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13.3 Shifting Equilibria: Le Châtelier’s Principle

3 min read•june 25, 2024

Chemical equilibrium is a dynamic balance between forward and reverse reactions. Factors like , , and can this balance. Understanding these influences helps predict and control reaction outcomes in various chemical processes.

The compares to the (K) to predict shifts. Temperature affects exothermic and endothermic reactions differently. Pressure impacts gas-phase equilibria, while speed up reactions without changing the .

Factors Affecting Equilibrium

Dynamic Equilibrium and Reaction Quotient

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  • occurs when forward and rates are equal
  • (Q) measures the relative amounts of products and reactants at any point in a reaction
  • Comparing Q to the equilibrium constant (K) predicts the direction of equilibrium shift:
    • If Q < K, the reaction will shift towards products
    • If Q > K, the reaction will shift towards reactants
    • If Q = K, the is at equilibrium

Effects of concentration on equilibrium

  • Increasing the of a
    • Shifts the equilibrium to the right, towards the products (forming more products)
    • Increases the rate of the until a new equilibrium is established to counteract the change
  • Decreasing the concentration of a reactant
    • Shifts the equilibrium to the left, towards the reactants (forming more reactants)
    • Increases the rate of the reverse reaction until a new equilibrium is established to counteract the change
  • Increasing the concentration of a
    • Shifts the equilibrium to the left, towards the reactants (consuming more products)
    • Increases the rate of the reverse reaction until a new equilibrium is established to counteract the change
  • Decreasing the concentration of a product
    • Shifts the equilibrium to the right, towards the products (forming more products)
    • Increases the rate of the forward reaction until a new equilibrium is established to counteract the change

Temperature changes in chemical equilibria

  • Exothermic reactions (ΔH<0\Delta H < 0) release heat to the
    • Increasing temperature shifts the equilibrium to the left, towards the reactants (consumes heat)
    • Decreasing temperature shifts the equilibrium to the right, towards the products (releases heat)
  • Endothermic reactions (ΔH>0\Delta H > 0) absorb heat from the surroundings
    • Increasing temperature shifts the equilibrium to the right, towards the products (absorbs heat)
    • Decreasing temperature shifts the equilibrium to the left, towards the reactants (releases heat)
  • Temperature changes affect the equilibrium constant (KK)
    • For exothermic reactions, increasing temperature decreases KK (favors reactants)
    • For endothermic reactions, increasing temperature increases KK (favors products)

Pressure impacts on gas-phase equilibrium

  • Increasing pressure
    • Shifts the equilibrium towards the side with fewer (reduces volume)
    • Favors the reaction that decreases the total number of gas molecules (forming fewer gas molecules)
  • Decreasing pressure
    • Shifts the equilibrium towards the side with more moles of gas (increases volume)
    • Favors the reaction that increases the total number of gas molecules (forming more gas molecules)
  • Pressure changes have no effect on equilibrium if
    • The total number of moles of gas is the same on both sides of the reaction (no net change in gas molecules)
    • The reaction involves only solid or liquid phases (incompressible, volume unaffected by pressure)

Catalysts and Equilibrium

  • A increases the rate of both forward and reverse reactions equally
  • Catalysts do not affect the position of equilibrium or the equilibrium constant
  • Catalysts help a system reach equilibrium faster without changing the final equilibrium composition

Key Terms to Review (28)

