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Predicting Shifts in Equilibrium: Q vs K (9 Favorites)

SIMULATION in Le Châtelier's Principle, Establishing Equilibrium. Last updated May 6, 2019.


Summary

In this simulation, students will take a 15 question quiz. Each quiz questions has two parts. The first part requires the student to calculate the value of the reaction quotient, Q. The second part requires students to compare the value of Q to the equilibrium constant, K, and predict which way the reaction will shift to reach equilibrium. The simulation includes five different reactions which each have three scenarios: Q > K, Q = K, and Q < K.

Grade Level

High School

NGSS Alignment

This simulation will help prepare your students to meet the performance expectations in the following standards:

  • HS-PS1-6: Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
  • Scientific and Engineering Practices:
    • Using Mathematics and Computational Thinking
    • Analyzing and Interpreting Data

AP Chemistry Curriculum Framework

This activity supports the following learning objectives:

  • Big Idea 6: Any bond or intermolecular attraction that can be formed can be broken. These two processes are in a dynamic competition, sensitive to initial conditions and external perturbations.
    • 6.1 The student is able to, given a set of experimental observations regarding physical, chemical, biological, or environmental processes that are reversible, construct an explanation that connects the observations to the reversibility of the underlying chemical reactions or processes.
    • 6.2 The student can, given a manipulation of a chemical reaction or set of reactions (e.g., reversal of reaction or addition of two reactions), determine the effects of that manipulation on Q or K.
    • 6.4 The student can, given a set of initial conditions (concentrations or partial pressures) and the equilibrium constant, K, use the tendency of Q to approach K to predict and justify the prediction as to whether the reaction will proceed toward products or reactants as equilibrium is approached.
    • 6.5 The student can, given data (tabular, graphical, etc.) from which the state of a system at equilibrium can be obtained, calculate the equilibrium constant, K.
    • 6.6 The student can, given a set of initial conditions (concentrations or partial pressures) and the equilibrium constant, K, use stoichiometric relationships and the law of mass action (Q equals K at equilibrium) to determine qualitatively and/or quantitatively the conditions at equilibrium for a system involving a single reversible reaction.

Objectives

By the end of this simulation, students should be able to:

  • Calculate the value of the reaction quotient given an initial set of concentrations for the reactants and product.
  • Compare the values of the reaction quotient and the equilibrium constant.
  • Predict the direction the reaction will shift to reach equilibrium.

Chemistry Topics

This simulation supports students’ understanding of:

  • Establishing equilibrium
  • Equilibrium constant
  • Reaction quotient
  • Le Chatelier's Principle

Time

Teacher Preparation: minimal

Lesson: 30–45 minutes

Materials

Safety

  • No specific safety precautions need to be observed for this activity.

Teacher Notes

  • This simulation should be used after a teacher has introduced the concept of equilibrium constants, reactions quotients, and shifts in equilibrium from a set of initial conditions.
  • Each quiz includes five different generic reactions.The quiz rotates through them.
  • Each reaction has three different conditions: Q > K, Q = K, and Q < K. The quiz rotates through them.
  • The quiz is not randomized. All students will see the 15 sets of reactions and conditions in the same order. However, your students may not need to complete all 15 if they have mastered the concept.
  • The simulation can be found at either of the following links (note that students can access the simulation without an AACT login):

For the Student

Background

If only reactants are present at the beginning of a reaction, you know that the reaction will “go right” and produce products until equilibrium is established.However, if both the reactants and products are present at the beginning of a reaction, you must compare the values of the reaction quotient, Q, to the value of the equilibrium constant, K, to determine which way the reaction will shift to reach equilibrium.The value of Q is calculated the same way that K is calculated, except you use the initial concentrations if the reactants and products.

A + B2 ↔ AB2
[AB2]final [AB2]initial
K = ------------- Q = -------------
[A]final[B2]final [A]initial[B2]initial

Figure 1. Sample equations for K and Q for the reaction A + B2 ↔ AB2

Comparing Q and K

  • If the value of Q is greater than that of K, it means that there are not enough reactants present and the reaction will shift left, towards the reactants until equilibrium is established.
  • If the value of Q is equal to that of K, the reaction is at equilibrium.
  • If the value of Q is less than that of K, it means that there are not enough products present and the reaction will shift right, towards the products until equilibrium is established.

Instructions

  • Log on to the simulation: teachchemistry.org/QvsK
  • Calculate the value of the reaction quotient and compare it to the value of the equilibrium constant for each of the reactions.Enter the value using scientific notation.
  • Predict which way the reaction will shift to reach equilibrium.
Write the reaction and the initial concentration of the reactants and products. Calculate the value of Q and enter it using scientific notation. In which direction will equilibrium shift? Explain your answer.