PGCC CHM 103 - An Investigation of Electrochemical Reactions

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Further Explorations in the Chemical World Prince George’s Community College 73 Name_________________________________ Section_________________ Partner(s)______________________________ Date___________________ AN INVESTIGATION OF ELECTROCHEMICAL REACTIONS PRE-LAB QUERIES 1. How does electricity or electrical current get from the power plant to your house? Can you get electrical current from any source other than an electrical outlet? If so, what? 2. Dissociate the following soluble salts into ions: NaCl à MgSO4 à Fe(NO3)3 à 3. If the following gases were produced during a reaction, how would you identify them? Consider chemical tests and any physical properties to aid in identification. hydrogen oxygen chlorine sulfur dioxide 4. For the spontaneous reactions below, predict the products and list evidence that would indicate reaction occurred. Mg(s) + HCl(aq) à Fe(s) + CuSO4(aq) àFurther Explorations in the Chemical World Prince George’s Community College 74 voltmeterbatteryprobebeaker with test solution+-PROBING THE CURRENT SITUATION In this activity we will explore some electrochemical properties of solutions and reactions. Careful observation is critical in this activity. You may want to review An Investigation of Chemical Reactions from your CHM 101 manual. The conductivity of a solution is a measure of its ability to carry electrical current. Obtain a conductivity probe, battery, and voltmeter and set them up as shown in the illustration. Your instructor will help with the wiring. The conductivity probe functions by measuring the relative ability of a solution to conduct electricity. A voltage is read on the voltmeter that is mathematically related to conductivity. (They are NOT directly proportional.). Do NOT change the spacing of the electrodes after you start making measurements with the probe. 1. To show that voltage on the voltmeter is related to the conductivity of the solution, hold a piece of pencil lead (graphite with a clay binder, a good conductor) across the two metal electrodes (paper clips) on the probe using plastic forceps. What happens? 2. What type of particles, ions or molecules, are required for a solution to be conductive? Explain your answer.Further Explorations in the Chemical World Prince George’s Community College 75 Predict whether each of the solutions in the table below will be conductive. Dip the probe into distilled water and then into each of the 0.01 M solutions listed below and record each reading on the voltmeter. Do NOT leave the probe sitting in any solutions. Rinse the probe in distilled water after each solution. Classify each of the solutes as non-electrolyte, weak electrolyte, or strong electrolyte. Solution Will Conduct? (Y or N) Voltmeter Reading Type of Electrolyte distilled water sugar NaCl CH3COOH HCl tap water 3. Now let's see what factors influence the conductivity of a solution. Dip the probe into the solutions of strong electrolytes listed in the table that follows and record the readings on the voltmeter. Remember to rinse the probe with distilled water after testing each solution. Look for trends or patterns in the readings.Further Explorations in the Chemical World Prince George’s Community College 76 Possible Factor Solution Reading 0.001 M NaCl 0.01 M NaCl Concentration 0.1 M NaCl 0.01 M NaCl Type of Salt 0.01 M CaCl2 0.01 M HCl 0.01 M NaCl Cation 0.01 M KCl 0.01 M NaCl 0.01 M NaOH Anion 0.01 M NaNO3 0.01 M NaCl @ 25oC Temperature 0.01 M NaCl @ 60oC Summarize your results for each factor and explain how each factor influences conductivity. Does any one ion seem to have an unusually high conductivity? Explain. 4. Conductivity measurements can be used to quickly determine total dissolved solids (TDS) in stream water, since the proportion of ions are fairly constant. TDS is usually measured by massing the residue remaining after evaporating a quantity of filtered stream water (suspended solids removed). Using the handheld meter, measure the TDS content of the stream water provided and tap water. Multiply the scale reading by 10 to get TDS in mg solids/L water.Further Explorations in the Chemical World Prince George’s Community College 77 Solution TDS stream water tap water Here are some typical TDS levels for various water sources. Source TDS Rainwater < 10 mg/L municipal water systems < 500 mg/L rivers and streams 100-2000 mg/L seawater 35,000 mg/L How do the stream water and tap water you measured above compare to the sources in the table? 5. Next we will pass an electrical current through a solution and observe if any chemical reaction occurs. What types of evidence would indicate that a chemical reaction is occurring?Further Explorations in the Chemical World Prince George’s Community College 78 ReservoirElectrodesObtain the small-scale Hoffman apparatus, illustrated below, and a battery. The electrodes in this version are made of pencil lead. Be careful they are fragile! Place distilled water into the reservoir, making sure the electrodes are submerged, and apply a current by connecting the battery to the electrodes. What happens and why? Empty the apparatus and place an aqueous solution of sulfuric acid into the apparatus. Apply the current. What happens and why? Empty the apparatus, rinse with distilled water, and place an aqueous solution of NaNO3 in the apparatus. Apply the current. What happen and why? Rinse the small-scale Hoffman apparatus with distilled water when finished. Observe the reaction again using the large Hoffman apparatus containing aqueous sulfuric acid. Do NOT change the solution in this apparatus! Test the products and record your results. What type of chemical reaction is occurring? Write an overall chemical reaction. This is called the electrolysis of water. What is the purpose of the sulfuric acid or NaNO3 in the apparatus?Further Explorations in the Chemical World Prince George’s Community College 79 6. In any electrochemical reaction, both oxidation (loss of electrons) and reduction (gain in electrons) occur simultaneously. The two electrodes, or half cells, are given names depending on which process occurs. Oxidation occurs at the anode and reduction occurs at


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PGCC CHM 103 - An Investigation of Electrochemical Reactions

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