CHEM 122L General Chemistry Laboratory Revision 3.2 Determination of the Equilibrium Constant for Ferric Thiocyanate • To learn about Chemical Equilibrium. • To learn about the Ferric Thiocyanate system. • To learn about Absorbance Spectroscopy. In this laboratory, we will determine the Equilibrium Constant for the reaction between the Ferric Ion (Fe 3+ ) and the Thiocyanate Ion (SCN - ) which produces Ferric Thiocyanate Ion (Fe(SCN) 2+ ): Fe 3+ (aq) + SCN - (aq) Fe(SCN) 2+ (aq) (Eq. 1) Ferric Thiocyanate is a brick-red Complex Ion which is stable in an aqueous environment. In this aqueous environment, the Ferric Ion (Fe 3+ ) actually exists as a hydrated octahedral complex, more correctly denoted Fe(H 2 O) 6 3+ . In the presence of Thiocyanate Ion (SCN - ), one of the water molecule ligands is displaced and the Ferric Thiocyanate Ion (Fe(H 2 O) 5 SCN 2+ ) is produced: (Fe 3+ in Water) (Fe(SCN) 2+ in Water) We will make use of the highly colored nature of this product in order to measure the Equilibrium Constant for the reaction of interest; (Eq. 1) . Chemical Equilibrium is established when a reaction exists in a state such that the forward reaction rate is equal to its reverse reaction rate. When the reaction is initiated, the concentration
P a g e | 2 of reactants is high and the rate of the forward reaction is great. As the reaction proceeds this rate typically slows. At the same time the concentration of products increases and the reverse reaction begins in earnest. Unless the system does something quirky, like oscillate, we will reach a point where these two reaction rates are equal and the system appears to remain static with fixed concentrations of reactants and products. For a general reaction, we denote this equilibrium as: a A + b B c C + d D (Eq. 2) The equilibrium concentration of each species can be denoted as: [A] = [A] e [B] = [B] e [C] = [C] e [D] = [D] e however, the subscript “ e ” is generally dropped for notational convenience. The ratio of the Product concentrations to Reactant concentrations, at the equilibrium point, defines the Equilibrium Constant K c for the system: K c = (Eq. 3) Large values of K c mean products dominate at equilibrium, whereas small values imply a predominance of reactants. It should be noted this definition of K c is a bit misleading. Each concentration is actually referenced to its Standard State concentration; [A] s , etc. Standard States are defined as: Solution: 1 M when the system is behaving Ideally Gas: 1 atm when the system is behaving Ideally Liquid: Pure Liquid Solid: Pure Solid So, we should really write K c as: K c = (Eq. 4) Since each of the Standard State concentrations is “ 1 ”, (Eq. 4) reduces notationaly to (Eq. 3) . If a Species is in its Standard State, such as a Pure Liquid or Pure Solid, it will not appear in the Equilibrium Constant expression.
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- Summer '19
- Equilibrium, Chemical reaction, Kc, Equilibrium point, equilibrium concentrations