Monthly Archives: December 2016

Emission

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Learning Goals

  1. Practice using computational chemistry software;
  2. Perform flame tests;
  3. Use Erlenmeyer flasks to mix solutions;
  4. Use a spectroscope to observe atomic emission;
  5. Manipulate units of wavelength and energy;
  6. Practice writing ground state and excited state electron configurations.

Background

By Comunicaciones CONICYT – María Teresa Ruiz, Premio Nacional de Ciencias Exactas., CC0, https://commons.wikimedia.org/w/index.php?curid=69688240

María Teresa Ruiz is an astronomer born in 1946. In 1997, Ruiz and coworkers discovered a free-floating brown dwarf, which is an unusual celestial body that is halfway between a planet and a star. She used optical spectroscopy to analyze both the light emitted from the brown dwarf and the light it absorbed. This data gave clues about the brown dwarf’s temperature and chemistry. Over the next two weeks, you will learn about optical spectroscopy, both emission and absorption.

Colored chemicals absorb and/or emit light in the visible portion of the electromagnetic spectrum, which has a wavelength of approximately 400 – 700 nm. The color of the absorbed or emitted light depends on the amount of energy the chemical absorbed or emitted. Wavelength and energy are negatively correlated. 

Absorption occurs when an electron in a chemical absorbs energy from the light, temporarily promoting the electron to a higher energy orbital. Light emission can occur when an electron relaxes back to the ground state and produces light, but emission is less common than absorption because there are many of non-radiative ways for the electron to relax. carrots

Most chemicals are colored because they absorb light and reflect only a portion of incident light. In this case, the color that a chemical absorbs is the opposite of the color that it appears. The color wheel shows you which colors are opposite of one another. The color wheel helps you to predict the color that a chemical absorbs based on the color it appears (and vice versa). For example, beta-carotene, a pigment found in many fruits and vegetables including carrots, absorbs purple and blue light (400 – 500 nm) and reflects all of the other colors, so it appears yellow/orange. It DOES NOT emit yellow/orange light.

If a chemical is colored due to emission, then the color it appears corresponds to the color it emits. Many processes can temporarily promote an electron to a higher energy orbital and induce emission; these include light, heat, electricity, and chemical reactions. Today, you will observe light emission from electricity in part 1, chemical reactions in part 2, and heat in part 3.

Objective

In part 1 of this lab, you will observe emission of atomic hydrogen which will be excited with electricity. Using the Rydberg formula, you should be able to figure out which lines in the emission correspond to which transitions in the hydrogen atom. Refer to Chapter 3.1 in your textbook to refresh yourself on the Rydberg formula. You will also model molecular hydrogen to better understand molecular orbital theory.

In part 2 of this lab, you will observe the emission from a light-producing chemical reaction. You will model the product and run calculations to determine the quantum mechanical source of the emission.

In part 3 of this lab, you will perform flame tests and propose the chemicals you could use to make a fireworks display.

You may perform parts 1, 2, and 3 in any order.

Procedure

Part 1: Hydrogen
Experimental component

Note: perform this section individually.

