Water Chapter 3

Hydrogen Bonding Water molecules are polar They have charges Like Magnets Charged molecules are attracted to each other Opposites attract! Hydrogen bond: the attraction between 2 water molecules. Positive H to Negative O

Polarity of water molecules In a water molecule, two hydrogen atoms form single polar covalent bonds with an oxygen atom. Because oxygen is more electronegative than hydrogen, the region around the oxygen atom has a partial negative charge. The regions near the two hydrogen atoms have a partial positive charge. Water has a variety of unusual properties because of the attraction between polar water molecules.

Emergent properties of water Cohesion and Adhesion Surface tension Heat Capacity Moderation of temperature Insulation of bodies of water Lower density of ice compared to liquid water Water’s ability to dissolve other substances

Cohesion Cohesion in Space Hydrogen bonds hold water together, a phenomenon called cohesion Adhesion: an attraction between molecules of different substances. Cohesion among water molecules plays a key role in the transport of water and dissolved nutrients against gravity in plants. Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion. Water has a greater surface tension than most other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface. Water behaves as if covered by an invisible film. Cohesion in Space

Temperature moderation Water stabilizes air temperatures by absorbing heat from warmer air and releasing heat to cooler air. Water can absorb or release relatively large amounts of heat with only a slight change in its own temperature. Water stabilizes temperature because it has a high specific heat. The specific heat of a substance is the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1°C. By definition, the specific heat of water is 1 cal per gram per degree Celsius or 1 cal/g/°C.

High specific heat of water Water’s high specific heat is due to hydrogen bonding. Heat must be absorbed to break hydrogen bonds, and heat is released when hydrogen bonds form. Investment of one calorie of heat causes relatively little change to the temperature of water because much of the energy is used to disrupt hydrogen bonds, not speed up the movement of water molecules.

Water’s high heat of vaporization Heat of vaporization is the quantity of heat that a liquid must absorb for 1 g of it to be converted from liquid to gas. Hydrogen bonds must be broken before a water molecule can evaporate from the liquid. Water’s high heat of vaporization moderates climate

Evaporative cooling As a liquid evaporates, the surface of the liquid that remains behind cools, a phenomenon called evaporative cooling. This occurs because the most energetic molecules are the most likely to evaporate, leaving the lower–kinetic energy molecules behind. Evaporation of sweat in mammals or evaporation of water from the leaves of plants prevents terrestrial organisms from overheating. Evaporative cooling moderates temperature in lakes and ponds.

Insulation Water is unusual because it is less dense as a solid than as a cold liquid. At 0°C, water becomes locked into a crystalline lattice, with each water molecule bonded to a maximum of four partners. ice floats on the cool water below. Water Expanding as it Freezes

Water is a versatile solvent Water is an effective solvent because it readily forms hydrogen bonds with charged and polar covalent molecules. Any substance that has an affinity for water is hydrophilic (water-loving). These substances are dominated by ionic or polar bonds. Substances that have no affinity for water are hydrophobic These substances are nonionic and have nonpolar covalent bonds.

Polar Covalent Bonding Electrons unequally shared between atoms Atom that pulls on the e- more strongly  slightly negative Atom that has a weaker pull on the e-  slightly positive Uneven charge across the molecule  polar Water is polar

Non-Polar Covalent Bonding Electrons equally shared between atoms No charge across the molecule  non- polar Oils are non-polar

Like Dissolves Like Polar substances dissolve in other polar substances Ionic Compounds are polar Example: Salt in Water Non-polar substances dissolve in other non-polar substances Example: Oil based paint and paint thinner Polar and non-polar won’t mix Example: Oil and Water

Like Dissolves Like Solvent Polarity Solute Ethyl acetate Non-polar Iodine Water Polar Copper (II) sulfate Chloroform

Acids and Bases

Properties of Acids Produce H+ ions in water Taste sour Corrode metals React with bases to form salts and water Change pH paper red

Properties of Bases Produce OH- ions in water Taste bitter, chalky Are electrolytes Feel soapy, slippery React with acids to form salts and water Changes pH paper blue

Learning Check Describe the solution in each of the following as: 1) acid 2) base or 3)neutral. A. ___soda B. ___soap C. ___coffee D. ___ wine E. ___ water F. ___ grapefruit

Solution Describe each solution as: 1) acid 2) base or 3) neutral. A. _1_ soda B. _2_ soap C. _1_ coffee D. _1_ wine E. _3_ water F. _1_ grapefruit

