A limiting reactant is the reactant in a reaction that “limits” the amount of product that can be produced. An everyday example of a limiting reactant would be a sandwich ingredient that limits the amount of sandwiches that can be made. If you are like me, your turkey sandwich must have: 2 teaspoons of Dijon mustard, 4 slices of turkey, 3 lettuce leaves, 2 slices of Swiss cheese, and 2 slices of bread. When preparing turkey sandwiches for a picnic you realize you have 26 slices of bread, 34 leaves of lettuce, 30 teaspoons Dijon mustard, 32 slices of turkey, and 18 slices of cheese. Which ingredient will limit how many sandwiches you can make? This is your limiting ingredient or “limiting reactant because when it runs out you cannot make any more sandwiches.
In chemistry a similar situation frequently arises. Once any one reactant is used up it is impossible to produce any more of the product(s). A balanced equation, like the recipe for a turkey sandwich, tells a chemist how much of each reactant is needed and how much product can be produced. The mole ratio is used in a conversion factor to calculate how much product can be produced from each reactant which allows identification of the reactant that is limiting.
The reactant that is left over is called the excess reactant. And the amount of product that can be produced when all of the limiting reactant is used is called the theoretical yield. The percent of the theoretical yield that is actually recovered is the percent yield. In this lab you will determine which reactant is the limiting reactant for the reaction below and you will calculate the percent yield of the product. zinc + iodine → zinc iodide Equation 1
Which reactant, zinc or iodine, will limit the production of zinc iodide? What is the theoretical yield of zinc iodide based on the amount of limiting reactant used? What is the percent yield obtained in your actual synthesis?
two 20 x 170 mm pyrex test tube
250 mL beaker boiling chips
1.00 g granular zinc
1.00 g iodine
10 mL beaker
10 mL graduated cylinder
Bunsen burner test tube holder
10 mL H
HC
2
H
3
O
2
2
O/6 drops 6M
1.
Rest a clean, dry test tube (20 x 170 mm) in a large beaker. Place on the electric balance and record its mass.
2.
Weigh out 1.00
±
0.05 grams of granular zinc using glassine weighing paper (don’t use the plastic weigh boats—fine powders stick due to static electricity) and transfer to the test tube.
3.
With a different spatula weigh out 1.00 ± 0.05 grams of iodine crystals and add to the same test tube.
4.
Pour 10 mL tap water into a 10-mL graduated cylinder. Take the water over to the hood and use a pipette to add 6 drops of glacial acetic acid to the water. This is your “acidified water” to be used in the reaction.
5.
Carefully pour out 2.5 mL of the acidified water and add this volume to the boiling tube containing the Zn and iodine. Once done, immediately begin vigorous swirling of the chemicals in a clockwise or counterclockwise motion. Do not shake up and down. Do not stop mixing until the solution is close to room temperature.
6.
Weigh together a dry large test tube (20 x 170 mm) and two boiling chip and record the mass.
7.
Leaving the gray zinc metal behind, slowly pour off all of the nearly colorless reaction mixture into the dry large test tube containing the boiling chips. This solution is aqueous zinc iodide.
Lab24-LimitingReactant

8.
Any traces of zinc iodide remaining behind with the zinc can be diluted with water and collected; this is called "washing" a substance. Wash the left over zinc metal, by first adding approximately 1 mL of the acidified water to the reaction tube. Swirl and pour the wash solution into the boiling tube containing the aqueous zinc iodide. Once again, leave the zinc behind in the reaction tube. Repeat this step 2 more times.
9.
For the meantime, ignore the solution of aqueous zinc iodide and focus attention on the zinc metal remaining in the boiling tube. Rinse the remaining granular zinc 3 more times with about 2-3 mL portions of regular tap water. After rinsing for a few seconds, carefully discard the liquid down the drain without losing any zinc.
10.
With a test tube holder, heat the reaction tube containing the zinc metal over a Bunsen burner flame in circular motions until all moisture is just driven off. POINT THE TEST TUBE AWAY FROM PEOPLE!
HEAT GENTLY TO AVOID SPATTERING. An indication that drying is complete is when the zinc granules will no longer adhere to the walls of the test tube. Flame-dry any condensed water vapor adhering to the sides of the test tube.
11.
Let the test tube containing the zinc metal reach room temperature and determine its’ mass.
12.
To make sure your evaporated all the water from the zinc metal, repeat the drying procedures (steps #10 and
#11) until the masses you recorded for each trial are within 0.05 grams.
13.
Now focus attention on the zinc iodide solution which was decanted from the excess zinc metal into the boiling tube. With a test tube holder, carefully heat the zinc iodide solution in the large test tube over a
Bunsen burner flame until all the water has been evaporated off and the white solid zinc iodide remains. Be sure to constantly swirl the solution, move the test tube in and out of the flame to maintain controlled simmering, and keep the walls of the test tube hot so that evaporating water does not re-condense on the sides of the test tube and fall back into the solution. Your goal is to evaporate the water while not allowing any of the solution to spurt out of the test tube. As more and more water evaporates, the test tube can be lowered into the flame to speed evaporation. Eventually the entire length of the test tube should be heated to evaporate any re-condensed water. This step takes approximately 10 minutes and must be done patiently.
14.
When all of the water has been evaporated off and only the white solid remains, continue heating the solid while moving the test tube in and out of the flame for about a minute. If the solid turns yellow in any spot, discontinue heating in that area of the test tube. The yellow color indicates that decomposition is taking place. If any condensed water is present on the inside of the test tube, evaporate it.
15.
Cool the test tube to room temperature. Determine the mass of the test tube, the zinc iodide, plus the boiling chips.
16.
Now, reheat the test tube containing the zinc iodide, cool it, weigh again, and record its mass; the difference between trials should be less than 0.05 grams, repeat if necessary.
Lab24-LimitingReactant

Name:
Lab Partners:
1.
Write a balanced chemical equation for the reaction of zinc with iodine to form zinc iodide. Classify the reaction as a precipitation reaction, oxidation-reduction, combustion, synthesis, or decomposition
(underline; more than one choice possible).
2.
Find the limiting reactant: a.
Based on the mass of zinc used, calculate the mass of zinc iodide that would be produced if all the zinc were converted to zinc iodide. b.
Based on the amount of iodine used, calculate the mass of zinc iodide that would be produced if all the iodine were converted to zinc iodide. c.
Which reactant is limiting?
3.
What is the theoretical yield, in grams, of zinc iodide, based on the limiting reactant?
Mass of empty test tube for Zn g
Mass of Zn
Mass of I
2
Mass of empty test tube + 2 boiling chips
Mass of dried Zn + test tube, 1 st heating
Mass of dried Zn + test tube, 2 nd heating
Mass of test tube + product + boiling chips, 1 st heating
Mass of test tube + product + boiling chips, 2 nd heating
Describe the reaction as it progresses.
Lab24-LimitingReactant g g g g g g

YOU MUST SHOW ALL CALCULATIONS !
1.
Calculate the mass of zinc iodide produced.
2.
What was your percent yield, based on the theoretical yield?
3.
Determine your material balance.
4.
Was this reaction exothermic (gave off heat) or endothermic (absorbed heat)?
5.
Why was the residual zinc washed with acidified water (step 7)? Why didn’t the wash water have to be measured exactly?
6.
How would each of the following errors affect the results of the experiment; would the actual yield be too high or too low? Explain. a.
The zinc iodide was not completely dry when weighed. b.
The unreacted zinc was not thoroughly washed.
Lab24-LimitingReactant