Honors Chemistry Stoichiometry of a Reaction and Limiting Reactant

Introduction

A chemical reaction can be described by an equation. The equation shows the relationship of the moles of reactants to moles of products. Since matter is neither created nor destroyed in chemical reactions (Law of Conservation of Mass), the equation must show equal numbers of atoms of each element in both reactants and products. Such an equation is said to be 'balanced."

Stoichiometry is the study of quantity relationships in chemical reactions. The stoichiometry of a reaction is based on the mole. The coefficients in the balanced equation represent the relative numbers of moles of substances that react or are formed in the reaction.

For practical reasons, the quantities of the reactants used in a reaction are generally not in the same mole proportions as the balanced chemical equation. It is important to be able to predict how much product will form under these conditions.

Hypothesis: 3-4 sentences

Purpose

To demonstrate that substances react in proportions related to the number of moles of reactants. To show that if the reactants are not present in stoichiometric proportions, the substance present in smallest stoichiometric amount (limiting reactant) will determine the amount of product(s) formed.

Safety

1. Wear protective goggles throughout the laboratory activity.

2. Carefully clean up any spills.

3. Exercise caution in lighting the burner. Loose hair should be tied back. Hair ignites easily especially if hair spray is used.

4. Always exercise caution when handling glassware especially if it is hot. Hot and cold glass looks the same!

Pre-Lab Analysis

  1. How many moles are present in a sample of 34.0 g of iron metal? How many moles are present in a sample of 34.0 g of copper metal? Show your work and units!
  2. Define the lab technique of decanting.
  3. Define single replacement reaction.
  4. Write two balanced chemical equations, one for each of iron’s oxidation number (2+&3+), for the reaction that occurs when solid iron reacts with lead (II) carbonate dissolved in water. Include the phase (s, l, g, or aq) for each substance.
  5. What is the Fe : Pb mole ratio in each of the equations you wrote in the question above?
  6. Experimentally, how would you determine the mass of iron lost from a nail during a single replacement reaction in which iron reacts with copper(II) chloride?
  7. If your experimental results for the mass of iron lost are 0.85 g, how would you calculate thepercent yield of copper expected, when the theoretical yield was 1.2g? Show your calculations.

Procedure

  1. Weigh accurately a clean, dry 50- or 100-ml beaker. Record the mass. Add to the beaker the assigned quantity (1.0 – 2.0 g) of Fe filings or zinc metal. Reweigh accurately the beaker and contents. Record the mass.
  2. Weigh accurately a 150- or 250-ml beaker. Add approximately 7.00 g of CuS04.5H20. Reweigh accurately and record the mass of beaker and CuS04.5H20. Add approximately 80 ml water (preferably distilled water) to the beaker. Record the solution's color.
  3. Once the copper sulfate has dissolved, carefully pour the Fe filings or Zn metal into the CuS04 solution a little at a time, while stirring vigorously. After each addition of Fe filings or Zn metal, wait for the reaction to subside, and then continue stirring. Use a thermometer to record changes in temperature over a five-minute time period, or until the reaction starts to cool. In the case of zinc metal, do not touch the beaker during this process. You must also record your observations.
  4. When the reaction is complete (no Fe filings or Zn metal remain; look from below if you have a problem seeing whether Fe filings remain), allow the solid to settle. Let the beaker cool until it is safe to touch. Carefully decant into a beaker (pour off the liquid leaving the solid behind). Record the appearance of the decant (the liquid poured off). You will not be able to pour off all the liquid, but pour off as much as possible without losing any of the solid. (If you accidentally pour some of the solid off with the decant, you can carefully pour the decant into another beaker, wash the particles back into the beaker with the solid reaction product and try decanting again.) Dispose of the decant as your teacher directs.
  5. Wash the solid by adding 10 ml water (preferably distilled water). Swirl very gently. (Be very careful not to spill the solid and ruin your results!) Let the solid settle. Decant (as described in Step 5). Continue the washing procedure several times until the wash water is colorless. After the last washing decant as much water as possible so drying will not take so long.
  6. Dry the solid by placing the beaker where it can remain undisturbed; allow to dry overnight. If the solid is allowed to stay at an elevated temperature for a long period of time, it may react with oxygen in the air and alter the results.
  7. Weigh the beaker and contents accurately and record the mass.
  8. Thoroughly wash your hands before leaving the laboratory.

Part I. Data Analysis: Stoichiometry of the Reaction

  1. Prepare a data table that includes the following: 1)mass of 50-mL beaker, 2) mass of 5O-mL beaker and metal used, 3)mass of metal used, 4)mass of l50-mL beaker, 5) mass of 150-mL beaker and CuSO4.5H2O, 6) mass of CuS04.5H20, 7) mass of 150-mL beaker and Cu, 8) mass of Cu
  2. Prepare a data table that contains the temperature vs. time data for the metal used in your experiment.
  3. Graph the temperature versus time data for your experiment.
  4. Write a balanced chemical equation for the reaction that occurred and identify the reaction type.
  5. Calculate the moles of metal used in your experiment and the number of atoms produced from the number of moles of reactant metal.
  6. Calculate moles CuSO4 used. Since one mole of CuSO4.5H2O contains one mole of CuSO4, the moles of CuSO4.5H2O equal the moles of CuS04. Then calculate the atoms of CuSO4 used.
  7. Calculate moles Cu formed in the reaction. Then calculate atoms of CuS04used.
  8. Calculate the mole ratio of metal used to copper. What does this ratio tell you about the relationship between to amount of metal used and the amount of copper produced in the chemical reaction?

