Mole Ratio of Chemical Reactions Essay

OBJECTIVE In this experiment, you will determine the number of moles of reactants and products present in the reaction of copper and silver nitrate, and calculate their mole-to-mole ratio. The mole-to-mole ratio relating to the disappearance of copper and the formation of silver metal will be used to write the balanced equation for the reaction. The reaction of copper metal with silver nitrate solution is a single replacement reaction, represented by the following unbalanced equation: Cu(s) + AgNO3(aq) → Cu(NO3)2(aq) + Ag(s) The values of the coefficients can be determined experimentally by measuring the mass of copper wire that reacts with the mass of silver that is produced in the above reaction. BACKGROUND MATERIALS NEEDED The reaction of copper and silver nitrate in aqueous solution provides an interesting display of chemistry in action – delicate silver crystals begin to grow on the wire surface and the colour of copper(II) ions gradually appears in the solution. Stoichiometry is the area of chemistry that deals with the numerical relationships and mathematical proportions of reactants and products in a chemical reaction. One of the most important lessons of stoichiometry is that the amounts of reactants and products in a chemical reaction are related to one another on a mole basis. Chemical reactions are normally represented by balanced chemical equations. The coefficients in a balanced chemical equation summarize the relative number of moles of each reactant and product involved in a chemical reaction. The ratios of these coefficients represent the mole-to-mole (or simply mole) ratios that govern the disappearance of reactants and the appearance of products. Knowing the mole ratios in a bala nced chemical equation is essential to solving stoichiometry problems. General Chemistry 1 Lab Acetone Copper wire 3 M HNO3 AgNO3, silver nitrate 50 mL beaker 100 or 150 mL beaker Toothpick Stirring rod Wash bottle 50 mL measuring cylinder SAFETY PRECAUTIONS Nitric acid is a corrosive liquid and strong oxidizer. Silver nitrate is a corrosive solid and is toxic by ingestion; it will stain skin and clothes. Acetone is a flammable liquid – avoid contact with flames and other sources of ignition. Avoid contact of all chemicals with eyes, skin, and clothing. Wash hands thoroughly with soap and water before leaving the laboratory area. Page | 1 PROCEDURE 1. Obtain a clean, dry 50 mL beaker. Carefully add 1.40 – 1.60 g of silver nitrate crystals to the beaker. Caution: use a spatula to transfer the solid. Do not touch the silver nitrate and carefully clean up any spilled material on the balance or the bench top. 2. Measure and record the exact mass of silver nitrate to the nearest 0.01 g. 3. Fill the beaker with 30 mL of distilled water and stir the mixture with a s tirring rod until all the solid has dissolved. Rinse the stirring rod over the beaker with a little distilled water. 4. Take a piece of copper wire and loosely coil it. 5. Find the initial mass of the copper wire to the nearest 0.01 g and record it. 6. Use a toothpick to suspend the copper wire in the silver nitrate solution. The copper wire should not be touching the bottom or sides of the beaker. 7. Carefully add 3 drops of 3 M HNO3 to the beaker. Do NOT stir the solution. 8. Allow the beaker to sit on the lab bench for 15 minutes. Try not to jostle or shake the suspended copper wire in any way. 9. Observe the signs of chemical reaction occurring in the beaker and record all observations. 10. While the reaction is taking place, label a 100 or 150 mL beaker with your name and class session. Measure and record the mass of this beaker. 11. After 15 minutes, gently lift the toothpick to remove the copper wire from the solution. 12. Holding the wire with the wooden toothpick, place the copper wire above the 100 or 150 mL beaker. Rinse the wire with a steady stream of distilled water from a wash bottle. The silver crystals should easily fall off the wire into the beaker. Gently shake the wire and rinse with General Chemistry 1 Lab water until no more silver adheres to the wire. Note: Use a total of about 40 mL of distilled water. 13. When all of the silver has been removed, lift the copper wire out of the beaker and place it in anot her beaker containing acetone. The acetone will clean the wire surface and allow it to dry more quickly. Note: Several groups can share this same rinse beaker of acetone. 14. Remove the copper wire from the acetone beaker and allow it to air dry for 3-4 minutes. 15. Measure and record the final mass of the copper wire. Note the appearance of the leftover wire and record your observations. 16. Examine the beaker containing the silver product. Most of the silver should have settled into a dense mass at the bottom of the beaker. Carefully decant the liquid into a waste flask to remove most of the water. Note: Try not to lose any of the solid in the process. 17. Rinse the solid with 5-10 mL of distilled water from a wash bottle. Decant the rinse water into the waste water flask as well. 18. Repeat the rinsing and decanting cycle with a second portion of distilled water. 19. Discard the waste water in the large beaker in the fume hood. 20. When all of the water has been decanted, place the labelled beaker containing th e silver product on the warming tray in the fume hood. 21. Allow the solid to dry overnight. 22. When the solid is dry, measure and record the final mass of the beaker plus silver solid. Page | 2 PRE-LAB QUESTIONS 1. Define: a. Law of Conservation of Matter b. Molar mass c. Mole d. Stoichiometry e. Limiting reagent 2. Solve: a. Using the balanced equation for the reaction of aluminum with iodine, answer the following questions: 2 Al (s) + 3 I2 (s) → Al2I6 (s) i. How many moles of Al2I6 are produced by the reaction of 4.0 mol of aluminum? ii. How many moles of I2 are required to react exactly with 0.429 mol of aluminium? b. What mass of sodium hydroxide, NaOH, would be required to produce 16 g of the antacid milk of magnesia [magnesium hydroxide, Mg(OH)2] by the reaction of magnesium chloride, MgCl2, with NaOH? MgCl2 (aq) + 2 NaOH (aq) → Mg(OH)2 (s) + 2 NaCl (aq) i. Which reactant is limiting? ii. How much water will the reaction produce? POST-LAB QUESTIONS 1. Calculate the mass and moles of copper wire that reacted in this experiment. Show your calculations. 2. Calculate the mass and moles of the silver produced in this experiment. Show your calculations. 3. Determine the mole ratio – the ratio of the number of moles of silver to the number of moles of copper. Round the result to the nearest whole number. Show your calculations. 4. Use the silver/copper mole ratio to write a balanced chemical equation for this reaction of copper and silver nitrate. 5. What was the limiting reagent in this experiment? Show all calculations and explain your answer. 6. Given what you know about copper, silver nitrate and this kind of single replacement reaction, what would you have predicted the balanced chemical equation to be? 7. Explain the possible differences between Question 4 and Question 6. 8. Do the results of your experiment support the Law of Conservation of Matter? Explain. c. Calculate the mass of oxygen gas, O2, required for the combustion of 702 g of octane, C8H18. 2 C8H18 + 25 O2 (g) → 16 CO2 (g) + 18 H2 O (l) d. A mixture of 5.0 g of H2 (g) and 10.0 g of O2 (g) is ignited. Water forms according to the following addition reaction: 2H2 (g) + O2 (g) → 2 H2O (g) General Chemistry 1 Lab