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Name: Bob
Status: educator
Grade: 9-12
Location: Outside U.S.
Country: Canada
Date: April 6, 2011

How can the molar enthalpy of gaseous water be given at standard conditions if water is a liquid at standard conditions?


Standard conditions are just a pre-agreed set of environmental conditions that we use as reference. Since enthalpy (like internal energy) is not something that we can measure - we can only measure the difference or the change in enthalpy (delta-H), then we have agreed that - for example: the enthalpy of formation of elements in their standard state is zero. This does not mean that there really is no energy needed or produced to make elemental O2, it is just a baseline agreement (like the way we say that a mole of C-12 is 12g, or that the half-cell standard reduction potential of hydrogen is 0 volts).

Greg (Roberto Gregorius)
Canisius College

The "standard state" of a substance is usually defined as the most stable form of the substance at 1 atm pressure and a stated temperature. This is usually "298.15" Kelvins, or less commonly "0" Kelvins. That is the good news.

However, there are circumstances where it is convenient to bend the definition a bit for computational purposes, and some other form of the molecule is selected. Such an example is atomic oxygen "O". This does not exist at room temperature, but it can be observed at high temperatures, say 2000 Kelvins. It is possible (without getting into the computational details) to extrapolate the high temperature data down to 298.15 or "O" Kelvins, and set its value. That is the value of the thermodynamic property at the stated temperature, "if "O" could exist at that temperature, based on the experimental data measured at high temperatures. Your observation is very perceptive.

When doing thermochemical computations it is important to know what the numbers are being referred to. The motive is to be able to compute thermodynamic properties in conditions that are not easily accessible from conditions that are more readily accessible.

Vince Calder

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