Question

When the following reactions come to equilibrium, does the equilibrium mixture contain mostly reactants or mostly products? (a) ${\mathrm{N}}_2(g)+{\mathrm{O}}_2(g)\rightleftharpoons 2\mathrm{NO}(g);K_c=1.5\times {10}^{-10}$ (b) $2{\mathrm{SO}}_2(g)+{\mathrm{O}}_2(g)\rightleftharpoons 2{\mathrm{SO}}_3(g);K_p=2.5\times {10}^9$

Short answer

For the given reactions: (a) ${\mathrm{N}}_2(g)+{\mathrm{O}}_2(g)\rightleftharpoons 2\mathrm{NO}(g)$ with $K_c=1.5\times {10}^{-10}$ - The equilibrium mixture will contain mostly reactants, due to the small equilibrium constant. (b) $2{\mathrm{SO}}_2(g)+{\mathrm{O}}_2(g)\rightleftharpoons 2{\mathrm{SO}}_3(g)$ with $K_p=2.5\times {10}^9$ - The equilibrium mixture will contain mostly products, due to the large equilibrium constant.

Step-by-step solution

The equilibrium constant (Kc or Kp) is the ratio of the concentrations or partial pressures of the products to the reactants at equilibrium. A larger value of the equilibrium constant indicates that the equilibrium is more towards the products, while a smaller value indicates that the equilibrium is more towards the reactants. #Step 2: Analyze Reaction (a)#

For reaction (a) ${\mathrm{N}}_2(g)+{\mathrm{O}}_2(g)\rightleftharpoons 2\mathrm{NO}(g)$, the equilibrium constant Kc is given as $1.5\times {10}^{-10}$. Since Kc is very small, this means that the equilibrium lies more towards the reactants. Therefore, the equilibrium mixture will contain mostly reactants for Reaction (a). #Step 3: Analyze Reaction (b)#

For reaction (b) $2{\mathrm{SO}}_2(g)+{\mathrm{O}}_2(g)\rightleftharpoons 2{\mathrm{SO}}_3(g)$, the equilibrium constant Kp is given as $2.5\times {10}^9$. Since Kp is very large, this means that the equilibrium lies more towards the products. Therefore, the equilibrium mixture will contain mostly products for Reaction (b). #Step 4: Summary of results#

Based on the given equilibrium constants, the equilibrium mixtures for the given reactions are as follows: (a) N2(g) + O2(g) ↔ 2 NO(g): Mostly reactants, due to a small equilibrium constant. (b) 2 SO2(g) + O2(g) ↔ 2 SO3(g): Mostly products, due to a large equilibrium constant.

Key concepts

Understanding Reaction Equilibrium

In any chemical reaction, reaching reaction equilibrium means the rate of the forward reaction equals the rate of the reverse reaction. At this stage, the concentration of reactants and products remains constant. This doesn't necessarily mean they are equal in concentration. It simply means they no longer change with time. Equilibrium does not imply that a reaction has halted; instead, both forward and reverse reactions continue but at the same rate.

• When the forward reaction produces products at the same rate they revert to reactants, equilibrium is achieved.
• Equilibrium is dynamic, meaning changes at the molecular level still occur even though macroscopic changes cease.

By examining equilibrium constants, students can deduce which direction a reaction favors at equilibrium.

The Role of Chemical Reactions

Chemical reactions involve the transformation of substances through the breaking and forming of bonds, resulting in different substances called products. In reversible reactions, this process can occur in both directions, leading to the establishment of an equilibrium position.
The initial concentration of the reactants, temperature, and the specific properties of the substances influence how the reaction progresses.

• In reversible reactions, the same substances can convert back to initial reactants from products.
• The speed of these changes depends on factors like temperature and pressure.

Chemical reactions form the basis for calculating the equilibrium constant, providing insights into the reaction's balance between products and reactants.

Characteristics of an Equilibrium Mixture

An equilibrium mixture is a combination of reactants and products in a chemical reaction that occurs at equilibrium. This mixture reflects the concentrations present when a reversible reaction reaches equilibrium. A major tool for understanding this is the equilibrium constant (K), which gives insight into the ratio of products to reactants.

• If the equilibrium constant is large, the equilibrium mixture contains mostly products, like in reaction (b) with a Kp of $2.5\times {10}^9$.
• A small equilibrium constant indicates a mixture with mostly reactants, as seen in reaction (a) with a Kc of $1.5\times {10}^{-10}$.

These constants help determine the "position" of the equilibrium, informing whether a mixture favors the formation of products or the retention of reactants at equilibrium.