Enthalpy change of solution

Enthalpy change of solution

The enthalpy of solution (or enthalpy of dissolution) is the enthalpy change associated with the dissolution of a substance in a solvent at constant pressure.

The heat of solution is one of the three dimensions of solubility analysis. It is most often expressed in kJ/mol at constant temperature. Just as the energy of forming a chemical bond is the difference between electron affinity and ionization energy, the heat of solution of a substance is defined as the sum of the energy absorbed, or endothermic energy (expressed in "positive" kJ/mol), and energy released, or exothermic energy (expressed in "negative" kJ/mol).

Because heating decreases the solubility of a gas, dissolution of gases is exothermic. Consequently, as a gas continues to dissolve in a liquid solvent, temperature will decrease, while the solution continues to release energy. This is an effect of the increase in heat or of the energy required to attract solute and solvent molecules—in other words, this energy outweighs the energy required to separate solvent molecules. When the gas is "completely" dissolved (this is purely theoretical as no substance can infinitely dissolve)—the heat of solution will be at its maximum. Dissolution can be viewed as occurring in three steps:
# Breaking solute-solute attractions (endothermic), see for instance lattice energy in salts.
# Breaking solvent-solvent attractions (endothermic), for instance that of hydrogen bonding
# Forming solvent-solute attractions (exothermic), in solvation.

The value of the overall enthalpy change is sum of the individual enthalpy changes of each step. For example dissolving ammonium nitrate in water will decrease the temperature of water (solvation does not weigh up against energy spent in breaking down the crystal lattice) while adding potassium hydroxide will increase it.

Solutions with negative heats of solution form stronger bonds and have lower vapor pressure.

ee also

* Heat
* Solution
* Thermometric titration

References


Wikimedia Foundation. 2010.

Игры ⚽ Поможем решить контрольную работу

Look at other dictionaries:

  • Standard enthalpy change of reaction — The standard enthalpy change of reaction (denoted ΔH ° or ΔH o) is the enthalpy change that occurs in a system when one mole of matter is transformed by a chemical reaction under standard conditions. For a generic chemical reaction:nA A + nB B + …   Wikipedia

  • Standard enthalpy change of neutralisation — The standard enthalpy change of neutralisation is the change in enthalpy that occurs when an acid and base undergo a neutralisation reaction to form one mole of water under standard conditions (298 K and 1 atm), i.e. react to produce water and a… …   Wikipedia

  • Enthalpy — Thermodynamics …   Wikipedia

  • Enthalpy of fusion — The standard enthalpy of fusion (symbol: Delta{}H {fus}), also known as the heat of fusion or specific melting heat, is the amount of thermal energy which must be absorbed or evolved for 1 mole of a substance to change states from a solid to a… …   Wikipedia

  • Solution — This article is about chemical solutions. For other uses, see Solution (disambiguation). Making a saline water solution by dissolving table salt (NaCl) in water. The salt is the solute and the water the solvent. In chemistry, a solution is a… …   Wikipedia

  • Metal ions in aqueous solution — A metal ion in aqueous solution is a cation, dissolved in water, of chemical formula [M(H2O)n]z+. The solvation number, n, determined by a variety of experimental methods is 4 for Li+ and Be2+ and 6 for elements in rows 3 and 4 of the periodic… …   Wikipedia

  • Flory-Huggins solution theory — is a mathematical model of the thermodynamics of polymer solutions which takes account of the great dissimilarity in molecular sizes in adapting the usual expression for the entropy of mixing. The result is an equation for the Gibbs free energy… …   Wikipedia

  • Ideal solution — In chemistry, an ideal solution or ideal mixture is a solution in which the enthalpy of solution is zero; [ A to Z of Thermodynamics Pierre Perrot ISBN 0198565569] the closer to zero the enthalpy of solution, the more ideal the behavior of the… …   Wikipedia

  • Solvation — Solvation, commonly called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute. As ions dissolve in a solvent they spread out and become surrounded by solvent molecules. The… …   Wikipedia

  • Calcium chloride — Calcium chloride …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”