Chapter 2 Physical and Chemical Properties of Water and pH PDF

Chapter 2 Physical and Chemical Properties of Water and pH Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Bond strength...

Chapter 2 Physical and Chemical Properties of Water and pH Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Bond strength Covalent - Polar & Non-polar Ionic Hydrogen Van der Waals  Polarity (1 of 2) Electronegativity is the measure of an atom’s attraction for the electrons it shares in a chemical bond with another atom The more electronegative an atom is, the more strongly it attracts shared electrons Covalent bonds differ widely in degree of sharing of valence electrons, depending on the difference in electronegativity between the bonded atoms Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Polarity (2 of 2) In a nonpolar covalent bond, electrons are shared equally In a polar covalent bond, electrons are shared unequally The atom that attracts the electrons more strongly carries a partial negative charge, δ− The atom deprived of electrons carries a partial positive charge, δ+ Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. Greek alphabet  Polarity (2 of 2) In a nonpolar covalent bond, electrons are shared equally In a polar covalent bond, electrons are shared unequally The atom that attracts the electrons more strongly carries a partial negative charge, δ− The atom deprived of electrons carries a partial positive charge, δ+ The whole molecule is polar because one end is partially positive and the other end is partially negative The nett charge of the molecule remains zero Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Water, a Polar Molecule In water, an oxygen atom forms polar covalent bonds with two hydrogen atoms The electrons are attracted much more strongly to the oxygen nucleus than to the hydrogen nuclei The water molecule is asymmetric The oxygen atom is located on one side (δ−) and hydrogen atoms on the other (δ+), making the water molecule strongly polar The polar nature of water is the basis of its ability to adhere to ions and weaken their attractions Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Polarity in Water Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Symmetry Oxygen, nitrogen, and sulfur share electrons unequally with hydrogen; —OH, —NH, and —SH groups tend to be located asymmetrically in biological molecules and make these regions polar Carbon-hydrogen bonds tend to be arranged symmetrically (as in methane), so their partial charges cancel each other and the molecule as a whole is nonpolar Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. Polar Molecules Associate and Exclude Nonpolar (1 of 2) Polar molecules attract other polar molecules and charged ions and molecules, forming polar associations that tend to exclude nonpolar molecules Polar molecules that associate readily with water are hydrophilic (“water loving”) Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. Polar Molecules Associate and Exclude Nonpolar (2 of 2) Excluded nonpolar molecules tend to clump together in nonpolar associations which reduce the surface area exposed to the surrounding polar environment Nonpolar substances that are excluded by water and other polar molecules are hydrophobic (“water fearing”) Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Hydrogen Bonds Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Hydrogen Bonds (1 of 3) Hydrogen bonds are attractions between partially positive hydrogen atoms (sharing electrons unequally with oxygen, nitrogen, or sulfur) and partially negative atoms sharing in a different covalent bond Hydrogen bonds may be intramolecular (between atoms in the same molecule) or intermolecular (between atoms in different molecules) Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Hydrogen Bonds (2 of 3) Individual hydrogen bonds are weak, but when numerous, hydrogen bonds are collectively strong Hydrogen bonds stabilize the three-dimensional structure of large biological molecules such as proteins Hydrogen bonds between water molecules are responsible for many of the properties that make water important to life Hydrogen bonds begin to break extensively at temperatures above 45°C Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Hydrogen Bonds (3 of 3) Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. Hydrogen Bonds Visualised Non-contact atomic force microscopy (NC-AFM) Zhang et al. , Science, 2013 Stabilizing Effect of Hydrogen Bonds Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. Hydrogen Bonds and the Properties of Water  2.4 Hydrogen Bonds and the Properties of Water 1. Hydrogen bonds between water molecules produce a water lattice that affects properties of density, heat absorption, cohesion, and surface tension 2. Polarity of water molecules contributes to formation of distinct polar and nonpolar environments critical to cell organization 3. Water is a solvent for charged or polar molecules 4. Water molecules separate into hydrogen and hydroxyl ions Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  A Lattice of Hydrogen Bonds Liquid water forms a water lattice Each water molecule constantly breaks and reforms hydrogen bonds with its neighbors (average 3.4 bonds) An ice lattice is a rigid, crystalline structure Each water molecule forms four hydrogen bonds, which spaces the water molecules farther apart than the water lattice Ice is about 10% less dense than liquid water, an unusual property that makes ice float Water reaches its greatest density at a temperature of 4°C Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Hydrogen Bonds and Water Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. Density of Water ice lattice 1.0 liquid water Density (g/cm3) 0.9 0 4 100 Temperature (ºC) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  A Pond in Winter Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ice layer Protists provide food for fish. River otters visit ice-covered ponds. Aquatic insects survive in air pockets. Freshwater fish take oxygen from water. Common frogs and pond turtles hibernate. 