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Oxygen gas and nitrogen gas are in their most stable form. They have standard enthalpy formation of 0 because nothing changes when they are formed from themselves. For example, in the reaction O₂ (g) --> O₂(g) the enthalpy change would equal 0.
Bernardo_Diaz_3H wrote:Why is it that diatomic molecules like oxygen gas and nitrogen gas have a standard enthalpy of formation of 0?
Recall that standard enthalpy of formation is the heat required to make one mole of something from its components in their most stable state (paraphrased from course reader page 17).
Oxygen gas is made up of the element oxygen, which appears as O2 in its most stable form. The heat required to turn a single O2 molecule into oxygen gas (O2) is 0. Also, since the reactant and product are the same here, there is no change in heat.
The visual representation that was provided in class (lecture 3) is shown below:
standard enthalpy of formation = 0
The same concept applies for nitrogen gas.
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