Question

How will the position of equilibrium in the reaction $$ \text { heat }+\mathrm{CH}_{4}(g)+2 \mathrm{H}_{2} \mathrm{~S}(g) \rightleftharpoons \mathrm{CS}_{2}(g)+4 \mathrm{H}_{2}(g) $$ be affected by the following changes? (a) Adding \(\mathrm{CH}_{4}(g)\) (b) Adding \(\mathrm{H}_{2}(g)\) (c) Removing \(\mathrm{CS}_{2}(g)\) (d) Decreasing in the volume of the container (e) Increasing the temperature

Short answer

(a) Right, to produce more products. (b) Left, to produce more reactants. (c) Right, to replace removed CS2. (d) Left, to decrease pressure from reduced volume. (e) Left, to absorb excess heat.

Step-by-step solution

- Adding CH4

To determine the effect of adding CH4, compare the mole ratio of CH4 on the reactants side to its products. Since CH4 is a reactant, adding CH4 would shift the equilibrium to the right to form more products (CS2 and H2) according to Le Chatelier's Principle.

- Adding H2

Similarly, for the addition of H2, it's a product in the given reaction. Increasing the concentration of a product will shift the equilibrium to the left to produce more reactants (CH4 and H2S).

- Removing CS2

Removing CS2, which is a product, will result in the system shifting to the right to compensate for the loss and produce more CS2.

- Decreasing Volume

Decreasing the volume of the container increases the pressure. Since there are more moles of gas on the right (5 moles) than on the left (3 moles), the equilibrium will shift to the left where there are fewer gas moles to reduce the pressure.

- Increasing Temperature

As the reaction is exothermic (releasing heat), increasing the temperature will shift the equilibrium to the left, favoring the reactants, in order to absorb the excess heat and decrease the temperature.

Key concepts

Le Chatelier's Principle

Le Chatelier's Principle is a cornerstone of chemical equilibrium, describing how a system at equilibrium responds to changes in concentration, temperature, or pressure. This principle essentially states that when a system in equilibrium experiences a disturbance, it will adjust to minimize that disturbance and re-establish equilibrium. For instance, when a reactant or product is added to a reaction at equilibrium, the system will shift to use up the added substance. Conversely, when a substance is removed, the system will shift to produce more of that substance.

The principle can be thought of as the system's way of balancing itself. This brings harmony back to the reaction, not by reversing changes, but by changing the position of equilibrium in response to them. Understanding this principle is critical to manipulating and predicting the behavior of chemical systems under a variety of conditions.

Equilibrium Shift

An equilibrium shift occurs when a system at equilibrium is disturbed, and the position of equilibrium moves to either the right (towards products) or the left (towards reactants). This is a direct consequence of Le Chatelier's Principle. Shifts can be caused by changes in concentration, pressure, or temperature. For example, adding more reactant to a system at equilibrium will shift the equilibrium position toward the products, as the system strives to use up the added reactant. Moreover, a change in temperature can significantly alter the position of equilibrium; with exothermic reactions, an increase in temperature will shift the equilibrium towards the reactants, while in endothermic reactions, it will shift towards products.

Understanding the conditions leading to an equilibrium shift is critical for controlling chemical reactions, such as in large-scale industrial processes where maximizing product yield is essential. It's also useful in environmental chemistry, where understanding how reactions shift can help in predicting the behavior of pollutants.

Reaction Quotient

The reaction quotient (Q) is a way to determine the direction in which a reaction at non-equilibrium conditions will proceed to reach equilibrium. It is calculated in the same way as the equilibrium constant (K), but for a system not at equilibrium. The formula includes the concentrations of the reactants and products at a particular moment.

If Q is less than K, the reaction will proceed in the forward direction to reach equilibrium, converting reactants into products. Conversely, if Q is greater than K, the reaction will proceed in the reverse direction, converting products back into reactants until equilibrium is achieved. In the context of the given problem, adding or removing substances changes their concentrations, affecting the reaction quotient and thereby influencing the direction of the equilibrium shift.

Exothermic Reactions

Exothermic reactions are chemical reactions that release heat to their surroundings. Since they give off energy, the products of an exothermic reaction are usually at a lower energy level than the reactants. An increase in temperature, as seen in part (e) of the exercise, supplies additional heat to the system. According to Le Chatelier's Principle, the reaction will adjust to this change by favoring the formation of reactants, as the system seeks to absorb the excess heat by shifting the equilibrium to the left.

Exothermic reactions are commonly used in everyday applications, like combustion engines or hand warmers, due to their heat-releasing properties. The manipulation of temperature in these reactions is a key aspect of various technologies and industries, including energy production, where control over exothermic reactions is critical for efficiency and safety.