Getting Started
Chemical reactions in a closed system often do not proceed to completion but instead reach a state of dynamic equilibrium. At the molecular level, this is a state of balance where the forward and reverse reaction rates are equal. This chapter explores how we can predict the behavior of a system at equilibrium when it is subjected to a disturbance, or "stress," and how it re-establishes a new equilibrium state.
What You Should Be Able to Do
After completing this section, you should be able to:
Calculate the reaction quotient, Q, for a system at any given moment.
Compare the value of Q to the equilibrium constant, K, to determine if a reaction is at equilibrium.
Predict the direction (forward or reverse) a reaction will shift to reach equilibrium based on the relationship between Q and K.
Explain how changes in concentration, pressure, or temperature affect a system at equilibrium, causing it to shift to a new equilibrium position.
Key Concepts & Analysis
Baseline Condition: The System at Equilibrium
A reversible reaction is at equilibrium when the rate of the forward reaction equals the rate of the reverse reaction. Macroscopically, the concentrations of all reactants and products appear constant. This state is quantified by the equilibrium constant (K), a temperature-dependent value representing the ratio of product concentrations to reactant concentrations, each raised to the power of its stoichiometric coefficient.
For a general reaction: aA + bB ⇌ cC + dD
The equilibrium constant expression is:
K = ([C]ᶜ[D]ᵈ) / ([A]ᵃ[B]ᵇ)
When a system is at equilibrium, this specific ratio of concentrations is maintained.
The Process or Stress: Disturbing Equilibrium
Equilibrium is a delicate balance. A stress is any change in conditions that disrupts this balance. According to Le Châtelier’s Principle, if a stress is applied to a system at equilibrium, the system will shift in a direction that partially counteracts the stress to re-establish equilibrium.
To analyze this shift quantitatively, we introduce the reaction quotient (Q). Q is calculated using the same mathematical expression as K, but it uses the concentrations of reactants and products at any given moment, not just at equilibrium.
If Q = K, the system is at equilibrium. The forward and reverse reaction rates are equal, and there is no net change.
If Q < K, the ratio of products to reactants is too small. To reach equilibrium, the system must increase the concentration of products and decrease the concentration of reactants. The reaction will proceed in the forward direction (shift right).
If Q > K, the ratio of products to reactants is too large. To reach equilibrium, the system must decrease the concentration of products and increase the concentration of reactants. The reaction will proceed in the reverse direction (shift left).
A disturbance to a system at equilibrium causes Q to no longer equal K. The system then responds by shifting until Q once again equals K.
The Resulting Change: Re-establishing Equilibrium
The way a system responds depends on the type of stress applied.
1. Changes in Concentration:
Stress: Adding a reactant or product.
Immediate Effect: The value of Q changes instantly. For example, adding a reactant decreases the value of Q (Q < K). Adding a product increases the value of Q (Q > K).
Response: The system shifts to consume the added species. If a reactant is added, the reaction shifts right. If a product is added, the reaction shifts left.
Continuity: The value of K remains unchanged because the temperature is constant. The system shifts until the concentration ratio brings Q back to the original value of K.
2. Changes in Pressure (for gaseous systems):
Changes in pressure, typically by changing the volume of the container, affect the concentrations of all gaseous species simultaneously.
Stress: Increasing pressure (decreasing volume).
Immediate Effect: Q changes. The system is no longer at equilibrium.
Response: The system will shift to reduce the total number of gas moles, thereby reducing the pressure. It shifts toward the side of the reaction with fewer moles of gas.
Continuity: K remains unchanged.
3. Changes in Temperature:
Temperature is unique because it is the only stress that changes the value of the equilibrium constant, K.
Stress: Increasing the temperature.
Immediate Effect: Both Q and K are affected. The system is no longer at equilibrium.
Response: The system will shift to absorb the added heat.
For an endothermic reaction (ΔH > 0), heat is a reactant. Increasing temperature will cause the reaction to shift right, favoring products. The value of K increases.
For an exothermic reaction (ΔH < 0), heat is a product. Increasing temperature will cause the reaction to shift left, favoring reactants. The value of K decreases.
Change: The value of K itself changes. The system establishes a new equilibrium state with a different ratio of products to reactants.
Key Models & Representations
The table below summarizes how a system at equilibrium responds to various stresses, using the Haber-Bosch process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (ΔH < 0), as an example.
| Stress Applied | Immediate Effect on Q or K | Direction of Shift | Resulting Change to Reach New Equilibrium |
|---|---|---|---|
| Add N₂(g) | Q decreases (Q < K) | Right (Forward) | [NH₃] increases; [H₂] decreases. |
| Remove NH₃(g) | Q decreases (Q < K) | Right (Forward) | [N₂] and [H₂] decrease to produce more NH₃. |
| Increase Pressure | Q changes | Right (toward fewer moles of gas) | [NH₃] increases; [N₂] and [H₂] decrease. |
| Increase Temperature | K decreases | Left (Reverse, endothermic direction) | [N₂] and [H₂] increase; [NH₃] decreases. |
Key Terms, Quantities, & Concepts
Equilibrium: A state in a reversible reaction where the rate of the forward reaction equals the rate of the reverse reaction, resulting in no net change in the concentrations of reactants and products.
