Equilibrio Químico Teoría 2: Constante de equilibrio Kc. Ley de acción de masas.

Quimitube Tu libro de Química

5 Mar 201519:33

Summary

TLDRThis video explains the concept of equilibrium constants in chemical reactions, detailing how they are derived from reaction rates and concentrations. It emphasizes that equilibrium constants depend solely on temperature and not on the initial concentrations of reactants or products. The video also discusses the relationship between equilibrium constants and the direction in which a reaction is shifted, whether toward reactants or products. Finally, it addresses special cases such as heterogeneous equilibria and reactions with multiple stages, making the content useful for students learning about chemical kinetics and thermodynamics.

Takeaways

⚖️ The equilibrium constant, denoted as K, is a relationship between the concentrations of products and reactants in a reversible reaction.
🔁 A reversible reaction can proceed in both directions, with forward and reverse rates represented by specific constants.
⚡ At equilibrium, the rates of the forward and reverse reactions become equal, which defines the state of balance between products and reactants.
🧪 For elementary reactions, the rate law can be expressed directly based on reactant concentrations, following stoichiometric coefficients.
🧮 The equilibrium constant formula involves the ratio of the concentration of products to reactants, each raised to the power of their stoichiometric coefficients.
🌡️ The equilibrium constant depends solely on temperature and is not influenced by initial concentrations of reactants or products.
🔬 High K values indicate a reaction favoring products, while low K values suggest that reactants dominate at equilibrium.
📉 A very high equilibrium constant implies the reaction is almost irreversible, producing mostly products.
🔄 For reversible reactions expressed in opposite directions, the equilibrium constants are reciprocal of each other.
🧱 Solids and pure liquids are not included in the equilibrium expression as their concentrations remain constant.

Q & A

What is the equilibrium constant, and how is it symbolized?
-The equilibrium constant is a relationship between the concentrations of products and reactants at equilibrium. It is symbolized as 'Kc'.
How do reaction rates differ for forward and reverse reactions in a reversible process?
-In a reversible reaction, the forward reaction has a rate constant 'k_forward', and the reverse reaction has a rate constant 'k_reverse'. These rates change until they equal each other at equilibrium.
What happens to the reaction rates at equilibrium?
-At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction. This balance means the concentrations of reactants and products remain constant over time.
How can we express the equilibrium constant in terms of reactant and product concentrations?
-The equilibrium constant expression is written as the ratio of the product of the concentrations of products (raised to their stoichiometric coefficients) over the product of the concentrations of reactants (also raised to their stoichiometric coefficients).
What does the magnitude of the equilibrium constant tell us about the reaction?
-If Kc > 1, the reaction favors the formation of products at equilibrium. If Kc < 1, the reaction favors the reactants, meaning there will be more reactants than products at equilibrium.
How does temperature affect the equilibrium constant?
-The equilibrium constant is dependent on temperature. Any change in temperature will alter the value of Kc but not the concentrations of reactants or products.
What are the units of the equilibrium constant?
-In most cases, the equilibrium constant is dimensionless. However, it is calculated using concentrations (in mol/L), but the final value of Kc does not have units.
When writing the equilibrium constant expression, which substances are excluded?
-In equilibrium expressions, pure solids and pure liquids are excluded because their concentrations do not change. Only gases and aqueous solutions are included.
How does the equilibrium constant change if the reaction is reversed?
-If the reaction is reversed, the equilibrium constant for the reverse reaction is the reciprocal of the equilibrium constant for the forward reaction (K_reverse = 1/K_forward).
What is the relationship between equilibrium constants for reactions involving multiple steps?
-If a reaction occurs in multiple steps, the equilibrium constant for the overall reaction is the product of the equilibrium constants for each step.

Outlines

00:00

🔬 Understanding Equilibrium Constant in Chemical Reactions

The paragraph explains the concept of equilibrium constant (K), which represents the relationship between the concentrations of products and reactants in a reversible reaction. It introduces the idea of reaction rates for both the forward and reverse reactions and shows how these rates balance at equilibrium. The paragraph details how the concentrations of reactants and products are raised to their stoichiometric coefficients in the rate equations. It also highlights that once equilibrium is reached, the rate of the forward reaction equals the rate of the reverse reaction, resulting in a constant ratio of products to reactants.

05:05

⚖️ Expression of the Equilibrium Constant and Its Temperature Dependence

This section discusses how to express the equilibrium constant for specific reactions. It demonstrates using an example of a reaction between iodine and hydrogen, describing the general form of the equilibrium constant expression as the ratio of product concentrations to reactant concentrations, raised to their stoichiometric coefficients. It emphasizes that the equilibrium constant depends on temperature, not on initial concentrations, and explains why concentration units are often omitted from the equilibrium constant's expression.

10:05

📈 Analyzing the Equilibrium Constant to Predict Reaction Direction

The paragraph focuses on interpreting the value of the equilibrium constant. A value greater than 1 indicates that products dominate at equilibrium, suggesting the reaction is displaced to the right. Conversely, a value less than 1 suggests that reactants dominate, meaning the reaction is displaced to the left. It provides examples, such as the decomposition of N2O4, with varying equilibrium constants to illustrate how the reaction shifts towards products or reactants depending on the constant's magnitude.

