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Electrochemistry Experiment. Objective: To investigate the effect of change in lead(II) ion concentration on the potential of the Pb2+(aq) ∣Pb(s) electrode

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Introduction

Tsuen Wan Public Ho Chuen Yiu Memorial College Form 6 Chemistry Practical Experiment 15: Electrochemistry Date of experiment: 8/4/2011 Introduction: Redox reactions are reactions where the oxidation states of the atoms change. The atoms are either oxidized or reduced, depending if they lose or gain electrons. Electrochemical cells are devices that cause a current from redox reactions. It is set up so that electrons lost from one of the reagents can travel to another reagent. This creates a voltage, which is also known as the electric potential difference. This voltage can be read if a high-resistance voltmeter or multimeter is connected to the circuit Salt bridge is used to allow migration of ions between two electric cells to maintain neutrality of solutions. It is usually made up of a filter paper moistened with an inert solution or an inert solution/gelatine salt bridge to prevent oxidation of certain ions. This experiment is divided into 2 parts: part A and part B. Part A Objective: To investigate the effect of change in lead(II) ion concentration on the potential of the Pb2+(aq) ...read more.

Middle

are independent of its amount present and can be considered as constant. This reduces the expression to Kc= [ Pb2+(aq)] /[Cu2+(aq)] = 1x1021 Conclusion The potential of the cell decreases ad the ion concentration of Pb2+ increases. Further Analysis: Using the Nernst equation: E=E?-0.059/n log[ox]/[red], Take [Pb2+]=0.1 M as an example, E=0.47-0.059/2 log(0.1/1)=0.4405(V) [Pb2+]/M log[Pb2+] E/V(calculated) E/V(measured) %difference 0.1 -1 0.4405 0.482 9.42% 0.01 -2 0.529 0.502 5.10% 0.001 -3 0.5585 0.521 6.71% 0.0001 -4 0.588 0.545 7.31% It is shown that difference is present between the calculated value and measured value. This may be because of different conditions, resistance of the multimeter or errors in preparation of various concentrations of solutions. Part B Objective: To find out the equilibrium constant by e.m.f. measurement Introduction: The equilibrium constant for the below reaction is found out: Ag+(aq) + Fe2+(aq) Fe3+ (aq) + Ag(s) By e.m.f. measurement on the cell Pt |Fe2+(aq), Fe3+(aq)|Ag+(aq)|Ag(s) Chemicals: 0.1 M Fe3+ solution, 0.2 M iron(II)sulphate, 0.2M barium nitrate, 0.4M silver nitrate, platinum electrode, silver electrode Apparatus: gelatine salt bridge, 250 cm3 beakers, 50cm3 beakers ,multimeter, distilled water bottle, electrical wires with electrode holders, forceps, 10ml pipette x2 Procedure: 1. ...read more.

Conclusion

-The same ammonium nitrate/gelatine salt bridge was used several times in part II of experiment. Ions of previous measurement may remain in the salt bridge and change the concentration of ions in the next measurement. -The apparatus including pipettes, beakers and volumetric flask may not be washed to be very clean. The error in concentration may be enormous when handling very dilute solutions (e.g.0.001M, 0.0001M, 0.025M) -The electrode touched the salt bridge once so that the accuracy of measurement of e.m.f. was affected. The solution is not exactly passed. -Air gap may be present in the ammonium nitrate/ gelatine salt bridge, increasing the resistance of ion migration. 2. Difference in effect of ion concentration on electrical potential of cell: -In part A, the cell e.m.f. increases as [Pb2+(aq)] decreases. However, in part B, the e.m.f. drops as [Ag+(aq)]. This suggests that when the species is a stronger oxidizing agent in the reaction and undergoes reduction, the presence of its ions tends to increase the e.m.f. whereas the one which undergoes oxidation tends to reduce the e.m.f. The species with more positive standard reduction potential has a positive effect but the one with less positive potential has a negative effect. Reference: Physical Chemistry II by TM Leung and CC Lee( p.295-298 &299-301) - 7 - ...read more.

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This is a very well written set of two experiments using electrochemical cells. It is set out and written in a clear and easy to understand way. Unfortunately, the graphs are not present, however the results tables are excellent and valid conclusions are made.

Overall, this piece of work is 4 stars out of 5.

Marked by teacher Brady Smith 10/04/2013

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