General Chem Chemical Equilibria lab

General Chem Chemical Equilibria lab - Chemical Equilibria...

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Chemical Equilibria Lab Introduction This experiment was performed in order to prove that the equilibrium constant of a reaction at a constant temperature is independent of the concentration of the reactants. The equation for the reversible equation studied in this experiment is shown below: Fe 3+ + SCN -  [Fe(SCN)] 2+ From the equation, the expression for the equilibrium constant for the reaction was determined. The equilibrium constant for this reaction was equal to the equilibrium concentration of the product, [Fe(SCN)] 2+ , divided by the product of the equilibrium concentrations of the reactants, Fe 3+ and SCN - . The mathematical formula for the equilibrium constant is shown below: K c = [Fe(SCN)] 2+ _________________ [NCS - ] [Fe 3+ ] The law firm Cheatham, Bunkham, Hoodwinkle, and Associates is looking for evidence that low levels of light can be accurately measured. With the calculated equilibrium constant for the reaction, it is possible to calculate the concentration of NCS - , and therefore the moles of NCS - . Along with φ, which is known, and the time of irradiation, t, the light absorbed, I, can be determined. I is measured in units of mol of photons per unit of time, which is exactly the kind of accurate measurement the law firm is in search of. The equation for I is shown below: I = n/(φxt) A Beer-Lambert curve was created for the reaction in order to calculate the change in concentration of the reactants and product, thus allowing the determination of the equilibrium concentrations. On the Beer-Lambert curve, the final concentration of [Fe(SCN)] 2+ in each standard solution was plotted on the x-axis while the absorbance at 460 nm of each standard solution was plotted on the y-axis. With Microsoft Excel, the equation of the linear regression line of the Beer-Lambert plot was determined as well as R 2 , the correlation coefficient. This curve acted as a calibration curve because it
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provided a way to calculate the change in concentration of the reactants and product with respect to the absorbances of the three equilibrium solutions. In order for the final concentration of [Fe(SCN)] 2+ in each standard solution to be calculated, the reaction was forced to proceed almost entirely to the product side. This shift in the reaction to the right was accomplished by applying a stress to the system in the five standard solutions from Part A. The concentration of Fe 3+ was very high, which led to a higher concentration of [Fe(SCN)] 2+ being formed as a way to reduce the stress. This formation of product in each of the standard solutions can be explained by LeChatelier's principle, which states that if a stress is applied to a system at equilibrium, the system will respond to reduce the stress in reaching a new equilibrium state. In words applicable to this experiment, a higher concentration of reactants will shift the equilibrium position to the right, causing more products to form.
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General Chem Chemical Equilibria lab - Chemical Equilibria...

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