Ammonia reacts with oxygen in the presence of a platinum catalyst to give nitric oxide and water, according to the following gas phase equilibrium:
4NH_3(g) + 5O_2(g) <---> 4NO(g) + 6H_2O(g) ÄH = −906 kJ mol^−1
What is an appropriate equilibrium expression for the reaction. What would the units of the equilibrium constant K be if the concentrations of all the gases in the equilibrium expression were measured in mol dm−3? And how do you determined your units for K using your equation.
The appropriate equilibrium expression for the reaction is:
K = [NO]^4[H2O]^6 / [NH3]^4[O2]^5
The units of the equilibrium constant K can be determined by analyzing the stoichiometry of the balanced equation.
In the equilibrium expression, the concentrations of gases appear as raised to their respective stoichiometric coefficients. For example, [NO]^4 means the concentration of NO is raised to the power of 4.
In this case, the stoichiometric coefficients of NO and H2O are 4 and 6 respectively. Therefore, the units for these gases in the equilibrium expression would be raised to their respective stoichiometric coefficients.
Since the concentrations of all the gases are measured in mol dm^−3, the units of K would be [(mol dm^−3)^4 * (mol dm^−3)^6] / [(mol dm^−3)^4 * (mol dm^−3)^5].
Simplifying this, we get [(mol^4 dm^−12) * (mol^6 dm^−18)] / [(mol^4 dm^−12) * (mol^5 dm^−15)].
This equation simplifies to mol^−2 dm^3.
Therefore, the units of the equilibrium constant K, when concentrations are measured in mol dm^−3, are mol^−2 dm^3.
The appropriate equilibrium expression for the given reaction is:
K = [NO]^4[H2O]^6 / [NH3]^4[O2]^5
In this equilibrium expression, the concentrations of the gases are raised to the power of their stoichiometric coefficients. The stoichiometric coefficients in the balanced chemical equation represent the number of moles of each substance involved in the reaction.
Now, let's determine the units for the equilibrium constant, K, using the equation. The units of K can be obtained by examining the units of concentration in the equilibrium expression.
For the reactants NH3 and O2, their concentrations are measured in mol dm^−3. Therefore, their units can be represented as mol dm^−3.
For the products NO and H2O, their concentrations are also measured in mol dm^−3, so their units are mol dm^−3 as well.
Thus, the equilibrium constant, K, has units of (mol dm^−3)^4(mol dm^−3)^6 / (mol dm^−3)^4(mol dm^−3)^5.
Simplifying the units: (mol dm^−3)^(4+6-4-5) = (mol dm^−3)^1 = mol dm^−3.
Therefore, the units of K, when the concentrations of all the gases in the equilibrium expression are measured in mol dm−3, are mol dm^−3.
Isn't this just the standard Keq expression? What's not to understand?
As far as the units, put the ones suggested into the equation and see what dimensional analysis gives you. Technically, the Keq has no unit since activities go into the calculation of Keq and activities have no units. However, problems use molarities, in most cases, and they often ask for provisional Keq in those cases.
In an industrial process for making nitric acid, the first step is the reaction of ammonia with oxygen at high temperature in the presence of a platinum gauze.
Nitrogen monoxide forms as follows.
4NH3 + 5O2 → 4NO + 6H2O
How many grams of nitrogen monoxide can form if a mixture initially contains 25.86 g of NH3 and 39.96 g of O2 ?