Shifting Reactions A

We will look at some chemical reactions at the microscopic level and at the macroscopic level to understand some new characteristics of reactions.

View the movie on the right and answer the following three questions:

1. Identify when you think the reaction is over/complete. Answer

2. Do either of the reactants completely react? Answer

3. Is there anything interesting about the amounts of reactants reacting and products forming in the reaction? Answer

Let's look at this reaction:

View the movie on the right and answer the following question:

4. How is this reaction related to the first reaction above? Answer

View the movie on the right and answer the following question:

5. Is this reaction similar to the first reaction, or different? How is this reaction similar, or different? Answer

View the movie on the right and answer the following question:

6. Is this reaction similar to the first reaction, or different? How is this reaction similar, or different? Answer

View the movie on the right and answer the following question:

7. Is this reaction similar to the first reaction, or different? How is this reaction similar, or different? Answer

Modified from JCE Software reactions video.

View the movie on the right and answer the following question:

8. Is this reaction similar to the first reaction, or different? How is this reaction similar, or different? Answer

Modified from JCE Software reactions video.

We will all admit that it is a little exciting to actually see some of these reactions occur. Another reaction that might be fun to watch can be expressed in the following equation;

8. Is this reaction, 2K(s) + Br2(l) ---> 2KBr(s), similar to the first reaction, or different? How is this reaction similar, or different? Answer

Modified from JCE Software reactions video.

Reactions fall into two categories: reversible and irreversible reaction. Reversible reactions have three characteristics: 1) the reaction proceed in both directions, left to right and right to left; 2) all reactants and products are present in the reaction container; 3) at the macroscopic level the reaction appears to stop, when in fact at the microscopic level the reaction does not stop. In irreversible reactions one of the reactants is called the limiting reagent because it reacts completely, and the other reactant is the excess reagent. We have investigated irreversible reactions in the discussion on stoichiometry, limiting reagents and solution stoichiometry.

Shifting Reactions B

In this next section we will investigate reversible reactions in more detail by observing adding or removing amounts of reactants or products. The reaction we will look at is the same reaction that we began with; BG + R RG + B. In place of the microscopic representation we will view the reaction by tracking the amount of reactants and products using a chart recorder.

Before starting the movie on the right note the Initial amounts of BG, R, RG and B. When you start the movie you will see a chart recording depicting the relative amounts of all four chemical spcies. View the movie on the right and answer the following question:

9. Can you set up the RICE table (Reaction, Initial Condition, Change Condition and Ending Condition) with the Initial amounts of BG, R, RG and B? Answer

10. What happens when the Resume button is selected? Answer

View the movie on the right and answer the following question:

11. Describe what happens when the Enable Reactions button is selected? Answer

12. Continue the RICE table started in Question 9 by including the amounts of BG, R, RG and B after the chart recording stops (Ending amounts). Answer

View the movie on the right and answer the following question:

11. ? Answer

View the movie on the right and answer the following question:

12. ? Answer

View the movie on the right and answer the following question:

13. ? Answer

View the movie on the right and answer the following question:

14. ? Answer

View the movie on the right and answer the following question:

15. ? Answer

In this next section we will investigate reversible reactions in more detail by observing increasing or decreasing the temperature of the reaction. The reaction we will look at is the same reaction that we began with; G + R RG. In place of the microscopic representation we will view the reaction by tracking the amount of reactants and products using a chart recorder.

View the movie on the right and answer the following question:

. ? Answer

View the movie on the right and answer the following question:

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View the movie on the right and answer the following question:

. ? Answer

In this next section we will investigate reversible reactions in more detail by observing increasing or decreasing the temperature of the reaction. The reaction we will look at is the same reaction that we began with; BG + R RG + B. In place of the microscopic representation we will view the reaction by tracking the amount of reactants and products using a chart recorder.

View the movie on the right and answer the following question:

. ? Answer

View the movie on the right and answer the following question:

. ? Answer

View the movie on the right and answer the following question:

. ? Answer

In this next section we will investigate reversible reactions in more detail by observing increasing or decreasing the volume of the reaction container. The reaction we will look at is the same reaction that we began with; G + R RG. In place of the microscopic representation we will view the reaction by tracking the amount of reactants and products using a chart recorder.

