Student Exploration: Equilibrium and Concentration (ANSWER KEY)
Student Exploration: Equilibrium and Concentration
Vocabulary: chemical equilibrium, concentration, equilibrium, equilibrium constant, reaction quotient, reversible reaction, Le Châtelier’s principle
Gizmo Warm-up
If Gary spends exactly as much as he earns, his savings will be in equilibrium. Equilibrium occurs when two opposing processes occur at the same rate, leading to no net change. In the Equilibrium and Concentration Gizmo™, you will investigate how equilibrium can occur in chemical reactions.
To begin, check that Reaction 1 is selected. Set Moles NO2 to 8 and Moles N2O4 to 0.
Click Play () and observe the colliding molecules.
In the Gizmo, a blue flash appears every time two reactants combine to form a product. A red flash appears every time a product dissociates into reactants.
Click Reset (), and set Moles NO2 to 0 and Moles N2O4 to 8. Click Play.
What do you notice now?
When a reaction can proceed in either direction, it is a reversible reaction. Based on what you have observed, is the synthesis of NO2 into N2O4 a reversible reaction? Explain.
Activity A:
Reversible reactions
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Get the Gizmo ready:
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Question: What are the characteristics of reversible reactions?
Predict: Suppose you began with 8 moles of NO2 in the chamber. What do you think will happen if you let the reaction go for a long time?
Test: Click Play. Select the BAR CHARTtab and check that Moles is selected. Observe the bar chart for about 30 seconds. As time goes by, what do you notice about the bars representing moles NO2 and moles N2O4?
Observe: Click Pause (). Select the GRAPH tab. Click the (–) zoom control on the horizontal axis until you can see the whole graph. What do you notice?
This situation, in which the overall amounts of reactants and products does not change significantly over time, is called a chemical equilibrium.
Record: On the BAR CHARTtab, turn on Show data values. How many moles of NO2 and N2O4 are there right now? Moles NO2 __________ Moles N2O4__________
Calculate: Suppose all the NO2 molecules were synthesized into N2O4. Given the equation 2NO2 ⇄ N2O4, how many moles of N2O4 would be produced? ____ ___________
Experiment: Click Reset. On the INITIAL SETTINGStab, set Moles NO2 to 0 and Moles N2O4 to 4. Click Play. Click Pause when the bars of the bar chartstop moving very much.
List the current amounts of each substance: Moles NO2 __Moles N2O4 ___
How do these results compare to starting with 8 moles of NO2?
(Activity A continued on next page)
Activity A (continued from previous page)
Activity A (continued from previous page)
Summarize: In each trial, you started with the same amounts of nitrogen and oxygen. In this situation, did the equilibrium amounts change depending on the direction of the reaction?
Set up the Gizmo: Click Reset and select the EXPERIMENTtab on the left. On the INITIAL SETTINGStab on the right, select Reaction 2. Set Moles NO to 5, Moles NO2 to 5, and Moles N2O3 to 0. What are the reactants and product of this reaction?
(Note: In this reaction, some of the NO2 reactants combine to form N2O4, as in reaction 1.)
Observe: Recall that a blue flash appears every time two reactants combine to form a product. A red flash appears every time a product dissociates into reactants. Click Play.
At first, do you notice more blue flashes or red flashes?
What do you notice about the frequency of blue and red flashes as time goes by?
Click Reset. This time, start the experiment with 0 moles of NO and NO2 and 5 moles of N2O3. Click Play. What do you notice about the red and blue flashes now?
Explain: Think about how the numbers of blue and red flashes reflect the rates of the forward (reactants à products) and reverse (products à reactants) reactions.
What happens to the rate of the forward reaction as the reactants are consumed?
What happens to the rate of the reverse reaction as the products are produced?
Why do reversible reactions always result in chemical equilibria?
Activity B:
The equilibrium constant
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Get the Gizmo ready:
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Introduction: When investigating the rates of reactions, it often is useful to consider the concentrations of reactants rather than the total number of moles. Concentrations are often expressed in moles per liter, or mol/L. Brackets are used to signify concentration. For example, “[H2] = 5.0 M” means the concentration of hydrogen gas in a chamber is 5.0 moles per liter.
Question: What are the characteristics of reactions in equilibrium?
Record: On the BAR CHARTtab, select Concentration. Check that Show data values is on. If necessary, use the arrows to adjust the scale of the chart.
What are the current concentrations of each compound?
[NO2] __________ [N2O4] __________
Click Play and wait for equilibrium to become established. Click Pause. What are the approximate equilibrium concentrations?