23  Water and Temperature The hydrogen-bond lattice of liquid water retards the escape of individual water molecules as water is heated Water remains liquid in a wide temperature range (0°C to 100°C) A large amount of heat must be added to break enough hydrogen bonds to make water boil Water has a relatively high specific heat It can absorb or release relatively large quantities of heat energy without undergoing extreme changes in temperature Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Specific Heat and Calories Specific heat Amount of heat energy required to increase the temperature of a given quantity of water Measured in calories calorie (or small calorie) Heat energy required to raise 1 g of water by 1°C Calorie (with a capital C) A kilocalorie (kcal) or 1,000 calories Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Heat of Vaporization A large amount of heat (586 calories per gram) must be added to give water molecules enough energy of motion to break loose from liquid water and form a gas This required heat, known as heat of vaporization, allows humans and many other organisms to cool off when hot Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. Temperature and Water 800 Gas 600 540 Calories of Heat Energy / g calories 400 200 Liquid 80 Solid calories 0 freezing occurs evaporation occurs 0 20 40 60 80 100 120 Temperature (°C) a. Calories lost when 1 g of liquid water freezes and calories required when 1 g of liquid water evaporates. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Cohesion and Adhesion Cohesion Tendency of water molecules to “stick” to each other, due to the hydrogen-bond lattice Adhesion Tendency of water molecules to “stick” to the walls of tubes by forming hydrogen bonds with charged and polar groups in molecules that form the walls Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. Water as a Transport Medium - Adhesion & Cohesion  Surface Tension Water molecules at surfaces facing air can form hydrogen bonds with water molecules beside and below them, but not on the sides that face the air This unbalanced bonding places the surface water molecules under tension (surface tension) making them more resistant to separation than the underlying water molecules Surface tension causes water to form water droplets, and can support small insects and other objects Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. Surface Tension in Water (1 of 2) Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. Surface Tension in Water (2 of 2) iStock.com/Alasdair Thomson Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Polar and Nonpolar Environments The water lattice resists invasion by other molecules unless the molecule contains polar or charged regions that can form competing attractions with water molecules Nonpolar molecules are excluded from the water lattice, forcing them to form nonpolar associations that expose the least surface area to the surrounding water Distinct polar and nonpolar environments created by water are critical to the organization of cells Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Biological Membranes Biological membranes (which form boundaries around and inside cells) consist of lipid molecules with dual polarity – one end of each molecule is polar, the other end is nonpolar Membranes are surrounded on both sides by strongly polar water molecules Exclusion by water molecules forces lipid molecules with dual polarity to associate into a double layer (bilayer) in which only the polar ends are exposed to water Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Formation of a Membrane Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Water as a Solvent Because water molecules are small and strongly polar, they tend to coat the surfaces of other polar and charged molecules and ions, forming a hydration layer Hydration layers reduce attraction between molecules or ions and promote their entry into a solution Water (solvent) surrounds the dissolved substance (solute) and prevents the polar molecules or ions from reassociating (e.g., sodium and chloride) Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. Water Forming a Hydration Layer Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. Water as solvent  Chemical Reactions in Cells Involve Aqueous Solutions Concentration is the number of molecules or ions of a substance in a unit volume, such as milliliter (mL) or liter (L) The number of molecules can be calculated indirectly First, use the mass number (number of protons and neutrons) to calculate the weight of a single atom Example: Carbon, mass number 12 (6 protons + 6 neutrons) Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Chemical Reactions in Cells Involve Aqueous Solutions Concentration is the number of molecules or ions of a substance in a unit volume, such as milliliter (mL) or liter (L) Avogadro’s number - 6.022 X 1023 atoms or molecules per