Sidebar: An Animated Look at VSEPR Molecules

Just found this animated video that shows more molecule structures as predicted by VSEPR theory than your textbook does. It includes the shapes of molecules that contain atoms with lone pairs of electrons as well the shapes of those with only full bonds.

It may be easier for you to see the underlying patterns by seeing the full array of potential molecule shapes. Enjoy!

(Video by Ashley Jennings)

Lab 13: Conductance of Ionic and Molecular Solutes

This week we are going to investigate the movement of cations and anions through solutions.

Reading:  The Home Scientist Lab Manual Topic II. Session II-2. Conductance of Ionic and Molecular Solutes, pp. 53-59.

Experimental Title: Lab 13:  Conductance of Ionic and Molecular Solutes

Date of laboratory:  August 26, 2014

Purpose: The purpose of this laboratory is to investigate the conductivity of distilled water, tap water, plus solutions of strong and weak acids, strong and weak bases, ionic salts and a molecular compound.

Introduction:

Read the background to the experiment on pp 53-55.

Briefly explain conductivity and resistivity. Write the equations for the reactions in your notebooks.

Special safety concerns for Lab 13:

  • This week the most hazardous materials are the 6M ammonia, 6M acetic acid, 6M hydrochloric acid, and 6M sodium hydroxide. Please use gloves and goggles at all times when handling these materials. (MSDS for 6M Ammonia, MSDS for 6M Acetic Acid, MSDS for 6M Hydrochloric Acid, and MSDS for 6M sodium hydroxide.
  • Remember to cover spills of acids with baking soda and spills of bases with vinegar.
  • If a chemical spills on you, wash it off immediately. Be especially careful when handling the multimeter probes!
  • Be sure to wash your hands very carefully when you are finished with this lab.

Materials:  See list page 53.

Procedures:

I will do Part I (boiling the distilled water) for you before lab. You don’t need to write it in your notebook, but be aware why boiling of the distilled water was necessary.

Write down the procedures for Part II and Part III  on pages 56-58 in the lab manual. Be sure to leave room for the data table to record your results. I will bring graph paper to graph your results, if you need it.

Template for the 24-well reaction plate:

24-well-reaction-plate-template

 

Conclusions:

Once you have completed the experiment and cleaned up, sit down and write a sentence or two to explain the results. It is always a good idea to do this part while the experiment is fresh in your mind.

Discussion:

Record any thoughts you have about the experiments, including:

  • Possible improvements to the procedures or how to tweak techniques
  • How the results differed from your expectations
  • Whether the goals were met
  • Suggestions for other experiments
  • The answers to the review questions he provides on page 59.

Please leave a comment or send an e-mail if you have any questions before our meeting.

Lesson 13: Chemical Bonding

Time to put those atom models to use and make molecules.

Textbook Reading: Chapter 10, pages 325-349.

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Lewis Dot Diagrams and Structures

Last week we learned that the outer electrons, called valence electrons, of an atom are the ones involved in bonding and chemical reactions.  Lewis structures consist of the element’s symbol surrounded by dots to indicate the valence electrons.

lewis-stuctures

There are a few rules for creating Lewis structures:

1. Find the number of valence electrons for a given element using the group numbers of the periodic table. There will never be more than 8.

2. Represent the valence electrons by placing dots on four sides around the symbol for the element.

3. Start filling with single dots. If there are 5 or more valence electrons, pair them after the four single dots have been placed. Exception:  Helium has a single pair of two dots because that is its stable configuration (see illustration above).

4. Exact location of dots can vary (which side placed on doesn’t matter).

Bozeman Science has an in depth explanation about Lewis structures. (Those who like their chemistry to “pop” will enjoy the first part of this video. The last part about Lewis himself is sad.)

When drawing molecules, the sharing of two electrons is represented by a line connecting the symbols. If the two atoms share two pairs of electrons, then the resulting double bond is shown as two lines.

Be aware that as powerful as it is, there are exceptions to the octet rule!

Valence Shell Electron Pair Repulsion (VSEPR)

Remember how the magnets of the same pole repelled each other, pushing away? The VSEPR model uses that concept to predict the shape of molecules.

Some of the structures we will investigate are:

  • linear
  • trigonal planar
  • tetrahedral
  • trigonal pyramidal
  • bent
  • octahedral

(See the Sidebar about VSEPR post to see these shapes)

Mr. Isaacs at IsaacsTeach has an introduction to molecular geometry that explains why water molecules are bent.

Bozeman Science pulls the Lewis Structures and VSEPR together for a comprehensive overall of drawing molecules. He goes into a bit more depth than is covered in your text.