View the movie on the right and answer the following question:

. ? Answer

View the movie on the right and answer the following question:

. ? Answer

View the movie on the right and answer the following question:

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In Chapter 17 we begin to look at chemical reactions in more detail. To afford this type of scrutiny we will be studying a particular type of reaction referred to as an equilibrium reaction. The term equilibrium is not unfamiliar to us because we used it in Chapter 13 and 14 to describe vapor pressure. The equilibrium vapor pressure for a liquid was attained when the rate of condensation of a liquid was equal to the rate of evaporation of the liquid. This equilibrium vapor pressure was dependent on the temperature of the system. We will study the idea of chemical equilibrium in more detail in this chapter and relate it to chemical systems. Let's begin by looking at an example of a chemical reaction that we know to exhibit some of the observable behavior that we associate with chemical equilibrium. I'm not providing you with a definition of chemical equilibrium just yet, rather I would like to show you some examples of chemical systems which exhibit the behavior that we associate with chemical equilibria.

The reaction we will study is;

Fe3+(aq) + SCN-(aq) ---> FeSCN2+(aq)

View the movie on the right and answer the following three questions:

1. Identify the chemical species present in the solution poured into the beaker on the left in the movie. Answer

2. Identify the chemical species present in the solution poured into the beaker on the right in the movie. Answer

3. What evidence is there for a chemical reaction when the two solutions are mixed? Answer

In the reaction the color of each species is;

Fe3+(aq) + SCN-(aq) ---> FeSCN2+(aq)

yellowish/colorless

colorless

reddish-brown

 

In the movie equal samples of the reaction mixture are added to two petri dishes.

To the solution in the petri dish on the left several drops of Fe3+ are added.

Describe what you observe when the Fe3+ is added to the solution in the petri dish.

What chemical species must have been formed to account for your observation?

What chemical species did the Fe3+, that was added to the solution in the petri dish, react with to produce the change you observed?

Answer

 

 

In this movie equal samples of the reaction mixture were added to two petri dishes. To the petri dish on the left several drops of Fe3+ have been added.

To the solution in the petri dish on the right several drops of SCN- are added.

Describe what you observe when the SCN- is added to the solution in the petri dish.

What chemical species must have been formed to account for your observation?

What chemical species did the SCN-, that was added to the solution in the petri dish, react with to produce the change you observed?

Answer

 

What is really interesting about adding Fe3+(aq) to the petri dish on the left and SCN-(aq) to the petri dish on the right are the results we observe. Adding some Fe3+(aq) and observing the solution get darker implies that there must be some unreacted SCN-(aq) in the petri dish on the left. So when the original sample of Fe3+(aq) and SCN-(aq) were mixed there is unreacted SCN-(aq) available to react with any additional Fe3+(aq) .

But wait, when some SCN-(aq) is added to petri dish on the right the color gets darker implying there is unreacted Fe3+(aq) in that solution. So when the original sample of Fe3+(aq) and SCN-(aq) were mixed there is unreacted Fe3+(aq) available to react with any additional SCN-(aq).

How can that be!!! How can it be possible to mix the original samples of Fe3+(aq) and SCN-(aq) split the resulting mixture and find there is still some unreacted Fe3+(aq) and SCN-(aq) in both solutions?!

The only way this could happen is if when the original solutions of Fe3+(aq) and SCN-(aq) were mixed, some product, FeSCN2+(aq) was formed, but the reaction did not completely use up either of the reactants. Some Fe3+(aq) and SCN-(aq) remained unreacted. So when the sample was split into two beakers the addition of more reactant resulted in more reaction.

What is even more interesting is what happens when F-(aq) is added to a fresh sample of FeSCN2+(aq). Before describing what is observed it is important to know that mixing F-(aq) with Fe3+(aq) produces the colorless compound FeF63-. So adding F-(aq) will react with any Fe3+(aq) in the solution.

Describe what you observe when the F- is added to the solution in the beaker.

Explain why the solution changes color.

Answer

 

 

 

 

Amazing, it is as though the reaction

Fe3+(aq) + SCN-(aq) ---> FeSCN2+(aq)

can also go backwards;

FeSCN2+(aq) ---> Fe3+(aq) + SCN-(aq)

This is unlike any reaction we have investigated before.

Actually this is not that unique, and we describe such a reaction as an equilibrium reaction. The equation is written;

Fe3+(aq) + SCN-(aq) FeSCN2+(aq)

Where the symbol represents a reaction that is capable of going in both directions.