  1. First, look through the spectroscope through the window. You should see a rainbow approximately spanning 4.0 – 8.0 on the scale in the spectroscope. The units on these instruments are in hundreds of nanometers, so 4.0 corresponds to 4.0 x 102 nanometers. If you don’t see a rainbow across this range, ask your TA or instructor for help.
  2. Turn on the hydrogen lamp and observe the emission through the spectroscope.
  3. Use the Rydberg formula to calculate the energy transition associated with each line. h = 6.626*10-34 Js, c = 2.9979*108 m/s
  4. Turn off light source once you are finished observing the emission. Record your observations.
  5. Extra lamps are provided for you to explore, but they are not required for this lab.
Computational component
  1. Open Chem3D 18.0
  2. Click on the white panel to the right of the main window. It is titled “ChemDraw-LiveLink.”
  3. In the text bar, type “Hydrogen,” then hit “enter.” A molecule should appear.
  4. Optimize the structure by hitting “control-m.”
  5. There are several things you can do to get a better look at the molecule.
    • You can click the third button from the left on the top toolbar to rotate the molecule. It looks like a circle with an arrow on it. After you click it, you can use the mouse to rotate the molecule.
    • You can click “View” on the main menu, then click “Model Display.” This will present you with many options to change the display of the molecule. For example, “Display Mode” gives you more modes. The “Ball & Stick” mode is most common, but “Wire Frame” is convenient for a very complicated molecule, and “Space Filling” is helpful for visualizing atom size differences.
  6. After making the model and optimizing the structure, click “Surfaces” on the main menu, then “Choose Surface,” then “Molecular Orbital.”
  7. The HOMO (highest occupied molecular orbital) will automatically be shown, but you can choose another molecular orbital from “Select Molecular Orbital.” You will also see the energies associated with each orbital. The labels are with respect to the HOMO and the LUMO (lowest unoccupied molecular orbital). Notice that the HOMO energy is usually negative, indicating a favorable state, but the LUMO energy is usually positive, indicating an unfavorable state (which is why it is UNOCCUPIED!)
  8. Draw on your worksheet the HOMO and the LUMO. Indicate color differences either by using color in your drawing or by shading.
Part 2: Luminol
Experimental component

Source: https://www.carolina.com/teacher-resources/Interactive/luminol-glowing-reaction/tr10786.tr

Note: perform this section with a partner.

Create Solution A
  1. Measure out the correct mass of NaOH pellets according to your pre-lab calculation. Place into a 250 or 300 mL Erlenmeyer flask.
  2. Measure out 100 mL of DI water in a 150 mL beaker. Add to the Erlenmeyer flask with the NaOH pellets. Swirl to dissolve.
  3. Measure out ~0.18 g of luminol in a weigh boat. Add to the Erlenmeyer flask. Swirl to dissolve. Pro-tip: an Erlenmeyer flask is a great choice when mixing up a solution from a solid because you can swirl without the solution splashing out of the flask. Always use an Erlenmeyer flask that has plenty of extra space to allow you to swirl.
  4. Record the appearance of Solution A.
Create Solution B
  1. Measure out 0.03 g of potassium ferricyanide.
  2. In a second 250  or 300 mL Erlenmeyer flask, add 100 mL of DI water, 1 mL of 3% hydrogen peroxide, and the 0.03 g of potassium ferricyanide. Swirl to dissolve.
Mix them together
  1. Go into the dark room.
  2. Slowly pour the two solutions simultaneously into the funnel at the top of the apparatus.
  3. Record your observations immediately after the two solutions mix.
  4. Dispose of the solution in the waste containers.
Computational component
  1. Open Chem3D 18.0
  2. Click on the white panel to the right of the main window. It is titled “ChemDraw-LiveLink.”
  3. In the text bar, type “3-aminophthalate,” then hit “enter.” A molecule should appear.
  4. Optimize the structure by hitting “control-m.”
  5. After making the model, click “Surfaces” on the main menu, then “Choose Surface,” then “Molecular Orbital.”
  6. The HOMO (highest occupied molecular orbital) will automatically be shown, but you can choose another molecular orbital from “Select Molecular Orbital.” You will also see the energies associated with each orbital. The labels are with respect to the HOMO and the LUMO (lowest unoccupied molecular orbital).
  7. You will need to record the difference between the MOs prescribed on your worksheet. Note that ChemDraw reports the band gap in terms of eV, so you will need to convert from those energy units to wavelength units to determine the color that corresponds to each of these potential transitions. This will require using Plank’s constant in different units than you typically use: h = 4.1357 × 10–15 eV s. I suggest that you use Excel or some other software to do this repeated calculation. Once you know the colors of each possible transition, you will be able to choose the most likely transition that corresponds to the color you observed.
Part 3: Flame tests

Note: perform this section individually in a fume hood.

  1. Observe sources one at a time by eye. If you are color-blind, you will need to work with a non-color-blind partner.
  2. Light a bunsen burner with a non-luminous blue flame using your amber tubing and your bunsen burner in  a fume hood.
  3. For the first metal, obtain a wooden splint that has been soaked in the metal solution.
  4. Wave the wet part of the splint in the hottest part of the flame (the top of the inner cone).
  5. The color you observe is the first color you see, not the yellow color of the wood burning. If you aren’t sure what color is wood burning, experiment by burning a dry splint.
  6. Write down your observations of the color(s) and intensity on your worksheet.