Electronegativity and Polarity

Another aspect of molecules that determines their shape and chemical properties is the ability of the atoms of an element within a bond to attract electrons, or its electronegativity.

You can read the electronegativity of an atom from a specially designed periodic chart like this one:

Periodic_Table_of_Electronegativity_

(Illustration from UC Davis ChemWiki is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.)

Non-polar bonds form when the difference in electronegativity between the two atoms is between 0 and 0.4, polar bonds form when the difference in electronegativity between the two atoms is between 0.4 and 2.0, and ionic bonds form when the difference in electronegativity between the two atoms is greater than 2.0.

Why does this matter? Knowing whether a molecule is polar helps predict its characteristics, such as whether it will be soluble in a given solute.

That’s it for this week. Let me know if you have any questions!

Lesson 12: Electrons, Atom Models, and the Periodic Table

After a hectic week, you will be happy to hear we don’t have a lab to write up in your notebooks for this lesson. That means you don’t have to bring your goggles or gloves either. You do, however, need to really focus on the readings in the textbook because this lesson is critical for your understanding of chemistry.

Textbook Reading: Finish Chapter 9, sections 9.5-9.9 on pages 294-315.

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Supplemental:

Introduction:

Electrons are where all the action is in chemistry. By developing sophisticated and complex models of atoms that explain electron behavior, chemists have a powerful tool to understand molecular structures and chemical reactions.

Our most recent model of the atom comes from the study of quantum mechanics. The theory and math behind the model are pretty complex, but chemists have been pulling out some basic concepts that can be extremely useful. Remember, however, that this is a model and may be modified as our understanding increases.

Electrons:

We have some awesome videos this week to supplement your textbook.

Let’s start out with a TED video about the uncertain location of electrons.

Although this video suggests orbitals are where electron can be found 95% of the time, other representations of orbitals are often where the electrons are found 90% of the time. In any case, you can think of orbitals as boxes or rooms where electrons are found most of the time. You should also remember that each orbital can only hold two electrons.

For the musicians in the class, this Crash Course video has an explanation of electrons that might just “resonate” with you 🙂

 

 

Okay, if you don’t know enough about music to understand his analogy, then we can use a simpler analogy.

Single_electron_orbitals

(Illustration of single electron orbitals from UC Davis ChemWiki, licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.)

Orbitals are grouped around the nucleus in very specific ways, based on the number of electrons and their energy states. The groupings are given names: shells and subshells. If orbitals are rooms where electrons are found most of the time, then subshells are clusters of rooms. You might think of them as one apartment in a large apartment building, or one set of offices in an office complex.

Continuing the building analogy, the shells would be the different floors of the building. The higher floors have more energy and contain more subshells.

The shells and subshells fill with electrons in a very orderly way, so that we can figure out the electron configuration of atoms of each element in the periodic table simply based on the number of electrons it contains. The electron configuration, although it looks complicated, is simply the arrangement of the electrons.

Bozeman Science has a serious explanation of electron configurations. He relates the configurations to “ionization energy” or how hard it is to pull an electron off an atom.

Isn’t the idea that the p orbitals fill like seats on a bus helpful?

Still unsure what all this means? Don’t worry, we’ll being going over it all in class. Be sure to write down your questions and bring them with you.

Sidebar: Burning Steel or Iron Wool

Remember our demonstration of burning iron wool as an example of a composition reaction (Session III-1 in the lab manual)? Here’s a bit more information about what you saw.

Chemical Equations

First, when the iron in the wool reacted with the oxygen in the air it formed iron oxide, but what was the equation? Was it 2 Fe + O2 → 2 FeO ?

Because iron can be in different states – iron (II) and iron (III) – there are actually three potential products. FeO is one product, but other reactions might be:

4 Fe + 3 O2 → 2 Fe2O3
3 Fe + 2 O2 → Fe3O4

We can not tell which products we obtained by merely looking at the results.

By the way, the second equation above is for rust. We were creating rust more quickly because we added heat to get the reaction started. You can also speed up rust with salt water (see a simple experiment at Growing With Science blog.)

Survival Tool

FYI: If you ever are in a survival situation and just happen to have steel wool and a 9-volt battery, you can use your chemistry knowledge to start a fire!

Want to see the reaction again? Check this NurdRage video:

 

Please let me know if you have any questions.

Lab 11: Photochemistry

For this lab, we are going to perform the photochemical reaction of iodine and oxalate. We will be running the experiment until two hours after we set it up, so we will likely go 15 or so minutes over our usual lab time.