15:07

🔄 Reciprocal and Adjusted Equilibrium Constants

This section explains the concept of reciprocal equilibrium constants, showing that if a reaction is reversed, the equilibrium constant becomes the reciprocal of the original. Additionally, the paragraph demonstrates how multiplying the coefficients of a reaction by a factor affects the equilibrium constant by raising it to the corresponding power. This allows for flexibility in adjusting equilibrium expressions based on how the reaction is written.

🧮 Combining Equilibrium Constants for Multi-Step Reactions

The paragraph explores how equilibrium constants behave when reactions occur in multiple steps. It explains that the overall equilibrium constant for a reaction that is the sum of two other reactions is the product of their individual equilibrium constants. This principle is useful when analyzing complex reactions broken into smaller steps. Additionally, it touches on heterogeneous equilibria, stating that pure solids and liquids are not included in the equilibrium expression, as their concentrations remain constant.

Mindmap

Keywords

💡Equilibrium constant (K)

The equilibrium constant, denoted as 'K,' represents the ratio of the concentrations of products to reactants at equilibrium, with each raised to the power of their respective stoichiometric coefficients. It indicates the extent to which a chemical reaction proceeds and only depends on temperature. In the video, the constant is used to understand how reactions shift between reactants and products at equilibrium.

💡Reaction rate

Reaction rate refers to the speed at which reactants are converted into products. The video discusses reaction rates in both directions of a reversible reaction, highlighting how at equilibrium, the rates of the forward and reverse reactions become equal, maintaining a constant concentration of both reactants and products.

💡Reversible reaction

A reversible reaction is one in which products can revert back to reactants. In the video, reversible reactions are depicted using the double arrow (⇌), indicating that both the forward and reverse processes occur, ultimately leading to equilibrium. Examples include the reaction between hydrogen and iodine to form hydrogen iodide.

💡Law of mass action

The law of mass action states that the rate of a chemical reaction is directly proportional to the concentrations of the reactants. The video explains this principle in the context of equilibrium, where the equilibrium constant is derived from the concentrations of products and reactants raised to the power of their stoichiometric coefficients.

💡Stoichiometric coefficients

Stoichiometric coefficients are the numbers placed in front of reactants and products in a balanced chemical equation, indicating the ratio in which substances react. The video emphasizes their importance in calculating both reaction rates and equilibrium constants, as these coefficients determine the exponents in the equilibrium expression.

💡Elementary reaction

An elementary reaction is a single-step process in which reactants directly form products. The video assumes that the reactions discussed are elementary, which simplifies the relationship between reaction rate and concentration. For such reactions, the stoichiometric coefficients and the exponents in the rate law match.

💡Temperature dependence

The video highlights that the equilibrium constant (K) is dependent on temperature. This means that changes in temperature can shift the position of equilibrium, favoring either the formation of products or reactants. The script provides examples, such as how the equilibrium constant changes for a specific reaction at different temperatures.

💡Concentration at equilibrium

The concentration at equilibrium refers to the amounts of reactants and products present once a reaction has reached equilibrium. The video clarifies that these concentrations are used to calculate the equilibrium constant, and they differ from the initial concentrations introduced into the system.

💡Heterogeneous equilibrium

Heterogeneous equilibrium occurs when reactants and products are in different phases, such as solids, liquids, and gases. The video explains that in such cases, the concentrations of pure solids and liquids are not included in the equilibrium constant expression, as their concentrations remain constant.

💡Reaction displacement

Reaction displacement refers to whether a reaction at equilibrium has more products or reactants. The video discusses how the magnitude of the equilibrium constant (K) determines the direction of displacement: if K is greater than 1, the reaction favors products, while if K is less than 1, it favors reactants.

Highlights

The relationship between the concentrations of products and reactants in a reversible reaction is known as the equilibrium constant.

Equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction.

The equilibrium constant expression is written as the ratio of the concentrations of products to reactants, each raised to the power of their stoichiometric coefficients.

The equilibrium constant (K) depends only on temperature and is independent of the initial concentrations of reactants.

The equilibrium constant can indicate the direction in which a reaction is favored: if K > 1, products are favored; if K < 1, reactants are favored.

In an equilibrium reaction with a large K value, the reaction behaves almost like an irreversible reaction, with nearly complete conversion to products.

When K is very small, the reaction barely proceeds, and the concentration of reactants dominates in the equilibrium state.

For reactions expressed in different directions, the equilibrium constants are reciprocals of each other.

If the reaction is scaled (e.g., coefficients are multiplied), the equilibrium constant is raised to the corresponding power.

The equilibrium constant for a multi-step reaction is the product of the equilibrium constants for the individual steps.

In heterogeneous equilibria, only gaseous and dissolved species are included in the equilibrium expression; solids and pure liquids are not.

At equilibrium, the relative amounts of products and reactants can be predicted using the equilibrium constant expression.

The value of K provides insight into how far the reaction proceeds before reaching equilibrium.

In cases of heterogeneous reactions involving solids, the concentration of solids remains constant and is not included in the equilibrium constant expression.

The equilibrium constant for a given reaction can help predict the shift in equilibrium when temperature changes.