Determination of Kc for an Equilibrium System Flashcards

Vocabulary terms and mathematical definitions regarding the use of spectrophotometry to determine the equilibrium constant (Kc) for the iron (III) thiocyanate system.

Complex ion

A species such as $FeSCN^{2+}$ formed when a new covalent bond is created between a metal ion (like $Fe^{3+}$) and a lone pair of electrons on another ion (like thiocyanate).

Dynamic equilibrium

A state established when the rate of the reverse reaction equals the rate of the forward reaction, resulting in no net change in the concentration of reactants or products.

Equilibrium constant ($K_c$) expression

For the reaction $Fe^{3+}(aq) + SCN^{-}(aq) \rightleftharpoons FeSCN^{2+}(aq)$, the expression is $K_c = \frac{[FeSCN^{2+}]}{[Fe^{3+}] [SCN^{-}]}$.

Spectrophotometer

An instrument containing a light source and electronic detector used to measure how much of a single wavelength of light passes through a sample solution.

Percent transmittance ($\%T$)

The measure of the amount of original light able to pass through a solution, recorded by the detector in a spectrophotometer.

Absorbance ($A$)

A measure of how much light was absorbed by a solution, mathematically defined as $A = \text{log}\frac{100}{\%T}$.

Beer's Law

The principle stating that absorbance is directly proportional to the concentration of a solution, expressed as $A \times c$.

Beer's Law constant (eta)

The proportionality constant in the equation A = eta c, which corresponds to the slope of a graph plotting concentration versus absorbance.

LeChatelier’s principle

A principle used to resolve the known concentration of $FeSCN^{2+}$ by adding an extremely large excess of one reactant to force essentially all of the limiting reactant to convert to product.

Wavelength ($\text{λ}$) for $FeSCN^{2+}$

The specific setting of $450\text{ nm}$ used in the spectrophotometer for the red-colored complex ion study.

Cuvette

A small tube used in a spectrophotometer to hold the reaction mixture sample, typically filled at least two-thirds full.

Blank

A solution used to calibrate the spectrophotometer to adjust the 100% transmittance before measuring samples.

Stoichiometry of rxn. 3

The $1:1:1$ ratio in the reaction $Fe^{3+} + SCN^{-} \rightleftharpoons FeSCN^{2+}$, meaning the molarity of $FeSCN^{2+}$ produced equals the molarity of $Fe^{3+}$ and $SCN^{-}$ consumed.

[$Fe^{3+}$]$_{equil}$ formula

$$[Fe^{3+}]_{\text{equil}} = [Fe^{3+}]_{\text{initial}} - [FeSCN^{2+}]_{\text{equil}}$$

[$SCN^{-}$]$_{equil}$ formula

$$[SCN^{-}]_{\text{equil}} = [SCN^{-}]_{\text{initial}} - [FeSCN^{2+}]_{\text{equil}}$$

Dilution equation

$M_1V_1 = M_2V_2$, the formula used to calculate the initial molarity of reactants after mixing different volumes.