Consider the equilibrium \(4 \mathrm{NH}_{3}(\mathrm{~g})+5 \mathrm{O}_{2}(\mathrm{~g})=\) \(4 \mathrm{NO}(\mathrm{g})+6 \mathrm{H}_{2} \mathrm{O}(\mathrm{g})\). (a) What happens to the partial pressure of \(\mathrm{NH}_{3}\) when the partial pressure of \(\mathrm{NO}\) is increased? (b) Does the partial pressure of \(\mathrm{O}_{2}\) decrease when the partial pressure of \(\mathrm{NH}_{3}\) decreases?

Chemical equilibrium is a state in a reversible chemical reaction where the rates of the forward and reverse reactions are equal, resulting in no net change in the amounts of the reactants and products over time. It's essential to note that while the macroscopic properties, such as color and density, remain constant, the microscopic processes of the reactions continue; reactants are transformed into products and vice versa at equal rates. The equilibrium can be represented using a chemical equation with double arrows pointing in opposite directions.
Our example involves the gases NH₃, O₂, NO, and H₂O. At equilibrium, 4 NH₃ molecules react with 5 O₂ molecules to form 4 NO molecules and 6 H₂O molecules continuously, but the overall concentrations remain steady.A fundamental principle to remember is that equilibrium does not mean the reactants and products are in equal concentrations, but rather that their concentrations have stabilized and will not change unless the system is disturbed.

Partial pressure is a measure of the pressure exerted by a single type of gas in a mixture of gases. It's directly proportional to the concentration of that gas in the mixture. According to Dalton's Law of Partial Pressures, the total pressure exerted by a mixture of non-reacting gases is equal to the sum of the partial pressures of individual gases.
For instance, in our chemical reaction equilibrium involving NH₃, O₂, NO, and H₂O, each gas has its own partial pressure contributing to the total pressure of the gaseous system. If we alter the partial pressure of one component, for instance, NO, this would represent a change in concentration and therefore a disturbance to the equilibrium.

An equilibrium shift, also known as Le Chatelier's Principle, is an adjustment that a system at chemical equilibrium undergoes in response to an external stress, such as change in concentration, temperature, or pressure. The principle essentially states that a system will respond to such a stress by shifting in a direction that counteracts the change.

• If the concentration of a product (like NO) is increased, the equilibrium will shift to the left (towards reactants) to decrease the concentration of NO, meaning more NH₃ and O₂ will be present. This is what we see in step 2 of the solution.
• If the concentration of a reactant (like NH₃) is decreased, the equilibrium will shift to the right (towards products) to increase the concentration of NH₃ by producing more NO and H₂O, consequently reducing the partial pressure of O₂. This behavior is explained in step 3 of the solution.

Understanding these shifts is crucial for predicting how the equilibrium will respond to various changes, allowing chemists and students to manipulate and control reactions effectively.