10th Class Chemistry full Notes of Chapter No. 9
The phenomenon of equilibrium plays a crucial role in sustaining life on Earth. It is responsible for the delicate balance in the atmosphere, allowing us to inhale oxygen and exhale carbon dioxide, while plants perform the reverse process, consuming carbon dioxide and releasing oxygen. This natural process is essential for the existence of life as we know it. Moreover, many environmental systems depend on equilibrium phenomena. For instance, the concentration of gases in lake water is governed by the principles of equilibrium, and the lives of aquatic plants and animals are indirectly related to the concentration of dissolved oxygen in water.
Long Questions
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Short Questions
Chemical equilibrium, a specific type of equilibrium, is a fascinating concept in chemistry. In a chemical reaction, reactants combine to form products. Irreversible reactions proceed to completion, with the products not recombining to form reactants, while reversible reactions allow products to recombine back into reactants. Reversible reactions are represented by a double arrow between the reactants and products. They consist of two reactions: the forward reaction (reactants to products) and the reverse reaction (products to reactants). These reactions occur simultaneously and eventually reach a dynamic equilibrium state, where the rates of the forward and reverse reactions become equal. At this state, the composition of the reaction mixture remains constant, and both reactions continue to take place.
For instance, a reversible reaction between hydrogen and iodine vapors can be observed in a closed flask. Initially, hydrogen and iodine react to form hydrogen iodide, causing the purple color of iodine to fade. However, when only hydrogen iodide is heated, purple iodine vapors reappear as the reverse reaction takes place. In a closed flask, both reactions proceed at the same rate, establishing a dynamic equilibrium where the concentration of various species remains constant.
MCQ’s
The law of mass action, proposed by Guldberg and Waage, describes the relationship between the rate of a reaction and the active mass of the reacting substances. It states that the rate of a substance’s reaction is directly proportional to its active mass, represented as molar concentration in square brackets []. The equilibrium constant (Kc) is derived from this law and represents the ratio of the products’ active masses to the reactants’ active masses at equilibrium. The equilibrium constant is a fundamental concept in understanding the extent of a reversible reaction at equilibrium.
Numerical and Examples
Overall, the concept of equilibrium, whether in the atmosphere, environmental systems, or chemical reactions, plays a crucial role in maintaining stability and balance in various natural processes. Understanding chemical equilibrium is essential in grasping the dynamics of reversible reactions and the conditions under which they occur. The pursuit of knowledge in these areas not only deepens our understanding of the natural world but also lays the foundation for further exploration and scientific advancements.
The concept of equilibrium constant is fundamental in understanding chemical reactions and their behavior. It is a ratio that relates the concentrations of reactants and products in a reversible reaction, as expressed in the balanced chemical equation. By knowing the balanced chemical equation for a reversible reaction, we can write the equilibrium expression and calculate the numerical value of the equilibrium constant (Kc) by substituting the actual equilibrium concentrations of the reactants and products.
The value of Kc depends solely on the temperature of the reaction and remains constant, regardless of the initial concentrations of the reactants and products. This constant value plays a crucial role in predicting the direction and extent of a reaction. By comparing the reaction quotient (Qc), which is the equilibrium expression with concentrations measured at a particular time, with the equilibrium constant (Kc), we can determine the direction in which the reaction will proceed. If Qc is less than Kc, the reaction will move in the forward direction, and if Qc is greater than Kc, the reaction will move in the reverse direction. When Qc equals Kc, the reaction has reached equilibrium.
The numerical value of Kc also provides valuable insights into the extent to which a reaction proceeds before establishing the equilibrium state. A large value of Kc indicates that the reaction has almost gone to completion, and the equilibrium mixture contains mostly products. On the other hand, a small value of Kc suggests that the reaction has established equilibrium with only a small conversion of reactants to products, with a significant amount of reactants remaining. Reactions with Kc values that are neither small nor large have comparable amounts of reactants and products at equilibrium, indicating that the rates of forward and reverse reactions are comparable.
Understanding the equilibrium constant and its implications allows chemists to make predictions about the behavior of chemical reactions and design processes for desired outcomes in various chemical systems. It is a powerful tool in the study of chemical equilibrium and plays a crucial role in many branches of chemistry.
What do you mean by the extent of a reaction?
Answer: The extent of a reaction refers to the degree to which reactants are converted to products in a chemical reaction. It is a measure of how far a reaction proceeds before reaching equilibrium. Reactions with a large extent have almost all reactants converted into products, while reactions with a small extent have only a small conversion of reactants to products at equilibrium.
Why do reversible reactions not go to completion?
Answer: Reversible reactions do not go to completion because they involve both forward and reverse reactions, and both reactions occur simultaneously. At equilibrium, the rates of the forward and reverse reactions become equal, resulting in a dynamic equilibrium where reactants are continuously forming products, and products are continuously forming reactants. As a result, a mixture of reactants and products is present at equilibrium, and the reaction does not fully convert all reactants into products.
If a reaction has a large value of Kc, will it go to completion, and why?
