Beer's Law

Many chemical reactions progress in two directions, forward and reverse. Over time, the forward and reverse reactions will occur at the same rate, and the concentration of the reactants and products will no longer change. This is known as chemical equilibrium.

At chemical equilibrium, the concentrations of each component are related to each other by the equilibrium constant, K, which is the ratio of product concentrations to reactant concentrations, each raised to the power of their stoichiometric coefficients.

But how do you determine equilibrium concentrations? One method measures the intensity of a wavelength of light that the product absorbs before and after it passes through a sample. The intensity difference is called absorbance, and it corresponds to the amount of the absorbing compound in the sample.

You might remember that electrons mostly occupy the ground state. When they absorb a certain amount of energy, they are excited to a higher energy level. That energy corresponds to a specific wavelength of light. You can find this wavelength and measure absorbance with a spectrophotometer, which directs a beam of light through the sample and measures the change in intensity at one or more wavelengths.

The absorbance is equal to the negative log of the intensity of the attenuated light over the intensity of the incident light. By plotting the absorbance values of multiple solutions with different known product concentrations, we observe a linear relationship between absorbance and concentration. This is an example of Beer's Law.

Beer's Law is expressed mathematically by this equation, where A is the absorbance, epsilon is the molar attenuation coefficient, a constant that varies for each compound, l is the path length of light through the sample, and c is the concentration of the compound.

By identifying the linear function for a given compound at a specific wavelength and path length, you can use the absorbance data of a solution at equilibrium to determine the product's equilibrium concentration. From there, you can calculate the equilibrium concentrations of the reactants and solve for the equilibrium constant. In this lab, you will prepare solutions of iron(III) isothiocyanate and use a spectrophotometer to determine its absorbance at various concentrations.