Le Châtelier’s Principle: A principle stating that if a change of condition (a stress) is applied to a system in equilibrium, the system will shift in a direction that relieves the stress.
Reaction Quotient (Q): A value calculated using the law of mass action expression with concentrations at any given moment; it is used to determine the direction a reaction must shift to reach equilibrium.
Equilibrium Constant (K): The value of the reaction quotient when the system has reached equilibrium. It is constant for a given reaction at a specific temperature.
Stress: A change in concentration, pressure, volume, or temperature that disturbs a system at equilibrium.
Shift Right: The forward reaction is favored, consuming reactants to form more products.
Shift Left: The reverse reaction is favored, consuming products to form more reactants.
Endothermic Reaction: A reaction that absorbs heat from its surroundings (ΔH > 0).
Exothermic Reaction: A reaction that releases heat into its surroundings (ΔH < 0).
Skill Snapshots
Causation
Cause: Adding more reactant to a system at equilibrium. Effect: The value of Q becomes less than K, causing the forward reaction rate to temporarily exceed the reverse rate, shifting the equilibrium to the right.
Cause: Increasing the temperature of an exothermic reaction. Effect: The value of K decreases, causing the system to shift left to consume products and absorb the added thermal energy.
Cause: Decreasing the volume of a gaseous equilibrium system. Effect: The partial pressures of all gases increase, causing the equilibrium to shift toward the side with fewer moles of gas to reduce the overall pressure.
Comparison
Q vs. K: Q is a measure of the product-to-reactant ratio at any point in a reaction, while K is the specific value of that ratio only at equilibrium.
Concentration Stress vs. Temperature Stress: A change in concentration or pressure alters Q but not K; the system shifts to restore Q to its original K value. A change in temperature alters the value of K itself, causing the system to shift to establish a new equilibrium state where Q equals the new K.
Shift Left vs. Shift Right: A shift right indicates the forward reaction is favored (Q < K), leading to a net increase in products. A shift left indicates the reverse reaction is favored (Q > K), leading to a net increase in reactants.
Change and Continuity Over Time (CCOT)
Baseline: A system is at equilibrium, where Q = K and the concentrations of all species are constant.
Change 1 (Concentration): A reactant is added. Q immediately drops below K. The system responds by shifting right, consuming reactants and forming products until Q once again equals the original K.
Change 2 (Temperature): The temperature is increased for an endothermic reaction. The value of K increases. The system responds by shifting right, consuming reactants to form more products until Q equals the new, larger K.
Continuity: Unless the temperature is changed, the value of the equilibrium constant (K) for a given reaction remains constant, regardless of changes in concentration or pressure.
Common Misconceptions & Clarifications
Misconception: Adding a catalyst changes the equilibrium position.
- Clarification: A catalyst increases the rates of both the forward and reverse reactions equally. It causes the system to reach equilibrium faster, but it does not change the value of K or the final concentrations of reactants and products at equilibrium.
Misconception: Adding an inert (non-reacting) gas to a constant-volume container will shift the equilibrium.
- Clarification: Adding an inert gas increases the total pressure, but it does not change the partial pressures or concentrations of the reacting gases. Since Q and K are based on partial pressures or concentrations, the equilibrium is unaffected.
Misconception: Changes in the amounts of pure solids or pure liquids can shift an equilibrium.
- Clarification: The concentrations (or activities) of pure solids and liquids are considered constant and are not included in the Q or K expressions. Therefore, adding or removing a pure solid or liquid does not change Q and does not shift the equilibrium.
Misconception: Le Châtelier's Principle means the system completely undoes the stress.
- Clarification: The principle states the system partially counteracts the stress. For example, if you add a reactant, the system will shift right to consume some of it, but the final concentration of that reactant will still be higher than it was at the initial equilibrium.
One-Paragraph Summary
The reaction quotient, Q, provides a snapshot of a reversible reaction's state by comparing the ratio of products to reactants at any moment to the equilibrium constant, K. If a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, Q will no longer equal K. Le Châtelier’s Principle dictates that the system will shift—either forward (if Q < K) or reverse (if Q > K)—to partially counteract the disturbance and re-establish equilibrium. While concentration and pressure changes cause shifts that restore the original K value, a temperature change alters K itself, leading the system to a fundamentally new equilibrium position. This predictive framework is essential for controlling the yield of chemical reactions in industrial and laboratory settings.