Report

Fill out this worksheet. Turn in either a paper or digital copy.

Data analysis in Excel

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Linear relationships are characterized by a slope (m) which tells the steepness of the line and an intercept (b) which tells the value of Y when x is zero.  Because of random measurement errors, experimental data will seldom fall exactly on a line given by the equation Y = m x + b.  A pertinent question is: What is the best slope and intercept to describe our linearly correlated but imprecise experimental data?

When it is important to make the best possible use of the data, the fitting must be done numerically.  The computational method most commonly used is the method of least squares.  The best linear line is the one that minimizes the distance of all the data points from the line.  That is the line which minimizes the squared difference (Y – yi)2 between the observed data points yi and the calculated data points Y = m xi + b.  Such a line is called a linear least squares line or a linear regression line.

The method of least squares is built into many computer programs for analyzing data.  We will be using Excel to analyze our data this semester.

Plotting and linear regression can be done in Excel. In one column of cells (let’s say column A starting with row 1), enter your x-data. In the cells of the neighboring column (B), enter the corresponding y-data. Select the data you entered. Then from the top menu, select Insert –> Chart –> X Y (scatter). Select the points on the graph, right click, and select “Add trendline”. Make sure the trendline options are set to “Linear” with the equation and R-squared value displayed on the chart.

Dressing up the figure:  You need axis labels.  Under Layout, click Axis Title and then “Primary Horizontal Axis Title” and finally “Title Below Axis”. Type in an appropriate title with appropriate units.  Do the same for the Primary Vertical Axis Title…used “Rotated Title”. You want your graph to be as large as possible, and you don’t really need the box on the right side that says Series 1 and Linear. Click this and delete it. Gridlines aren’t normally placed on a figure, so click on one of them and delete them. If your equation line and R2 value landed on top of the line, click on that information and move the box to a good location. Save this file.

There is error in the linear regression data that will determine how many significant figures you can report.  The linearity of the line is shown by the R2 value. If you got above 0.98, pat yourself on the back or thank your lucky stars that you have the world’s best lab partner.

Don’t Eat the Yellow Snow

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Learning goals: maintain safety in a chemistry laboratory,  follow instructions to complete a laboratory experiment, collect experimental data, explain likely sources of experimental error

Introduction

A chemical reaction occurs when electrons and/or bonds rearrange. A chemical reaction can be described with a chemical equation. You may notice a chemical reaction has occurred by one of the following indicators:

  • A color change (eg. a colorless solution turns red),
  • A change of state (eg. a solid precipitates in a solution),
  • A change in temperature (eg. a solution gets hot).

Today’s lab activity is an example of a chemical reaction. You will form a golden solid from two colorless solids.

Background

Lead nitrate and potassium iodide are both highly soluble in water. This means:
Pb(NO3)2 (s) → Pb2+(aq) + 2NO3–(aq)
KI(s) → K+(aq) + I–(aq)

When you mix those two solutions together, all four ions encounter each other, and that’s when the chemical reaction occurs:

Pb2+(aq) + 2I–(aq) → PbI2(s)

The potassium and nitrate ions are not involved in the chemical reaction; they are called “spectator ions.” They are present in the same amounts before and after the reaction (on both sides of the arrow).

Pb2+(aq) + 2NO3–(aq) + 2K+(aq) + 2I–(aq) → PbI2(s) + 2NO3–(aq) + 2K+(aq)

When the solid forms, it first looks like yellow clouds because the tiny solid particles are suspended in the solution (like mud). We have to take advantage of its slight solubility in order to form a sparkling, golden solid. When you first add the lead nitrate to the potassium iodide, you will notice the cloudy precipitate forms and then dissolves. This is because lead nitrate is “slightly soluble.” 0.08 g of it can dissolve in 100 mL of water: the yellow clouds only remain visible when there is more than 80 mg present per 100 mL of water.