Catalyst: A catalyst is a substance that increases the rate of a chemical reaction without being consumed or altered itself. Catalysts play a crucial role in various chemical processes, including reaction mechanisms, catalysis, chemical equilibria, and shifting equilibria.
Catalysts: Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process. They work by lowering the activation energy needed for the reaction to proceed.
Concentration: Concentration is the measure of the amount of a solute that is dissolved in a given quantity of solvent. It is commonly expressed in terms of molarity (M), which is moles of solute per liter of solution.
Concentration: Concentration is a measure of the amount of a substance present in a given volume or mass of a solution or mixture. It is a fundamental concept in chemistry that is closely related to the study of chemical reactions, equilibrium, and the behavior of solutions.
Dalton’s law of partial pressures: Dalton's law of partial pressures states that the total pressure exerted by a mixture of non-reacting gases is equal to the sum of the partial pressures of each individual gas. Each gas in a mixture behaves independently and contributes to the total pressure in proportion to its mole fraction.
Dynamic equilibrium: Dynamic equilibrium occurs when the rates of the forward and reverse processes are equal, resulting in no net change in the system. It is a key concept in phase transitions where phases coexist at equilibrium.
Dynamic Equilibrium: Dynamic equilibrium is a state in which opposing chemical processes occur at equal rates, resulting in a stable and unchanging overall system. It is a fundamental concept in chemistry that describes the balance between forward and reverse reactions in a closed system.
Endothermic Reaction: An endothermic reaction is a type of chemical reaction in which the system absorbs energy from the surroundings in the form of heat. This means that the products of the reaction have a higher energy content than the reactants, and the reaction requires an input of energy to proceed.
Equilibrium Constant: The equilibrium constant is a quantitative measure of the extent of a chemical reaction at equilibrium. It represents the ratio of the concentrations of the products to the reactants, raised to their respective stoichiometric coefficients, and is a fundamental concept in understanding the behavior of chemical systems at equilibrium.
Equilibrium constant, K: The equilibrium constant, $K$, is a ratio that quantifies the concentrations of reactants and products in a chemical reaction at equilibrium. It provides insight into the position of the equilibrium and the extent to which reactants are converted into products.
Equilibrium Position: Equilibrium position refers to the state in which the forward and reverse reactions in a chemical system occur at equal rates, resulting in a constant composition of the reactants and products. This term is central to understanding the behavior and characteristics of chemical equilibria.
Exothermic Reaction: An exothermic reaction is a chemical reaction that releases energy in the form of heat to the surrounding environment. These types of reactions are characterized by a decrease in the overall energy of the system, with the products of the reaction having less energy than the reactants.
Forward Reaction: The forward reaction refers to the direction of a chemical reaction that proceeds from the reactants to the products. It is one of the key components of understanding equilibrium and how chemical systems respond to changes in conditions, as described in Le Châtelier's Principle.
Le Châtelier’s principle: Le Châtelier’s principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. This principle helps predict how changes in concentration, temperature, or pressure affect chemical systems at equilibrium.
Le Châtelier's Principle: Le Châtelier's Principle states that when a system at equilibrium is subjected to a change in one of the factors (concentration, temperature, or pressure) determining the equilibrium, the system will shift to counteract the change and establish a new equilibrium.
Moles of Gas: The mole is the fundamental unit of amount of substance in the International System of Units (SI). It is defined as the amount of a substance that contains the same number of particles, such as atoms, molecules, or ions, as there are atoms in 12 grams of pure carbon-12. The moles of gas refer to the quantity of a gaseous substance expressed in terms of the number of moles present.
Pressure: Pressure is a measure of the force exerted per unit area on a surface or object. It is a fundamental concept in physics and chemistry that plays a crucial role in understanding the behavior of gases, liquids, and solids, as well as the principles of equilibrium and the shifting of equilibria.
Product: In the context of chemical equilibria, the product refers to the substance(s) formed as the result of a chemical reaction. It is the desired outcome of the reaction and is present on the right side of the balanced chemical equation.
Reactant: A reactant is a substance that participates in a chemical reaction, undergoing transformation to produce new substances known as products. Reactants are the starting materials that interact and rearrange their chemical bonds to form the final outcome of a reaction.
Reaction Quotient: The reaction quotient, denoted as Q, is a measure of the relative concentrations of the products and reactants in a chemical reaction at any given time, regardless of whether the system has reached equilibrium or not. It is a useful tool for understanding the direction and extent of a reaction as it progresses towards equilibrium.
Reaction quotient (Q): The reaction quotient, Q, is a measure of the relative amounts of products and reactants present in a reaction mixture at any given point in time. It is calculated using the same expression as the equilibrium constant but with current concentrations or partial pressures.
Reverse Reaction: A reverse reaction is the opposite of the forward reaction in a chemical equilibrium system. It describes the process where the products of a reaction are converted back into the original reactants, moving the equilibrium in the opposite direction.
Shift: A shift refers to a change or adjustment in the position or state of an equilibrium system in response to a change in one of the conditions affecting the system. This concept is central to understanding Le Châtelier's Principle, which describes how a system at equilibrium will respond to external disturbances.
Standard temperature and pressure (STP): Standard Temperature and Pressure (STP) is a reference point used in chemistry to define a set of conditions for experimental measurements. It is defined as a temperature of 0°C (273.15 K) and a pressure of 1 atm.
Stress: Stress is a state of mental or emotional strain or tension resulting from adverse or demanding circumstances. In the context of 13.3 Shifting Equilibria: Le Châtelier's Principle, stress refers to the external factors that can disrupt the equilibrium of a chemical system.
Surroundings: The surroundings refer to the external environment or conditions that exist around a system of interest, such as a chemical reaction or a physical process. The surroundings play a crucial role in understanding the behavior and equilibrium of a system, as described by Le Châtelier's Principle.
System: A system is a set of interconnected components or elements that work together to achieve a common goal or purpose. It is a conceptual framework that helps us understand and analyze complex phenomena by breaking them down into manageable parts.
Temperature: Temperature is a measure of the average kinetic energy of the particles (atoms or molecules) in a substance. It is a fundamental physical quantity that reflects the degree of hotness or coldness of an object or system, and it plays a crucial role in various chemical and physical processes.
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13.4 Equilibrium Calculations