Quantitative Yield of a Redox Reaction

Experiment (9) Quantitative Yield of a Redox Reaction Objectives: 1- To observe an oxideantion-reduction reaction. 2- To calculate the molar concentration of Cu2+ solution by using a redox reaction. Introduction: In a redox reaction, electrons are transferred from one substance to another, resulting in changes in oxidation number: the substance achieving a higher oxidation number by losing electrons is oxidized and is called a reducing agent, the substance achieving a lower oxidation number by gaining electrons is reduced and is called an oxidizing agent. Electrons are never considered as reactants or products – the total number gained must equal the total number gained must equal the total number lots. For example, copper metal oxidized by silver ions, the equation is: According to this equation, Cu increases its oxidation number form 0 to +2 by losing 2 moles of electrons per mole of Cu, it is oxidized and is called a reducing agent: Ag+ decrease its oxidation number from +1 to 0 by gaining 1 mole of electrons per mole of Ag+; it is reduced and its called an oxidizing agent: Now the question is, why does a reaction such as Equation 1 occur, whereas another reactions such as, reaction, does not occur? To answer this kind of question, we can get some help from what is called an activity series, which is a list of metal arranged in order of decreasing ease of oxidation, (table 1). The activity series can be used to predict the outcome of reactions between metals and either metal salts or acids. [Any metal on the list can be oxidized by the ions of elements below it]. For example, copper is above silver in the series. Thus, copper metal will be oxidized by silver ions, Eq. 1. Whereas, copper is below hydrogen in the series. Thus, copper metal will not be oxidized by acids. Table (1). Activity Series of Metals: Metals arranged in order of decreasing ease of oxidation. Metals Sodium Zinc Iron Hydrogen Copper Silver Gold Oxidation reaction From Table (1), Zn is above Cu and H2, Thus, Zn metal will be easily oxidized by both Cu2+ and H+ ions. However, H2 is above Cu in the series, and therefore Cu metal cannot be oxidized by H+ ions. Thus, if solid zinc is added in excess to a solution of blue copper sulfate, the copper ions are precipitated out of solution as solid copper, and zinc metal dissolves to form aqueous zinc ions. As the copper ions are removed, the solution becomes less blue, finally appearing colorless when all Cu2+ is precipitated. Any unreactred zinc metal can be dissolved with dilute HCl, leaving copper as the only solid phase in the mixture. The copper can then be separated, dried and weighted. By determining the mass of copper precipitated from a known volume of Cu2+ solution, it will be possible to calculate the molarity of Cu2+ solution. If the mass of a known volume of Cu2+ solution is obtained, it is additionally possible to calculate the density of the solution and the % by mass of copper in it. In this experiment you will analyze a sample of Cu2+ of unknown molarity. As a reminder, the relationships involving concentration, density, and % by mass are listed below (in the present experiment, the solute is Cu2+). Molar concentration = moles of solute / liters of solution. Density = mass of solution (g) / solution volume (mL). And by mass = mass of solute (g) / mass of solution (g). C- Practice Exercise: A student pipeted 10.000 ml of CuSO4 solution, having an unknown molarity and a density of 1.040 g/mL at 25oC,into a beaker and added 1.50 g of zinc powder. The mixture was stirred occasionally over a period of about 5 minutes, and then 10 mL of 6 M HCl was added. When bubbles stopped evolving, the mixture was quantitatively transferred to a pre-weighed filtering crucible (mass = 10.8275g) which was connected to an aspirator. The reddish metal that separated was washed with water and acetone. The crucible-with-metal was dired in an overn at 100oC for 5 minutes, cooled, and found to weight 11.3459 g. Calculate the molarity and % by mass of Cu2+ in the CuSO4 solution. Mass of Cu = 11.3459 g. Calculate the molarity and % by mass of Cu2+ in the CuSO4 solution. Mass of Cu = 11.3459 – 10.8275 = 0.5184g = mass of Cu2+ in the solution moles of Cu2+ = mass/molar mass = 0.5184/63.55 = 9.157 x 10-3 mol. Concentration of CuSO4 solution = mole/volume = 8.157 x 10-3 / 10 x 10-3 = 0.8157 M. % by mass = mass of Cu2+ in the solution / total mass of solution. = mass of Cu2+ in the solution / (Volume x density)solution. = 0.5184g / 10 ml x 1.040 g/ml) x 100% = 4.985. D- Experimental Procedure: 1- Pick up a CuSO4 solution of unknown concentration from the Instructor. 2- Pipette 100 ml of unknown solution into a clean, dry, and preweighed 50ml beaker. Weigh the beaker and solution. Measure the room temperature. Record your measurements on the Report Sheet. 3- Add about 1 g (an excess) of granulated zinc to the Cu2+ solution and stri with the glass rod. The mixture should be stirred occasionally until the blue color disappears. Note: solid zinc should still be present at this time. 4- Add approximately 15 ml of 6 M HCL to the beaker and stir occasionally until the bubbles stop evolving. The solid zinc should all be dissolved by this time, leaving brown copper solid. 5- Weigh a filter paper, and set-up the suction filtration apparatus. 6- Transfer the reaction mixture from the beaker to the filtering Buchner funnel. 7- Wash the copper twice with distilled water, and conce with about 5 ml of acetone. Then let the aspirator pull air through the funnel until all obvious liquid is gone. 8- Dry the copper solid in the oven for about 10 minutes. Allow to cool to room temperature, and weigh the solid copper. Record the mass of solid copper on the report sheet. 9- Calculate the density, the morality, and the % by mass of copper in the solution. Data and Results: A: Data Unknow NO:……………… n Volume of CuSO4 solution …………… ml Mass of CuSO4 …………… g Room temperature …………… Co Mass of filter paper …………… g Mass of Filter paper + Cu …………… g Mass of Cu …………… g B- Results (show your calculation methods, with units): Density of CuSO4 solution …………… Molarity of CuSO4 solution …………… % by mass of Cu in solution …………… * Write the net ionic equation for the replacement of Cu2+ by Zn2+ in aqueous solution: ……………………………………………………………………………………………...... ………………………………………………………………………………………………… ………………………………………………………………………………………………… * Write the net ionic equation for the dissolution of zinc by dilute HCl: ……………………………………………………………………………………………...... ………………………………………………………………………………………………… …………………………………………………………………………………………………