Water Chemistry All living organisms are dependent on water. The structure of water is the basis for its unique properties. The most important property of water is the ability to form hydrogen bonds.

Hydrogen bonding

Water Chemistry Within a water molecule, the bonds between oxygen and hydrogen are highly polar. Partial electrical charges develop: - oxygen is partially negative - hydrogen is partially positive

Water Chemistry Hydrogen bonds are weak attractions between the partially negative oxygen of one water molecule and the partially positive hydrogen of a different water molecule. Hydrogen bonds can form between water molecules or between water and another charged molecule.

Water Chemistry

Water Chemistry The polarity of water causes it to be cohesive and adhesive. cohesion: water molecules stick to other water molecules by hydrogen bonding adhesion: water molecules stick to other polar molecules by hydrogen bonding

Water Chemistry

Surface tension

Properties of Water 1. Water has a high specific heat. - A large amount of energy is required to change the temperature of water. 2. Water has a high heat of vaporization. - The evaporation of water from a surface causes cooling of that surface.

Properties of Water 3. Solid water is less dense than liquid water. - Bodies of water freeze from the top down. 4. Water is a good solvent. - Water dissolves polar molecules and ions.

Properties of Water

Properties of Water 5. Water organizes nonpolar molecules. - hydrophilic: “water-loving” -hydrophobic: “water-fearing” - Water causes hydrophobic molecules to aggregate or assume specific shapes. 6. Water can form ions. H2O  OH-1 + H+1 hydroxide ion hydrogen ion

Ice, water, vapor

Hydrogen bonding (electric attraction) Ice Liquid

“Dissociation” of water

pH [H+]= 10-1M pH = – log [H+] Log scale means 10X change per unit!

Buffers Buffers resist changes to the pH of a solution when H+ or OH- is added to the solution. Buffers accept hydrogen ions from the solution when they are in excess and donate hydrogen ions when they have been depleted.

Overall lessons: Many properties of water are emergent properties due to hydrogen bonding. The cohesion of water molecules to each other is exploited by plants and animals. Water resists temperature changes by absorbing lots of heat. Lower density of ice causes it to float & insulate the water below. The polarity of water allows it to dissolve other polar molecules. Non-polar compounds are hydrophobic and not easily dissolved in water. A mole of a compound has a constant # of molecules. Adding or removing hydrogen ions changes the pH of a solution. Buffers resist pH changes by accepting or donating H ions when [H+] changes.

Quiz 1. How would you make a 0. 1M solution of glucose Quiz 1.    How would you make a 0.1M solution of glucose? (I want to know the # of grams of glucose used)   What accounts for the movement of water up xylem vessels in a plant? cohesion hydrogen bonding adhesion hydrophilic vessel walls all of these   The following are pH values: cola-2; orange juice-2; beer-4; coffee-5; human blood-7.4. Which of these liquids has the highest molar [OH-] cola orange juice beer coffee human blood   Which of the following is a molecule capable of forming hydrogen bonds and why? CH4 H2O NaCl H2 A, b, and d can form hydrogen bonds.   Taking into account the bonding capacities or valences of C, O, how many hydrogen must be added to complete the structural diagram of the molecule on the board.   A radioactive isotope of carbon has the mass number 14. How many neutrons does this isotope have?   Explain evaporative cooling to me.