction H+(aq) + OH–(aq) → H2O(l), equal volumes of 0.1 M solutions of HCl and of NaOH can be combined initially at 25°C.
    • The number of joules of heat released into each gram of the solution is calculated from the product of the rise in temperature and the specific heat capacity of water (assuming that the solution is dilute enough so that its specific heat capacity is the same as that of pure water's).

  • Standard States and Standard Enthalpy Changes

    • The standard enthalpy of formation refers to the enthalpy change when one mole of a compound is formed from its elements.
    • In chemistry, the standard state of a material, be it a pure substance, mixture, or solution, is a reference point used to calculate its properties under different conditions.
    • The standard enthalpy of formation, or standard heat of formation, of a compound is the change in enthalpy that accompanies the formation of one mole of the compound from its elements in their standard states.
    • For example, the standard enthalpy of formation for carbon dioxide would be the change in enthalpy for the following reaction:
    • Note that standard enthalpies of formation are always given in units of kJ/mol of the compound formed.

  • Change in Enthalpy

    • Enthalpy (H) is a measure of the total energy of a thermodynamic system.
    • By absorbing heat, the temperature, and thus the enthalpy of a substance increases.
    • Hess's law states that the standard enthalpy change of the overall reaction is the sum of the enthalpy change of all the intermediate reactions that make up the overall reaction.
    • This lesson introduces Enthalpy and the energy of chemical bonding.
    • We discuss where the energy in chemical bonds comes from in terms of internal energy and enthalpy, as well as how to approximate the overall heat of reaction using bond enthalpies.

  • Standard Enthalpy of Reaction

    • The standard enthalpy of reaction is the enthalpy change that occurs in a system when a chemical reaction transforms one mole of matter under standard conditions.
    • The standard enthalpy of reaction, $\Delta H^\ominus _{rxn}$, is the change in enthalpy for a given reaction calculated from the standard enthalpies of formation for all reactants and products.
    • In order to calculate the standard enthalpy of a reaction, we can sum up the standard enthalpies of formation of the reactants and subtract this from the sum of the standard enthalpies of formation of the products.
    • Calculate the standard enthalpy of reaction for the combustion of methane:
    • A calculation of standard enthalpy of reaction (∆H°rxn) from standard heats of formation (∆H°f)

  • Bond Enthalpy

    • The total enthalpy, H, of a system cannot be measured directly.
    • Generally, a positive change in enthalpy is required to break a bond, while a negative change in enthalpy is accompanied by the formation of a bond.
    • Bond enthalpy, also known as bond dissociation energy, is defined as the standard enthalpy change when a bond is cleaved by homolysis, with reactants and products of the homolysis reaction at 0 K (absolute zero).
    • For instance, the bond enthalpy, or bond-dissociation energy, for one of the C-H bonds in ethane (C2H6) is defined by the process:
    • Describe the changes in enthalpy accompanying the breaking or formation of a bond

  • Internal Energy and Enthalpy

    • The enthalpy of reaction measures the heat released/absorbed by a reaction that occurs at constant pressure.
    • To correct for this, we introduce the concept of enthalpy, which is much more commonly used by chemists.
    • The enthalpy of reaction is defined as the internal energy of the reaction system, plus the product of pressure and volume.
    • Due to this relation, the change in enthalpy is often referred to simply as the "heat of reaction."
    • An explanation of why enthalpy can be viewed as "heat content" in a constant pressure system.

  • Exothermic and Endothermic Processes

    • We will explore these concepts in more detail after introducing the concept of enthalpy.
    • Enthalpy (signified as H) is a measure of the total energy of a system and often expresses and simplifies energy transfer between systems.
    • Since the total enthalpy of a system cannot be measured directly, we most often refer to the change in enthalpy for a particular chemical reaction.
    • Due to this relation, the change in enthalpy, $\Delta H$, is often referred to as the "heat of reaction."
    • In an exothermic reaction, the total energy of the products is less than the total energy of the reactants.