Question

Smog contains formaldehyde that is responsible for an eyeburning sensation. Formaldehyde, \(\mathrm{CH}_{2} \mathrm{O},\) is produced from the reaction of ozone and atmospheric ethylene, \(\mathrm{C}_{2} \mathrm{H}_{4},\) as follows. $$ 2 \mathrm{C}_{2} \mathrm{H}_{4}(g)+2 \mathrm{O}_{3}(g) \rightleftarrows 4 \mathrm{CH}_{2} \mathrm{O}(g)+\mathrm{O}_{2}(g)+\text { heat } $$ Predict the direction of equilibrium shift for each of the following stresses: (a) increase \(\left[\mathrm{C}_{2} \mathrm{H}_{4}\right]\) (b) decrease \(\left[\mathrm{O}_{3}\right]\) (c) increase \(\left[\mathrm{CH}_{2} \mathrm{O}\right]\) (d) decrease \(\left[\mathrm{O}_{2}\right]\) (e) increase temperature (f) decrease temperature (g) increase volume (h) decrease volume (i) add Ne inert gas (j) ultraviolet light

Short answer

(a) right, (b) left, (c) left, (d) right, (e) left, (f) right, (g) right, (h) left, (i) no change, (j) typically no change without specified photoreaction.

Step-by-step solution

Le Chatelier's Principle

The principle states that if a stress is applied to a system at equilibrium, the system adjusts to counteract the imposed change and a new equilibrium is established. This fundamental principle will guide us in predicting the shift in equilibrium for each stress listed.

Analyze Increase of [C2H4]

According to Le Chatelier's Principle, increasing the concentration of ethylene \(\mathrm{C}_{2}\mathrm{H}_{4}\) will shift the equilibrium to the right, favoring the forward reaction to consume the added \(\mathrm{C}_{2}\mathrm{H}_{4}\) and produce more products.

Analyze Decrease of [O3]

Decreasing the concentration of ozone \(\mathrm{O}_{3}\), a reactant, will shift the equilibrium to the left. This shift seeks to produce more \(\mathrm{O}_{3}\) by favoring the reverse reaction.

Analyze Increase of [CH2O]

Increasing the concentration of formaldehyde \(\mathrm{CH}_{2}\mathrm{O}\), a product, will shift the equilibrium to the left. This shift occurs as the system tries to reduce the amount of added product by favoring the reverse reaction.

Analyze Decrease of [O2]

Decreasing the concentration of oxygen \(\mathrm{O}_{2}\), a product, will shift the equilibrium to the right, favoring the forward reaction to produce more \(\mathrm{O}_{2}\) and offset the decreased concentration.

Analyze Increase in Temperature

For an exothermic reaction (heat is a product), increasing temperature shifts the equilibrium to the left, favoring the endothermic reverse reaction to absorb the added heat.

Analyze Decrease in Temperature

Decreasing the temperature will shift the equilibrium to the right, favoring the exothermic forward reaction since it produces heat, thereby counteracting the reduction in temperature.

Analyze Increase in Volume

Increasing the volume of the container decreases pressure. As there are more moles of gas on the right side (products: 5 moles) than the left side (reactants: 4 moles), the equilibrium will shift to the right to increase pressure by producing more gas.

Analyze Decrease in Volume

Decreasing volume increases pressure. The equilibrium will shift to the side with fewer moles of gas (left side, reactants) in order to decrease pressure.

Analyze Addition of Inert Gas (Ne)

Adding an inert gas like neon at constant volume does not affect the partial pressures of reactants or products because it does not participate in the reaction. Consequently, the equilibrium position remains unchanged.

Analyze Effect of Ultraviolet Light

Ultraviolet light can affect chemical reactions, particularly if it provides energy that can break bonds or alter kinetics, but typically it does not cause a shift by itself without a photo-sensitive component directly involved. Here, no direct impact on equilibrium is assumed unless specified otherwise related to photochemical reactions.

Key concepts

Le Chatelier's Principle

Le Chatelier's Principle is a key concept in understanding chemical equilibria. It tells us how a system at equilibrium responds to changes. Specifically, if we apply a stress to a chemical system in equilibrium, the system will adjust to counteract the stress and establish a new equilibrium. This response aims to minimize the effect of the change imposed.
For instance, if we add more of a reactant to the system, the system will "want" to consume the additional reactant by shifting the equilibrium towards the products. Conversely, if a product is added, the equilibrium will shift towards the reactants to reduce the extra product present.
This principle helps us predict changes in a system's behavior without needing to perform empirical experiments. It serves as a predictive tool for understanding how changes in concentration, temperature, or pressure affect the equilibrium position of a reaction.

Exothermic Reaction

In chemistry, reactions are classified based on how they interact with heat. An exothermic reaction is one that releases heat as a product. The given reaction where formaldehyde is produced is exothermic because it generates heat along with formaldehyde and oxygen.
In the context of Le Chatelier's Principle, if we increase the temperature of an exothermic reaction, we are essentially adding more "product" in the form of heat. The equilibrium will then shift to the left, favoring the reverse reaction to consume excess heat.
Conversely, if we decrease the temperature, the system compensates by shifting to the right, favoring the production of heat and thus stabilizing at a new equilibrium. Understanding this concept is pivotal for predicting how temperature fluctuations can influence the direction of equilibrium shifts.

Gas Laws

Gas laws, such as those described by Boyle's and Charles's laws, are fundamental in understanding reactions involving gases. They describe the relationship between the volume, temperature, pressure, and amount of gas.
In the context of chemical equilibrium, changes in volume or pressure can lead to shifts in equilibrium according to the number of gaseous molecules involved.
For the reaction producing formaldehyde, an increase in volume (which reduces pressure) will cause the equilibrium to shift toward the side with more gas molecules to restore pressure. Conversely, decreasing the volume increases pressure, shifting equilibrium towards the side with fewer gas molecules.
These concepts provide an insight into how physical conditions impact gaseous equilibrium systems without changing the actual chemical concentrations.

Equilibrium Shifts

Equilibrium shifts describe how an equilibrium balance changes in response to external stresses like concentration, pressure, and temperature. It's the observable outcome predicted by Le Chatelier’s Principle.
Different types of stresses cause the equilibrium to shift differently:

• Increasing concentration of a reactant shifts equilibrium toward products.
• Increasing concentration of a product shifts equilibrium toward reactants.
• Increasing pressure due to decreased volume shifts toward fewer gas moles if gases are involved.
• Temperature changes can favor either exo- or endothermic reactions, depending on whether the temperature is increased or decreased.

This dynamic behavior ensures that the chemical system remains balanced, adapting to external changes while maintaining a form of internal consistency. Understanding these shifts is fundamental in optimizing industrial chemical processes and in comprehending natural phenomena.