Arrhenius Acids & Bases Acids produce H+ in aqueous solutions HCl  H+(aq) + Cl- (aq) Bases produce OH- in aqueous solutions NaOH  Na+(aq) + OH- (aq)

Bronsted-Lowry Acids Acids are hydrogen ion (H+) donors Bases are hydrogen ion (H+) acceptors HCl + H2O  H3O+ + Cl- donor acceptor + - + +

Strength of Acids determined by the concentration of [H+1] ion in the solution greater the [H+1] is, stronger the acid is Strong acids ionize completely, (100%) in aqueous solution strong electrolytes Strong Acids HCl, HNO3 , H2SO4 Most other acids are weak.

Strength of Bases Strong Bases: the bases that ionize 100% (completely) in aqueous solution Strong Bases NaOH, KOH, and Ca(OH)2 Most other bases are weak

pH Scale We use this scale to measure the strength of an acid or base. From the French pouvoir hydrogene “hydrogen power” or power of hydrogen pH can use the concentration of hydronium ions or hydrogen ions.

7 Acid Base pH Scale 14

pH of Common Substances

pH of Common Substance 14 1 x 10-14 1 x 10-0 0 13 1 x 10-13 1 x 10-1 1 pH [H1+] [OH1-] pOH 14 1 x 10-14 1 x 10-0 0 13 1 x 10-13 1 x 10-1 1 12 1 x 10-12 1 x 10-2 2 11 1 x 10-11 1 x 10-3 3 10 1 x 10-10 1 x 10-4 4 9 1 x 10-9 1 x 10-5 5 8 1 x 10-8 1 x 10-6 6 6 1 x 10-6 1 x 10-8 8 5 1 x 10-5 1 x 10-9 9 4 1 x 10-4 1 x 10-10 10 3 1 x 10-3 1 x 10-11 11 2 1 x 10-2 1 x 10-12 12 1 1 x 10-1 1 x 10-13 13 0 1 x 100 1 x 10-14 14 NaOH, 0.1 M Household bleach Household ammonia Lime water Milk of magnesia Borax Baking soda Egg white, seawater Human blood, tears Milk Saliva Rain Black coffee Banana Tomatoes Wine Cola, vinegar Lemon juice Gastric juice More basic 7 1 x 10-7 1 x 10-7 7 More acidic

Acid – Base Concentrations 10-1 pH = 3 pH = 11 H+ OH- pH = 7 10-7 concentration (moles/L) H+ OH- OH- H+ 10-14 [H+] > [OH-] [H+] = [OH-] [H+] < [OH-] acidic solution neutral solution basic solution Timberlake, Chemistry 7th Edition, page 332

pH pH = -log [H+]

Neutralization Reactions When acid and bases with equal amounts of hydrogen ion H+ and hydroxide ions OH- are mixed, the resulting solution is neutral. NaOH (aq) + HCl(aq)  NaCl + H2O base acid salt water Ca(OH)2 + 2 HCl  CaCl2 + 2H2O base acid salt water

Neutralization H+ and OH- combine to produce water H+ + OH-  H2O from acid from base neutral

Dissociation of water molecules Occasionally, a hydrogen atom participating in a hydrogen bond between two water molecules shifts from one molecule to the other. The hydrogen atom leaves its electron behind and is transferred as a single proton—a hydrogen ion (H+). The water molecule that lost the proton is now a hydroxide ion (OH−). The water molecule with the extra proton is now a hydronium ion (H3O+). Hydrogen and hydroxide molecules are highly reactive H2O  H+ + OH-

Acid/Base Summary

Acid rain Unpolluted rain has a pH of 5.6 Rain with a pH below 5.6 is “acid rain“ CO2 in the air forms carbonic acid CO2 + H2O  H2CO3 Adds to H+ of rain H2CO3  H+ (aq) + HCO3-(aq)

Sources of acid rain Power stations Oil refineries Coal with high S content Car and truck emissions

Formation of acid rain Reactions with oxygen in air form SO3 2SO2 + O2  2 SO3 Reactions with water in air form acids SO3 + H2O  H2SO4 sulfuric acid NO + H2O  HNO2 nitrous acid HNO2 + H2O  HNO3 nitric acid

Effects of Acid rain Leaches Al from soil, which kills fish Fish kills in spring from runoff due to accumulation of large amounts of acid in snow Dissolves waxy coatings that protect leaves from bacteria Corrodes metals, textiles, paper and leather