2022 Live Review 4 | AP Chemistry | Equilibrium Multiple-Choice and Free-Response Questions

The video begins with an introduction to the topic of equilibrium, emphasizing its significance in the AP Chemistry exam. Dr. Catchatory from Charlestown High School in Boston introduces the concept of equilibrium and outlines the key areas that will be covered, including writing equilibrium constant expressions, interpreting the equilibrium constant (K), applying Le Chatelier's principle, calculating equilibrium concentrations using ICE tables, and interpreting graphs and particle diagrams related to equilibrium. Additionally, the video will touch on bonus topics such as lattice energy trends and the relationship between intermolecular forces and boiling points.

The segment focuses on interpreting a constant concentration-time graph for the reaction of H2 gas and N2 gas to form NH3 gas. The reaction is given with its enthalpy value, and a graph displays the changing concentrations of each species over time. The question posed is to determine what was true for the system between times t1 and t2. The analysis involves understanding that at equilibrium, concentrations of species remain constant, indicating that the rates of the forward and reverse reactions are equal. This understanding leads to the correct answer, which is that the rates of the forward and reverse reactions were equal during the time interval.

This part delves into the interpretation of the equilibrium constant (K_eq) and its implications on the favorability of a reaction at equilibrium. Given the chemical equation for the reaction of Fe3+ and SCN- to form Fe(SCN)2+, the equilibrium constant is provided, and the task is to deduce the correct statements about the reaction at 25°C. The discussion clarifies that the size of K_eq does not indicate reaction rate, but a K_eq greater than one suggests that the products are favored at equilibrium. The correct deduction from the given information is that the product is favored over the reactants at equilibrium.

The focus shifts to calculating the reverse K for a given reaction and interpreting particle diagrams. A multiple-choice question presents a reaction between H2 gas and Br2 gas to form 2 HBr gas, with the equilibrium constant K given for the forward reaction. The task is to find the value of K for the reverse reaction at the same temperature. By understanding the inverse relationship between forward and reverse K values, the correct calculation is made. Following this, a particle diagram representing the reaction between X gas and Y gas is used to determine when the system reaches equilibrium. The analysis involves counting the particles at different time points to identify when the concentrations stabilize, indicating equilibrium.

The video continues with the interpretation of K values for different reactions and predicting the outcomes at equilibrium. A reaction involving H2S and CH4 is given with a small K value, and the task is to predict which species will have the highest concentration at equilibrium. The analysis shows that a small K value favors reactants, and the concentration of H2S will be greater than CH4. A twist on this scenario is then presented, where only the reactants are initially present, and the prediction changes based on the expected shift towards product formation at equilibrium. The segment also includes a question about calculating the equilibrium constant K from a particle diagram, where each particle represents a specific concentration.

This section discusses Le Chatelier's principle and its application to predict shifts in equilibrium. A reaction involving H2 gas and N2 gas to form NH3 gas is used as an example, with a focus on what happens when more NH3 is added to the system at equilibrium. The discussion emphasizes that the value of K does not change when the concentration of a species is altered, and the system will shift to counteract the disturbance. The correct prediction is that the amount of N2 will increase as the system shifts to consume the added NH3 and make more reactants.

The concept of the common ion effect on solubility is explored in this part. Two beakers with different solutions are considered, and solid AgCl is added to each. The task is to determine the solubility of AgCl in the presence of a common ion. The discussion highlights that the presence of a common ion reduces the solubility of a slightly soluble salt. The correct conclusion is that the beaker with more chloride ions (from NaCl solution) will have less silver ion concentration due to the common ion effect, leading to lower solubility of AgCl.

The segment transitions to free response questions, starting with a calculation involving the solubility product constant (Ksp) for calcium carbonate. The task is to calculate the amount of calcium carbonate dissolved in a saturated solution at 25°C. The problem involves using the Ksp value and the molar mass of calcium carbonate to find the moles dissolved in a given volume, and then converting this to grams. The next part examines the solubility of calcium carbonate in different solutions, considering the common ion effect. The conclusion is that the solubility in a 0.1 M calcium chloride solution is less than in pure water due to the common ion effect, while the solubility in a 0.1 M sodium chloride solution is expected to be the same as in pure water because there is no common ion affecting the solubility equilibrium.

The video concludes with bonus topics not directly related to equilibrium but important for a comprehensive understanding of chemistry. The first topic is lattice energy, which is the energy required to separate the ions in a crystal lattice. The discussion uses Coulomb's law and periodic properties to explain why magnesium carbonate has a greater lattice energy than calcium carbonate. The second topic is intermolecular forces, specifically focusing on H2, I2, and HI. The task is to identify the intermolecular forces present in each substance and order them from weakest to strongest based on boiling points and the types of intermolecular forces involved.

The final part of the video provides a summary of the key takeaways from the session. It emphasizes the importance of understanding and applying equilibrium concepts, the use of K and Ksp expressions, the impact of common ions on solubility, and the application of Le Chatelier's principle. The video also highlights the significance of intermolecular forces and their relationship with boiling points. The summary encourages viewers to review the entire session and to continue practicing with the provided resources.