Answer: A large value of Kc indicates that at equilibrium, the reaction mixture consists mainly of products, and the reactants are almost completely converted into products. However, this does not necessarily mean that the reaction will go to completion. Reversible reactions, even with large Kc values, still involve the presence of both reactants and products at equilibrium due to the continuous interconversion between them.
Which types of reactions do not go to completion?
Answer: Reversible reactions do not go to completion. These reactions involve both forward and reverse reactions, allowing for the establishment of an equilibrium state. In contrast, irreversible reactions proceed in one direction only, converting all the reactants into products and going to completion.
Why does the reaction mixture not have 50% reactants and 50% products at the equilibrium position?
Answer: At the equilibrium position, the reaction mixture does not have 50% reactants and 50% products because the rates of the forward and reverse reactions become equal. The concentrations of both reactants and products remain constant, but their values are not necessarily equal. The actual concentration of reactants and products at equilibrium depends on the specific equilibrium constant (Kc) for the reaction.
What are irreversible reactions? Give a few characteristics of them?
Answer: Irreversible reactions are chemical reactions that proceed in one direction only and do not form products back from the products. In these reactions, reactants are completely converted into products, and the reaction goes to completion. Characteristics of irreversible reactions include:
They are represented by a single arrow (→) in the balanced chemical equation.
The reactants are consumed entirely, and the reaction cannot be reversed under normal conditions.
The reaction proceeds in one direction until all the reactants are converted into products.
The reaction rate is usually faster due to complete conversion of reactants into products.
Define chemical equilibrium state.
Answer: Chemical equilibrium state refers to the point in a reversible reaction where the rate of the forward reaction is equal to the rate of the reverse reaction. At this state, the concentrations of reactants and products do not change over time, and the system is in a dynamic balance. Both forward and reverse reactions continue, but their rates are equal, resulting in no net change in the concentration of reactants and products.
Give the characteristics of reversible reactions.
Answer: Reversible reactions are chemical reactions in which reactants can react to form products and products can react to form reactants. Characteristics of reversible reactions include:
They are represented by a double arrow (⇌) in the balanced chemical equation.
The reaction proceeds in both forward and reverse directions.
The reaction does not go to completion, and there is an establishment of dynamic equilibrium.
Concentrations of reactants and products remain constant at equilibrium.
The rates of the forward and reverse reactions become equal at equilibrium.
How is dynamic equilibrium established?
Answer: Dynamic equilibrium is established in a reversible reaction when the rates of the forward and reverse reactions become equal. Initially, the forward reaction is faster than the reverse reaction, but as the reaction proceeds, the concentrations of reactants decrease and products increase. Eventually, the rate of the forward reaction slows down, and the rate of the reverse reaction speeds up. At equilibrium, both reactions occur at the same rate, and the concentrations of reactants and products remain constant.
Why at equilibrium state reaction does not stop?
Answer: At equilibrium, the reaction does not stop because both the forward and reverse reactions continue to occur at the same rate. Although there is no net change in the concentrations of reactants and products, the individual reactions are still active. The system is in a dynamic balance, with reactants converting to products and products converting back to reactants at equal rates. This continuous movement of reactants and products maintains the equilibrium state.
Why is the equilibrium state attainable from either way?
Answer: The equilibrium state is attainable from either way in a reversible reaction because the reaction can proceed in both forward and reverse directions. Initially, reactants react to form products, and later products can react to produce reactants. As the reaction proceeds, the concentrations of reactants and products change until they reach a point where their rates become equal. The equilibrium state is established when the forward and reverse reactions occur at the same rate, regardless of the direction in which the reaction started.
What is the relationship between active mass and the rate of reaction?
Answer: The rate of a chemical reaction is directly proportional to the product of the active masses of the reacting substances. The active mass of a substance is represented by its molar concentration, expressed in mol dm^-3 and enclosed in square brackets [ ]. In the rate equation, the concentration of each reactant is raised to the power of its coefficient in the balanced chemical equation. This relationship is known as the Law of Mass Action, which helps in understanding the rate of a reaction and the establishment of equilibrium.
Derive the equilibrium constant expression for the synthesis of ammonia from nitrogen and hydrogen.
Answer: The synthesis of ammonia from nitrogen and hydrogen is a reversible reaction represented by the balanced chemical equation:
N2(g) + 3H2(g) ⇌ 2NH3(g)
The equilibrium constant expression (Kc) for this reaction is derived as follows:
Kc = [NH3]^2 / [N2][H2]^3
How can the direction of a reaction be predicted?
Answer: The direction of a reaction can be predicted by comparing the reaction quotient (Qc) with the equilibrium constant (Kc). If Qc is less than Kc, the reaction will proceed in the forward direction to attain equilibrium. If Qc is greater than Kc, the reaction will proceed in the reverse direction to attain equilibrium. When Qc is equal to Kc, the reaction is already at equilibrium, and both the forward and reverse reactions occur at the same rate.
How can you know that a reaction has achieved an equilibrium state?
Answer: A reaction has achieved an equilibrium state when the concentrations of reactants and products remain constant over time, and the rates of the forward and reverse reactions become equal. At equilibrium, there is no net change in the amount of reactants and products, and the reaction system is in dynamic balance. The concentrations of the species involved in the reaction do not change, and physical properties such as color, density, etc., remain constant.