Enthalpy change values that are tabulated under standard conditions are indicated with a superscripted ° symbol, as in ΔH°rxn. Standard conditions for enthalpy values are:
Notice that standard conditions do not specify a temperature. Most standard enthalpy values are tabulated at 25 °C (298 K), but it is possible to report standard enthalpy change values at other temperatures.
Creating a list of all possible standard enthalpy values is impossible. Fortunately, tabulating only one type of standard enthalpy change is all that is needed to calculate the standard enthalpy change for almost any chemical reaction. The standard heat of formation (or standard enthalpy of formation) for a species is the enthalpy change for the formation of one mole of a species from its constituent elements in their most stable form. Standard heat of formation values are denoted with the symbol ΔHf°.
For example, the standard heat of formation for solid N2O5 is the enthalpy change when one mole of the compound is formed from its constituent elements, nitrogen and oxygen, both in their standard states.
||ΔH° = ΔHf° = -43.1 kJ/mol|
Notice that the chemical reaction for the standard heat of formation of N2O5 includes a fractional coefficient. It is often necessary to include fractional coefficients in chemical reactions representing ΔHf° values because the enthalpy change must be for the formation of one mole of a species.
A chemical reaction that does not represent a standard heat of formation is the formation of gaseous N2O4 from nitric oxide and oxygen. The enthalpy change for this reaction is not a ΔHf° value because NO is not a constituent element of N2O4.
|2 NO(g) + O2(g) → N2O4(g)||ΔH° = -171.3 kJ|
Some standard heat of formation values are shown in Table 5.6.1 and a more complete table is found in the appendix.
|Al2O3(s)||Aluminum oxide||-1675.7||HF(g)||Hydrogen fluoride||-271.1|
|BaCO3(s)||Barium carbonate||-1219.0||HCl(g)||Hydrogen chloride||-92.3|
|CaCO3(s)||Calcium carbonate||-1206.9||HBr(g)||Hydrogen bromide||-36.3|
|CaO(s)||Calcium solid||-635.1||KCl(s)||Potassium chloride||-436.7|
|CCl4(ℓ)||Carbon tetrachloride||-135.4||KClO3(s)||Potassium chlorate||-397.7|
|CO(g)||Carbon monoxide||-110.5||NaCl(s)||Sodium chloride||-411.2|
|CO2(g)||Carbon dioxide||-393.5||NaBr(s)||Sodium bromide||-361.0|
|C2H2(g)||Acetylene (ethyne)||226.7||NaI||Sodium iodide||-288.0|
|CuCO3(s)||Copper(II) carbonate||-595.0||SiCl4(g)||Silicon tetrachloride||-657.0|
|Fe2O3(s)||Iron(III) oxide (hematite)||-824.2||SiO2||Silicon dioxide (quartz)||-910.9|
|FeSO4(s)||Iron(II) sulfate||-929.0||SnCl4(ℓ)||Tin(IV) chloride||-511.3|
|H2O(g)||Water (vapor)||-241.8||SO2(g)||Sulfur dioxide||-296.8|
|H2O(ℓ)||Water (liquid)||-285.8||SO3(g)||Sulfur trioxide||-395.7|
|H2O2(ℓ)||Hydrogen peroxide||-187.8||ZnCl2(s)||Zinc chloride||-415.1|
There are some important details that should be noted about ΔHf° values.
The ΔHf° value for an element in its standard state is equal to 0 kJ/mol. For example, the reaction for ΔHf° of elemental bromine is written
Br2(ℓ) → Br2(ℓ)
There is no change from reactants to products, so ΔHf° = 0 kJ/mol.
The standard enthalpy change for the following reaction is 2261 kJ at 298 K.
|2 Na2CO3(s) → 4 Na(s) + 2 C(graphite) + 3 O2(g)||ΔH°rxn = 2261 kJ|
What is the standard heat of formation of Na2CO3(s)?
You are asked to write an equation for the standard heat of formation of a compound.
You are given the compound formula.
The standard heat of formation (or standard enthalpy of formation) of a substance in a specified state at 298 K is the enthalpy change for the reaction in which one mole of the substance is formed from the elements in their stable forms at 1 bar and 298 K. Potassium is a solid and chlorine and oxygen are diatomic gases at 1 bar and 298 K.
Is your answer reasonable? The equation shows the formation of only one mole of product and the reactants are all elements in their standard states.
You are asked to determine the standard heat of formation of a compound.
You are given the enthalpy change under standard conditions for a reaction involving that compound.
The equation that represents the standard heat of formation of Na2CO3(s) is
Reversing the equation in the problem and multiplying by ½ results in the equation for the standard heat of formation of Na2CO3, so
Is your answer reasonable? The given equation shows sodium carbonate as a reactant, and the standard heat of formation is the energy change for the formation of one mole of a compound. The given reaction is endothermic as written, so the standard heat of formation for this compound is exothermic.