Polar Covalent Bonds (Allison Hastings)
Title: Polar Covalent Bonds
Defying Physics: Bending Water
Principle(s) Investigated:
1. Polarity of Water
2. Static Electricity
3. Charges of Molecules
4. Chemical Bonds
Chemistry 1d: Students know how to use the periodic table to determine the number of electrons available for bonding.
Chemistry 2a. Students know atoms combine to form molecules by sharing electrons to form covalent or metallic bonds or by exchanging electrons to form ionic bonds.
Chemistry 2f. Students know how to predict the shape of simple molecules and their polarity from Lewis dot structures.
Materials:
1. Water stream from a faucet/tap
2. Plastic Balloon & Hair/Wool/Fur for a Negative Charge
3. OR Glass Rod & Silk for a Positive Charge
Procedure:
1. Go over or review premises pertaining to lesson.
a. Bohr model (understand atomic structures, electron configurations)
b. Valence electrons (understand they are the ones used in reactions)
c. Chemical bonds (understand that molecules share or exchange electrons in a bond)
d. Remind students that like charges repel and opposite charges attract.
2. Break the students into groups (each group at one sink)
3. Have students make predictions about what is going to happen. Have volunteers run the water, rub the balloon on wool or hair, discuss, then demonstrate.
4. Explain. Have the students come back together for a discussion. Use the premises to get students to understand what happened in the demo. Go through each one until students show understanding. Essential questions: What happened to the water? What is static electricity? What charge is the balloon/your hair? What charge is water? If water has a neutral charge, how can this happen?
5. Use students' bodies to demonstrate further. Whiteboard is a pos/neg charge, their faces are a pos/neg charge, switch off scenarios until they understand that their faces turning to and away from the charged whiteboard is how a polar covalent bond moves in attraction to the pos/neg charge present.
6. Talk about the difference between ionic, covalent, and polar covalent chemical bonds, relate them to each other, venn diagram the similarities and differences, relate to demo.
Student prior knowledge:
Students need to understand Bohr’s Model, and how electrons are set up in orbitals, with Valence electrons on the outer orbitals. Students need to understand that valence electrons are often engaged in chemical reactions, and can be procured by other atoms/elements. Students need to understand hydrogen and polar covalent bonding, and the concept of chemical bonds in general in that they share or transfer electrons.
Explanation:
This discrepant event of water bending from the faucet toward the balloon works by using static electricity, or creating a negative charge on the balloon by rubbing in hair to take advantage of water’s polarity. This requires knowledge of chemical bonds, which is defined as a transfer or sharing of electrons between two atoms. This occurs when you rub the balloon on hair, the balloon picks up extra electrons, leaving a positive charge on the hair (cation), and creating a negative charge on the balloon (anion), which is an ionic bond. Water has an overall neutral charge, but being a polar covalent bond, it is partially sharing electrons, creating both positively and negatively charged particles on opposite 'poles' in the molecule. Since water is fluid, the molecules are able to rotate themselves so the positive polar end of water can be attracted to the negative charge of the balloon. When the balloon is near enough, the water bends towards the balloon, showing that opposite charges attract, and what you see is water bending toward the balloon.
Questions & Answers:
1. If you created a positive charge on an object and conducted the same procedure, would the results be the same, using water’s negatively charged pole attracted to the positively charged object?
A: Yes. The attractive forces will always exceed the repulsive forces, therefore water will always bend towards both positive and negative objects.
2. Could you procure the same results in the experiment using repulsion? i.e. negative charged object bending the water away from it?
A: No. Electrostatic forces decrease with distance, and the repulsive force will always be less than the attractive force. The attractive forces are closer together, a smaller distance; whereas the negative forces are over a greater distance, so the electrostatic forces are stronger in the attraction versus the repulsion. Therefore, a negatively charged object will always attract the positive pole, and never repel the negative pole.
3. If water has a neutral charge, why does it have attractive/repulsive properties?
A: Water has an overall neutral charge, but it has both positively and negatively charged particles in each molecule, which is why it is a polar covalent bond. The positive pole is attracted to negatively charged objects like the balloon, and the negative pole is attracted to positively charged objects like the glass rod.
Applications to Everyday Life:
1. Household Ammonia: Ammonia's molecular shape gives the molecule a dipole moment (separation of positive and negative electric charge) and makes it polar. The molecule's polarity and, especially, its ability to form hydrogen bonds (which is found often in polar covalent bonds), makes ammonia highly miscible with water, which is what you find with household ammonia.
2. Solubility: Due to the polar nature of H2O itself, polar molecules are generally able to dissolve in water, which is why sugar dissolves. Sucrose has many polar oxygen–hydrogen (-OH) groups and is overall highly polar. Water is a polar solvent.
3. High Melting/Boiling Points: High melting and boiling points are due to the high number of hydrogen bonds each molecule can form relative to its low molecular mass. Because of the difficulty of breaking these bonds, water has a very high boiling point, melting point, and viscosity compared to otherwise similar liquids not conjoined by hydrogen bonds. Hydrogen bonds are often found in covalent bonds, and these hydrogen bonds cause higher boiling and melting points.
Photographs:
Videos: Ihttp://www.youtube.com/watch?feature=endscreen&NR=1&v=LKAjTE7B2x0
References:
1. http://www.sciencekids.co.nz/experiments/bendingwater.html
2. Herr, N., & Cunningham, J. (1999). Hands-On Chemistry Activities with real-Life Applications. San Francisco, CA: Jossey-Bass. Nonpolar and Polar Covalent Bonds. Pgs 214-215.
3. http://www.homework-help-secrets.com/electronegativity.html