Activity 2: Temperature Effects
Major Chemical Concept
This activity is designed to develop the concept that a change in temperature can affect the equilibrium position of a chemical system.
Level
Basic, general or honors students
Expected Student Background
General knowledge of solutions and concentrations, formula writing, equation writing, balancing equations, effect of indicators in a solution, and acids and bases in solution.
Time
One 50-min for the laboratory activity and the discussion in which the concept is developed from the observations.
Safety
See safety suggestions in Student Version .
Materials (For 24 students working in pairs)
12 Beakers, 600-mL
36 Beakers, 100-mL
12 Burners
12 Test-tube holders
48 Test-tubes, 18- x 150-mm
12 Test-tube racks
12 Medicine droppers
Consumables (For 24 students working in pairs)
Phenolphthalein, in dropper bottle
Concentrated NH3 , ammonia, small dropper bottle(in Procedure A, two drops of dilute NH3 can be used.)
Ice
0.2 M Iron(III) nitrate, Fe(NO3)3, small dropper bottle (see Activity 1)
0.005 M Potassium thiocyanate, KSCN, 60 mL (see Activity 1 )
Advance Preparation
It is advisable to prepare a solution of the concentrated ammonia and phenolphthalein, that shows the desired color. Display the desired color of the solution to students before they start the laboratory procedure. Directions for preparation of the other solutions are in Laboratory Activity 1.
Pre-Laboratory Discussion
A discussion of the effect of indicators in basic and acidic solutions may be helpful to students. You may want to do Part A, Steps 1-3, as a class demonstration, and then make the excess ammonia solution available to students. You may want to do Part A, Steps 1-3, as a class demonstration, and then make the excess ammonia solution available to students to complete the rest of the activity.
Teacher-Student Interaction
During the laboratory activity, it is advisable to move from group to group to insure that students have obtained the desired color with the phenolphthalein indicator. Have students look down the tube for a better color comparison. Make sure they do not look down the tube while heating it.
When students have gathered the data, have them attempt to write a chemical equation for what happened. They can be led to this equation by answering the questions in the activity. Many students, however, may need the class discussion to develop the equation unless they learned about the interaction of acids with water in previous acid-base studies.
Anticipated Student Results
The pink color of the aqueous ammonia solution should disappear when the solution is heated. The pink color returns when the solution is cooled. The color of FeSCN 2+ fades as the solution is heated. Cooling reverses this effect.
Answers to Implications and Applications
1. If the ammonia concentration is too high, the added heat may not lower the hydroxide ion concentration enough to change the color due to phenolphthalein.
2. As a rule of thumb, increasing the temperature of a solid/liquid mixture will increase the solubility of the solid. This is true because most dissolution processes of solids in water are endothermic. (There are exceptions. Increasing the temperature of a gas/liquid solution will decrease the solubility of the gas because the entropy factor is much more important in gaseous systems.)
Post-Laboratory Discussion
Use the ÒData Analysis and Concept DevelopmentÓ questions as the basis of a discussion to develop the ideas of temperature effects on systems at equilibrium.
Extensions
1. Have students predict the effect that cooling would have on a system at equilibrium. After predictions are made, perform a laboratory activity in which the solution is cooled.
2. Possible Laboratory Activity: Boil 200 mL cold, saturated CoCl2 (aq), cobalt(II) chloride solution. The cherry red solution turns blue. Pour the solution into a 15- x 60-mm test-tube. Place the lower quarter in salt-ice bath and heat the upper portion. The lower end becomes red, while the upper portion becomes blue (see Demonstration 2).
Assessing Laboratory Learning Questions
1. What is a reversible reaction? [A reaction where products can recombine to form reactants.]
2. Compare the rates of forward and reverse reactions for a chemical system at equilibrium. [Forward rate = Reverse rate]
3. How is chemical equilibrium related to a reversible reaction? [A reversible reaction is an example of chemical equilibrium.]
4. What is chemical equilibrium? [Behavior of a chemical reaction where forward and reverse reactions proceed at the same rate.]
5. Using Le ChatelierÕs Principle, explain the effect of adding heat to a system at equilibrium. [The system will respond by favoring the reaction that counteracts the heat increase; the endothermic reaction predominates.]
6. Present a demonstration in which an equilibrium is shifted due to
a change in temperature. Have students write an equation for the reaction,
and explain the shift in equilibrium [e.g., Demonstration 1 based on 2NO2
(g)
N2 O4
(g)].
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