We will also do other labs and activities as demonstrations to fill in while we wait between observations of the experiment. You won’t be asked to write those up in your lab notebook.

Reading:  The Home Scientist Lab Manual Topic XI. Photochemistry Session XI-1: Photochemical Reaction of Iodine and Oxalate pp. 182-186. This will be the lab we are writing up. 

Two demonstrations we will be doing from the Home Scientist:  Topic III Session 1 and 2, read pp. 70-74. You do not need to write these up.

Experimental Title: Lab 11:  Photochemical Reaction of Iodine and Oxalate

Date of laboratory:  August 12, 2014

Purpose: The purpose of this laboratory is to observe photochemical initiation of the oxidation of iodine to iodide by oxalate, as well as to determine the effect of wavelength on photochemical initiation of a reaction.

Introduction:

Read the background to the experiment on page 183-top 0f page 184.

Write the equations for the reactions in your notebooks. Also, summarize what the color changes or lack of color changes mean about how far the reaction has progressed.

Special safety concerns for Lab 11:

  • This week the most hazardous material is the 6M Ammonia.Please use gloves and goggles at all times when handling this material. It is corrosive and the vapor can be irritating. (MSDS for 6M Ammonia)
  • You may remember from the oscillating reaction that iodine can stain. Sodium thiosulfate makes it colorless again.
  • If glass test tube breaks, do not pick it up with your bare hands. Notify your instructor immediately.
  • Be sure to wash your hands very carefully when you are finished with this lab.

Materials:  See list page 183.

light-bulb-turn

(Public domain image by George Hodan)

 

Procedures:

Write down the Procedure on pages 184-186 the lab manual. Be sure to leave room for the data table to record your results.

Conclusions:

Once you have completed the experiment and cleaned up, sit down and write a sentence or two to explain the results. It is always a good idea to do this part while the experiment is fresh in your mind.

Discussion:

Record any thoughts you have about the experiments, including:

  • Possible improvements to the procedures or how to tweak techniques
  • How the results differed from your expectations
  • Whether the goals were met
  • Suggestions for other experiments
  • The answers to the review questions he provides on page 186.

Please leave a comment or send an e-mail if you have any questions before our meeting.

Lesson 11: The Electromagnetic Spectrum

Seems like quite a jump from quantifying chemical reactions to learning about the electromagnetic spectrum, but it will soon become clear that in order to understand chemical reactions, we really need to understand electrons and to understand electrons, we need to learn about the electromagnetic spectrum.

Textbook reading:  Chapter 9, Sections 9.1 – 9.4. The Electromagnetic Spectrum and the Bohr Model of the Atom, pp. 285-294.

________________________________

Supplemental

Before we jump into the electromagnetic spectrum, let’s spend a minute learning about waves. You may have read that light and its relatives in the spectrum travel in waves. What does that mean?

Think about physical waves, like the waves in the ocean. Electromagnetic waves exhibit a similar motion. Check this interactive website from Colorado to see how you can create varying waves with different motions.

Of course, Bozeman Science also has an introduction to waves with cool animations to help you understand what you did at the interactive site.

EM_Spectrum3-new

(Illustration from NASA)

Although electromagnetic radiation forms a continuous ribbon of wavelengths, we cut the ribbon into segments and name different regions based on their size and related properties. Electromagnetic radiation has been mined over the years to produce many useful gadgets, which helps us relate to the different types.

Moving from left to right, we find that the first piece of the ribbon, radio waves, are the largest (the illustration suggests as big as buildings). Besides sending sound to radios, radio waves are are used in astronomy. For example, radar and radio waves were used to investigate the recent asteroid that passed by the Earth. Currently the largest telescope to detect radio waves from space is the Arecibo Radio Telescope in Puerto Rico.

Microwaves consist of wavelengths shorter than radio waves. They are used for all sorts of things besides cooking, including remote sensing of weather for forecasting, as well as for monitoring space. Some forms are also used for communications.

Nearer to the visible spectrum (forms of electromagnetic radiation we can see), are the infrared waves. We can’t see infrared radiation with our eyes, but we can sometimes feel it is as heat. Certain snakes, called pit vipers, have organs on their heads that can detect infrared radiation as a way to find their prey in the dark. Humans rely on equipment, such as specially designed cameras, to observe infrared radiation. Your TV remote works using infrared waves.

We are most familiar with the wavelengths in the visible spectrum. Once again, the wavelengths are continuous, not discrete regions. Sir Isaac Newton paved the way by breaking white light into its components with a prism and naming bands of wavelengths as colors. Although realistically they could be broken into any number of colors, he gave us seven: red, orange, yellow, green, blue, indigo and violet.

