Exam (elaborations) GIZMOS Equilibrium Concentration Student Lab Sheet

Exam (elaborations) GIZMOS Equilibrium Concentration Student Lab Sheet Vocabulary: chemical equilibrium, concentration, equilibrium, equilibrium constant, reaction quotient, reversible reaction Prior Knowledge Questions (Do these BEFORE using the Gizmo.) Gary has $5,000 in his bank account and earns a modest salary. Every month he pays for rent, food, utilities, and entertainment. A. How will Gary’s account change if he saves more than he spends? _it will increase. B. How will Gary’s account change if he spends more than he saves? It will decrease. C. What happens if Gary spends exactly as much as he saves? It will stay the same. 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. 1. Click Play ( ) and observe the colliding molecules. What do you notice? the molecules move and collide. Occasionally there is a blue or red flash. 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. 2. Click Reset ( ), and set Moles NO2 to 0 and Moles N2O4 to 8. Click Play. What do you notice now? There are more red flashes than blue. 3. 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. I know the synthesis of NO2 into N2O4 is a reversible reaction because both the forward reaction (blue flashes) and the reverse reaction (red flashes) are observed. GIZMOS Equilibrium Concentration Student Lab Sheet Activity A: Reversible reactions Get the Gizmo ready:  Click Reset. Reaction 1 should be selected.  Set Moles NO2 to 8 and Moles N2O4 to 0.  Move the Sim. speed slider all the way to the right. Question: What are the characteristics of reversible reactions? 1. 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? 2. Test: Click Play. Select the BAR CHART tab 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? After a while, the bars do not move as much as at first. 3. 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? At first, the amounts of NO2 and N2O4 change rapidly. After a while the amounts do not change as much. This situation, in which the overall amounts of reactants and products does not change significantly over time, is called a chemical equilibrium. 4. Record: On the BAR CHART tab, turn on Show data values. How many moles of NO2 and N2O4 are there right now? Moles NO2 ___4.0_______ Moles N2O4 ___2.0_______ 5. Calculate: Suppose all the NO2 molecules were synthesized into N2O4. Given the equation 2NO2 ⇄ N2O4, how many moles of N2O4 would be produced? _4 moles of N2O4 6. Experiment: Click Reset. On the INITIAL SETTINGS tab, set Moles NO2 to 0 and Moles N2O4 to 4. Click Play. Click Pause when the bars of the bar chart stop moving very much. A. List the current amounts of each substance: Moles NO2 _4.0_____ Moles N2O4 __2.0____ B. How do these results compare to starting with 8 moles of NO2? _the resulting amounts of NO2 and N2O4 in this experiment are very similar to the results obtained when starting with 8 moles NO2 and 0 moles N2O4._______________ ___________________________________________________________________ Activity A (continued from previous page) 7. 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? The equilibrium amounts were about the same in both situations. 8. Set up the Gizmo: Click Reset and select the EXPERIMENT tab on the left. On the INITIAL SETTINGS tab 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? Reactants: NO NO2 Product: __N2O3________ (Note: In this reaction, some of the NO2 reactants combine to form N2O4, as in reaction 1.) 9. 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. A. At first, do you notice more blue flashes or red flashes? __blue flashes B. What do you notice about the frequency of blue and red flashes as time goes by? After a while, the frequency of blue and red flashes is about the same. _________________________________________________________________ C. 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? At first, there are more red flashes than blue flashes. Later, the frequencies are even. 10. Explain: Think about how the numbers of blue and red flashes reflect the rates of the forward (reactants  products) and reverse (products  reactants) reactions. A. What happens to the rate of the forward reaction as the reactants are consumed? The rate of the forward reaction decreases as reactants are consumed. What happens to the rate of the reverse reaction as the products are produced? The rate of the reverse reaction increases as products are produced. Why do reversible reactions always result in chemical equilibria? Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal. If there is an excess of reactants, the forward reaction will occur more quickly than the reverse reaction until there is no longer an excess of reactants. At this point, the rates of the forward and reverse reactions are