Chapter 14 Chemical Equilibrium Part 1 The concept of chemical equilibrium

Summary

This video script explains the concept of equilibrium in reversible chemical reactions, highlighting how reactions proceed in both forward and reverse directions. It introduces the equilibrium constant (Kc) and reaction quotient (Qc), which are essential in understanding how a system reaches equilibrium. The equilibrium constant remains constant at a given temperature, but varies with the concentrations of reactants and products. Examples of equilibrium expressions, including one for the nitrogen-hydrogen reaction forming ammonia, demonstrate how Kc is calculated. The script emphasizes that while equilibrium composition can differ, Kc remains unchanged unless the temperature changes.

Takeaways

• 😀 Reversible reactions can proceed in both directions: forward (reactants to products) and reverse (products to reactants).
• 😀 Equilibrium is reached when the rates of the forward and reverse reactions are equal, and the amounts of reactants and products remain constant.
• 😀 At equilibrium, reactions continue, but no net change occurs in the concentration of reactants and products.
• 😀 Some reactions are considered irreversible for practical purposes due to the significant energy required to reverse them, such as the combustion of glucose.
• 😀 Photosynthesis is an example of a reaction where carbon dioxide and water are converted back into glucose, though this requires energy input.
• 😀 The equilibrium constant (Kc) is a unitless numerical value that represents the ratio of product concentrations to reactant concentrations at equilibrium.
• 😀 The equilibrium expression includes reactant and product concentrations raised to powers based on their stoichiometric coefficients.
• 😀 The reaction quotient (Qc) is calculated using non-equilibrium concentrations and can be used to predict the direction the reaction will go to reach equilibrium.
• 😀 The reaction quotient (Qc) equals the equilibrium constant (Kc) when equilibrium is reached, and they approach each other as the system progresses toward equilibrium.
• 😀 The equilibrium constant (Kc) is constant for a given reaction at a specific temperature, although equilibrium concentrations can vary based on initial amounts.
• 😀 Only temperature changes can affect the value of the equilibrium constant, not changes in concentration or pressure.

Q & A

What is the concept of chemical equilibrium?

Chemical equilibrium occurs when a reversible reaction reaches a state where the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products remain constant over time.

Why are some reactions considered irreversible for practical purposes?

Some reactions, like the combustion of glucose, are highly exothermic and release large amounts of energy. Reversing these reactions would require an immense amount of energy, making them practically irreversible.

What does the double arrow (⇌) in a chemical equation indicate?

The double arrow indicates that the reaction is reversible, meaning both the forward and reverse reactions are occurring simultaneously.

What is meant by the term 'equilibrium composition'?

Equilibrium composition refers to the constant concentrations of reactants and products once a reversible reaction has reached equilibrium.

What is the difference between dynamic equilibrium and static equilibrium?

In dynamic equilibrium, both the forward and reverse reactions continue to occur at equal rates, while in static equilibrium, no reactions are taking place. Chemical equilibrium is dynamic, as the reactions continue but at equal rates.

How is the equilibrium constant (Kc) calculated?

The equilibrium constant (Kc) is calculated using the concentrations of the reactants and products at equilibrium. The expression is the ratio of the product concentrations to the reactant concentrations, each raised to the power of its stoichiometric coefficient.

What is the reaction quotient (Qc), and how is it different from the equilibrium constant (Kc)?

The reaction quotient (Qc) is calculated using the concentrations of reactants and products at any point in time, not necessarily at equilibrium. If Qc equals Kc, the system is at equilibrium; if Qc is different from Kc, the system will shift to reach equilibrium.

Why does the equilibrium constant (Kc) not change with concentration or pressure?

The equilibrium constant (Kc) is only affected by temperature. Changes in concentration or pressure may shift the position of equilibrium, but they do not change the value of Kc.

How does the reaction progress towards equilibrium in a system that starts out of equilibrium?

If a system is not at equilibrium, the reaction will spontaneously move toward equilibrium. The system will shift in the direction that reduces the free energy until the rates of the forward and reverse reactions are equal.

How do you calculate the equilibrium constant for a reaction when the number of moles is given?

To calculate the equilibrium constant when given the number of moles, you first convert the moles into molarity by dividing by the volume of the container. Then, substitute these values into the equilibrium expression to calculate Kc.