MCB 100 Introductory Microbiology January 28 2019 Electronegativity

MCB 100 Introductory Microbiology January 28, 2019

Electronegativity of The Most Common Elements Found in Biological Molecules H 2. 1 C 2. 5 N 3. 0 Examples: Non-Polar Covalent Bonds H–H H / H–C–H H O 3. 5 P 2. 1 S 2. 5 Polar Covalent Bonds H–O H H / H–N H

A Bit About Water molecules are held together by polar covalent bonds, because oxygen atoms have a greater electronegativity than hydrogen atoms.

Redox Values of Elements in Covalent Compounds Of the 6 elements that are most common in biological macromolecules: C, H, O, N, P and S, Oxygen has the highest electronegativity (3. 5) & Hydrogen has low electronegativity (2. 1). Since oxygen atoms most often form 2 covalent bonds and have a relatively strong tendency to attract electrons, by convention we say that the redox level of an oxygen atom in a compound is -2 (more electrons than protons). Since hydrogen atoms form 1 covalent bond and have a relatively low affinity for electrons, we say that hydrogen atoms in a compound have a redox level of +1 (more proton than electron). The redox level of a carbon atom or a nitrogen atom in a compound varies, depending on how many oxygen atoms or hydrogen atoms it is associated with.

Redox Values of Elements in Covalent Compounds What is the redox value of the nitrogen atom in the compound that is underlined? N 2 A. -3 NH 2 OH B. 0 HNO 3 HNO 2 C. -1 D. +3 ----------- NH 3 E. +5

Redox Values of Elements in Covalent Compounds What is the redox value of the nitrogen atom in the compound that is underlined? N 2 NH 2 OH A. -3 B. 0 HNO 3 HNO 2 C. -1 D. +3 ----------- NH 3 E. +5 H = +1, O = -2, 3 oxygens = -2 x 3 = -6, What’s N? HNO 3 = 0, 1 + N – 6 = 0, N = +5

Redox Values of Elements in Covalent Compounds In which one of the compounds given below does the carbon atom have a redox value of zero? A. CH 4 B. CH 3 OH D. HCOOH E. CO 2 C. H 2 CO

Redox Values of Elements in Covalent Compounds In which one of the compounds given below does the carbon atom have a redox value of zero? A. CH 4 -4 B. CH 3 OH -2 D. HCOOH +2 E. CO 2 +4 C. H 2 CO 0

These weak non-covalent bonds are called hydrogen bonds. Because oxygen has a greater electronegativity than hydrogen, water is a polar solvent.

Significance of Polar and Non-Polar Compounds A solvent is a liquid that is used to dissolve something. Water is a polar solvent. Hydrocarbons and oils are non-polar solvents. Like dissolves like. Some compounds dissolve freely in water, but poorly in oils. Other compounds dissolve freely in oils but poorly in water. Since oils are non-polar, they dissolve molecules that include hydrophobic alkyl groups. Water is polar, it dissolves molecules with polar groups such as alcohols, aldehydes, carboxylates and amines.

There isn’t much difference in the electronegativity of hydrogen and carbon so the bonds in hydrocarbons have little polarity. Hydrocarbons don’t form hydrogen bonds with other compounds.

Organic Functional Groups Which one of the organic compounds depicted below is hydrophobic? . .

Organic Functional Groups Which one of the organic compounds depicted below is hydrophobic? propyl. C-H bonds are relatively non-polar.

A Bit About Water molecules are polar enough that a small fraction of water molecules will always ionize. Water constant = 1 x 10 -14 M = [H+] x [-OH] A high concentration of H+ causes a low concentration of –OH, and vice -versa. p. H = -log [H+] p. OH = -log[-OH] p. H + p. OH = 14 if p. H = 1. 0, the [H+] = 1 x 10 -1 M or 0. 1 M if p. H = 4. 0, the [H+] = 1 x 10 -4 M or 0. 0001 M if p. H = 7. 0, the [H+] = 1 x 10 -7 M or 0. 0000001 M if p. H = 10. 0, the [H+] = 1 x 10 -10 M or 0. 000001 M A low p. H = a higher concentration of free protons (stronger acid). A high p. H = a higher concentration of free hydroxyl ions (stronger base)

Organic Functional Groups Which one of the organic compounds depicted below is an acid? . .

Organic Functional Groups Which one of the organic compounds depicted below is an acid? R – COOH RR – COO- + H+. .

Organic Acids and Bases Carboxylic Acid Carboxylate anion + proton Amine + Protonated Amine

p. H Which one of the following statements about p. H and water is FALSE? A. In pure water at a p. H of 7. 0 the concentration of hydroxyl ions (OH-) is 0. 0000001 M. (1 x 10 -7 M) B. A solution with a p. H of 4. 0 has a proton concentration that is 10 times greater than a solution with a p. H of 5. 0. C. When bacteria ferment a sugar under anaerobic conditions they make acidic waste products. D. Adding an acid to a solution causes p. H to go down. E. It is possible to make a solution with a p. H of -3. 0.

p. H Which one of the following statements about p. H and water is FALSE? A. In pure water at a p. H of 7. 0 the concentration of hydroxyl ions (OH-) is 0. 0000001 M. (1 x 10 -7 M) B. A solution with a p. H of 4. 0 has a proton concentration that is 10 times greater than a solution with a p. H of 5. 0. C. When bacteria ferment a sugar under anaerobic conditions they make acidic waste products. D. Adding an acid to a solution causes p. H to go down. E. It is possible to make a solution with a p. H of -3. 0. If the p. H was -3, then the concentration of free protons would be 1 x 103 molar. It’s impossible to make a 1000 M solution of anything!

A 1. 0 molar solution equals 1. 0 mole of substance dissolved per liter of solution. A 1. 0 molar solution of hydrofluoric acid (HF, f. w. = 20. 01 g/mole) is 20. 01 grams of HF per liter of solution. What is the concentration of water molecules in pure water?

1 mole of water has a mass of 18. 0 grams (H 2 O, f. w. = 18. 015 g/mole). 1 liter of water (at standard temperature and pressure) has a mass of 1000 grams/liter 18. 0 grams/mole = 55. 5 moles/liter