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Standard Enthalpy of Formation Standard Enthalpy 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 the compound from its elements, with all substances in their standard states. Its symbol is ΔHfO or ΔfHO. The superscript theta (zero) on this symbol indicates that the process has been carried out under standard conditions. Standard States are as follows: 1. For a gas: standard state is a pressure of exactly 1 bar 2. For a substance present in a solution: a concentration of exactly 1 M at a pressure of 1 bar 3. For a pure substance in a condensed state (a liquid or a solid): the pure liquid or solid 4. For an element: the form in which the element is most stable in under 1 atm of pressure and the specified temperature. (Usually 25 degrees Celsius or 298.15 K) but One exception is Phosphorus. most stable under 1atm in standard state is black phosphorus, but use zero enthalpy at white phosphorus.

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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) Note that all elements are written in their standard states, and one mole of product is formed. This is true for all enthalpies of formation. 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. Enthalpy of Vaporization It is also called as Heat of Vaporization. It can be defined as: Note :- The net energy which is required to convert a given specific mass of substance from liquid state into gaseous state is called enthalpy of vaporization. This is considered to be done at a particular pressure which is mostly called as atmospheric pressure. The normal boiling point of the substance can be said to reference of measurement. It is expressed in Joule per mole or KJ/mole. For example: The energy changes when water is converted into steam.

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Bond Enthalpy :Note :- The energy that Is basically required to break a bond is called Bond enthalpy. It is expressed in KJ/mole. The precise value for it depends on the surrounding environment in which the bond is made. If we are given the bonding energy or the energy required to make or break the bonds then the formula is given as: HReaction = ∑ BEReactants – ∑ BEProducts. This is because when actually a bond is formed then energy is released. For example : F – F bond has a bond length of 0.142 nm and the bond enthalpy is 158 kJ per mole.

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Standard Enthalpy of Formation  
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