Equilibrium expression for oxidation of Hydrogen to form water vapor: 2H2+O22H2O

The equilibrium expression for any chemical reaction can
be determined if the phase of each substance that constitutes the chemical equation is
known and the equation is balanced with the same number of atoms of each element on both
the sides.

The equilibrium expression can be in terms of
the concentration of each substance that makes up the equation or if there are gases
involved it can be terms of the partial pressure.

For the
equation given: 2H2 + O2<------->2H2O, the reactants as well as products
are gases.

2H2(g) +
O2(g)<------->2H2O(g)

The equation is
balanced with two molecules of hydrogen reacting with a molecule of oxygen to give two
molecules of water. The equilibrium expression is written in terms of the partial
pressures of all the constituent
compounds.

Kp = P(H2O)^2/P(H2)^2 *
P(O2), where P(X) is the partial pressure of X and it is raised to the
power equal to the number of molecules in the balanced chemical
equation.