Ultraviolet radiation has a slightly shorter wavelength than visible light. Humans can’t see ultraviolet light, but many other animals can, including honey bees and butterflies. In fact, many flowers have spots we can not see in visible light, but that show under ultraviolet light. Scientists call these spots and patterns “nectar guides” because they are thought to attract bees and other pollinators to the flower.

X-rays are a form of electromagnetic radiation with a very short wavelength. X-rays are considered to be “ionizing radiation,” which means its energy levels are high enough that when it strikes a molecule it can remove an electron, thus forming an ion. Because of this property, exposure to X-rays should be limited.

Certain gamma rays have the highest level of energy of all the forms of electromagnetic radiation, as well as the shortest wavelength. Here on earth gamma rays are only produced by lightning, radioactive decay of certain radioactive elements, and nuclear explosions. In space they are produced by high-energy events such as supernovas. Gamma rays are mostly harmful, but they can also be helpful because they are used to treat certain cancers and to kill bacteria in food.

Particles

The various forms of electromagnetic radiation were long known to behave like waves, but also they also act like particles. Scientists call these elementary particles photons. They are odd particles because they have no mass or charge, and could be thought of as bundles of energy or “quanta.” At motion in a vacuum, photons travel at a constant rate: the speed of light c = 2.998 x 108 m/s. One way photons behave like particles is when they collide with electrons.

Bohr Model of the Atom

Where is all this going? Niels Bohr used data from light emission spectra of elements to develop his famous model of the atom.

Emission_spectrum-H

Visible lines in the hydrogen emission spectrum (public domain image from Wikimedia)

Bohr’s observations that atoms of different elements emit light at only certain wavelengths lead him to conclude that electrons are fixed positions from the nucleus. Check out the ladder analogy in Figure 9.10 on page 293 in your text.

340px-Bohr_Model.svg

(Public domain image from Wikimedia)

Although Bohr’s model has been widely adopted in popular culture, it only had the power to explain the hydrogen emission spectrum, not those for elements. Obviously, something more was needed. We will learn about how the model has be modified over the years in our next lesson.

Sidebar: Mass Spectrometry

molecule-imageDid you ever wonder how chemists figure out the atomic mass units for the periodic table? The answer comes from a piece of equipment that has been central to chemical exploration:  the mass spectrometer. We will be seeing a mass spectrometer on our field trip, so let’s take a few minutes to find out more about them.

Mass spectrometers are used find out what atoms are present in a sample based on size. They are so sensitive that they can even detect differences between isotopes of the same element.

How do they work? That depends on the type of mass spectrometer, but in general they:

1. Pull apart molecules and convert all the atoms into a gas.

2. Knock off electrons and turns the atoms into ions.

3. Line up the ions and shoot them through a magnetic field. The magnetic field deflects lighter ions more than heavier ones, separating them by mass.

4. The separated ions hit a detector, which counts them.

5. The results are compared to known samples.

If you’d like to read more, there is a detailed explanation at Chem Guide.

 
Bozeman Science has a good discussion of Mass Spectrometry in this video. (Note: He starts with a discussion of Dalton, but hang in there. He will tie it into mass spectrometry fairly quickly).


Simple Explanation of the Mass Spectrometer has a cool animation of how it works. (A snooker ball is pool or billiard ball.)

Now you know where atomic mass units come from, how scientists do radiocarbon dating, how chemists figure out the chemical formula of unknown molecules, etc. A mass spectrometer is a handy and versatile machine.

Have you ever seen one? Have you ever used one? Where?

 

Lab 10: Double Displacement Reactions

For today’s lab, we are going to build on some of the information about classifying chemical reactions we learned last week.

Reading:  The Home Scientist Lab Manual Topic III-4. Observe Double Replacement Reactions p.77-82,

Experimental Title: Lab 10:  Double Displacement Reactions

Date of laboratory:  August 5, 2014

Purpose: The purpose of this laboratory is to observe a series of double displacement reactions.

Introduction:

Read the background to the experiment on page 77-78.

It is a bit confusing, but chemists use the terms “double-replacement” and “double displacement” interchangeably, and this type of reaction may also be called a “metathesis” reaction. Extra credit to anyone who can find out which term is better to use or if one term is “displacing” the other. For our class, I will accept all these terms, but I will follow the textbook and call it double displacement in my own writing.

A double displacement reaction involves two compounds switching ions. If A and B are cations and X and Y are anions, then the reaction is AX + BY -> AY + BX.

