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Investigation of the effect of changes in ionic concentration on the e.m.f of a cell

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Introduction

S.K.H. Lam Woo Memorial Secondary School F.7 Chemistry Laboratory Report Name: Chan Ching Wai Class: F.7H (2) Experiment 5: Investigation of the effect of changes in ionic concentration on the e.m.f of a cell Date of Experiment: 2-12-2010 Objective: To investigate the effect of changes in ionic concentration on the e.m.f of a cell and determine the number of electrons involved in the Fe2+ (aq)/ Fe3+ (aq) equilibrium. Introduction: In this experiment, with the Cu(s)/ Cu 2+(aq) half-cell being the reference half-cell, the variation in cell e.m.f was determined by varying the concentration ratio of Fe2+ (aq)/ Fe3+ (aq). In this way, the effect of changes in ionic concentration on the e.m.f of a cell and the number of electrons involved in the Fe2+ (aq)/ Fe3+ (aq) equilibrium could be found out. A reference half-cell was needed as the absolute electrode potential for a particular half-cell could not be determined. Yet, the potential difference between the two half-cells connected to form an electrochemical cell could be determined. Then, the relative contributions of any half cell to the cell e.m.f could be measured by arbitrarily choosing one system as the standard. Therefore, in this case, the Cu(s)/ Cu 2+(aq) ...read more.

Middle

This showed that with decreasing concentration ratio of Fe2+ (aq)/ Fe3+ (aq), the e.m.f measured decreased. This agreed with the Nerst equation, Ecell = 0.43 + {0.059 x2 x log [Fe3+(aq)]/ [Fe2+ (aq)] } / n , when [Fe3+(aq)]/ [Fe2+ (aq)] decreased, log [Fe3+(aq)]/ [Fe2+ (aq)] decreased and Ecell decreased. This could also be proved from the equilibrium equation. 2Fe3+ + Cu(s) 2Fe2+ (aq) + Cu2+ (aq) When the ratio [Fe3+(aq)]/ [Fe2+ (aq)] decreased and concentration of [Cu2+ (aq)] being unchanged, the proportion of [Fe3+(aq)] to [Fe2+ (aq)] decreased. Thus, the equilibrium position would shift to the left. The standard electrode potential of the system to be measured refer to reduction and a more positive value of electrode potential meant the reaction tend to proceed in the direction of reduction. Therefore, when the equilibrium position shifted to the left, the electrode potential of cathode and hence the cell e.m.f became less negative. From the graph, the slope = 0.104 ?slope = 0.0592 x 2 / n 0.104 = 0.0592 x 2 / n n = 1.14 ~ 1 ?The number of electrons involved in the Fe2+ (aq)/ Fe3+ (aq) equilibrium was 1. This agreed with the equation Fe3+ + e- Fe2+ (aq). ...read more.

Conclusion

sulphate solution in the beaker could be discarded each time and use the iron (II) sulphate solution which was freshly poured out. 3. As the determination of cell e.m.f depended on the concentration of ions inside. Contamination may varied the concentration ratio and hence affect the accurate determination of cell e.m.f. To minimize this error, different plastic containers were used for different trials. Or the plastic containers should be rinsed with distilled water thoroughly after each trial. Otherwise, the ions from the previous trial may remained and affect the concentration ratio. After rinsing, the containers should be dried such that no water remained and diluted the solution. Then, the concentration ratio of [Fe3+(aq)]/ [Fe2+ (aq)] only depended on the specific proportion of Fe(NO3)3 and FeSO4 added. 4. The volume of Fe(NO3)3 and FeSO4 added were expressed in term of the number of drop added. Yet, the size of each drop was not necessarily the same. Therefore, the actual volume of solutions added may not be the same as stated in the table. To minimize this error, the force when squeezing the dropper should be approximately constant and the dropper should be held vertically during addition of solution. Conclusion From the experiment, it was found out that a decrease in ionic concentration would decreased the cell emf and vice versa. Moreover, the number of electrons involved in the Fe2+ (aq)/ Fe3+ (aq) equilibrium was 1. ...read more.

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