H2s Standard Enthalpy Of Formation

Cadre Concepts

In this article, you will learn the basics of enthalpy, as well every bit how to use enthalpy of germination to calculate enthalpies of reaction and enthalpies of combustion.

Topics Covered in Other Articles

Computing Enthalpy
Bond Enthalpy
The Laws of Thermodynamics
Gibbs Free Energy
Combustion Reaction
Entropy

Enthalpy

Chemists and physicists tend to define change in enthalpy equally the rut substitution of a system at constant pressure.

Enthalpy is an important thermodynamic concept considering information technology informs whether a procedure is likely to occur, including chemical reactions. Specifically, chemists often use Gibbs free energy to correspond the favorability, or spontaneity, of a reaction. Enthalpy has a directly relationship with Gibbs free energy, equally indicated past the equation:

fundamental equation for gibbs free energy, used to illustrate the direct relationship between enthalpy and free energy

Since negative changes in Gibbs indicate spontaneous reactions, many "exothermic" reactions, involving negative changes in enthalpy, tend to be spontaneous. The opposite goes for "endothermic" reactions that have positive changes in enthalpy. Due to the importance of enthalpy in thermodynamically describing chemical reactions, chemists have determined many ways of measuring and calculating enthalpy.

Enthalpy of Formation

One of the most mutual means of calculating the enthalpy of a reaction (or "heat of reaction") involves using what chemists telephone call enthalpies of formation (or "heat of formation"). For context, each molecule has a characteristic enthalpy of formation. This enthalpy essentially represents the sum total energy of each bail in the molecule. Expressed differently, a molecule'south enthalpy of germination is the heat associated with forming it from its near basic components.

For example, take the reaction forming hexane (C6H14) from half-dozen moles of elemental carbon and seven moles of H2. The heat given off by this reaction, nether constant force per unit area, is equal to the enthalpy of formation of hexane.

reaction forming hexane with associated enthalpy of formation, showing the origin of enthalpy of formation

Information technology's of import to note that the above value for enthalpy of formation just applies nether "standard atmospheric condition". Specifically, a temperature of 20 degrees Celsius (or 298.15K) and a pressure level of one atm. In context, you will often see standard conditions abbreviated to "STP" for "standard temperature and pressure". If the reaction takes identify under nonstandard conditions, the enthalpy of formation would change. All enthalpies that assume standard weather have a small-scale circle in superscript, similar to the notation for degrees.

Importantly, all enthalpies are what chemists phone call state functions, in that the modify in enthalpy betwixt two states is equal to the difference between united states of america' enthalpies, regardless of the intermediate steps taken between the two states. Thus, regardless of mechanism between the carbon and hydrogen, an enthalpic alter of -199kJ/mol always occurs nether standard conditions.

Enthalpy of Reaction

Using enthalpies of formation, you can calculate the enthalpic alter of any chemical reaction at a given temperature. Chiefly, there are many means of calculating an enthalpy of reaction. Some examples include using bond enthalpies or the temperature change to the environs. However, if your reaction involves well known reactants nether familiar conditions, then the necessary enthalpies of formation exist online. In such cases, with a fairly simple formula, enthalpies of formation most easily yield the reaction enthalpy.

To summate enthalpy of reaction, you lot need to multiply the enthalpies of germination of each of your reactants by the stoichiometric coefficients of those reactants in the counterbalanced chemical equation. Then, yous demand to add the multiplied enthalpies of the products and reactants separately. Finally, you decrease the combined enthalpy of the reactants from the products to yield the overall enthalpy of reaction.

formula showing how to calculate enthalpy of reaction using enthalpies of formation and a balanced chemical reaction

Interestingly, chemists as well use this process of subtracting the combined values of the reactants from the combined values of the products to calculate an overall reaction value for many other state variables, such as entropy of reaction and Gibbs gratis free energy of reaction. Broadly, chemists phone call this method of calculating change in a state variable Hess'south Law, after Swiss chemist Germain Hess. Many chemical science students memorize the phrase "products minus reactants" to remember the formula associated with Hess's Law.

hess's law, with relation to enthalpy, entropy, and gibbs free energy

Allow'south await at a worked instance.

Enthalpy of Reaction Instance: Nitrogen Dioxide Decomposition

Nitrogen dioxide sometimes decomposes into nitrogen monoxide and diatomic oxygen according to the following chemical reaction:

balanced equation of the decomposition of nitrogen dioxide

Nether standard weather, nitrogen dioxide and nitrogen monoxide have enthalpies of germination of 33.18kJ/mol and 90.25kJ/mol, respectively. Considering elemental oxygen occurs naturally as diatomic oxygen, O2 has an enthalpy of formation of zero.

enthalpies of formation for nitrogen dioxide, nitrogen monoxide, and diatomic oxygen for purposes of calculating the enthalpy of reaction for the decomposition of nitrogen dioxide

To calculate the enthalpy of reaction, we need to multiply the enthalpies of germination of both nitrogen dioxide and nitrogen monoxide by 2, because both have a stoichiometric coefficient of 2 in the balanced chemic equation. Then, nosotros take the multiplied enthalpy of nitrogen dioxide (the "products") and subtract the multiplied enthalpy of nitrogen monoxide (the "reactants") to go our overall enthalpy of reaction of 114.14 kilojoules per "one mole of reaction". This positive reaction enthalpy reveals that the decomposition of nitrogen dioxide is endothermic.

hess's law with the example numbers for the decomposition of nitrogen dioxide

Importantly, "one mole of reaction" hither refers to the decomposition of 2 moles of nitrogen dioxide. This is because we calculated the reaction enthalpy using a balanced equation which gave nitrogen dioxide a coefficient of 2. If you lot instead wanted to know the reaction enthalpy of the decomposition of one mole of nitrogen dioxide, yous can simply divide the enthalpy nosotros calculated by two, since that would exist a "half mole of reaction." Similarly, if y'all wanted to do the aforementioned for 4 moles of nitrogen dioxide, or "two moles of reaction," you would multiply our value by two.

enthalpies of reaction at different moles of reaction, one half and two respectively

Enthalpy of Combustion

Combustion reactions provide 1 of the nearly mutual reaction types for which chemists use Hess'south Law to calculate enthalpy of reaction from enthalpies of formation. Importantly, the term "enthalpy of combustion" is used for such enthalpies of reaction, specifically apropos the molecule being combusted. For instance, chemists would use the phrase "hexane's enthalpy of combustion" to describe the standard reaction enthalpy associated with the combustion reaction of hexane.

The same rules as enthalpy of reaction apply when calculating enthalpy of combustion, with the added benefit that dissimilar combustion reactions often accept the same products. For hydrocarbons, the products by and large merely involve carbon dioxide and water, though the quantities may differ depending on the number of carbons and oxygens in the molecule. Combustion may also produce nitrogen dioxide and hydrogen sulfide if the combusted molecule has nitrogens or sulfurs.

table of common combustion product enthalpies of formation — Table of enthalpies of formation for mutual combustion products

Let's expect at some other example, this time a combustion reaction.

Enthalpy of Combustion Reaction Case: Ethanol Combustion

When one molecule of ethanol combusts under standard weather condition, the reaction produces two carbon dioxides and three h2o molecules according to the following equation:

chemical equation for the combustion of ethanol

Thus, to yield the enthalpy of the combustion reaction, nosotros sum the enthalpies of germination of the products, weighted by their stoichiometric coefficients, and decrease the enthalpy of formation of ethanol. The upshot is ethanol's standard enthalpy of combustion of -1234.79kJ/mol.