Consider the reaction $$\begin{array}{rl}{4 \mathrm{NH}_{3}(g)+5 \mathrm{O}_{2}(g)} & {\rightleftharpoons} \\ {4} & {\mathrm{NNO}(g)+6 \mathrm{H}_{2} \mathrm{O}(g), \Delta H=-904.4 \mathrm{kJ}}\end{array}$$ Does each of the following increase, decrease, or leave unchanged the yield of \(\mathrm{NO}\) at equilibrium? (a) increase \(\left[\mathrm{NH}_{3}\right] ;(\mathbf{b})\) increase \(\left[\mathrm{H}_{2} \mathrm{O}\right] ;(\mathbf{c})\) decrease \(\left[\mathrm{O}_{2}\right] ;(\mathbf{d})\) decrease the volume of the container in which the reaction occurs; (e) add a catalyst; (f) increase temperature.

When a chemical reaction occurs, the reactants are converted into products, but under certain conditions, the products can also revert to reactants. Chemical equilibrium is the state in which the rate of the forward reaction equals the rate of the reverse reaction, meaning the concentrations of reactants and products remain constant over time.

This does not mean the amounts of reactants and products are equal, but that their concentrations no longer change. It's like a tug-of-war where both teams are equally strong; they pull with the same force, and the rope doesn't move. Understanding equilibrium is crucial because it tells us about a reaction's efficiency and how much product we can expect under given conditions.

The reaction yield refers to the amount of product formed in a chemical reaction. It is an essential factor in determining a chemical reaction's efficiency. In the context of our exercise, the amount of NO generated from the reaction involving NH3 and O2 is the reaction yield. The reaction yield of NO can change depending on various factors such as concentration of reactants, volume changes, addition of catalysts, or temperature shifts.

For example, increasing the concentration of NH3 directly impacts the yield of NO by driving the reaction towards more product formation. On the other hand, increasing the concentration of a product like H2O will drive the reaction back towards reactants, thus lowering the NO yield. Reaction yield is a vital concept as it influences industrial manufacturing processes, economic viability, and research directions in chemistry.

Le Châtelier's principle provides a prediction on how a system at equilibrium responds to changes in concentration, temperature, or pressure, leading to an 'equilibrium shift.' It essentially tells us that if a system at equilibrium is disturbed, it will adjust itself to minimize that disturbance.

Increasing reactants or decreasing products typically shifts equilibrium to the right, producing more products. Conversely, increasing products or decreasing reactants shifts equilibrium to the left, forming more reactants. Changes in volume and pressure also affect the equilibrium position; reducing volume (which increases pressure) shifts the balance toward the side with fewer moles of gas. Temperature changes can shift equilibrium too; for exothermic reactions, increasing temperature shifts equilibrium to the left, decreasing product yield. These shifts are at the heart of understanding how to control and manipulate chemical reactions for desired outcomes.