Equilibrium

Unit Overview

Chemical equilibrium is perhaps the most important concept developed in Chemistry 30. The topics solubility, acids and bases, and oxidation/reduction can be treated as examples of equilibrium. Since the establishment of an equilibrium depends on equal rates in the forward and reverse reactions in a system, understanding the factors which influence rates of reaction, and why they influence the reaction, is essential to understand how equilibria become established and how Le Chatelier's principle works. Laboratory activities, independent research, and the case study could all revolve about chemical equilibrium. The study of equilibrium serves as an overall organizing theme for Chemistry 30.

Factors of scientific literacy which should be emphasized

• A2 ..........historic
• A5 ..........empirical
• A7 ..........unique

• B1 ..........change
• B3 ..........orderliness
• B5 ..........perception
• B8 ..........quantification
• B9 ..........reproducibility of results
• B11 ..........predictability
• B13 ..........energy-matter
• B14 ..........cycle
• B16 ..........system
• B23 ..........invariance
• B28 ..........equilibrium

• C1 ..........classifying
• C2 ..........communicating
• C3 ..........observing and describing
• C5 ..........measuring
• C6 ..........questioning
• C7 ..........using numbers
• C8 ..........hypothesizing
• C9 ..........inferring
• C10 ..........predicting
• C11 ..........controlling variables
• C12 ..........interpreting data
• C13 ..........formulating models
• C14 ..........problem solving
• C15 ..........analyzing
• C16 ..........designing experiments
• C17 ..........using mathematics
• C20 ..........defining operationally

• D1 ..........science and technology
• D4 ..........science, technology, and the environment

• E3 ..........using equipment safely
• E4 ..........using audiovisual aids
• E5 ..........computer interaction
• E7 ..........manipulative ability
• E8 ..........measuring time
• E9 ..........measuring volume
• E10 ..........measuring temperature
• E13 ..........using quantitative relationships

• F2 ..........questioning
• F3 ..........search for data and their meaning
• F5 ..........respect for logic
• F7 ..........demand for verification
• F8 ..........consideration of premises

• G1 ..........nterest
• G2 ..........confidence
• G3 ..........continuous learner
• G8 ..........explanation preference

Foundational Objectives for Chemistry and the Common Essential Learnings

Recognize the characteristics and dynamics of equilibrium reactions .

• Observe and describe some reactions which are easily reversible and some which are not easily reversible.
• Consider the implications for a system when the rates of the forward and the reverse reactions that define the system are equal.
• Discuss non-chemical analogies which illustrate or simulate equilibria.
• Distinguish between dynamic equilibria and steady-state processes.
• Discuss the influence of free energy on the spontaneity of reactions.
• Understand why Le Chatelier's principle works.
• Use Le Chatelier's principle to predict how various equilibrium systems will shift in response to external stress.
• Discuss industrial applications of Le Chatelier's principle.

Understand some quantitative aspects of equilibrium systems.

• Write the equilibrium constant expression for a chemical reaction using the general equation:
  aA_(aq) + bB_(aq) cC_(g) + dD_(aq)
• Recognize that K_eq values are dependent upon temperature but are independent of concentration.
• Analyze graphs of the concentrations of reactants and products with respect to time in a chemical reaction which is approaching equilibrium.
• Interpret K_eq values to determine whether products or reactants are favoured once equilibrium has been reached.
• Solve problems involving the equilibrium constant expression for a chemical reaction, with concentrations expressed in mol×litre^-1 and kPa.

Use a wide range of language experiences for developing knowledge of equilibrium systems. (COM)

• Show understanding of equilibrium by rephrasing text or classroom definitions and explanations, creating models, drawing diagrams and concept maps.
• Discuss the relationships between the activities and analogies used to illustrate equilibrium and the principles of equilibria.
• Ask pertinent questions (prior questions, contextual questions, evaluative questions) and discuss multiple responses to those questions.

Strengthen knowledge and understanding of how to compute, measure, estimate and interpret mathematical data, when to apply these skills and techniques, and why these processes apply within chemistry. (NUM)

• Use the language of estimation.
• Understand the quantitative nature of equilibria.
• Understand that divergent thinking and reasoning often precede convergent thinking and solutions to problems.
• Verify answers by referring to problem parameters, checking the validity of each step of the method of solution, looking for errors in reasoning and in information and searching for alternative methods of solution.
• Use problem solving tools such as tables and calculators.

Develop a contemporary view of technology. (TL)

• Understand the influence of the underlying values or assumptions of a society on the support of technological development.
• Understand how scientific understanding can support technological developments.

Suggested activities and ideas for research projects

• An oscillating reaction is a good way of illustrating the reversibility of reactions. On a magnetic stirrer, prepare a solution containing 25 mL of concentrated H₂SO₄ in 250 mL of water in a beaker. Add 2 spoons of malonic acid and 1 spoon of KBrO₃ (quantities are not critical). Add a pinch of MnSO₄ .

  Use a white background behind the beaker. The oscillation may continue for a half an hour or longer. (This activity was adapted from CHEM13 NEWS, #81, November 1976, page 9, based on an idea contributed by J. Eix, Toronto, ON)

• Analogies are useful to reinforce the concept of a dynamic equilibrium. One way to illustrate the concept is with the dual aquaria analogy. Fill one aquarium 3/4 full with water. Leave the other aquarium empty. Alternating turns, a student using a 500 mL beaker pours water from the full aquarium (representing the initial reactants in a reaction) to the empty aquarium. Each student fills the beaker as much as possible without tipping the aquarium. Continue until no further macroscopic changes are evident.

