Enthalpy change of solution
Encyclopedia
The enthalpy of solution, enthalpy of dissolution, or heat of solution is the enthalpy
change associated with the dissolution of a substance in a solvent
at constant pressure resulting in infinite dilution.
The enthalpy 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 values and unit kJ/mol, and energy released, or exothermic
energy (negative value).
The value of the overall enthalpy change is the sum of the individual enthalpy changes of each of these steps. For example, dissolving ammonium nitrate in water decreases the temperature of the solution. Solvation does not compensate energy spent in breaking down the crystal lattice, while adding potassium hydroxide
will increase it.
Solutions with negative enthalpy changes of solution form stronger bonds and have lower vapor pressure.
The enthalpy of solution of an ideal solution
is zero since the attractive and repulsive properties of ideal fluids are equal, irrespective of the compounds, thus mixing them does not change the interactions.
Enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...
change associated with the dissolution of a substance in a solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
at constant pressure resulting in infinite dilution.
The enthalpy of solution is one of the three dimensions of solubility
Solubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...
analysis. It is most often expressed in kJ
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
/mol
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
at constant temperature. Just as the energy of forming a chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
is the difference between electron affinity
Electron affinity
The Electron affinity of an atom or molecule is defined as the amount of energy released when an electron is added to a neutral atom or molecule to form a negative ion....
and ionization energy
Ionization energy
The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...
, the heat of solution of a substance is defined as the sum of the energy absorbed, or endothermic
Endothermic
In thermodynamics, the word endothermic describes a process or reaction in which the system absorbs energy from the surroundings in the form of heat. Its etymology stems from the prefix endo- and the Greek word thermasi,...
energy, expressed in positive values and unit kJ/mol, and energy released, or exothermic
Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...
energy (negative value).
Energetics
Dissolution of most gases is energetically favorable. That is, when a gas dissolves in a liquid solvent, energy is released into the surroundings as heat in an exothermic reaction. As energy leaves the system, the temperature of the solution decreases. Thus, decreasing the temperature of solution increases the solubility of a gas. Alternatively, when a solution is heated, the unfavorable reverse reaction occurs more readily and gas evolves. When the gas is completely dissolved (at a very low temperature, although this is purely theoretical as no substance can infinitely dissolve), the heat of solution is said to be at its maximum (a large negative enthalpy).Steps in dissolution
Dissolution can be viewed as occurring in three steps:- Breaking solute-solute attractions (endothermic), see for instance lattice energyLattice energyThe lattice energy of an ionic solid is a measure of the strength of bonds in that ionic compound. It is usually defined as the enthalpy of formation of the ionic compound from gaseous ions and as such is invariably exothermic. Lattice energy may also be defined as the energy required to completely...
in salts. - Breaking solvent-solvent attractions (endothermic), for instance that of hydrogen bonding
- Forming solvent-solute attractions (exothermic), in solvationSolvationSolvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...
.
The value of the overall enthalpy change is the sum of the individual enthalpy changes of each of these steps. For example, dissolving ammonium nitrate in water decreases the temperature of the solution. Solvation does not compensate energy spent in breaking down the crystal lattice, while adding potassium hydroxide
Potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...
will increase it.
Solutions with negative enthalpy changes of solution form stronger bonds and have lower vapor pressure.
The enthalpy of solution of an ideal solution
Ideal solution
In chemistry, an ideal solution or ideal mixture is a solution with thermodynamic properties analogous to those of a mixture of ideal gases. The enthalpy of solution is zero as is the volume change on mixing; the closer to zero the enthalpy of solution is, the more "ideal" the behavior of the...
is zero since the attractive and repulsive properties of ideal fluids are equal, irrespective of the compounds, thus mixing them does not change the interactions.
| Enthalpy change of solution for some selected compounds | ||
| hydrochloric acid Hydrochloric acid Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid.... |
||
| ammonium nitrate | ||
| ammonia Ammonia Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or... |
||
| potassium hydroxide Potassium hydroxide Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive... |
||
| caesium hydroxide Caesium hydroxide Caesium hydroxide is a chemical compound consisting of an atom of caesium and a hydroxide group . It is a powerful base, much like other alkali metal hydroxides such as sodium hydroxide and potassium hydroxide... |
||
| sodium chloride Sodium chloride Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms... |
||
| potassium chlorate Potassium chlorate Potassium chlorate is a compound containing potassium, chlorine and oxygen atoms, with the molecular formula KClO3. In its pure form, it is a white crystalline substance. It is the most common chlorate in industrial use... |
||
| acetic acid Acetic acid Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell... |
||
| sodium hydroxide | ||
| Change in enthalpy Enthalpy Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a... ΔH Joule The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second... /mol Mole (unit) The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value... in water Water Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a... at 25°C |
||