moles

Página 1

Study Guide: Chemical Equilibrium

This study guide summarizes key concepts and principles related to chemical equilibrium, using the example of the reaction between hydrogen and iodine gases to form hydrogen iodide (HI). It includes essential details about equilibrium, the equilibrium constant, reaction quotient, and how changes in conditions affect equilibrium.

1. Chemical Equilibrium Basics

Equilibrium Definition: Chemical equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction. At this point, the concentrations of reactants and products remain constant, even though the reactions are still occurring.

Example Reaction:

H2(g)+I2(g)⇌2HI(g)\text{H}_2(g) + \text{I}_2(g) \rightleftharpoons 2 \text{HI}(g)H2​(g)+I2​(g)⇌2HI(g)

Forward Reaction: H2(g)+I2(g)→2HI(g)\text{H}_2(g) + \text{I}_2(g) \to 2 \text{HI}(g)H2​(g)+I2​(g)→2HI(g)

Reverse Reaction: 2HI(g)→H2(g)+I2(g)2 \text{HI}(g) \to \text{H}_2(g) + \text{I}_2(g)2HI(g)→H2​(g)+I2​(g)

Equilibrium Process:

Initially, only the forward reaction occurs (since no HI is present).

As HI forms, it begins to decompose into H2 and I2.

Eventually, the rate of the forward reaction slows while the rate of the reverse reaction increases.

At equilibrium, the rate of formation of HI equals the rate of its decomposition, and the concentrations of all species become constant.

2. Conditions for Equilibrium

Closed System: The system must be closed so no substances can enter or leave.

Dynamic Process: Even though concentrations are constant at equilibrium, both the forward and reverse reactions are still occurring at equal rates.

Equilibrium Characteristics:

The rates of the forward and reverse reactions must be equal.

The concentrations of reactants and products do not have to be equal at equilibrium, but they remain constant.

The position of equilibrium (relative concentrations of reactants and products) is characteristic of the particular reaction and is independent of how equilibrium was achieved.

3. Types of Equilibrium

Chemical Equilibrium: This refers to the equilibrium between reactants and products in a chemical reaction (like the H2 + I2 ⇌ 2HI reaction).

Phase Equilibrium: Occurs when a substance is in equilibrium between two phases. For example, when the rate of evaporation equals the rate of condensation in a sealed container of water.

Solution Equilibrium: Occurs when a solid is in equilibrium with its dissolved ions in a saturated solution. The rate of dissolution equals the rate of recrystallization.

4. The Equilibrium Constant (Keq)

Definition: The equilibrium constant (Keq) expresses the ratio of the concentrations of products to reactants at equilibrium. It is determined experimentally and is a constant value for a given reaction at a specific temperature.

General Expression: For the reaction:

aA+bB⇌cC+dDaA + bB \rightleftharpoons cC + dDaA+bB⇌cC+dD

The equilibrium constant is:

Keq=[C]c[D]d[A]a[B]bK_\text{eq} = \frac{[C]^c [D]^d}{[A]^a [B]^b}Keq​=[A]a[B]b[C]c[D]d​

where:

[C], [D], [A], [B] are the molar concentrations of the substances at equilibrium.

The exponents correspond to the coefficients from the balanced chemical equation.

Interpreting Keq:

If Keq > 1: Products are favored, meaning the concentration of products is greater than that of the reactants.

If Keq < 1: Reactants are favored, meaning the concentration of reactants is greater than that of the products.

If Keq ≈ 1: The concentrations of reactants and products are approximately equal.

Temperature Dependence: The value of Keq is temperature-dependent. Changing the temperature of the system affects the equilibrium constant.

5. Reaction Quotient (Q)

Definition: The reaction quotient (Q) is calculated in the same way as Keq but is used to determine whether a reaction is at equilibrium. It compares the ratio of product and reactant concentrations at any point in the reaction to the equilibrium constant.

Comparing Q to Keq:

Q = Keq: The system is at equilibrium.

Q < Keq: The system is not at equilibrium, and the reaction will proceed to the right (towards products) to reach equilibrium.

Q > Keq: The system is not at equilibrium, and the reaction will proceed to the left (towards reactants) to reach equilibrium.

6. Le Châtelier's Principle

Principle Overview: If a system at equilibrium is disturbed by changing the conditions (such as concentration, pressure, or temperature), the system will adjust to counteract the change and restore equilibrium.

Effect of Changes:

Concentration: Increasing the concentration of reactants or products shifts the equilibrium in the direction that reduces the added substance.

Temperature: Increasing the temperature shifts the equilibrium in the direction that absorbs heat (endothermic direction).

Pressure: Changes in pressure affect equilibria involving gases. An increase in pressure shifts the equilibrium toward the side with fewer moles of gas.

7. Example: Hemoglobin and Carbon Monoxide Poisoning

Reaction:

Hb(CO)4(aq)+4O2(g)⇌Hb(O2)4(aq)+4CO(g)\text{Hb(CO)}_4(aq) + 4 \text{O}_2(g) \rightleftharpoons \text{Hb(O}_2)_4(aq) + 4 \text{CO}(g)Hb(CO)4​(aq)+4O2​(g)⇌Hb(O2​)4​(aq)+4CO(g)

Explanation:

In the presence of carbon monoxide (CO), hemoglobin binds more strongly to CO than to oxygen (O2), impairing the ability of red blood cells to transport oxygen.

Treatment involves adding excess O2, which shifts the equilibrium toward the formation of Hb(O2)4, displacing CO and restoring oxygen transport.

8. Graphs of Concentration vs. Time

Changes in Concentration:

Graph A: Starts with only reactants (H2 and I2), and the concentration of products (HI) increases over time as the reaction proceeds toward equilibrium.

Graph B: Starts with only product (HI), and the concentration of reactants (H2 and I2) increases over time as the reverse reaction occurs until equilibrium is reached.

At Equilibrium:

The concentration of each species becomes constant.

The relative position of equilibrium (the ratio of reactants to products) is the same in both cases.

Key Takeaways:

Chemical equilibrium is dynamic; both forward and reverse reactions continue, but at equal rates.

The equilibrium constant (Keq) is specific to each reaction and is influenced by temperature.

The reaction quotient (Q) can help predict the direction of the reaction based on the current concentrations of reactants and products.

Le Châtelier's Principle helps predict how a system will respond to changes in concentration, temperature, or pressure.