2604 Chemical Equilibrium

1.0 OBJECTIVES

• Students will investigate a reaction between calcium nitrate and potassium iodate to determine if the reaction goes to completion or reaches a state of equilibrium.
• Students will gain experience producing a precipitate and filtering it out of its solution.

1.1 INTRODUCTION

While many reactions go to completion, many do not. The reactants begin to form products, but the products then begin to collide with one another and reform the reactants. While the initial forward reaction is the fastest with the higher concentration of reactants, as the concentration of products begins to increase, the rate of the reverse reaction begins to increase as well. Eventually, the forward reaction and the reverse reaction are moving at the same rate and a state of equilibrium is reached. There are some reactants changing into products and some products changing into reactants all the time, but the concentrations of the reactants and products become steady.

In this experiment, the reaction between calcium nitrate, Ca(NO3)2, and potassium iodate, KIO3, will be examined to determine if it is a reaction that goes to completion or if it is a reaction that reaches a state of equilibrium.

Ca+2 + 2IO3- → Ca(IO3)2 (s) Equation 1

Ca+2 + 2IO3- ↔ Ca(IO3)2 (s) Equation 2

The difference between Equations 1 and 2 is the arrow: → means a reaction that goes to completion and ↔ means a reaction that reaches equilibrium, implying it goes forwards and backwards.

If the reaction goes to completion, there may still be excess reagent, either the calcium or the iodate, if we did not mix the chemicals in their stoichiometric ratio: 1 calcium to 2 iodates. In an equilibrium situation, there would always be some of all the chemicals present, calcium and iodate and solid calcium iodate.

2.0 SAFETY PRECAUTIONS AND WASTE DISPOSAL

3.0 CHEMICALS AND SolutionS

Chemical/Solution	Concentration	Amount	Notes
Ca(NO3)2	0.200M	40 mLs
KIO3	0.100M	55 mLs
Na3PO4	0.3M	5 mLs
HCl	1.0M	5 mLs
Solid KI	xxxx	0.25 inch on spatula (0.5 cm3)
Laboratory water	xxxx	25 mLs

4.0 GLASSWARE AND APPARTUS

2 100- OR 150-mL beakers	2 filter funnels
2 250-mL beakers	2 Erlenmeyer flasks
2 graduated cylinders	Several small test tubes
Several disposable pipettes	Small spatula
2 filter papers	2 watch glasses

5.0 INVESTIGATION

5.1 Interaction of Calcium Nitrate and Potassium Iodate

1. Wash and label two 100 or 150 mL beakers. Obtain about 40 mL of 0.2 M calcium nitrate, Ca(NO3)2, and 55 mL of 0.1 M potassium iodate, KIO3, stock solutions in the clean labeled beakers – one solution in each beaker. Record the molarity of each stock solution.

2. Wash and label two 250 mL beakers. Prepare the following two solutions – one in each beaker – using a graduated cylinder and using the calcium nitrate and potassium iodate solutions you obtained previously and put into your 100 or 150 mL beakers.

Solution 1: 25 mL Ca(NO3)2 + 25 mL KIO3 (50 mL total)

Solution 2: 15 mL distilled H2O + 10 mL Ca(NO3)2 + 25 mL KIO3 (50 mL total)

3. Mix each solution with a separate stirring rod, rubbing the tip of the rod against the bottom or sides of the beakers. This is a process called scratching. Scratching produces an unpleasant sound something like imperfections on the surface of the beaker and provides a surface for precipitation of a solid to occur thus speeding up the precipitation process. Scratch the beaker with the glass rod frequently until a solid has begun to form. Then let the two solutions stand for 20-30 minutes. Begin filling in the data in Part 6.0 and do the calculations for the theoretical yield.

4. Label and weigh two pieces of filter paper and record their masses in your laboratory notebook.

5. Fold the weighed filter paper as illustrated by your instructor. Insert the folded filter paper into a filter funnel. Place the filter funnel in a clean Erlenmeyer flask. Seat (or seal) the filter to the funnel surface by filtering approximately 5 mL of distilled water through the filter.

6. Gently swirl your solution to suspend the solids. Quickly and carefully pour the solution into the filter funnel. Make sure you pour the solution from beaker 1 into filter 1. Use a stirring rod and rubber policemen to transfer (push) as much of the precipitate as possible into the filter funnel. Use a wash bottle of distilled water to wash any remaining precipitate out of your beaker and into the filter funnel.

7. Wash the precipitate and the surface of your filter with approximately 2 mL of distilled water from a wash bottle and allow the water to completely drain from the filter. Repeat this washing step at least 4 more times. Gently remove the filter paper from the funnel, place it on a watch glass and allow it to air dry. Save your filtrates for further analyses.

5.2 Tests of the Supernatant/Filtrate Identities

A supernatant is the solution standing above a settled precipitate in the bottom of a container.

1. Measure and transfer about 5 mL of 0. 3 M Na3PO4 solution into a small test tube. Measure about 5 mL of 1 M HCl into a separate small test tube.

2. Place about 1 mL of the stock 0.2 M Ca(NO3)2 solution from your 100 or 150 mL beaker in a test tube. Place about 1 mL of 0.1 M KIO3 stock solution from your 100 or 150 mL beaker in a separate test tube. Note that 20 drops is equal to about 1 mL.

3. Calcium cation, Ca2+, test: Add about 10 drops (0.5 mL) of 0.3 M Na3PO4 solution from your small test tube to the test tube containing 1 mL of stock 0.2 M calcium nitrate solution. Note what happens.

4. Iodate, IO3-, test: Dissolve about 0.5 cm3 (about 1/4 inch on a spatula) of KI in the 1 mL of stock 0.1 M potassium iodate solution in your test tube; then add about 1 mL of 1 M HCl to the test tube. Note what happens.

5. Repeat the calcium cation test using 1 mL of solution 1 filtrate rather than 1 mL of 0.1 M Ca(NO3)2 stock solution. Repeat the test again using 1 mL of solution 2 filtrate. Compare your results for the calcium cation test using solution 1 and solution 2 filtrates to each other and to the results using the 0.2 M Ca(NO3)2 stock solution. Note what happens to each.

6. Repeat the iodate test using 1 mL of solution 1 filtrate rather than 1 mL of 0.1 M KIO3 stock solution. Repeat the test again using 1 mL of solution 2 filtrate. Compare your results for the iodate test using solution 1 and solution 2 filtrates to each other and to the results using the 0.1 M KIO3 stock solution. Note what happens to each.

6.0 DATA RECORDING SHEET

Last Name

First Name

Partner Name(s)

Date

Molarity of stock Ca(NO3)2 solution ___________ M (moles/L)

Molarity of stock KIO3 solution_______________ M

			Solution 1					Solution 2
mL Ca(NO3)2 solution:			_________					_ ________
mL KIO3 solution:			_________					__________
mass of filter paper:			_________					__________
mass of filter & precipitate:			_________					__________
mass of precipitate:			_________					__________
moles of Ca(NO3)2:			_________					__________
moles KIO3:			_________					__________
expected mass of precipitate, Ca(IO3)2: (or theoretical yield)			_________					__________
% yield:			_________					__________
Observations of tests of filtrates: Calcium test Iodate Test

7.0 POST-LAB QUESTIONS

1. How do your actual precipitate masses compare to the masses you predicted by calculation, in other words were your percent yields high or low? If your percent yields were low make a list of at least 4 possible explanations for the low yield.

2. Are your test results for the presence of Ca2+ and IO3- in the supernatants consistent with the limiting reagent and excess reagent? Or are they consistent with an equilibrium reaction? Give an explanation for your answer.