Video Transcript
The equilibrium of a mixture of gases can be evaluated by 𝐾 𝑝, the equilibrium constant for partial pressures. Consider the following equation: two NO2 gas in equilibrium with N2O4 gas. What is the correct equation of 𝐾 𝑝 for the equilibrium between the two different oxides of nitrogen shown?
When we have a mixture of gases in a container, gas particles collide with the walls of the container, which exerts a pressure. Each gas contributes a partial pressure towards the total pressure in the container. We can use these partial pressures to calculate 𝐾 𝑝, the equilibrium constant for partial pressures. For a general equilibrium reaction, where 𝑎 plus 𝑏 is in equilibrium with 𝑐 plus 𝑑, 𝐾 𝑝 can be defined as the partial pressures of the products divided by the partial pressures of the reactants all raised to the powers of the stoichiometric coefficients.
In this problem, we need to create an expression for 𝐾 𝑝 for the equilibrium reaction between nitrogen dioxide and dinitrogen tetroxide. The partial pressure for dinitrogen tetroxide will go in the numerator of our 𝐾 𝑝 expression because it’s the product. The partial pressure of nitrogen dioxide will go on the bottom because it’s the reactant. Now, we’ll raise the partial pressures to the power of the stoichiometric coefficients.
The stoichiometric coefficient for dinitrogen tetroxide is one, but raising something to the power of one doesn’t change its value. So, we can remove this power from our expression. This leaves us with the correct equation for 𝐾 𝑝 for the equilibrium between the two different oxides of nitrogen, which is the partial pressure of dinitrogen tetroxide divided by the partial pressure of nitrogen dioxide squared.
Consider the following equation: four NH3 gas plus seven O2 gas in equilibrium with four NO2 gas plus six H2O gas. What is the correct equation of 𝐾 𝑝 for the equilibrium between ammonia and oxygen shown?
In this reaction, ammonia reacts with oxygen to form nitrogen dioxide and water. We need to create the equilibrium expression for this reaction. Though this reaction involves more chemical species, the way to create the 𝐾 𝑝 expression will be the same. We’ll start off with the partial pressures of our products, nitrogen dioxide and water, in the numerator. The partial pressures of our reactants, ammonia and oxygen, will be in the denominator.
Now, we need to raise each partial pressure to the power of the stoichiometric coefficient. Nitrogen dioxide has a stoichiometric coefficient of four. Water has a coefficient of six. Ammonia has a coefficient of four, and oxygen has a coefficient of seven. We’ve completed the 𝐾 𝑝 expression. So, the correct equation of 𝐾 𝑝 for the equilibrium between ammonia and oxygen is PNO2 to the fourth PH2O to the sixth divided by PNH3 to the fourth PO2 to the seventh.
