Water and Polarity Chapter 2

What is electronegativity? The tendency of atoms to attract electrons to itself in a chemical bond – electronegativity A measure of the force of an atom’s attraction for electrons it shares in a chemical bond with another atom – electronegativity C-H bonds in CH4 form non polar bonds – electronegativity difference is small.

Electronegativity Oxygen and Nitrogen - more electronegative than Carbon and Hydrogen Fluorine is most electronegative (4)

Formation of nonpolar bonds Atoms of same element share electrons equally in a bond – equal electronegativity When atoms with same electronegativity form a bond – there is equal sharing of electrons - nonpolar bonds C-H bond in CH4

Formation of polar bonds Atoms of different elements may not share electrons equally – unequal electronegativity When atoms with different electronegativity form a bond – there is unequal sharing of electrons - polar bonds Water - of polar bond

What is polarity? Oxygen is an "electronegative" or electron "loving" atom. Water is a "polar" molecule - there is an uneven distribution of electron density giving rise to partial positive charge ∂+ and ∂- Hydrogen bonds (dipole-dipole bond) are weak attractions between the partially negative oxygen of one water molecule and the partially positive hydrogen of a different water molecule (Polarity) Compared to hydrogen, oxygen is more electronegative. The structure of water is the basis for its unique properties. The most important property of water is the ability to form hydrogen bonds Water is composed of one oxygen atom and two hydrogen atoms. Each hydrogen atom is covalently bonded to the oxygen via a shared pair of electrons. Oxygen has two unshared pairs of electrons. Thus there are 4 pairs of electrons surrounding the oxygen atom, two pairs involved in covalent bonds with hydrogen, and two unshared pairs on the opposite side of the oxygen atom. Oxygen is an "electronegative" or electron "loving" atom compared with hydrogen. Water is a "polar" molecule, meaning that there is an uneven distribution of electron density. Water has a partial negative charge () near the oxygen atom due the unshared pairs of electrons, and partial positive charges () near the hydrogen atoms.

What are dipoles? What is a polar bond: • Electrons are unequally shared, more negative charge found closer to one atom. • Due to difference in electronegativity of atoms involved in bond.

Polar Bonds & Molecules CO2 have polar bonds Geometry - nonpolar molecules CO2 have a zero dipole moments

Solvent Properties of H2O Hydrophilic: water-loving tend to dissolve in water Hydrophobic: water-fearing tend not to dissolve in water Amphipathic: has characteristics of both properties molecules that contain one or more hydrophobic and one or more hydrophilic regions, e.g., sodium palmitate

Why do some chemicals dissolve in water? Hydrophillic (water loving/polar) Polar covalent compounds (eg: alcohols - ethanol and ketones – acetone Sugars Ionic compounds-KCl Amino acids, phosphate esters Hydrophobic (water fearing/nonpolar) Nonpolar covalent compounds (eg: hydrocarbons hexane) Fatty acids Cholesterol Small organic molecules contain electronegative atoms as oxygen and nitrogen – interaction between dipoles of these molecules and water dipoles makes them to dissolve. For ionic compounds there is interaction between ion and dipole that leads to solubility of ionic compounds. Covalent compounds – dipole-dipole interaction The interactions between nonpolar molecules and water molecules are weaker than dipolar interactions. Hydrocarbons tend to sequester themselves from an aqueous environment . Nonpolar solids leaves undissolved material in water.

Ion-dipole and Dipole-dipole Interactions Electrostatic attraction of unlike charges - ion-dipole interaction: e.g., KCl dissolved in H2O dipole-dipole interactions: e.g., ethanol or acetone dissolved in H2O dipole induced-dipole interactions: weak and generally do not lead to solubility in water

Ion-dipole and Dipole-dipole Interactions • Ion-dipole and dipole-dipole interactions help ionic and polar compounds dissolve in water

Amphipathic molecules • Polar (carboxylic acid group ) and nonpolar (hydrocarbon group)