[NO2] ___ [N2O4] __________
Calculate: The value Kc represents the ratio of products to reactants in a reaction at equilibrium. The greater the amount of products relative to reactants, the higher the resulting value of Kc. For a general reaction between gases: aA(g) + bB(g) ⇌ cC(g) + dD(g), Kc is calculated as follows:
For the current reaction, 2NO2 ⇌ N2O4, we have:
Based on the current concentrations of NO2 and N2O4, what is Kc? ____________________
Show your work here:
(Activity B continued on next page)
Activity B (continued from previous page)
Gather data: Experiment with a variety of initial concentrations of NO2 and N2O4. For each set of initial concentrations, use the Gizmo to determine the equilibrium concentrations of each substance. In the last column, find Kc for that trial. Run three trials for each set of initial conditions.
Calculate: Find the average value of Kc for each set of three trials.
Trials 1-3: __________ Trials 4-6: ___ Trials 7-9: __________
Analyze: What do you notice about the values of Kc?
In general, the value of Kc will be constant for a given reaction at a constant temperature, no matter the starting concentrations. That is why Kc is known as the equilibrium constant. In this Gizmo, the values of Kc will vary somewhat because there is a very limited number of molecules in the chamber.
On your own: Use the Gizmo to find Kc for Reaction 4: H2 + I2 ⇌ 2HI. Collect data at least 10 times and average your results to get the best approximation of Kc. Show your data and work on a separate sheet of paper.
(Hint: Because of the coefficient “2” in front of HI, you will have to square the concentration of HI to find Kc.)
Kc = _________
Activity C:
Reaction direction
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Get the Gizmo ready:
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Introduction: For a reversible reaction with equilibrium constant Kc, it often is useful to know in which direction the reaction will proceed given the starting amounts of reactants A and B and products C and D. This is done by calculating the reaction quotient, Qc:
Question: How can you predict the direction of a reversible reaction?
List: Select the BAR CHARTtab. What are the initial concentrations of each substance?
[H2] _______ [I2] _______ [HI] __
Calculate: Use the equation above to find Qc for the current reaction.
What is the current value of Qc?
In activity B, what value of Kc did you arrive at for this reaction?
How does Qc compare to Kc?
Analyze: Recall that Qc is equal to the ratio of product concentrations to reactant concentrations.
If there is an excess of products, will Qc be greater than or less than Kc? ___ _______
If there is an excess of reactants, will Qc be greater than or less than Kc? _ ________
In the current situation, is there an excess of products or reactants? _____ ________
Explain:
When the reaction begins, do you expect [HI] to increase or decrease? ___ ________
Explain:
Test: Click Play. What happens to [HI]?
Activity C:
Le Châtelier’s principle
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Get the Gizmo ready:
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Introduction: In the Equilibrium and Concentration Gizmo, you learned that you can predict the direction of a reaction by comparing the reaction quotient (Qc) with the known equilibrium constant Kc. You can do the same thing using partial pressures:
Question: How do changes in pressure affect the reaction equilibrium?
List: Click Play (). Select the BAR CHARTtab, allow the reaction to proceed to equilibrium, and then click Pause (). What are the equilibrium partial pressures?
PNO _ PNO2 ___ PN2O3 _______
Calculate: Calculate Kpfor this reaction. Kp = ____
Predict: If you add weight to the lid, it increases the pressure on the chamber.
How do you expect increasing pressure on the chamber to affect the partial pressures of the gases? __________________________________________
How do you expect the increased pressure to affect the equilibrium?
Calculate: Add all five weights to the lid.
What is the partial pressure of each substance now?
PNO _______ PNO2 _______ PN2O3 __
Calculate Qp for this setup. Qp = ___
In the current situation, is there an excess or products or reactants? ____ _________
Explain:
(Activity C continued on next page)
Activity C (continued from previous page)
Test: Click Play. Allow the reaction run until it reaches equilibrium again.
What is the partial pressure of each substance now?
PNO _______ PNO2 _______ PN2O3 _______
Calculate Kp for this setup. Kp = ___ _______
How does this value of Kp compare to the value you found before? ______________
As the experiment reached equilibrium again, did the reactants or products increase?
As the pressure on the system increased, the reaction shifted toward the side with fewer molecules—in this case the products. You may have noticed the volume of the chamber decreased slightly as the reaction proceeded. This is an example of Le Châtelier’s principle, which is the tendency of a system in equilibrium to shift in response to a change.
Apply: Select Reaction 5. If you were making water (H2O) from oxygen (O2) and hydrogen (H2), would it be an advantage to increase the pressure? Explain your answer, and then use the Gizmo to check your work.
Think and discuss: Why do you think increasing the pressure has the effect of shifting the equilibrium toward the side with fewer molecules? If possible, discuss your answer with your classmates and teacher.
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