State in words how \(K_{\mathrm{p}}\) is written.

In the realm of chemical reactions, the equilibrium constant for gases, represented as Kp, is a fundamental concept. This value encapsulates the relationship between the partial pressures of the reactants and products when a reaction has reached a state where the rate of the forward reaction is equal to the rate of the backward reaction, a condition known as chemical equilibrium. The Kp expression is derived from the law of mass action which states that for a balanced chemical reaction at equilibrium and in a closed system, the ratio of the product of concentrations (or pressures for gases) of the products raised to their stoichiometric coefficients to that of the reactants also raised to their stoichiometric coefficients remains constant at a given temperature.

Partial pressures play a key role in understanding gaseous systems. For a gas in a mixture, its partial pressure is the pressure that the gas would exert if it alone occupied the entire volume of the mixture at the same temperature. In the context of a chemical reaction, the partial pressures are important because they indicate the concentration of each gaseous reactant and product at equilibrium. When calculating the equilibrium constant Kp for a gas-phase reaction, it's these partial pressures that are used in the equation rather than molar concentrations. This is particularly useful because in a closed system at a constant temperature, according to Dalton's Law, the total pressure of a gas mixture is the sum of its components' partial pressures.

Stoichiometry is the branch of chemistry that involves the quantitative relationships between reactants and products in a chemical reaction. Stoichiometric coefficients are the numbers written in front of species in a balanced chemical equation. They indicate the mole ratios in which substances react and are formed. These coefficients are pivotal in calculating the Kp expression since they determine the exponent to which the partial pressure of each reactant and product is raised. It's essential to understand that these coefficients reflect not just a balance in mass, but a balance in the number of particles engaged in the reaction, which translates to how their pressures will influence the equilibrium state.

Chemical equilibrium is a state in a reversible reaction where no net change is observed. This does not mean the reactions have ceased, but that the rates of the forward and reverse reactions are equal, leading to a static condition where the concentrations of reactants and products remain constant over time. This concept is vitally important in the study of chemical reactions as it helps predict the extent of a reaction and the concentrations of substances at equilibrium. The equilibrium constant Kp, derived from the general equilibrium expression, enables us to quantify this equilibrium status for reactions involving gases, providing a window into how temperature, pressure, and concentration changes can shift the equilibrium position.