Question

Suppose the reaction system $$ \mathrm{CH}_{4}(g)+2 \mathrm{O}_{2}(g) \rightleftharpoons \mathrm{CO}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(l) $$ has already reached equilibrium. Predict the effect of each of the following changes on the position of the equilibrium. Tell whether the equilibrium will shift to the right, will shift to the left, or will not be affected. a. Any liquid water present is removed from the system. b. \(\mathrm{CO}_{2}\) is added to the system by dropping a chunk of dry ice into the reaction vessel. c. The reaction is performed in a metal cylinder fitted with a piston, and the piston is compressed to decrease the total volume of the system. d. Additional \(\mathrm{O}_{2}(g)\) is added to the system from a cylinder of pure \(\mathrm{O}_{2}\).

Short answer

a. The equilibrium will shift to the right. b. The equilibrium will shift to the left. c. The equilibrium will shift to the right. d. The equilibrium will shift to the right.

Step-by-step solution

Case (a) Water removal

If we remove liquid water present in the system, we decrease the concentration of the reaction product. According to Le Chatelier's principle, the shift will occur in the direction that will increase the concentration of the product (to oppose the change). In this case, the equilibrium will shift to the right.

Case (b) Adding CO2

In this case, we add more CO2 (product) to the system by dropping a chunk of dry ice into the reaction vessel. This action increases the concentration of the CO2 in the system. Based on Le Chatelier's principle, the equilibrium will shift to the left to decrease the concentration of CO2 (opposing the change).

Case (c) Compression

The reaction is performed in a metal cylinder fitted with a piston. When we compress the piston to decrease the total volume of the system, we effectively increase the overall pressure in the system. According to Le Chatelier's principle, the equilibrium will shift in the direction which reduces pressure. In this scenario, the reaction consuming more molecules is the forward reaction (3 moles of gas reacting to produce 1 mole of gas and 2 moles of liquid). Therefore, the equilibrium will shift to the right (increasing the concentration of products and decreasing the concentration of reactants).

Case (d) Adding O2

When we add more O2 (reactant) to the system from a cylinder, we increase the concentration of O2. According to Le Chatelier's principle, the equilibrium will shift in the direction that will consume more of the added substance, thus opposing the change. In this case, the equilibrium will shift to the right, as it results in the consumption of O2, forming more CO2 and H2O.

Key concepts

Chemical Equilibrium

Imagine a crowded dance floor where dancers represent molecules in a chemical reaction, moving and interacting in a balanced flurry of activity. This is the essence of chemical equilibrium, a dynamic state in which the rates of the forward and reverse chemical reactions are equal, leading to no net change in the concentration of reactants and products.

When the methane reacts with oxygen, it reaches a point where the formation of carbon dioxide and water occurs at the same rate as their reconversion back into methane and oxygen gases. This state of equilibrium is like dancers seamlessly changing partners with no one left waiting. It's important to realize that although the concentrations of the substances are stable, the reaction is not static but perpetually adjusting as if the dancers never stop – they simply change partners at a consistent rate.

Understanding equilibrium allows us to predict how a reaction will respond to various changes, much like predicting how a dance might change if new dancers join or others leave.

Reaction Shifts

When a chemical system at equilibrium faces new dancers— I mean, changes, it responds just like a well-choreographed group adjusting to the rhythm. This response is known as a reaction shift. Let's take each situation from our exercise as an example:

• Removing liquid water is akin to taking dancers off the floor. The reaction adapts by shifting right, seeking to fill the void with new water dancers.
• Adding dry ice (CO₂) drops extra performers into the mix. To maintain balance, the reaction shifts left, limiting the number of CO₂ dancers.
• Compressing the piston squeezes more into the floor, increasing the pressure. The reaction moves right, favoring fewer molecule-dancers to ease the squeeze.
• Introducing more O₂ gas is like adding more oxygen dancers. The reaction shifts to the right, consuming the oxygen to restore harmony.

Recognizing how the equilibrium tilts with the addition or removal of reactant and product dancers is the key to mastering the concept of reaction shifts.

Equilibrium Perturbations

Imagine our chemical dance party subjected to external influences – dimming lights, a change in music tempo, or an unexpected conga line. These are equilibrium perturbations, and Le Chatelier's principle is our DJ, ensured to counteract to maintain the groove.

When the party faces a perturbation like concentration changes, pressure variations, or temperature shifts, the principle states that the equilibrium will adjust in a way that partly counteracts the perturbation.

• When we remove water (dim the lights), equilibrium seeks to make it more vibrant by producing more water (brightening the party).
• Adding CO₂ is the conga line nobody asked for, and equilibrium responds by reducing CO₂, asking some dancers to leave.
• When we compress volume, like speeding up the tempo, the system favors the reaction that takes up less space, slowing the pace back to normal.
• Finally, infusing more O₂ adds an extra beat to the music. Equilibrium responds by consuming O₂, restoring the original rhythm.

Through these adjustments, a chemical system maintains balance even as external factors try to throw off its steady beat.