Enthalpy of Formation, ∆Hf

•The change of heat when 1 mole of a  compound is formed from its elements at their  standard states.

      

      H2 (g) + ½ O2(g) → H2O (l)        ∆Hf = -286 kJ mol-1

 

•The standard enthalpy of formation of any element in its most stable state form is ZERO.

  ∆H (O2 ) = 0              ∆H (Cl2) = 0

f.o = A degree signifies that it's a standard enthalpy change.change of formation:change of formation issum of the standard enthalpies of formation of the products subtracted by the sum of the standardreactants.fo[A] = 433 KJ/molfo[B] = -256 KJ/molfo[C] = 523 KJ/mol

The standard enthalpy of formation is a measure of the energy released or consumed when one mole of a substance

is created under standard conditions from its pure elements.

The symbol of the standard enthalpy of formation is ?H

• ? = A change in enthalpy

•

• f = it is a reaction from a substance that's formed from its elements

For most chemistry problems involving standard enthalpies of formation, you will need the equation for the

standard enthalpy

Although this equation looks complicated, it essentially states that the standard enthalpy

equal to the

enthalpies of formation of the

For example, if we have a simple chemical equation with the variables A, B and C representing different

compounds:

And we have the standard enthalpy of formation values as such:

?H

?H

?H

The equation for the standard enthalpy change of formation is as follows:

?H

?H

reactiono = ?Hfo[C] - (?Hfo[A] + ?Hfo[B])reactiono = (1 mol)(523 kJ/mol) - ((1 mol)(433 kJ/mol) + (1 mol)(-256 kJ/mol)") }}

(Since we have one mole of A, B and C, we multiply the standard enthalpy of formation of each reactant and product by 1

mole, which eliminates the mol denominator)

?H

reactiono = 346 kJ

We get the answer of 346 kJ, which is the standard enthalpy change of formation for the creation of variable

"C".

An important point to be made about the standard enthalpy of formation is that when a pure element is in

its reference form it is zero.

This can be seen with the element carbon. Carbon naturally exists as graphite and diamond. The enthalpy difference

between graphite and diamond is too large for both to have a standard enthalpy of formation of zero. To determine

which form is zero we choose the reference form of the more stable form of carbon. Which is also the one with the

lowest enthalpy, so this is why graphite has the standard enthalpy of formation equal to zero.

Sample Table of Standard Enthalpy of Formation Values

This table provides a few sample values of the standard enthalpies of formation of various compounds:

Compound ?H

fo

O

C(graphite) 0 kJ/mol

CO(g) -110.5 kJ/mol

CO

H

H

HF(g) -271.1 kJ/mol

NO(g) 90.25 kJ/mol

NO

N

SO

SO

2(g) 0 kJ/mol2(g) -393.5 kJ/mol2(g) 0 kJ/mol2O(g) -241.8 kJ/mol2(g) 33.18 kJ/mol2O4(g) 9.16 kJ/mol2(g) -296.8 kJ/mol3(g) -395.7 kJ/mol

All values are in the units of kJ/mol and the physical conditions of 298.15 K and under 1 atm of pressure, which is

referred to as the "standard state", and are the conditions in which you will generally find values of standard

enthalpies of formation.

The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states (the most stable form of the element at 1 bar of pressure and the specified temperature, usually 298.15 K or 25 degrees Celsius). Its symbol is ΔH_f^O or Δ_fH^O.
For example, the standard enthalpy of formation of carbon dioxide would be the enthalpy of the following reaction under the conditions above:

C(s,graphite) + O₂(g) → CO₂(g)

The standard enthalpy of formation is measured in units of energy per amount of substance. Most are defined in kilojoules per mole (kJ mol⁻¹), but can also be measured in calories per mole, joules per mole or kilocalories per gram (any combination of these units conforming to the energy per mass or amount guideline). In physics the energy per particle is often expressed in electronvolts which corresponds to about 100 kJ mol⁻¹.
All elements in their standard states (oxygen gas, solid carbon in the form of graphite, etc.) have a standard enthalpy of formation of zero, as there is no change involved in their formation.

Contents 1 Mechanics 2 Standard enthalpy of reaction 3 Subcategories 4 Examples: Inorganic compounds (at 25 °C) 5 See also 6 External links 7 References

Mechanics

The standard enthalpy of formation is equivalent to the sum of many separate processes included in the Born-Haber cycle of synthesis reactions. For example, to calculate the standard enthalpy of formation of sodium chloride, we use the following reaction:

Na_(s) + (1/2)Cl_2(g) → NaCl_(s)

This process is made of many separate sub-processes, each with their own enthalpies. Therefore, we must take into account:

Standard enthalpy change of formation Born-Haber diagram for lithium fluoride.

