How does the effect of a catalyst differ between reaction rate and equilibrium position?

A catalyst increases the rate of both forward and reverse reactions equally, allowing equilibrium to be reached faster. However, it has no effect on the final position of equilibrium or the relative concentrations of reactants and products.

When comparing temperature and pressure, which factor's effect depends on the enthalpy change ($\Delta H$)?

Temperature's effect is determined by whether the reaction is exothermic or endothermic. Pressure's effect, by contrast, depends only on the change in the number of gaseous molecules between reactants and products.

In a reaction where the number of moles of gas is equal on both sides, how does an increase in pressure affect the yield compared to the rate?

The yield (position of equilibrium) remains unchanged because neither side is favored by a pressure change. However, the rate of reaction increases because the higher density of gas molecules leads to more frequent successful collisions.

What common mistake do students make when applying pressure rules to a reaction involving a solid reactant and a gaseous product?

Students often count the moles of the solid reactant when predicting the shift. Pressure only affects gaseous molecules, so the solid's coefficient should be ignored when determining which side has 'fewer molecules'.

Why is it incorrect to say that 'increasing temperature always increases the yield of a reaction'?

Increasing temperature only increases yield for endothermic reactions. For exothermic reactions, increasing temperature shifts the equilibrium to the left, actually decreasing the yield of products.

What error occurs if a student predicts an equilibrium shift in an open container?

Dynamic equilibrium can only be established and maintained in a closed system. In an open system, products or reactants can escape, preventing the reverse reaction from matching the forward rate and making Le Chatelier's Principle inapplicable.

Define Le Chatelier's Principle.

It is the principle stating that if a change in conditions (temperature, pressure, or concentration) is applied to a system at equilibrium, the system will shift its position to oppose and minimize that change.

What does a 'shift to the right' signify in a reversible reaction?

A shift to the right indicates that the forward reaction is favored, resulting in an increase in the concentration of products and a decrease in the concentration of reactants until a new equilibrium is reached.

What is meant by an 'endothermic direction' in the context of equilibrium?

It refers to the direction of a reversible reaction that absorbs thermal energy from the surroundings. Shifting in this direction helps the system counteract an external increase in temperature.

Why does increasing the pressure favor the side with fewer gas molecules?

According to Le Chatelier's Principle, the system tries to oppose the pressure increase. By shifting to the side with fewer molecules, the system effectively reduces the number of particles hitting the walls, thereby lowering the internal pressure.

Library Podcasts

Courses

Referral & Rewards

3. Physical Chemistry

The Position of Equilibrium

Summary

The position of equilibrium describes the relative proportions of reactants and products in a dynamic system at balance. Governed by Le Chatelier's Principle, the system responds to external changes in temperature, pressure, or concentration by shifting its balance to counteract the disturbance and establish a new state of equilibrium.

1. Definition & Le Chatelier's Principle

Position of Equilibrium: This term refers to the relative concentrations of reactants and products when a reversible reaction reaches a steady state in a closed system. If the 'position' lies to the right, the concentration of products is greater than that of reactants; if it lies to the left, reactants predominate.
Le Chatelier's Principle: This fundamental rule states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract or oppose the change. It provides a qualitative way to predict how a system will behave when subjected to external stress.
Predictive Power: By understanding this principle, chemists can manipulate reaction conditions such as temperature and pressure to maximize the yield of a desired product in industrial processes.

2. Underlying Principles of Temperature

Thermochemical Context: To predict the effect of temperature, one must know the enthalpy change ( $\Delta H$ ) of the reaction. In any reversible system, if the forward reaction is exothermic (releases heat), the reverse reaction must be endothermic (absorbs heat) by the same magnitude.
Temperature Increase: When heat is added to a system, the equilibrium shifts in the endothermic direction to absorb the excess thermal energy. This shift reduces the temperature rise that would otherwise occur.
Temperature Decrease: If the system is cooled, the equilibrium shifts in the exothermic direction to release heat and counteract the cooling effect. Consequently, cooling an exothermic forward reaction will increase the yield of products.

Graph showing how yield changes with temperature for endothermic and exothermic reactions.

3. Methods: Managing Gaseous Pressure

4. The Role of Catalysts

5. Key Distinctions: Yield vs. Rate

6. Exam Strategy & Tips