Temperature dependence of solubility for several solids. Source: http://wps.prenhall.com

Many solids are more soluble in hot water than cold. Heating the solution will cause the lead nitrate to dissolve completely so that it can slowly crystallize as it cools, which forms large crystals.

Procedure

Note: do this lab individually.
Prepare solutions
  1. Tare a 250 or 300 mL Erlenmeyer flask labeled “lead nitrate”. Add approximately 0.3 g of lead nitrate to the flask. Record the exact mass on the worksheet.
  2. Measure out 100 mL of deionized water (DI H2O) with a graduated cylinder. Pour the water into the flask containing the solid lead nitrate. Swirl to dissolve.
  3. Repeat this procedure with another flask for the Potassium Iodide.
  4. Add a few drops of 0.5 M hydrochloric acid to each solution.
Form Crystalline Lead Iodide
  1. Add the lead nitrate to the potassium iodide one drop at a time, swirling to dissolve after each drop. How many drops can dissolve? Once the yellow clouds start to form and will not dissolve when swirling, pour in the remainder of the lead nitrate. Record your observations.
  2. Prepare a hot water bath with a Bunsen burner, a ring stand, a ring, a wire mesh, and an 800 mL or 1000 mL beaker – similar to the picture to the right. Alternatively, you can use a tripod instead of the ring stand and ring. Put approximately 500 mL of water in the beaker, then heat the water up to boiling. (Hint: If you read through these instructions before coming to lab, you will probably start with this step so you do not have to wait for your water to heat up.)
  3. Carefully place the Erlenmeyer flask containing the lead iodide in the hot water bath. Heat the lead iodide solution up to about 80oC or until the lead iodide dissolves. Record your observations.
  4. Once the lead iodide has completely dissolved, remove the Erlenmeyer flask from the hot water bath. Set it on the bench top and watch the crystals form. Wait about half an hour for the crystals to precipitate slowly at room temperature. (The longer you wait, the nicer your crystals will look.) Record your observations. This may take a few minutes, so while you’re waiting, you should work on cleaning up the hot water bath, completing the worksheet, inventorying your glassware, and preparing for the next steps:
    • Prepare an ice bath. With the same 800 mL beaker, fill it about halfway with ice and water.
    • Obtain a Büchner funnel, a filter flask, and an appropriately sized piece of filter paper. Record the mass of the filter paper.
    • Set up a filter system like the one shown to the left. Wet the filter paper with a few mLs of water so that it sticks to the bottom of the funnel.
  5. After you have waited about half an hour, put the flask into the ice bath for about ten minutes.
  6. After that, turn on the vacuum source and pour the crystals and solution into the Büchner funnel. Use a few mLs of ice water to rinse away any crystals that may have stuck to the sides of the flask.
  7. Turn off the vacuum source once the crystals and paper appear dry.
  8. Weigh the filter paper and crystals on a tared watch glass. Record the mass on your worksheet and calculate the final mass of crystals.
  9. Ask your instructor or TA to check your data and observations before you clean up or leave.
Clean up

Pour the filtrate into the waste beaker labeled “lead iodide waste liquid.” Take a picture of your pretty crystals to show your roommate, then put the crystals and filter paper into the waste beaker labeled “solid lead iodide waste.” Pour hot water and ice baths down the sink. Rinse all glassware and return to them to where you found them.

Report

Complete the worksheet. Turn in either a paper or digital copy. This article will help you with the conclusion question. Optional survey.

Introduction to chemistry lab

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Welcome to the chemistry laboratory!

Before we get started, we need to cover a few topics:

Safety

Please read through the safety instructions. You are responsible for knowing this general information and working safely in the laboratory. You will receive more specific instructions before the start of lab each week.

Equipment

You will be assigned a cabinet of glassware and hardware. You are responsible for ensuring that you start and end the semester with all of the right equipment. You will receive an inventory list to check over.

Scientific Measurements and Calculations

For your first lab activity, you will familiarize yourself with making scientific measurements and performing simple scientific calculations. The first lab activity is Penny Statistics.