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To observe the effect of different concentrations of ferric nitrate on the equilibrium between ferric chloride and sodium thiocyanate solutions.

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Introduction

Chemistry Lab Report - Equilibrium Vivek Salgaocar Chemistry SL Ms. Pagdiwalla 10/9/2003 Objectives: 1. To observe the effect of different concentrations of ferric nitrate on the equilibrium between ferric chloride and sodium thiocyanate solutions. 2. To observe the effect of different concentrations of potassium thiocyanate on the equilibrium between ferric chloride and sodium thiocyanate solutions. 3. To observe the effect of different concentrations of potassium chloride on the equilibrium between ferric chloride and sodium thiocyanate solutions. Introduction and Hypothesis: The state of dynamic equilibrium, in which a chemical system is reacting to form and to destroy products, is a stable state. If a system is not in equilibrium, the driving force toward equilibrium is greater than the driving force away from equilibrium and the system has a net driving force toward equilibrium. Alternatively, if a system is not in equilibrium the rate of the reaction toward equilibrium is greater than the rate of the opposite reaction so there is a net movement always toward the equilibrium state. In other words, a system at equilibrium will tend to stay at equilibrium and a system not at equilibrium will tend to more toward equilibrium. This principle was first clearly enunciated by Henri Le Chatelier (1850 - 1936) and bears his name. Its elegant formulation is: If, to a system at equilibrium, a stress be applied, the system will react so as to relieve the stress. There are 3 factors that affect equilibrium - temperature, pressure (only for gasses) and concentration. In this lab, we will investigate the manner in which concentration affects the chemical equilibrium between solutions of ferric chloride and potassium thiocyanate. In this reaction, the equilibrium is formed between Fe+++ ions and SCN- ions which combine to form a Fe(SCN)3 2+ complex. The reaction can be represented as: FCl3(aq) + 3KSCN(aq) <=> Fe(SCN)3(aq) + 3KCl(aq) In order to accurately record any changes that may occur in the system that is in equilibrium, we will use a spectrophotometer which can accurately determine the optical densities of solutions placed in it. ...read more.

Middle

Dependent Variable: * Optical Density of the solution which shows the shift in the equilibrium of the system i.e. the Spectrophotometric Reading - this is dependant upon the volume of the independent variable added. Controlled Variables: * Temperature at which the system is - conducting the experiment in the same room allows the system at equilibrium to remain at a constant temperature. The system will remain at room temperature. * Pressure applied to the system - conducting the experiment in the same room allows the system at equilibrium to remain at a constant temperature. The system will remain at standard atmospheric pressure of 1 atm. * Concentration of Ferric Chloride Solution - this can be controlled by accurately measuring the amount of salt added to a specific amount of distilled water. * Concentration of Potassium Thiocyanate Solution - this can be controlled by accurately measuring the amount of salt added to a specific amount of distilled water. * Volume of Ferric Chloride Solution - this can be controlled by accurately measuring the volume of the solution that we add using a pipette and the graduated cylinder * Volume of Potassium Thiocyanate Solution- this can be controlled by accurately measuring the volume of the solution that we add using a pipette and the graduated cylinder * Concentration of potassium thiocyanate Solution - this can be controlled by accurately measuring the amount of salt added to a specific amount of distilled water. * Spectrophotometric Reading for the Fe(SCN)3 2+ complex solution - by first preparing a large amount of solution and then using parts, we can ensure that the solution is the same and so the Spectrophotometric readings will also be constant. * Size of the test tubes - this can be controlled by using test tubes that have been manufactured and calibrated so that their volumes are the same. * Physical states of the reactants - by keeping temperature and pressure constant, the physical states can also be kept constant. ...read more.

Conclusion

Firstly, the spectrophotometer must be considered. The original value that is set on the instrument was 450 nm. However, this could be inaccurate and therefore, the spectrophotometer readings must be considered with an error of 0.001. If the test tubes used have not been mechanically tested, the light that is emitted by the spectrophotometer could be absorbed to a certain extent by the walls of the test tube, thereby affecting the readings obtained from the spectrophotometer. Secondly, we must consider the human errors associated with measuring the amount of salt, the volume of the solutions added and the amounts of solution taken for measurement. These human errors can be reduced to a large extent by using instruments that are more sensitive. For example, using a thinner calibrated cylinder would reduce the error associated with it because a small change in amount would be seen by a large change in the cylinder. Thirdly, the salts and solution used could have been impure. There could have been a number of substances chemically combined with them that would have affected the readings. These could have been reduced by using chemicals that have been stored in conditions that prevent them from coming in contact with any impurities. Fourthly, the weights of the salts used could have been faulty and inaccurate. These could have been negated by measuring the salts repeatedly till constant values are obtained. Fifthly, the chemicals that were used might not have been of the specified concentrations and strengths. Again, these could have been negated by using chemicals stored in good conditions. Also, the tests tubes used could have had impurities which would have affected the readings. These could have been avoided by cleaning the test tubes well. Furthermore, impurities from the air could have reacted with the solutions when the experiment was being conducted. This could have been avoided by conducting the experiment. In general, the accuracy of results could have been increased by repeating the experiment many times. However, time constraints might prevent this from being possible. ...read more.

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