Question

Consider the following system at equilibrium: $$ \mathrm{NO}(g)+\mathrm{SO}_{3}(g) \rightleftharpoons \mathrm{NO}_{2}(g)+\mathrm{SO}_{2}(g) $$ For each of the following changes, predict whether the equilibrium concentration of \(\mathrm{NO}_{2}\) will increase or decrease. (a) a decrease in the concentration of NO (b) an increase in the concentration of \(\mathrm{SO}_{3}\) (c) an increase in the concentration of \(\mathrm{SO}_{2}\) (d) a decrease in the concentration of \(\mathrm{SO}_{2}\)

Short answer

(a) The concentration of \(\mathrm{NO}_{2}\) will decrease. (b) The concentration of \(\mathrm{NO}_{2}\) will increase. (c) The concentration of \(\mathrm{NO}_{2}\) will decrease. (d) The concentration of \(\mathrm{NO}_{2}\) will increase.

Step-by-step solution

Analyze the first change

A decrease in the concentration of NO means there's less of this reactant available for the reaction. According to Le Châtelier's Principle, in an attempt to offset this change, the equilibrium will shift to the left (towards the reactants). Consequently, the concentration of \(\mathrm{NO}_{2}\) (product) will decrease.

Analyze the second change

Increaing the concentration of \(\mathrm{SO}_{3}\) means there's now more of this reactant for the reaction. In order to offset this excess, the system shifts its equilibrium to the right (towards the products). As a result, the concentration of \(\mathrm{NO}_{2}\) will increase.

Analyze the third change

An increase in the concentration of \(\mathrm{SO}_{2}\) means a larger amount of this product is present. According to Le Châtelier’s Principle, to counterbalance this excess of \(\mathrm{SO}_{2}\), the system shifts left (towards the reactants). Consequently, the concentration of \(\mathrm{NO}_{2}\) will decrease.

Analyze the fourth change

A decrease in the level of \(\mathrm{SO}_{2}\) implies there's less product available. According to Le Châtelier's principle, the system tries to offset this by shifting right (towards the products). Therefore, the concentration of \(\mathrm{NO}_{2}\) will increase.

Key concepts

Chemical Equilibrium

Chemical equilibrium is a state in a reversible chemical reaction where the rate of the forward reaction equals the rate of the reverse reaction, resulting in no net change in the amount of reactants and products. At this point, the reaction has not necessarily stopped, but continues with reactants turning to products and vice-versa at an equal rate, maintaining a stable concentration of all chemical species involved.

Understanding this balance is crucial because it can be influenced by external conditions such as pressure, temperature, and concentration changes. These influences can cause the equilibrium to shift, thus changing the concentration of the involved compounds.

Equilibrium Concentration

Equilibrium concentration refers to the amounts of reactants and products in a chemical reaction that are present at equilibrium. These concentrations remain constant over time as long as the system's conditions are stable. However, if an external stress is applied to the system, such as changing the concentration of one of the reactants or products, the equilibrium will shift according to Le Châtelier's Principle in order to re-establish equilibrium.

Through careful measurements and calculations, chemists can determine the equilibrium concentration of each substance in the reaction mixture, which helps in understanding the reaction's dynamics and predicting the outcome of any changes to the system.

Equilibrium Shifts

Equilibrium shifts or changes in the balance of a chemical equilibrium occur in response to external stresses, as described by Le Châtelier's Principle. This principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. External factors that can cause a shift include changes in concentration, pressure, or temperature.

• If the concentration of a reactant is increased, the system shifts toward products to reduce the additional reactant.
• If the concentration of a product is increased, the system shifts toward reactants to decrease the additional product.
• A change in pressure or volume affects the equilibrium in reactions involving gases, generally favoring the side of the reaction with fewer moles of gas.
• An increase in temperature typically shifts the equilibrium position in the endothermic direction, as the system absorbs the added heat.

Reactants and Products

Reactants are substances consumed in the course of a chemical reaction, while products are the substances formed as a result. In a balanced chemical equation, the number and type of atoms are conserved from reactants to products, following the law of conservation of mass. The rate of a chemical reaction and the direction of the equilibrium shift can be affected by changes in the concentration of reactants or products.

For example, reducing the concentration of a reactant or product will shift the equilibrium towards the respective side to increase its concentration again. Conversely, adding more of a reactant or product will shift the equilibrium away from the added substance to regain balance. These adjustments in equilibrium concentrations underscore the dynamic nature of chemical reactions, even in a state of equilibrium.