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. For example, the standard enthalpy of formation of carbon dioxide would be the enthalpy of the following reaction under the conditions above:
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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. Standard Enthalpy of Formation Standard enthalpy of formation: Standard enthalpy of formation is defined as the enthalpy change that occurs when one mole of substance is formed from its constituent elements. All the involved substances must be in their standard states. Standard state of a substance is taken as its natural state at 250C and 1 atmosphere pressure. Standard enthalpy of formation is represented as ΔH0f . Substances having negative standard enthalpy of formation are called endothermic compounds and those with positive standard enthalpy of formation are called exothermic compounds. Standard Enthalpy Change :- Standard enthalpy change is more correctly termed as Standard molar enthalpy change.
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A substance is termed to be in its standard state when it is at 1 atmosphere pressure and 250C temperature. When such substances in their standard states react completely and form products in their standard states, the enthalpy change for one mole of product formed is called the standard enthalpy change, ΔH0. Thus standard enthalpy change is defined as the enthalpy change in the system when the reactants in their standard states are converted to the products in their states for one mole of reaction. For example one mole of carbon (graphite) is completely converted to one mole of carbon di oxide, with reactants and products at 1 atmosphere pressure and 25 degree C, evolve 393.51 kJ of heat. This is called the standard enthalpy change of the reaction and ΔH0 of the reaction is 393.51kJ mol-1. Standard Enthalpy of Reaction Standard enthalpy of reaction (ΔH0r) : The reactions and their values are dependent on pressure and temperature. When the values are given under standard conditions i.e., 1 atmosphere pressure and 298.17K temperature, it is termed as Standard enthalpy of reaction. Certain factors influence the enthalpy of reaction. They are. Temperature : The enthalpy of reaction depends on the temperature at which the reaction is carried out. Heat capacity of a system depends on the temperature and many substances change their physical state at different temperatures. Changes in physical state involve changes in energy.
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