Experimental Determination of Avogadro’s Number, Lab Reports of Chemistry

Download Experimental Determination of Avogadro’s Number and more Lab Reports Chemistry in PDF only on Docsity! Experiment 18 Experimental Determination of Avogadro’s Number Objectives:  To construct an electrochemical cell (also referred to as battery).  To measure the current produced in this cell.  To use stoichiometry to determine the theoretical change in mass of electrode.  To use the previous objectives to determine Avogadro’s number and the percentage difference from the accepted value. To prepare for this experiment:  Carefully read the entire experimental guide (below).  Answer all the prelaboratory practice problems.  Since some of this material is not covered in lecture, a short introductory lesson providing the necessary fundamental concepts will be provided. So listen closely!  Review dimensional analysis. Introduction: An electrochemical cell is a device that uses chemical reactions to convert chemical energy into electrical energy. These types of devices are seen often in the form of batteries, which power everyday electronics, such as laptops and cell phones. In this lab, you will be creating a variation of what is known as a Galvanic Cell. In this type of electrochemical cell, two different metal electrodes are submerged in a solution that contains the aqueous ionic form of the same two metals. This is demonstrated below in Figure 1. Figure 1: Electrochemical Cell Electrodes Wire Connecting Electrodes Solution Containing Metal Ions Electrochemical cells utilize Redox (Reductions-Oxidation) reactions. During reduction, an aqueous ion accepts electrons and becomes neutral, thus becoming solid metal. In oxidation, electrons are liberated by solid metals, thus turning them into ions that become dissolved in solution. In this experiment, Copper is Reduced and Zinc is Oxidized. So as the reactions proceed, Cu(s) is being formed and Zn(s) is being consumed. The wire shown in Figure 1 attaches the electrodes to allow for the flow of electrons, which is known as a current. The unit of current is Amperes (often called Amps for short). This unit measures the charge of the electrons over time, where charge is measured in Coulombs. Using this, you can obtain the following equation to relate charge to time: Charge = Current * Time where charge is measured in Coulombs (C), current is measured in Amps (A = Coulomb/Second), and time is measured in seconds. As you can see, this is consistent with the dimensional analysis. ∗ ∗ It may not be obvious, but all of this information can be used to determine Avogadro’s number, which gives the number of molecules or atoms in a mole of any substance. All electrons have a constant charge of 1.602*10-19 C. If you measure the current of this system for a certain amount of time, you can determine the total charge passed through the wire, thus determine the total number of electrons that were involved in the reaction.  This number of electrons can be correlated to the theoretical number of Zinc or Copper atoms involved using stoichiometry. For this experiment, we will choose to correlate the number of electrons involved to the number of Copper atoms involved. Since Copper metal is deposited, you can measure the change in mass of the Copper electrode, which can be converted to moles. The ratio of atoms of Copper to moles of Copper should tell you how many atoms are in a mole of Copper, which is Avogadro’s number. ′ Data Table 1 Trial 1 Trial 2 Initial Mass of Copper Electrode (g) Final Mass of Copper Electrode (g) Average Current (mA) Total Time (sec) 300 300 Data Table 2 Trial 1 Trial 2 Trial 1 Trial 2 Trial 1 Trial 2 Time (s) Current (mA) Current (mA) Time (s) Current (mA) Current (mA) Time (s) Current (mA) Current (mA) 0 105 210 15 120 225 30 135 240 45 150 255 60 165 270 75 180 285 90 195 300 Average Current Data Analysis: 1. Use the average value for your current measurements (in Amps) over the course of the reaction to find the total amount of charge (in Coulombs) passed through the circuit. 2. Calculate the number of electrons involved in the reaction. (Reminder: The charge of an electron is 1.602*10-19C) 3. Using the answer from #2, determine the number of Copper atoms deposited. This will be based on the stoichiometry of the reaction. (2 electrons are used to reduce 1 atom of Cu 2+) 4. Find the difference in the final and initial mass of the electrode, and use this to determine the number of moles gained from the electrolysis reaction. (For the molar mass of Cu use 63.546) 5. Using the ratio described in the Introduction and previous calculations, determine your experimental Avogadro’s Number 6. Calculate percent error from the theoretical value of 6.02*1023. For your percent error calculation you may use the formula given below: 100 7. Repeat these calculations (#1-#6) for your second trial. Pre-lab Questions 1. Galvanic cells convert __________ energy to __________ energy. a) chemical; electrical c) chemical; mechanical b) thermal; electrical d) ionic; mechanical 2. What are the two metals that will be used as electrodes? a) Copper and iron c) Copper and zinc b) Nickel and iron d) Nickel and zinc 3. The “redox” reaction gets its name because it involves: a) A red colored bovine c) A red colored oxide compound b) Reduction and oxidation d) Rhenium, deuterium, and oxygen 4. An ampere is a unit of: a) Charge c) Energy b) Time d) Current 5. Avogadro’s number is the number of __________ in a __________. a) atoms; mole c) atoms; gram b) grams; mole d) moles; gram