BCHM 307 1st Edition Lecture 2 Outline of Last Lecture • Basic Chemistry• Definition of Valence Electrons• Water and its Properties• Definition of Electronegativity• Chemical Bonds• Dipole Movements• Hydrogen Bonds• Covalent Bonds• Functional Groups Outline of Current Lecture • Oxidation levels• Definition of Oxidation • Hydrophilic and Hydrophobic Molecules• Definition of Hydrophilic• Definition of Hydrophobic• Definition of Hydrophobic Interactions• The Hydrophobic Effect• Amphiphilic Molecules in Water• The Equilibrium Constant and pH• Acid Dissociation Contant and pKa• Definition of Conjugate Base• BuffersCurrent LectureOxidation of a substance refers to the loss of electrons. The relative oxidation levels of substances changes based on what bonds are attached to the central carbon molecule. The central carbon atom becomes more oxidized with the addition of a single bond with oxygen. The addition of a double bond with oxygen to the central carbon atom creates an even greater oxidation level. This is due to the greater electronegativityof oxygen as compared with carbon. The oxygen tends to pull the electrons closer to itself, which creates the greater oxidation state. The most oxidized form of carbon is carbon dioxide, O=C=O. Hydrophilic molecules are those that love water. Example of hydrophilic molecules include amino acids, salts, and sugars. They are polar molecules that can formhydrogen bonds with water. Hydrophobic molecules are water fearing molecules. They are nonpolar molecules that can’t form hydrogen bonds due to a lack of certain functional groups. Example of hydrophobic molecules include fats, oils, and alkanes. Hydrophobic molecules can interact with each through hydrophobic interactions. Hydrophobic molecules are not soluble in water, and are prevented from entering the aqueous phase. This segregation is known as the hydrophobic effect. Waterwill surround the hydrophobic molecule and form a “cage” around it. It takes a great deal of energy for this caged molecule to interact with another caged molecule. Instead, to save energy, the hydrophobic molecule will interact with another hydrophobic molecule and water will form one giant cage around the two. Amphiphilic molecules are those that have both hydrophobic and hydrophilic regions to them. One example of such molecules are fatty acids. A fatty acid has a polar “head” region and a nonpolar “tail”. Amphiphilic molecules will segregate themselves and form a giant ball, when in water. The head region will face outwards towards the region, while the tails will stay facing within. Water will form and break hydrogen bonds with itself, making it a non-static solvent. This concept is represented by the reaction: H2O ⇋ H3O+ + OH-. The equilibrium constant for this equation is represented by the equation Keq = [H+] [OH-] / [H2O]. The brackets represent concentration, or moles/liter, with a unit of M. Pure water will alwayshave a Keq of 1.8 x 10-16 M at STP. When water dissociates, it yields an equal concentration of H+ and OH-, at 1.0 X 10-7 M.Due to the difficulty of working with exponents, the pH system was developed. pH is the –log[H+]. A low pH is indicative of a high proton concentration and vice-versa. The pH scale runs from 0 to 14, where 0-6 is acidic, 7 is neutral, and 8-14 is basic. Each step up the pH scale is a 10 fold increase and each step down is a 10 fold decrease. The pH of a substance will get smaller with the addition of an acid. The pH of a substance willget larger when a base is added. The acid dissociation constant, or Ka , is equal to [H+] [A-] / [HA]. HA is the acid, H+ is the proton, and A- is the weak acid formed by the dissociation. Every acid will have a complimentary conjugate base when it dissociates. The conjugate base is formed when a proton is removed from the acid. The easier way to talk about Ka is in terms of pKa. The pKa = -log Ka. These tell you the strength of a given acid. Ka and pKa are inverses of each other. Therefore, a large Ka and a small pKa both indicate a strong acid. A small Ka and large pKa both indicate a strong base. Examples of weak acids are those common in biological systems. Buffers help a system maintain a constant pH. They consist of acid-conjugatebase pairs in order to do this. Buffers help to either bind or release protons. This is important for metabolic reactions in biology. Many biological reactions would normally alter the pH if buffers were not
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