Everything about Standard Enthalpy Change Of Formation totally explained
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 101.325 kPa of pressure and the specified temperature, usually 298 K or 25 degrees Celsius). Its symbol is Δ
HfO.
A similar type of enthalpy change, known as the
standard enthalpy change of hydrogenation is defined as the enthalpy change observed when 1
mol of an
unsaturated compound reacts with an excess of hydrogen to become fully saturated, all elements within the reaction being within their
standard states.
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) + O
2(g) → CO
2(g)
The standard enthalpy change of formation is measured in units of energy per amount of substance. Most are defined in kilojoules per
mole, or 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's no change involved in their formation.
The standard enthalpy change 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 from the sum of the standard enthalpies of formation of the products, as shown in the equation below.
Δ
HreactionO = ΣΔ
HfO (Products) - ΣΔ
HfO (Reactants)
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:
- The standard enthalpy of atomization of solid sodium
- The first ionization energy of gaseous sodium
- The standard enthalpy of atomization of chlorine gas
- The electron affinity of chlorine atoms
- 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's hypothetical. For instance carbon and hydrogen won't 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's negative shows that the reaction, if it were to proceed, would be
exothermic; that is, it's 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.
Examples: Inorganic compounds (at 25 °C)
» (State: g = gaseous; l = liquid; s = solid; aq = aqueous)
Further Information
Get more info on 'Standard Enthalpy Change Of Formation'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://standard_enthalpy_change_of_formation.totallyexplained.com">Standard enthalpy change of formation Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