Why do oil and water mixed together separate into layers? Ionized polar groups are in contact with aqueous environment and nonpolar tails are sequestered from water -Micelles formation Oils and salad dressing form oil/sphere droplets Van der Waals interactions Polar group has two oxygen, carbon and hydrogen. Nonpolar group has carbon and hydrogen Van der Waals interactions between nonpolar molecules lead to temporary dipole moments and allows for such spontaneous molecular arrangement

Why does water have such interesting and unique properties? Tetrahedral arrangement Two pairs of partial positive charge and two of partial negative charge Each water molecule can be involved in 4 hydrogen bonds: 2 as donor, and 2 as acceptor

Strength of hydrogen Bonding Hydrogen bonds are weaker than covalent bonds

Why does water have such interesting and unique properties? High melting and boiling point Solid water is less dense than liquid water. Aquatic organisms survive in cold climates - density difference Good solvent – polar solutes can serve as donor or an acceptor of hydrogen bonds The water molecules are attracted to each other because of the hydrogen bonding – number and strength of hydrogen bonding is high. This strength should be overcome to melt ice or boil water. Amount of heat that must be absorbed or lost by 1 gm of substance to change its temperature by 1 degree C. Water has high specific heat therefore it takes time to heat up and stays hot for some time before cooling down. Hence large amount of energy is required to change the temperature of water. Less dense because hydrogen bonds in ice space water molecules apart therefore icebergs float. Bodies of water freeze from the top down and fish survive below the freezing levels in water bodies

Other Biologically Important Hydrogen bonds • Hydrogen bonding is important in stabilization of 3-D structures of biological molecules such as: DNA, RNA, proteins.

Acids and Bases Acids are proton (hydrogen ion) donors and bases are proton acceptors. When an acid is dissolved in water, its strength is measured by the amount of hydrogen ions released by acid – Acid dissociation constant (Ka) Higher the Ka, stronger the acid and more the hydrogen ions released H2O = H+ + OH-

What is pH? H2O  OH-1 + H+1 hydroxide ion hydrogen ion Kw= (H+1 ) (OH-1 ) = (10-7) (10-7) = 10-14 pH – negative logarithm of hydrogen ion concentration, pH = -log10 (H+) pH- measure of acidity of solution pH change of one unit – tenfold change in hydrogen ion concentration

Why do we want to know the pH? Various biological reactions require a vast range of pH pH should be controlled to make an experiment function properly Subcellular organelles undergo variations in pH – maintain neutrality to stay alive Naturally occurring pH buffers – phosphate and carbonate buffers maintain pH 7.0

Titration Curves a monoprotic acid releases one H+ per mole Titration: an experiment in which measured amounts of acid (or base) are added to measured amounts of base (or acid) Equivalence point: the point in an acid-base titration at which enough acid has been added to exactly neutralize the base (or vice versa) a monoprotic acid releases one H+ per mole a diprotic acid releases two H+ per mole a triprotic acid releases three H+ per mole

Buffers Buffer: a solution whose pH resists change upon addition of either more acid or more base consists of a weak acid and its conjugate base Examples of acid-base buffers are solutions containing CH3COOH and CH3COONa H2CO3 and NaHCO3 NaH2PO4 and Na2HPO4

How do buffers work? Based on nature of weak acids and their conjugate bases – buffers work Addition of extra hydrogen ion – reacts with conjugate base to form weak acid Addition of hydroxide ion – reacts with weak acid to form water and conjugate base Addition of either H+ or OH- buffers solution-maintains pH

Naturally Occurring Buffers H2PO4-/HPO42- is the principal buffer in cells H2CO3/HCO3- is an important (but not the only) buffer in blood

How do we make buffers in the laboratory? Add weak acid form or weak base form of the buffer to a container + add water + measure pH Should be low or high Add strong acid or strong base till the desired buffer pH is obtained Bring the solution up to final volume – correct concentration Based upon experiment - choose pH

Henderson-Hasselbalch Henderson-Hasselbalch equation When the concentrations of weak acid and its conjugate base are equal, the pH of the solution equals the pKa of the weak acid when pH < pKa, the weak acid predominates when pH > pKa, the conjugate base predominates

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