BIO282 Recitaion 1 KEY PDF

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BIO 282

Recitation



Fa2019

KEY BIO282 Rec1: Chemistry of Life 1.

Define polar and nonpolar bonds using complete sentences. In your definition, make it clear what criteria you use to determine if a bond is polar or nonpolar. Polar: When electrons being shared in a covalent bond are shared relatively unequally. Nonpolar: Occurs when electrons being shared in a covalent bond are shared relatively equally.  2. How are ionic bonds and covalent bonds different? Ionic bonds are bonds that occur when the electronegativity difference between two atoms in a bond is so great that there is a complete transfer of electrons from one atom to another, while covalent bonds occur when electrons are shared in a bond between two atoms.  3. What does electronegativity mean? How well/strongly an atom attracts electrons in a bond to themselves.  4. How do electronegativity values help us determine the type of bond created? Electronegativity values of atoms enable us to determine if and to what extent electrons are being shared equally or unequally and thus allows us to determine if a bond is ionic, nonpolar covalent, or polar covalent.  5. For each of the following molecules in the table below, determine if it is polar, nonpolar, or ionic. Show your work by listing the electronegativities of each element in the bond. 

Molecule

Electronegativity Values

Difference in Electronegativity

Bond Type

H - Cl

H: 2.1 Cl: 3.0

0.9

Polar Covalent

H - H

H: 2.1 H: 2.1

0

Nonpolar covalent

H - I

H: 2.1 I: 2.5

0.4

Nonpolar covalent borderline polar covalent

Cl - Cl

Cl: 3.0 Cl: 3.0

0

Nonpolar covalent

C - O

C: 2.5 O: 3.5

1.0

Polar covalent

Ca - O

Ca: 1.0 O: 3.5

2.5

Ionic

H2O

H: 2.1 O: 3.5

1.4

Polar covalent

Al - Fe

Al: 1.5 Fe: 1.8

0.3

Nonpolar covalent

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 6. For each of the four molecules below, complete the following: a. Draw in the pair of electrons shared between atoms. Draw the electrons closest to more electronegative atoms or in the middle of the bond if the atoms are equally electronegative. b. For each ATOM, determine if it has any electrons that are shared unequally, if so, write a partial charge (+ or -) next to that atom. c. Label all POLAR bonds with an asterisk. d. Label each of the molecules below as polar or nonpolar.           Polar or nonpolar? Nonpolar   

Polar or nonpolar? Nonpolar

       Polar or nonpolar? Polar  Polar or nonpolar? Polar   7. Label the following molecules as polar or nonpolar.             Polar/nonpolar? Nonpolar Polar/nonpolar? P  olar Polar/nonpolar? P  olar   2

 8. For each of the molecules below, draw the compound using a Lewis structure and then indicate whether that molecule is polar or nonpolar.  CH4

NH3









OH- 



polar

 

nonpolar



polar

 9. Answer the following questions using the diagram below:

 a. Which molecule(s) are polar? What led you to this conclusion? Electronegativity: O > N > C ≅ H Methanol - hydroxyl group is polar Water - polar,  b. Which molecule(s) are nonpolar? What led you to this conclusion? Ethane - Nonpolar, all C-H or C-C bonds  c. Which molecule is “water-fearing” or “hydrophobic”? What led you to this conclusion? Ethane - no partial charges for water to interact with.

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  d. Which molecule is “water-loving” or “hydrophilic”? What led you to this conclusion? Methanol - polar hydroxyl group interacts with water   10. What is the relationship between polarity and hydrophobicity, in general? Polar molecules are hydrophilic, while nonpolar molecules are hydrophobic.   11. For each of the following biological molecules, indicate whether the molecule is primarily hydrophobic or hydrophilic. For each molecule, justify why you indicated it was hydrophilic or hydrophobic. a. .  Hydrophilic. The presence of the -OH groups is indicative that this molecule will be soluble in water.    b. .  Hydrophilic. The presence of the -OH groups and NH2 group is indicative that this molecule will be soluble in water.    c. .  Hydrophilic. The presence of the -OH groups and the phosphate groups is indicative that this molecule will be soluble in water.  d. .     MOSTLY hydrophobic because of the long hydrocarbon chain, but the carboxyl group can interact with water.      4

 12. Answer the following questions using the diagram below: 

 a. Draw in the shared electrons for the bonds that are shown in bold (like for the unbolded bond above with the arrow pointing at it). b. Draw the partial charges on the atoms involved in the bonds that are bolded. c. Why is an attraction possible between the H and O involved in the hydrogen bond (indicated with the dotted line) shown above? Electrostatic attraction between partial δ+  (H) and δ- (O)

      d. Draw an alternative hydrogen bond between different atoms of these two molecules in the space below: See diagram above  e. Go back to Question 9 model and draw in some of the possible hydrogen bonds in both mixtures. See Question 9   

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 13. For the following pairs of molecules, draw their structures and at least one hydrogen bond connecting them. If no hydrogen bond is possible between these two molecules, indicate that. a. Water and CH3NH2 

  b. Methanol (CH3OH) and Carbon dioxide           No hydrogen bond, carbon dioxide is nonpolar, however it does present δ- which

can interact with water.

  c. Methanol and CH3NH2       

6

 14. Hydrogen bonds can occur between molecules (intermolecularly), or within different parts of a single molecule (intramolecularly). Hydrogen bonds are a crucial component of biomolecular structures. In the following drawings, draw in any hydrogen bonds that are possible between atoms within the same molecule or between different molecules: 

A: 



B: 

C: 





7



D:     



15. Biomolecules like DNA, proteins, and carbohydrates depend on hydrogen bonds to maintain their overall structure. However, each individual hydrogen bond is very weak. Using cellulose from question 14 as an example, explain how to resolve the apparent contradiction between these two statements.  Velcro, Baby. (Many hydrogen bonds = strength) 

  https://youtu.be/RkLn2gR7SyE?t=180    

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