0.3 Beer's law and data analysis  (Page 21/3)

Beer’S law and data analysis

Objectives

• Learn or review typical data analysis procedures–plotting data with excel, performing linear regression analysis, etc.
• Explore the concepts and applications of spectrophotometry

Grading

• Pre-lab (10%)
• Lab Report Form–including plot (80%)
• TA points + Pop Quiz (10%)

Before coming to lab…

• Read the lab instructions
• Print out the lab instructions and report form.
• Complete the pre-lab, due at the beginning of the lab

Introduction

When describing chemical compounds, scientists rely on their chemical and physical properties. In lab, we might observe that a metal reacts violently with water, that a reactant is liquid at room temperature, or that a powder is yellow. Chemical and physical properties can be used qualitatively to identify a material or to predict its behavior, or quantitatively to determine how much of that material is present in a solution. In this lab, we will develop a scheme to determine the concentration of copper sulfate in aqueous solution using spectrophotometry.

To start, we will consider light and its interaction with matter. Chemicals exhibit a diverse range of colors, especially when they contain transition metal ions. In order for a compound to have color, it must absorb visible light. Visible light consists of electromagnetic radiation with wavelengths ranging from approximately 400 nm to 700 nm, a small section of the electromagnetic radiation spectrum shown below.

Light is characterized by its frequency ( $\nu$ ), the number of times the crest of the wave passes some point in space per second, or by its wavelength ( $\lambda$ ), the distance between two successive crests. These two quantities are related by the speed of light, a fundamental constant: $\text{λν}=c=3\times {\text{10}}^8\text{m/s}$ . Planck related the frequency of light to its energy (E) according to $E=\text{hν}$ , where h is Planck's constant, $h=6\text{.}\text{626}\times {\text{10}}^{-\text{34}}\text{J/s}$ .

A compound will absorb light when the radiation posesses the energy needed to move an electron from its lowest energy (ground) state to some excited state. The particular energies of radiation that a substance absorbs dictate the colors that it exhibits. Conversely the color of a compound can help us to determine its electronic configuration.

White light contains all wavelengths in this visible region. When a transparent sample (like most aqueous solutions) absorbs visible light, the color we perceive is the sum of the remaining colors that are transmitted by the object and strike our eyes.If an object absorbs all wavelengths of visible light, none reaches our eyes, and it appears black. If it absorbs no visible light, it will look white or colorless. If it absorbs all but orange, the material will appear orange. We also perceive an orange color when visible light of all colors except blue strikes our eyes. Orange and blue are complementary colors; the removal of blue from white light makes the light look orange, and vice versa. Thus, an object has a particular color for one of two reasons: It transmits light of only that color or it absorbs light of the complementary color.