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Determination of the Equilibrium Constant for a Chemical Reaction

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Tamer Almajid




Experiment 4:

Determination of the Equilibrium Constant for a Chemical Reaction


Every chemical reaction has a characteristic condition of equilibrium at a given temperature. If two reactants form products until a state is reached where the amounts of reactants and products n longer change. such conditions the and products are in chemical equilibrium and will remain so until the system is altered in some way. Associated with the equilibrium state there is a number called the equilibrium constant, Keq, the necessary condition on the concentrations of reactants and products for the reaction.

The extent of the equilibrium for a given reaction is expressed by the equilibrium constant, Kc. For the general reaction:

aA + bB cC + dD (1)

The expression for the equilibrium constant is as follows:

Keq= (C)c (D) d/(A) a (B)b (2)

Where [A], [B], [c], and [D] are the concentrations that expressed in terms of molarity. any given temperature, the value of Kc for a reaction is fixed. If A, B, C and D are mixed in arbitrary amounts in a container they will tend to react until their concentrations satisfy the equilibrium expression. In this experiment we will study the equilibrium properties of the reaction between the iron (ll) ion and thiocyanic acid (HSCN):

Fe3+ (aq) + HSCN(aq) = FeSCN2+(aq) + H+(aq) (3)

When solutions containing Fe3+ n and HSCN acid are mixed, reaction occurs, forming the FeSCN2+ complex ion, which has a red color, and H+ ion while, all other species are colorless. According to the general law, Keq for reaction 3 takes the following form.

Keq= (FeSCN2+)(H+)/ (Fe3+) (HSCN)

Objective: is to measure keq for several mixtures of Fe3+ and HSCN-made up in different ways.

Experimental: In the following experiment, 5 different volumes of the HSCN will be reacted with the same amount of Fe3+, to show the relationship between the concentration of the reactants and products, and the equilibrium constant.

Data and result: See data sheet.


The equilibrium concentration of the [FeSCN2+] is found by creation a calibration curve, comparing absorbance (y-axis) vs. [FeScN2+](x-axis) the slope of the graph can be used find the concentration at any absorbance of each of the 5 solutions.

y=4245x.157=4245x x=[FeSCN2+]= 3.70 X 10-5M

With this concentration once it is converted to moles and since the initial moles of the reactants are known, the final moles of the product are subtracted from the initial moles of the product, which is 0 because no product has formed, to find the change in moles, shown as:

Equilibrium moles FeSCN2+ = O.010 L X (3.70 X 10-5M) = 3.70 X 10-7 mols

Change FeSCN2+ = 3.70 X 10-7 mols - 0 mols = 3.70 x 10-7 mols

When the change in moles is known it is added to the reactants, since the reactants are being used up the change is negative, giving you the equilibrium moles for each reactant:

Equilibrium moles Fe3+ = 1.00 X 10-5mols Fe3+ + (-3.70 X 10-7) = 9.30 X 10-6mols Fe3+

This is done for both reactants of the reaction. When all equilibrium moles are found, the

equilibrium molarity can then be found by dividing by the total mixture in liters:



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