CHE 105 – Week 2

Formative Assessment

Instructions: 

• Each student will work on this assignment alone. 
• Assignments with identical answers will earn grades of zero. 
• If you have questions, you are to ask me, your instructor, not your peers.
• The assessment will be printed out, completed, scanned, or photographed and uploaded to the upload area.  Only assignments uploaded to the class will be graded.
• You must show ALL work.  Submissions with answers only will not be evaluated.

Formats: 

The following formats will be evaluated:

• Microsoft Word

Note: if you cannot scan your work before submitting, you are welcomed and encouraged to do the following:

• Take an image of each page of your work with your phone. 
• Copy/paste those images to a Word document.
• Make sure the image size is the same as the page. Submit one image per page.
• Save the file as a Word document.

Submitted files in the following formats will NOT be evaluated:

• Pages
• Lone Jpeg images
• Any other file formats

Polar, Nonpolar, and Ionic Bonds

(Why Is Salt Ionic but Sugar Is Not?)

Part I. Pre-Lab Questions

Electronegativities by Linus Pauling.

Linus Pauling examined bonds between homonuclear diatomic molecules (such as H2 and Clz) and bonds in heteronuclear molecules (such as HCl). Bonds between different elements appeared to be stronger. He proposed that the bonding electrons in heteronuclear molecules were not shared equally. That is, he reasoned that in heteronuclear molecules one atom attracted the electrons in the bond more strongly than the other atom. Pauling called the ability of an atom (in a molecule) to attract electrons the electronegativity (EN) of the atom. In 1937, he devised a quantitative scale for electronegativity in which fluorine was assigned a value of about 4. This scale has since been refined based on more recent experimental evidence.

Nonpolar, and Ionic Bonds.

The chemical bond formed between two atoms is generally classified as one of three possible types, as shown in Figure 1.

• A nonpolar covalent bond formed between identical atoms.
• A polar covalent bond generally formed when there is a small electronegativity difference between the atoms.
• An ionic bond generally formed when there is a large electronegativity difference between the atoms. No sharing of electrons.

Experimentally, we find that binary compounds usually exhibit ionic behavior (high melting point, 500- 3000 J’C; conduct electricity as liquids) when the electronegativity difference between the constituent atoms is greater than 1.5.

Questions

1. For HF and HBr, the partial positive charge on H atom is 0.29 and 0.09, respectively. Use electronegativities (EN) to explain why the partial charge on H in HF is more positive than the partial charge on H in HBr.

2. Describe the trend in EN as one moves from left to right across a period of the periodic table.

3. Describe the trend in EN as one moves down a group of the periodic table.

4. Why do homonuclear molecules (H₂, Cl₂, N₂, and so on) have nonpolar bonds?

5.  When an ionic bond is formed, what type of atom (in terms of relative electronegativity) is likely to:

a) lose one or more electrons?

b) gain one or more electrons?

Explain your reasoning.

6.  Classify the bond in each of the following molecules (or ions) as nonpolar, polar, or ionic.

O2	I2	CO	CuO
NaF	KCl	NO	ICl

7.  Without referring to a table of electronegativities, identify the most electronegative atom in each case:

a) Al, P, S, Se, Te                   b) P, Sr, Cu, As, Pb                 c) K, Na, P, As, Si

______________                    _________________              ______________­­­­­__

• Based on the formula, predict whether each of the following compounds is primarily ionic or primarily covalent.

sodium iodide (NaI)	methane (CH4)	ammonia (NH3)	calcium chloride (CaCl2)

Part II. Data Collection through Simulated Lab Activity

Go to http://phet.colorado.edu/en/simulation/sugar-and-salt-solutions

Procedure

1. Macro Tab

1. Open the “Sugar and Salt Solutions” simulation on the PhET website by following the link below and clicking, “Run Now!”:   http://phet.colorado.edu/en/simulation/sugar-and-salt-solutions
2. Drag the conductivity tester, labeled A, into the beaker of water.  Place the negative and positive electrodes into the water, but not touching the bottom of the beaker.  When electricity is conducted by the solution, the light bulb will light up – the stronger the electric current, the brighter the bulb will glow.  Record any observations.
3. Click on the salt shaker, labeled B, and drag your mouse back and forth.  This will “shake” the salt into the water.  As you add salt, pay attention to the light bulb on the conductivity tester.  Continue to add salt until the shaker is empty.  Record observations.  Then click the “Remove salt” button to reset the simulation
4. Click the “sugar” bubble in the solute selector, labeled C, to switch from salt to sugar.  Then repeat step 3, this time using the sugar shaker.  Record observations.
5. Click on the “Micro” tab at the top of the simulation, labeled D, to switch to a new simulation for part B.

• Micro Tab

• Click on the salt shaker, labeled A, and drag your mouse back and forth.  This will “shake” the salt into the water.  Pressing pause, labeled C, will stop the salt crystals mid-air so you can see them before they dissolve.
• Observe how the crystal behaves once in the water.  Record observations in the “Ionization in Water” section of the data table.
• Click the “sucrose” bubble in the solute selector, labeled D, to switch from salt to sugar.  Then repeat steps 6 and 7, this time using the sugar shaker.  Record observations.
• You can test other solutes by clicked the right arrow in the solute selector, labeled D.
• To get an even further “zoomed in” view, click on the water tab, labeled E and repeat the simulation by dragging salt and/or sugar into the water.

• MSDC Information Search

1. Use the Flinn MSDS search to locate the MSDS for “sodium chloride” (salt) and “sucrose” (sugar): http://www.flinnsci.com/msds-search.aspx
2. Using the MSDS sheets for salt and sugar, locate their melting points.  Record in the data table below.
3. Using the MSDS sheets for salt and sugar, locate the information on solubility.  Record in the data table below.

Data Table

Substance	Melting Point	Solubility in Water (yes or no)	Electrical Conductivity (yes or no)	Ionization in Water (yes or no)
Pure Water H2O		N/A		N/A
Salt NaCl
Sugar C12H22O11

Part III. Analysis Questions

1. Based on their chemical formulas, state whether the substances tested above are covalent or ionic.

• A data table is provided below with information on substances not tested in the simulation.  Complete any missing information by looking up the MSDS sheets for each chemical using the same link provided in step 11 of the procedures (for part C).

Substance	Phase at 20°C (solid or liquid)	Melting Point	Solubility in Water	Electrical Conductivity in Water	Type of Bond
potassium chloride KCl	solid		Yes	Yes
benzoic acid C6H5COOH	solid	122.4°C		None detected	covalent
ethyl alcohol C2H5OH	liquid		Yes	None detected
iron (III) sulfate Fe2(SO4)3		480°C		Yes	ionic
oleic acid C18H34O2		13.4°C	No	None detected
nitrogen N2	gas	-210°C	Slightly	None detected

• Using the data you gathered during the simulation, as well as the data table from the previous question, what are some properties exhibited by covalent (molecular) compounds?

• What are some properties exhibited by the ionic compounds tested in the simulation and/or shown in the data table in question 2?

• Although both sugar and salt are soluble in water, the way in which they dissolve is not shown the same in the simulation.  How is their dissolving process different?  Explain why these differences exist. 

• What would the simulation have shown if oleic acid was added to water?  Would it look different than what was shown for sugar and/or salt?  Explain.

• When some ionic compounds dissolve, not all of their bonds dissociate. What kind of conductivity would you expect such a solution to have?  Explain.