Stoichiometry Lab: Determine the balanced chemical equation for a reaction

Background

A chemical equation like 2 H₂ + O₂ → 2 H₂O is balanced when both the reactant side and the product side of the equation show exactly the same number of each type of atom. A balanced reaction shows the molar relationship between the amount of each reactant used and the amount of each product produced. The use of the coefficients in a balanced chemical equation as a ratio to predict how much reactant is consumed or how much product is formed is called Stoichiometry. A correctly balanced equation describes the ideal ratio of reactants required in order to form products to the greatest possible extent, with no excess reactant.

The coefficients in the above reaction tell you the relative amounts of the reactants consumed, i.e. that two moles of hydrogen are consumed every time one mole of oxygen in consumed. It also allows you to answer questions about reactions, for instance, if you started with 15 moles of hydrogen and 15 moles of oxygen, would all of the reactants be consumed? Since, according the balanced equation, you need twice as many hydrogens as oxygens, you would not have enough hydrogens to use 15 moles of oxygen (you would have needed 30 moles of hydrogen). In this case, all of the hydrogen would be used (called the limiting reagent because it limits the amount of product that can be produced) and you would have excess oxygen. The production of water would stop when the hydrogen was used up even though there is still oxygen.

In the reaction you will perform in this lab, one of the products is energy. When one of the reactants is used up, all production of products, including energy, is stopped. When one of the reactants is limiting (and the other is "wasted" as excess) the reaction will produce less than the maximum amount of product possible. In this lab, you will try to determine the coefficients that produce no "wasted" reactants and therefore produce the maximum amount of released energy. You will mix a variety of volumes of sodium hypochlorite and sodium thiosulfate. The solutions will have the same molarity (they will be equimolar) so that the same volume of each solution will contain equal numbers of moles of the reactants. As a result, the ratio of volumes that produces the most energy, will be the same ratio as in the balanced equation. We will also be careful to maintain the same total volume of solution for each reaction so that the energy produced will be directly proportional to the temperature change.

Purpose

• To study the reaction between sodium hypochlorite, NaClO, and sodium thiosulfate, Na₂S₂O₃.

• To observe the effect of changing the volumes (and molar amounts) of each reactant.

• To determine the coefficients of the balanced chemical equation using a temperature sensor.

Chemicals Solution A: 0.5 M sodium hypochlorite (NaClO), 55.0 mL

Solution B: 0.5 M sodium thiosulfate (Na₂S₂O₃), in 0.2 M NaOH, 55.0 mL

Measuring Instruments PASPORT Xplorer GLX PASPORT Temperature probe

graduated cylinders (2), 10-mL graduated pipettes (2), 2- mL

Lab Equipment foam cups or small beakers (11) wash bottle and waste container protective gear

Safety Precautions

• Sodium hypochlorite (NaClO) is the chemical name for household bleach: beware of spills onto clothing or skin.

• The solutions in this lab are basic, and harmful to skin and eyes. In case of spills, rinse the affected area with LOTS of water.

• Be cautious of fumes; work in a fume hood or with plenty of ventilation. Avoid breathing in vapors.

• Wear safety glasses and follow standard laboratory safety procedures.

Procedure

Equipment Setup

• Label one graduated cylinder or graduated pipette “Solution A” and the other “Solution B”

• use the same graduated cylinder or graduated pipette for measuring the same solution throughout the proce­dure.

• Be sure a rinse bottle and waste container are nearby.

• Label every foam cup or beaker with the amounts of each reactant you will be mixing in each.

• Line up all 9 cups or beakers from left to right, by label, in order of decreasing concentration of NaClO (Solution A) as shown below.

• Work with your lab partner(s) to closely track which ratio of the reactants you are testing each time. Do NOT mix chemicals until directed to do so! Data recording must begin before mixing begins.

Xplorer GLX Setup

• Connect a temperature probe to
  or
  on the left side of the GLX.

Record Data

1. Measure and pour 9.0 mL of NaClO (Solution A) into the first cup/beaker Pour 8 mL of A into the second beaker, 7 mL of A into the third beaker, etc. (Be sure you are using the correctly labeled graduated cylinder.) Place the temperature probe into the first solution.

2. Press
   to begin recording the temperature. Watch the Graph display for the temperature to stabilize.

3. Once the temperature remains constant, press
   again to end data collection.

4. Press
   then
   to turn on Statistics. The maximum temperature is displayed below the graph. Record this value in your data table for the corresponding reactant concentrations.

5. Set aside the cup/beaker you were just measuring. Rinse the temperature probe before proceeding. Use the wash bottle and waste container provided.

6. Using your second graduated cylinder labeled “B”, measure the volume needed of Na₂S₂O₃ (Solution B); 1.0 mL for the first reaction. Mix the two solutions in the next cup/beaker.

Note: Remember to check the label to make sure you have the right container.

7. Again, place the temperature probe in the cup, press
   , and monitor the temperature of the mixture using the Graph display.

When the temperature stabilizes, press
to end data collection.

8. As before, read the maximum temperature value and enter this value in the data table.

9. Repeat steps #5 - 8 in this manner, measuring the maximum temperature reached for each of the remaining reactant ratios.

10. Repeat the procedure as needed to confirm results.

Analysis

• Record calculations in your data table as you complete your analysis.

• Create a graph of the data, using the template provided. Plot the data points you recorded in the data table.

• Note that the maximum temperature of the reaction mixture will start to increase with decreasing amounts of Solution A, up to a certain volume ratio, and then will start to decrease after that point. We know that the optimal mole ratio lies between this data point and the next data point, where the maximum temperature of the reaction starts to decrease.

• Draw a best-fit line through the data points where the maximum temperature of the reaction is increasing.

• Draw a best-fit line through the data points where the maximum temperature of the reaction is decreasing.

• The intersection of the two best-fit lines indicates the maximum increase in the temperature for the reaction. From the intersection of lines, identify the relative volumes of Solution A and Solution B that correspond to the greatest temperature change for the reaction. From these volumes, calculate the correct mole ratio for this reaction's ideal condition. Record this information in your lab notebook.

• Write and balance the correct net ionic equation representing this reaction in your lab notebook.

Table 1. The temperature changes for various volumes of sodium hypochlorite, NaClO, and sodium thiosulfate, Na₂S₂O₃, solutions

Volume / mL ±		Temperature / °C ±
Solution A (NaClO)	Solution B (Na2S2O3)	initial	Maximum	Change
10.0	0.0
9.0	1.0
8.0	2.0
7.0	3.0
6.0	4.0
5.0	5.0
4.0	6.0
3.0	7.0
2.0	8.0
1.0	9.0
0.0	10.0

Figure 1. Graph of temperature change against volumes of sodium hypochlorite, NaClO, and sodium thiosulfate, Na₂S₂O₃, solutions.

Graph of Temperature versus Volume of Reactants

Analysis and Synthesis Questions

1. Find the location on the graph that corresponds to the largest temperature change. (Hint: Where do the 2 best-fit lines intersect?)

2. What volume of each reactant solution corresponds to this point?

1. mL Solution A (sodium hypochlorite):

2. mL Solution B (sodium thiosulfate):

3. Based on the ratio of reactants (volume Solution A: volume Solution B) needed for the largest temperature change, what is the correct stoichiometry for the reaction?

4. What was the limiting reagent in reaction 5? How do you know?

5. Why was it important to maintain a constant volume of reactants despite changing the relative amounts of reactants?

6. Explain why the correct stoichiometric ratio is determined by the mole ratio that generated the greatest temperature change.

---