Consider the reaction in which a water molecule (H2O) is produced from hydrogen (H2) and oxygen (O2).
You probably know that this reaction proceeds spontaneously in the direction of H2 + O2 → H2O. We can verify this according to the previous equation.
Example: Determine by calculation whether hydrogen and oxygen molecules dissociate spontaneously from water molecules or whether water molecules are formed spontaneously from hydrogen and oxygen molecules. However, assume that the partial pressure of H2 is 0.1 (atm) and the partial pressure of O2 is 0.2 (atm). If liquid H2O is produced, the H2O activity is 1.
(Previously, the molar concentration was substituted into the equation for solution reactions. For gas-phase reactions, substitute the partial pressure.)
【Original form】 【Product form】
2H2O (liquid) ⇆ 2H2 + O2
Gf0 －229 (kJ/mol) 0 (kJ/mol) 0 (kJ/mol)
The difference between the total gibbs energy of the original and the product form is expressed as follows.
⊿∑ΣGProduct form－Original form
= ΣGfProduct form + RT・ln【Activity product of the product form】－ ΣGfOriginal form + RT・ln【Activity product of the original form】
= ⊿∑Gf0 + RT・ln（【Activity product of the product form】／【Activity product of the original form】）
= ( 0 + 0 ) － (－2×229×1000) + RT・ln （【0.1×0.1×0.2】／1）
※Since two H2 are generated, 0.1 x 0.1
※The activity of the liquid in the gas phase is assumed to be 1.
To be able to calculate by hand, let R = 8, T =300, ln(0.002) = -6,
⊿ΣGProduct form－Original form = 2×229000－14400 = 443600
⊿ΣGProduct form－Original form > 0, so
the above reaction proceeds naturally from 【Product form】 to 【Original form】 .
(However, the hydrogen combustion reaction will proceed if it does not ignite, since the activation energy mountain must be crossed.)