  Before beginning, explain this procedure to the class and ask each student to record a prediction of the final level of water in each aquarium. After "equilibrium" has been reached, discuss general principles regarding equilibrium.

  Repeat the activity, but change the amount of water in each of the aquaria at the starting point. All of the water can be in the second aquarium, or it can be distributed in some proportions in both aquariums. Regardless of the initial volumes in each aquarium, as long as the total amount of water used is the same, when a dynamic equilibrium has been attained the amount of water in each aquarium will be the same.

  Repeat the activity, changing the size of one beaker. This simulates altering the rate of the forward or reverse reaction. Have students predict the effect of this change.

  Once equilibrium is re-established, new "concentrations" of reactants and products will be present (i.e., the level of water in each aquarium remains unchanged, but will be different from what it was initially when different sized beakers were used.) This is a nice way of illustrating LeChatelier's principle.

  Have the students switch beakers and again transfer water. Once equilibrium is re-established, the levels of water in the two aquaria is reversed to what it was before the beakers were switched. This activity also helps to reinforce the idea that at equilibrium, there are not necessarily going to be equal amounts of reactants and products present.

  As a follow-up activity, some students may be able to write a computer program which presents the data in graphical form, showing what happens to the concentrations of "reactants" and "products" as time progresses. A computer program also allows the equations representing the forward and reverse reaction rates to be changed, allowing for rapid analysis without the tedium of recalculating and replotting the data.

  Developing a computer simulation of equilibrium can be a challenging activity that some students may wish to undertake as part of their contract in the independent research section of core unit 3. If a really good program is submitted, obtain the student's permission to use it in other classes. Connect a computer to an overhead projection display system to run the program so that the entire class can see it. (Computers have excellent potential for use in science classes. Consider other ways that their use can be incorporated into the chemistry program.)

• Identify and describe several industrial processes which rely on manipulating the equilibrium point with external stresses to make the processes economically viable?

• When acetic acid (CH₃COOH) and ethanol (C₂H₅OH) are mixed, water and ethyl acetate (CH₃COOC₂H₅) form. They reach equilibrium slowly. Devise a way to monitor the progress of this reaction until it reaches equilibrium, and to determine the equilibrium constant of the reaction.

• What is chemical reaction is involved in the Haber process? When was the process developed? What was the urgent social reason for developing the process?

• Medium strength orange pekoe tea (or any other black tea) can be used to illustrate how colour can be used to estimate the concentration of a solution. Brew a 250 mL beaker of medium strength tea. Put 50 mL into each of two 100 mL beakers. Put the beakers side by side on the stage of an overhead projector. Turn the projector on and compare the colours displayed on the screen. Ask the students to predict what will happen to the colours if water is added to one of the beakers. Add 25 mL water and see what happens. Ask the class to create an explanation for the observation.

  Ask the students to predict what will happen to the colours if the volume of diluted tea is reduced to 50 mL. Remove 25 mL of diluted tea and save it. Have the class agree on an explanation for this effect. Return the diluted tea removed to the beaker.

  Ask the students to assume that the concentration of the tea in the original beaker is 1.00. What is the concentration of the tea in the diluted beaker

  • as estimated by the dilution effect of 25 mL water added
  • as estimated by the relative heights of solutions in the beakers

  Predict whether the colours will match when the colour from 40 mL of the original strength tea is compared to the colour from 40 mL of a dilution made with 20 mL original strength tea and 20 mL water. Compare the two colours on the screen.

  Predict how much tea will have to be removed from the beaker with original strength tea to make the colours match.

  Ask the class to identify the two factors which influence the colour on the screen. From this discussion develop the relationships that colour is proportional to the depth of the solution and colour is proportional to the strength of the solution.

  If you have a class which is capable of abstract mathematical expression, create a mathematical expression which includes these two factors: Colour = kCh, where k is a constant, C is the concentration, and h is the height of the solution.

  Using the data from the demonstation, test this formula for the cases where a colour match existed (Colour₁ = Colour₂).

  Discuss how this principle is used in qualitative and quantitative analysis. Students might be assigned to find applications of this colour matching technique and report on these applications to the class. This has been adapted from an activity by Al Kabatoff, Saskatoon.

Sample ideas for evaluation and for encouraging thinking

• Given the information on the graph about the system A_(aq) + B_(aq) C_(aq) + D_(aq) , explain why it is a good inference that the system has reached equilibrium.

  

• What is equal in a chemical reaction which has reached a state of equilibrium?

• Contrast systems in physical equilibrium, physical steady state, chemical equilibrium and chemical steady state.

• To sustain burning for an indefinite period requires an open system. Why? To maintain a constant vapor pressure above a pool of propanol in a saucer requires a closed system. Why? Which system of the two described would most accurately be classified as a chemical reaction system? Why? Which could be described as a chemical equilibrium system?

• By common agreement, the concentrations of solid components of a reaction system are left out of the equilibrium constant expression. Why is this done?

• Which graph represents what goes on in an equilibrium system? Explain why you picked that one.