How does a spectrophotometer measure absorbance?

A spectrophotometer directs a beam of light through a sample and measures the change in light intensity before and after passing through. It calculates absorbance as the negative log of transmitted light intensity divided by incident light intensity. The instrument uses a diffraction grating or prism to separate light into component wavelengths and a CCD detector to record intensity at specific wavelengths.

Why do electrons transition to higher energy levels when absorbing light?

When a molecule absorbs light with energy equal to the difference between two electron energy states, electrons become excited and transition to higher energy levels. According to Bohr's model, electrons occupy fixed, discrete energy states and only transition when absorbed photon energy exactly matches the energy difference between states. This excitation moves electrons from the ground state to excited states like n = 2 or n = 3.

What role does the molar attenuation coefficient play in Beer's Law?

The molar attenuation coefficient, represented by epsilon in Beer's Law, is a constant that measures how strongly a specific compound absorbs light at a given wavelength. It varies for each compound and directly affects absorbance values. A greater molar attenuation coefficient indicates stronger light absorption, making it essential for calculating unknown concentrations from measured absorbance data.

How is a spectrophotometer calibrated before measuring samples?

A spectrophotometer is calibrated using a blank, which is a cuvette containing only the solvent without the solute. This calibration subtracts the absorbance from the cuvette and solvent, ensuring that measured absorbance reflects only the species of interest. Standard cuvettes have a pathlength of 1 cm, which is used in Beer's Law calculations.

How can Beer's Law determine equilibrium concentrations in chemical reactions?

By measuring absorbance of a product at equilibrium using a spectrophotometer, Beer's Law allows calculation of the product's equilibrium concentration. Once the product concentration is known, equilibrium concentrations of reactants can be determined using stoichiometry. These values then enable calculation of the equilibrium constant, K, which is the ratio of product concentrations to reactant concentrations.

What is a standard curve and how does it apply Beer's Law?

A standard curve is generated by plotting absorbance versus concentration for solutions with known solute concentrations. This produces a linear relationship matching Beer's Law equation, with slope equal to the product of pathlength and molar attenuation coefficient. Using this curve, an unknown sample's concentration can be determined by measuring its absorbance and reading the corresponding concentration value.

Beer's Law

Concepts Instructor Prep Student Protocol

83.4K views02:53 min

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.