Chemical Equilibrium

Chemical Equilibrium: How do changes affect the equilibrium point of a reaction?

Introduction:    

It is often useful to think of a reaction as a process that consists of two parts acting in opposite directions. From this view, a reaction begins with all reactants and no products. The reactants then begin to interact with each other and transform into products. The rate at which the reactants transform into products will begin to decrease over time as the concentration of the reactant decreases. At this point, some of the products will begin to revert back into reactants. The rate at which the products revert back into reactants will increase as the concentration of the product increases. There is a point, as a result, where the forward and reverse components of a reaction are happening at equal rates. This point is called chemical equilibrium. At equilibrium, the rates of the forward and reverse components of the reaction are equal but the concentrations of reactants and products are not.

                Chemical equilibrium, therefore, can be defined as the point in a reaction where the rate at which reactants transform into products is equal to the rate at which products revert back into reactants. The equilibrium point of a chemical reaction occurs when the amount or concentration of the products and reactants in a closed system is stable. Chemists use a specific property, such as color, concentration, or density, to determine when a reaction is in equilibrium. It is important to note, however, that chemists view the state of chemical equilibrium as dynamic because reactants continue to transform into products and products continue to revert back into reactants even though the amount of reactants and products in the closed system is stable.

                The equilibrium point of a reaction can change because chemical equilibrium is not static. There are a number of different factors that can change the equilibrium point of a reaction by changing the rate at which reactants transform into products or by changing the rate at which products revert back into the reactants. These factors include a change in temperature, pressure, reactant concentration, and product concentration. When any of these factors are changed, the equilibrium point of the reaction will move and the concentration of products and reactants in the system at the new equilibrium point will be different.

To control the amount of product or reactant present at the equilibrium point of a reaction in a closed system, chemists need to understand how various factors affect chemical equilibrium and why these various factors change the equilibrium point of a reaction. You will therefore explore how three specific factors affect the equilibrium point of chemical reaction. You will then develop a conceptual model that you can use to explain your observations and predict how the equilibrium point of a different reaction will change when the equilibrium point is disturbed by changing these same three factors.

Your task:

Determine how changes in temperature and the addition of extra reactant and product affect the equilibrium point of the reaction between iron(III) nitrate and potassium thiocyanate. The guiding question of this investigation is:  Why do changes in temperature, reactant concentration, and product concentration affect the equilibrium point of a reaction?

1. Explain what is the goal of the experiment; what are you trying to show?

Materials:

Consumables:                                                                   Equipment:

-iron (III) nitrate, Fe (NO₃)₃                                          -9 test tubes

-potassium thiocyanate, KSCN                                   -test tube rack

-copper (II) chloride, CuCl₂                                           -graduated cylinder (10 mL)

-sodium chloride, NaCl                                                  -pipettes

-silver nitrate, AgNO₃                                                     -beaker (250 mL)

-water                                                                                  -beakers (250 mL) for hot and cold water baths

-ice                                                                                         -thermometer                 

                                                                                                -hot plate

Procedure:

The first step in developing your model is to design and carry out three experiments. The goal of the first experiment will be to determine how a change in reactant concentration affects the equilibrium point of a reaction. The goal of the second experiment will be to determine how a change in product concentration affects the equilibrium point of a reaction. The goal of the third experiment will be to determine how temperature affects the equilibrium point of a reaction. For these three experiments, you will focus on the reaction of iron(III) nitrate and potassium thiocyanate. Iron(III) ions react with thiocyanate ions to form FeSCN²⁺ complex ions according to the following reaction:

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

yellow               colorless                  orange-red                

To determine what type of data you need to collect, think about the following questions:

• What type of measurements or observations will you need to record during each experiment?

• When will you need to make these measurements or observations?

To determine how you will collect the data, think about the following questions:

• What will serve as your independent variable in each experiment?

• How will you change the independent variable in each experiment?

• What types of comparisons will you need to make?

• What will you do to reduce measurement error?

• How will you keep track of the data you collect and how will you organize it?

To determine how you will analyze the data, think about the following questions:

• What type of calculations will you need to make?

• What type of graph could you create to help make sense of your data?

2. Write your procedure as a group of what steps you will take to conduct the each of the three experiments. Label them experiment 1, 2 and 3. Detail the steps that you will take in each experiment.

3. Write out data tables for each of your three experiments including any measurements or observations that you have made.

4. Explain in your own words what changes each of your experiments caused to the equilibrium point of the reaction. (Explain using shifts in the reaction, if it produced reactants or products, changes in colors and so forth.

5. Modify your models (of the three equilibrium points) from the beginning of the laboratory and revise it to show what the effect your experimental factors had on the reaction. (Use the paragraph below to help you with this model)

Once you have carried out your three experiments, your group will need to develop a conceptual model. This conceptual model will need to be able to provide an underlying reason for your findings about the effect of temperature, changes in reactant concentration, and changes in product concentration on the equilibrium point of a reaction. Your model should also include an explanation of what is happening at the submicroscopic level between and within molecules during a reaction. The collision theory of reaction rates and the concept of chemical equilibrium should serve as the theoretical foundation for your model.

The last step in this investigation is to test your model. To accomplish this goal, you can use a different reaction to determine if your model leads to accurate predictions about how the equilibrium point changes in response to different factors. If you can use your model to make accurate predictions about how the equilibrium point of a different reversible reaction changes, then you will be able to generate the evidence you need to convince others that the conceptual model you developed is valid.

                You can use the reversible formation of copper(II) complexes to test your model. When copper(II) chloride (CuCl₂) is dissolved in water, two different solutes are present in the solution. These solutes include Cu²⁺ ions and Cl^– ions. These solutes interact with water molecules to form two different complex ions. One complex ion is Cu(H₂O)₆²⁺ and the other is CuCl₄^2–. The reversible equation for the formation of the two complex ions is

                                Cu (H₂O)₆²⁺ (aq)     + 4Cl^- (aq)   ↔    CuCl₄^2- (aq)   + 6H₂O (l)

                                                       blue                                                green

You can change the equilibrium point by adding NaCl, or AgNO₃ or by changing the temperature of the solution. To change the concentration of the reactants or the products, simply add 2 ml of the copper(II) chloride solution to a test tube and then add up to eight drops of NaCl or AgNO₃. The addition of NaCl will increase the number of Cl– ions in the system. The addition of AgNO₃, in contrast, will decrease the number of Cl– ions in the system (because the Ag+ ions react with Cl– ions to form AgCl). To change the temperature of the system, use a hot-water bath or an ice bath.

	What are you adding: (reactant/product)	Prediction (color):	Result (color):
Addition of NaCl
Addition of AgNO3-

6. How does the copper chloride experiment compare to the FeSCN reaction? What is similar (how did they shift the same way?)? Could you predict what could change in the copper chloride from your previous observations?

7.  What is important to keep in mind when designing your own experiment?

8. What would you do differently if you would conduct the experiments and design the procedure again?