In this lab session, we will test dilute solutions of the following eight cations:
□ Barium(II) or Ba2+ (from barium nitrate solution)
□ Calcium(II) or Ca2+ (from calcium nitrate solution)
□ Copper(II) or Cu2+ (from copper(II) sulfate solution)
□ Hydrogen or H+ (from hydrochloric acid solution)
□ Iron(II) or Fe2+ (from iron(II) sulfate solution)
□ Iron(III) or Fe3+ (from iron(III) chloride solution)
□ Lead(II) or Pb2+ (from lead(II) acetate solution)
□ Magnesium or Mg2+ (from magnesium sulfate solution)
against the following twelve anions:
□ Bromide or Br- (from potassium bromide solution)
□ Carbonate or CO32- (from sodium carbonate solution)
□ Chloride or Cl- (from hydrochloric acid solution)
□ Dichromate or Cr2O72- (from potassium dichromate solution)
□ Ferricyanide or [Fe(CN)6]3- (from potassium ferricyanide solution)
□ Ferrocyanide or [Fe(CN)6]4- (from sodium ferrocyanide solution)
□ Hydroxide or OH- (from sodium hydroxide solution)
□ Iodide or I- (from potassium iodide solution)
□ Oxalate or C2O42- (from oxalic acid solution)
□ Phosphate or PO43- (from phosphoric acid solution)
□ Sulfate or SO42- (from magnesium sulfate solution)
□ Sulfide or S2- (from sodium sulfide solution)
to determine if a reaction occurs.

If you have time, do some research and see if you can make some predictions as to what might happen in each of the pairings.

Special safety concerns for Lab 10:

This week we have to take lab safety very seriously. We are only doing this lab because you have shown you can be responsible up to now. Keep up the good work.

  • Important:  Be sure to wear gloves and goggles for the entire lab this week. Also, please wear long pants and closed-toe shoes. No exceptions this week!
  • We will be doing the experiments in trays this week. Keep all the chemicals in the trays, please.
  • The most dangerous chemicals are :  6M hydrochloric acid and the 6M sodium hydroxide. Those are strong concentrations. Barium, lead and dichromate can also be toxic. Use extra caution with those chemicals.
  • If an acid spills, especially the 6M hydrochloric acid, cover the area of the spill with baking soda immediately! If it on you, go wash in the sink. Notify your instructor.
  • If a base spills, especially the 6.0 M Sodium hydroxide, please cover it immediately with acetic acid (vinegar). If it on you, go wash in the sink. Notify your instructor.
  • If glass breaks, do not pick it up with your bare hands. Notify your instructor immediately.
  • Be sure to wash your hands very carefully when you are finished with this lab.

Materials:

See the list page 77.

Procedures:

Write down the Procedure on pages 78-81 the lab manual. Be sure to leave room for the data table to record your results. It might be useful to underline, star, or otherwise note the 6M hydrochloric acid, 6M sodium hydroxide, barium, lead and dichromate whenever you work with them so you know those are chemicals to use with extra caution.

Edit:

24-Well Reaction Plate:

24-well-reaction-plate-template

Conclusions:

Once you have completed the experiment and cleaned up, sit down and write a sentence or two to explain the results. It is always a good idea to do this part while the experiment is fresh in your mind.

Discussion:

Record any thoughts you have about the experiments, including:

  • Possible improvements to the procedures or how to tweak techniques
  • How the results differed from your expectations
  • Whether the goals were met
  • Suggestions for other experiments
  • The answers to the review questions he provides on page 82.

Please leave a comment or send an e-mail if you have any questions before our meeting.

Lesson 10: Quantities in Chemical Reactions

Pull out those calculators as we delve back into math problems.

Textbook Reading:  Chapter 8, pp. 249 to 271.

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Supplemental:

After reading through this chapter, I realized using ratios is a good way to do these calculations, but when they get long and complicated there is always the chance of missing units or accidentally flipping one of the conversion factors.

55 g Na x 2 moles Na/ 33 g Na x 56.7 g Na/1 mole NaCl….. It goes on and on.

One way to add structure to these calculations is to use proportions. To help you get started, I put together a video explaining the method. Even if you don’t choose to use the ratios and proportions, you may find some of the information about determining molar mass and moles helpful.

You can also use this method for finding percentages and even for the heat of reaction calculations.

Bozeman Science explains some of the concepts from the chapter as well (he uses the strings of ratios like the textbook):

Remember, the answers to the odd-numbered problems at the end of the chapter are given in the back of the book. You might want to try a few to make sure you understand them.

If you choose, let me know if you have any questions.

Atoms rule!