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.

1. 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)).
2. 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.

OR

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:

Experimentally.

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

1. Determination of Enthalpy Change DH – at TutorVista
2. Standard enthalpy change of formation – Wikipedia, the free encyclopedia
3. How to find the enthalpy of formation of water ? – Yahoo! Answers

Heat of reaction is easy to measure because it simply represents the amount of heat that is given off if the reactants are mixed together in a beaker and allowed to react freely without doing any useful work. The above definition for enthalpy and its physical significance allow the equation for î”. To be written in the particularly illuminating and instructive form. Both terms on the right-hand side represent heats of reaction but under different sets of circumstances.

Heat of reaction is negative then the reaction is exothermic, if it is posititve then the reaction is endothermic.

Heat of reaction is a difference between the intrinsic energy in the products of a chemical reaction and the intrinsic energy in the reactants, and it is either adsorbed or released during the course of the chemical reaction.

Heat of reaction is the heat liberated or absorbed when a chemical reaction takes place. Reaction liberates heat, temperature of the reaction mixture increases. Reaction absorbs heat, temperature of the reaction mixture decreases. The heat of reaction for a neutralisation reaction is known as the heat of neutralisation.

Heat of reaction is determined on-line for a simulated reaction with first order kinetics and for the hydrolysis of acetic anhydride.

Heat of reaction is substantially greater than the amount of sensible heat which would be absorbed by simply passing the materials through the fuel cell, a large amount of heat can be removed with a small amount of material, thus reducing the size of the fuel cell passages, and the size of the fuel cell, while maintaining optimum temperature conditions.

References – Calculation Of Standard Heat Of Reaction

1. Heat of Reaction of Hydrogen and Coal – From Industrial & Engineering Chemistry Process Design and Development (ACS Publications)
2. Calculating Heat of Reaction – Calculate HOR
3. Enthalpies of Reaction – Enthalpy of a Reaction