The standard enthalpy of formation “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).
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.
Terms Related to Enthalpy Change Of Formation Of Water
- Enthalpy Change Formation Methane
- Enthalpy Change Of Reaction
- Enthalpy Change Combustion Hydrogen
- Enthalpy Change Calorimeter
- Calculate Enthalpy Change Combustion
How to find the enthalpy of formation of water ?
H2(g)+1/2 O2(g)----------------> H2O(l)
There are several ways.
- When one mole of water forms, one mole of H–H bonds & 1/2 moles of O==O bonds are broken, & two moles of O–H bonds are made. So dH = 2x(O–H) – ( H–H + 1/2(O==O)).
- Formation of water may be considered as combstion of H2, so dH = std enthalpy of combustion for H2
Other methods are there, all give the same answer, 285.0 kJ/mol, roughly.
Just open any table with enthalpies of formation and look it up.
Look up the delta H for the reaction, enthalpy of formation of H2(g) and of1/2 O2(g). Then set up the following:
dH (rxn) = x - (dH0 (H2) + 1/2 dH0 (O2)). Solve for x.
Remember, there is no such thing as an “absolute enthalpy of formation.” You can only measure delta H, never an absolute value. Thus all those 0 values in the tables that you see are zero only because we said so. For liquid water, it is possible that delta H0 is also 0, thats why look it up.
It is a tricky question, and another correct answer to it is:
Set up an experiment and measure how much heat is absorbed/released in a reaction. Good luck!
Websites / Refernces on Enthalpy Change Of Formation Of Water
- Determination of Enthalpy Change DH – at TutorVista
- Standard enthalpy change of formation – Wikipedia, the free encyclopedia
- How to find the enthalpy of formation of water ? – Yahoo! Answers