DEPARTMENT OF CHEMISTRYFACULTY OF SCIENCE AND MATHEMATICS UNIVERSITI PENDIDIKAN SULTAN IDRIS LABORATORY REPORT SKU3023: CHEMISTRY II Semester II Session 2016/2017 ID NO AND NAME 1.Abdul Rahim Bin Md Saad (D20162075593) 2.Mohd Hafiz Aiman Bin Abdullah (D20162075602) LECTURER Prof. Madya Dr. Ismail Zainol EXPERIMENT NO. 4 TITLE Electrochemistry DATE & DAY 23rd October 2017, Monday CHECK LIST (Please tick) Title Objective(s) Introduction Methods (in diagram) Observations (in table) Data & calculation Discussions Questions & answers (if appropriate) Conclusion(s) References (at least 2) EXPERIMENT 4: ELECTROCHEMISTRY 1.0 OBJECTIVES 1.1. To explain the concept of electrons flow, anions and cations. 1.2. To determine the relative potential of reduction for the redox reactions. 1.3. To describe the effect of concentration to the cell potential. 2.0 INTRODUCTION Electrochemical reaction, any process either caused or accompanied by the passage of an electric current and involving in most cases the transfer of electrons between two substancesβone a solid and the other a liquid. (Bockris & DespiΔ, 2011) Oxidation reaction occurred at the anode and reduction reaction occurred at the cathode Since electrons transfer occurs spontaneously, one set of galvanic cell can be designed by separations of the redox pair. Electrons transfer does not occur in the inter phase anymore, but through an external circuit. The two halves of the redox reaction are separated in two different containers (such as beaker) and named half-cell. A common kind of half-cell consists of a piece of metal (the electrode) immersed in a solution of its ions. The electrons are connected with a wire. The circuit between the two solutions is completed with a salt bridge. A salt bridge allows electrical contact between the two half- cells, internal circuit. It maintains the electrical neutrality in each half-cell as ions flow into and out of the salt bridge. "Pored pot" also serves the same function as the salt bridge. 3.0 ACTIVITIES Section A: Galvanic Cell Chemicals Apparatus Copper metal 50 mL beaker Zinc metal Magnesium metal Wire Iron metal Sand paper 0.1M copper sulphate solution Voltmeter 0.1M zinc salt solution Filter paper 0.1M magnesium salt solution 0.1M iron salt solution Procedures Refer to jotter 1.0 Section B: Cell Potential Chemicals Apparatus Zinc metal 50 mL beaker Copper metal Wire 0.1M CuSO4 solution Sand paper 0.01M CuSO4 solution Voltmeter 0.001M CuSO4 solution Filter paper 6 M NH3 solution 0.2M Na2S solution Procedures Refer to Jotter 2.0 4.0 RESULTS AND DATA Section A Electrochemical Cell Cell Potential/emf, Volt(v) CuSO4 and ZnSO4 0.46V Section B Electrolyte Concentration, M Emf, V 0.1M & 0.001M 0.03V Addition of 5ml NH3 0.37V Addition of 4ml Na2S 0.97V Action Changes Addition of 5ml NH3 Solution turn from light blue to light purple Addition of 4ml Na2S Solution turn from light purple to brown Section A: Galvanic Cell 1. Identify anode and cathode electrodes for each reaction. voltmeter 2. Write half-cell equation for each reaction. ππ(π ) β ππ2+ (ππ) + 2π β (anode) πΆπ’2+ (ππ) + 2π β β πΆπ’(π ) (cathode) 3. Write overall reaction equation for each reaction ππ(π ) + πΆπ’2+ (ππ) β ππ2+ (ππ) + πΆπ’(π ) 4. Give two reasons why reduction potential readings are difference from theory. ο· The metal used as a cell may be contaminated ο· The concentration of chemical used may not be precise Section B: Cell Potential 1. Identify anode and cathode electrode in step 1. 2. Write half-cell equation. πΆπ’(π ) β πΆπ’2+ (ππ) + 2π β (anode) πΆπ’2+ (ππ) + 2π β β πΆπ’(π ) (cathode) 3. Write overall reaction equation. πΆπ’(π ) + πΆπ’2+ (ππ) β πΆπ’2+ (ππ) + πΆπ’(π ) 4. Explain why there is a potential in step 2? According to Le Chatelier's principle, increase in concentration must cause the equilibrium to move to the left-hand side making the electrode potential more positive. And a decrease in concentration must have the opposite effect i.e. make it more negative. (The Student Room website, 2007 ) 5. Explain why potential changed with the addition of NH 3(aq)? NH3 will bind with the Cu2+ ion to form the complex ion Cu(NH3)42+. This has the effect of reducing the concentration of the Cu2+. Therefore, it will increase the potential between the two concentrations. 6. Explain why potential changed with addition of Na 2S? S2- will bind with the Cu2+ ion to form the insoluble salt of CuS. This has the effect of reducing the concentration of the Cu2+. Therefore, it will increase the potential between the two concentrations. 5.0 DISCUSSION For section A, a galvanic cell was set up by connecting a half cell of CuSO4 with half cell of ZnSO4 using salt bridge. The electrode for the cells are copper rod and zinc rod respectively. The salt bridge used to connect between the two half cell is a filter paper soaked in 0.1M KMnO4. The cathode rod in this reaction is copper rod where reduction process occurs while the anode is Zinc rod where oxidation occurs. The electrons will flow from anode to cathode resulting electrical potential detected using voltmeter. The theorotical electrical potential between the two electrolyte is 1.10V while the electrical potential we collected from the experiment is 0.46V. the experimental result is much lower from the theoretical value because of several reasons. The first one is the electrode may have been contamianted. We cannot conform that the electrode are the pure form of the metal. It may have been oxidized due to exposure to air and water. The second is the solution produced may have not the exact of measuremnet. The concentration may have been lower form what we expected. Therefore, the result should be lower than the theoretical value. For section B, the interaction of half cell by changing the means of its concentration was studied for the first step. From the equation, ππ(π ) + πΆπ’2+ (ππ) β ππ2+ (ππ) + πΆπ’(π ) , we can predict that there should be no potential between the two cells because there is no change in the system. Due to the difference in the concentration between the half-cell, the solution reacted to balance the concentration thus creating the electrical difference. When we added NH3 to the 0.001M solution, we are reacting the Cu2+ in the solution thus decreasing the amount of Cu2+ available. The decreasing of Cu2+ thus increasing the difference of concentration. To balance the system more Cu(s) was corroded from the plate thus increasing the electrical potential. The same case happens when we added Na2S into the solution. The changes in colour from adding of NH3 is due to the formation of copper ammonia complex. The colour changes when Na2S was added are due to the formation of CuS salt. 6.0 CONCLUSIONS From the experiment, we are able to explain the concept of electrons flow, anions and cations. Besides that, we also able to determine the relative potential of reduction for the redox reactions which in this experiment is between Zn and Copper which is 0.46V. we cannot get the theoretical value due to several errors. We also able to describe the effect of concentration to the cell potential. 7.0 REFERENCES Bockris , O. J., & DespiΔ, R. A. (2011, December 15). electrochemical reaction. Retrieved November 24, 2017, from EncyclopΓ¦dia Britannica: https://www.britannica.com/science/electrochemical- reaction Shakhashiri, Z. B. (1983). Chemical Demonstrations: A Handbook for Teachers of Chemistry, Volume 3. Wisconsin: University of Wisconsin Press. The Student Room website. (2007 , March 7). Effect of concentration on electrode potential . Retrieved from The Student Room website: https://www.thestudentroom.co.uk/showthread.php?t=360208