1. The standard enthalpy of atomization of solid sodium
2. The first ionization energy of gaseous sodium
3. The standard enthalpy of atomization of chlorine gas
4. The electron affinity of chlorine atoms
5. The lattice enthalpy of sodium chloride

The sum of all these values will give the standard enthalpy of formation of sodium chloride.
Additionally, applying Hess's Law shows that the sum of the individual reactions corresponding to the enthalpy change of formation for each substance in the reaction is equal to the enthalpy change of the overall reaction, regardless of the number of steps or intermediate reactions involved. In the example above the standard enthalpy change of formation for sodium chloride is equal to the sum of the standard enthalpy change of formation for each of the steps involved in the process. This is especially useful for very long reactions with many intermediate steps and compounds.
Chemists may use standard enthalpies of formation for a reaction that is hypothetical. For instance carbon and hydrogen will not directly react to form methane, yet the standard enthalpy of formation for methane is determined to be -74.8 kJ mol⁻¹ from using other known standard enthalpies of reaction with Hess's law. That it is negative shows that the reaction, if it were to proceed, would be exothermic; that is, it is enthalpically more stable than hydrogen gas and carbon.
It is possible to predict heat of formations for simple unstrained organic compounds with the Heat of formation group additivity method.

Standard enthalpy of reaction

The standard enthalpy of formation is used in thermochemistry to find the standard enthalpy change of reaction. This is done by subtracting the sum of the standard enthalpies of formation of the reactants (each being multiplied by its respective stoichiometric coefficient, ν) from the sum of the standard enthalpies of formation of the products (each also multiplied by its respective stoichiometric coefficient), as shown in the equation below:

ΔH° = Σ(ν * ΔH_f°) (products) - Σ(ν * ΔH_f°) (reactants)

For example, for the reaction CH₄ + 2 O₂ → CO₂ + 2 H₂O:

ΔH_r° = [(1 * ΔH_f°_(CO2)) + (2 * ΔH_f°(H₂O))] (products) - [(1 * ΔH_f°(CH₄)) + (2 * ΔH_f°_(O2))] (reactants)

If the standard enthalpy of the products is less than the standard enthalpy of the reactants, the standard enthalpy of reaction will be negative. This implies that the reaction is exothermic. similarly for an endothermic reaction, the standard enthalpy of reaction will be positive.

Subcategories

• Standard enthalpy of neutralization is the change in enthalpy that occurs when an acid and base undergo a neutralization reaction to form one mole of water under standard conditions, as previously defined.
• Standard enthalpy of sublimation, or heat of sublimation, is defined as the enthalpy required to sublime one mole of the substance under standard conditions, as previously defined.
• Standard enthalpy of solution (or enthalpy change of dissolution or heat of solution) is the enthalpy change associated with the dissolution of a substance in a solvent at constant pressure under standard conditions, as previously defined.
• Standard enthalpy of hydrogenation is defined as the enthalpy change observed when one mole of an unsaturated compound reacts with an excess of hydrogen to become fully saturated under standard conditions, as previously defined.

 Examples: Inorganic compounds (at 25 °C)

Chemical Compound	Phase (matter)	Chemical formula	Δ Hf0 in kJ/mol
Ammonia (Ammonium Hydroxide)	aq	NH3 (NH4OH)	-80.8
Ammonia	g	NH3	-46.1
Copper (II) sulfate	aq	CuSO4	-769.98
Sodium carbonate	s	Na2CO3	-1131
Sodium chloride (table salt)	aq	NaCl	-407
Sodium chloride (table salt)	s	NaCl	-411.12
Sodium chloride (table salt)	l	NaCl	-385.92
Sodium chloride (table salt)	g	NaCl	-181.42
Sodium hydroxide	aq	NaOH	-469.6
Sodium hydroxide	s	NaOH	-426.7
Sodium nitrate	aq	NaNO3	-446.2
Sodium nitrate	s	NaNO3	-424.8
Sulfur dioxide	g	SO2	-297
Sulfuric acid	l	H2SO4	-814
Silica	s	SiO2	-911
Nitrogen dioxide	g	NO2	+33
Nitrogen monoxide	g	NO	+90
Water	l	H2O	-285.8
Water	g	H2O	-241.82
Carbon dioxide	g	CO2	-393.5
Hydrogen	g	H2	0
Fluorine	g	F2	0
Chlorine	g	Cl2	0
Bromine	l	Br2	0
Bromine	g	Br2	+31
Iodine	s	I2	0
Iodine	g	I2	+62
Zinc sulfate	aq	ZnSO4	-980.14

(State: g = gaseous; l = liquid; s = solid; aq = aqueous)