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 HfO or fHO. The superscript theta (zero) on this symbol jay cutler youth jersey indicates that the process has been carried out under youth stephen paea jersey standard conditions. Standard States are as follows:
For a gas: standard stephen paea youth jersey state is antrel rolle youth jersey a pressure of exactly 1 bar
For an element: the form in which the element is most stable under 1 bar of pressure authentic tim jennings youth jersey and the specified temperature. (Usually 25 degrees Celsius or 298.15 K) One exception is phosphorus: most stable under 1 bar willie young youth authentic jersey is black phosphorus, but white phosphorus is used as the reference for zero enthalpy of formation[1]
For example, the standard enthalpy of formation of carbon dick butkus youth authentic jersey dioxide would be the enthalpy of youth jeremiah ratliff jersey the following reaction under the conditions above:
C(s,graphite) + O2(g) chris conte youth jersey youth brian urlacher jersey CO2(g)
Note that all elements are jay cutler youth authentic jersey written martellus bennett youth jersey 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 youth martellus bennett jersey measured in units of energy per amount of substance. Most are defined in kilojoules per mole (kJ mol1), 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 youth gale sayers jersey particle is often expressed in electronvolts which corresponds to about 100 kJ 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.
The sum of all these jared allen youth authentic jersey values will youth matt forte jersey 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 authentic marquess wilson youth jersey the number of steps or intermediate reactions involved. This is because enthalpy is a state function. In the example above the standard enthalpy change of authentic jon bostic youth jersey 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 jim mcmahon youth jersey 74.8 kJ mol1 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, richard dent youth authentic jersey would be exothermic; that is, it is enthalpically more stable than hydrogen gas and carbon.
It is possible to tom waddle youth jersey predict heat of formations lance briggs youth authentic jersey for simple unstrained organic compounds with the Heat of formation group additivity method.
Standard enthalpy of youth jay cutler jersey reaction authentic mike singletary youth jersey
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, ) from the sum of the standard enthalpies alshon jeffery youth jersey of formation of the products (each also multiplied by its respective stoichiometric coefficient), as shown in youth richard dent jersey the equation below:
H marquess wilson youth authentic jersey = ( Hf) (products) ( youth dan hampton jersey Hf) (reactants)
For authentic richard dent youth jersey example, for the reaction CH4 + 2 O2 CO2 + 2 authentic dick butkus youth jersey H2O:
Hr = matt forte youth jersey [(1 authentic walter payton youth jersey Hf(CO2)) + (2 Hf(H2O))] (products) [(1 Hf(CH4)) + (2 Hf(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 antrel rolle youth authentic jersey negative. This implies that the reaction is exothermic. The converse is also true; the standard enthalpy of youth eddie royal jersey reaction will be positive for an endothermic reaction.
Key concepts for doing enthalpy calculationWhen a reaction is reversed, the magnitude jeremiah ratliff youth jersey of H stays the same, but authentic jared allen youth jersey the sign changes.
When the balanced equation for a reaction is multiplied by an integer, the corresponding value of H youth jon bostic jersey must be multiplied by that integer as martellus bennett youth authentic jersey well.
The change in enthalpy for a reaction can be calculated from the enthalpies of formation shea mcclellin youth jersey of the reactants youth kyle fuller jersey and the productElements in their standard states are not included in the enthalpy calculations for the reaction since the enthalpy of an element in its standard state is zero.
Standard enthalpy of neutralization is the change william perry youth jersey in enthalpy that occurs when an acid and base undergo a neutralization reaction to form one chris conte youth authentic jersey mole of water under standard conditions, as previously defined.
Standard enthalpy of sublimation, or heat of sublimation, is defined as the authentic lamarr houston youth jersey enthalpy required to sublime richard dent youth jersey 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 authentic dan hampton youth jersey the dissolution of a substance in a solvent at authentic shea mcclellin youth jersey constant pressure under standard conditions, as previously defined.
Standard enthalpy of hydrogenation is authentic robbie gould youth jersey defined as ryan mundy youth authentic jersey 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.