Part II. Data Analysis: Limiting Reactant

  1. Calculate the theoretical mol of copper made in your experiment.
  2. Using the theoretical mol of copper calculated, find the theoretical mass of copper that should have been produced in your reaction.
  3. Compare the theoretical (calculated) mass Cu with the mass Cu obtained in the reaction. Determine the percent yield of Cu you obtained in this reaction.
  4. The reactant (CuSO4 or Fe) that gives the number of moles Cu closest to the experimental value is called the limiting reactant since this substance limits the yield of products. Using the measured mass of Fe and the measured mass of CuSO4, calculate which reactant is the limiting reactant?
  5. Based on your observations and mathematical calculations, calculate the mol of excess reactant.
  6. Based on your observations and mathematical calculations, calculate the mass of excess reactant.
  7. Calculate the percent error from the theoretical mass of copper that should have been produced and the experimental mass that was produced.

Part III. Implications and Applications

  1. How would your experimental value for moles Cu be affected in the following situations?
  2. If 8.5-g CuSO4.5H2O were added instead of 7.0-g CuSO4.5H2O?
  3. If the solid did not settle before decanting and some solid product was poured off during decanting?
  4. if the solid product was not dried thoroughly and weighed wet?
  5. Why was the mixture of CuSO4stirred? Suggest changes that might result if the mixture is not agitated.
  6. State two reasons why you may not recover the theoretical amount of copper in this experiment?
  7. Calculate the mass of the aqueous productcreated.
  8. How would your experimental results differ if you used less than the quantity of CuS04 calculated in Part I?
  9. List three possible sources of error in this experiment and explain how it would affect the Fe:Cu or Zn:Cu ratio.
  10. Would the reverse reaction occur? Explain your rational.

Part IV. Determination of Water in a Hydrate

Safety:

1.Wear safety goggles!

2.Always handle the crucibles with tongs. A hot crucible looks exactly like a cold crucible.

3.Allow the crucible to cool for 5 minutes before placing it on the balance.

Procedure:

1.Find the mass of an empty, dry crucible (without lid) and record in the data table.

2.Place one spatula tip full of the hydrate into the crucible and determine the mass (to 3 decimal places) of the crucible and sample and record this in the data table.

3.Place the crucible with sample on the clay triangle and heat for 10 minutes with the Bunsen burner.

4.Allow sample to cool for 5 minutes. While you are waiting for the crucible to cool, calculate the theoretical % of water in the hydrate.

5.Once the crucible is cool, determine the mass of the anhydrous sample and record in the data table.

6.Clean the crucible in the sink (notice how the anhydrous compound regains its color when water is added).

Data:

Mass of dry crucible:______g

Mass of crucible and hydrate:______g

Mass of crucible and anhydrous sample ______g

Sample name: ______

Chemical Formula:______

Calculations and Analysis

1. Calculate moles of hydrated CuSO4.5H2O placed inside the crucible.

2. Calculate moles of anhydrous CuSO4 produced.

  1. Calculate mass of water removed from the hydrate, CuSO4.5H2O, during the reaction. Find the difference between the mass of hydrated CuSO4.5H2O used and the mass of the anhydrous CuSO4 produced. This is the mass of the water produced.
  2. Determine the experimental moles of anhydrous CuSO4and water produced. This is done by dividing the experimental mass of CuSO4 placed in the crucible by 159.5g/mole. The experimental moles of water are calculated by dividing the mass of water produced by the molar mass of water.
  3. Determine the percent error of water produced in your hydrate. This is determined by subtracting the experimental number of moles in CuSO4.5H2O from the theoretical moles of water produced in the experimental ratio of CuSO4 to H2O. Divide the absolute value of the difference between the experimental moles and the theoretical moles by the theoretical moles of CuSO4 to get the percent error.

Concluding Paragraphs: 1-2 paragraphs: This should explain the validation of your hypothesis through the use of your experimental data and analysis, explain your sources of error, provide insight into the experimental process and what you discovered, and provide suggestions for improvement.

Analysis of a Metal: 2 paragraphs explaining the uses of stoichiometry in industry and the practical applications stoichiometry has in today’s world.

Bibliography: 3-5 citations minimum, MLA format, with footnotes as needed.

This lab must follow the criteria below: 1) each report is individualized – responses to questions must be unique and in your original voice, have all resources cited, and 3) include the following:

  1. Answers to all questions
  2. Completed data tables
  3. CuSO4 Spreadsheet
  4. Water in Hydrate Spreadsheet
  5. Hypothesis and Conclusions
  6. Analysis of a Metal and Bibliography