mole of a substance n = m/M Number of mol = mass/ molecular mass c1v1 = c2v2 Concentration in a volume = concentration in another volume  Concentration The number of moles of a substance dissolved in 1 L of solution is known as the molarity (M) of the solution Two solutions having the same volume and molarity contain the same number of molecules Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  2.5 Water Ionization and Acids, Bases, and Buffers Water dissociates into positively charged hydrogen ions (H+ or protons) and negative hydroxide ions (OH−) The reaction is reversible In pure water, concentrations of H+ and OH− are equal 1 x 10-7 moles/L of each 0.0000001 mol/L Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. Dissociation  Acids and Bases (1 of 2) Acids are proton donors that release H+ (and anions) when they are dissolved in water, increasing the H+ concentration Example: Hydrochloric acid (HCl) dissociates into H+ and Cl− when dissolved in water (HCl ↔ H+ + Cl− ) Bases are proton acceptors that reduce the H+ concentration of a solution; most release a hydroxide ion (OH−) and a cation Example: NaOH → Na+ + OH−; excess OH− combines with H+ to produce water (OH− + H+ → H2O) Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Acids and Bases (2 of 2) Some bases do not dissociate to produce hydroxide ions directly Example: Ammonia directly accepts a proton from water to produce an ammonium ion and releasing a hydroxide ion: + Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  pH (1 of 4) The concentration of H+ ions in a water solution compared with the concentration of OH− ions determines the acidity/alkalinity of the solution The concentration of H+ ions in a water solution compared with the concentration of OH− ions determines the pH of the solution Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  pH (2 of 4) pH is a measure of hydrogen ion concentration in a solution (potential of Hydrogen) Acidity pH is measured using the pH scale – ranging from 0 to 14 – based on logarithms of the number of H+ ions in solution Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  pH (3 of 4) The pH of pure water is 7 or neutral (neither acidic nor basic) – the concentration of both H+ and OH− ions is 1 X 10−7M Acidic solutions have pH values less than 7, with pH 0 being the value for 1M hydrochloric acid (HCl) Basic (alkaline) solutions have pH values greater than 7, with pH 14 being the value for 1M sodium hydroxide (NaOH) Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  pH (4 of 4) Each whole number on the pH scale represents a value 10 times greater or less than the next number pH 4 10x more acidic (10x more [H+]) than pH 5 PH 11 1000x more basic (1000x less [H+]) than pH 8 pH is important to cells because even small changes (0.1 or even 0.01 pH unit) can drastically affect biological reactions All living organisms have homeostatic systems that control internal pH by regulating H+ concentration near the neutral value of pH 7 Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. pH scale  pH and the Environment Pollution combines with atmospheric water to produce acid precipitation with a pH as low as 3 Acid precipitation can sicken and kill wildlife such as fishes and birds, as well as plants and trees Acid precipitation can contribute to human respiratory diseases such as bronchitis and asthma Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Buffers Buffers are substances that compensate for pH changes by absorbing or releasing H+ When H+ ions are released in excess, buffers combine with them and remove them from the solution If the concentration of H+ decreases greatly, buffers release additional H+ Most buffers are weak acids or bases that dissociate reversibly in water solutions to release or absorb H+ or OH− Buffering capacity Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Carbonic Acid – Bicarbonate Buffer System (1 of 3) A carbonic acid–bicarbonate buffer system buffers blood pH In a reversible reaction, carbonic acid (H2CO3), a weak acid, dissociates into bicarbonate ions (HCO3-) and H+: Each buffer has a specific range of greatest buffering capacity – interestingly, normal blood pH (7.4) is outside the region of greatest buffering capacity for this buffer system Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.  Carbonic Acid – Bicarbonate Buffer System (2 of 3) H2CO3  H+ + HCO3- Carbonic acid Bicarbonate Under acidic conditions H+ + HCO3-  H2CO3 Under alkaline conditions OH- + H2CO3  HCO3- + H2O Russell, Biology: The Dynamic Science, 5th edition. © 2021 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. Carbonic Acid – Bicarbonate Buffer System (3 of 3)  Acidification: A Threat to Water Quality & Life Human activities such as burning fossil fuels threaten water quality CO2 is the main product of fossil fuel combustion About 33% of human-generated CO2 is absorbed by the oceans CO2 dissolved in sea water forms carbonic acid; this process is called ocean acidification  Ocean acidification As seawater acidifies, H+ ions combine with carbonate ions (CO32-) to produce bicarbonate (HCO3-) Carbonate is required for calcification (production of calcium carbonate) by many marine organisms, including reef-building corals  CO2 CO2 + H2O H2CO3 H2CO3 H+ + HCO3 H+ + CO32 HCO3 CO32 + Ca2+ CaCO3 (a) (b) (c)  The burning of fossil fuels is also a major source of sulfur oxides and nitrogen oxides These compounds react with water in the air to form strong acids that fall in rain or snow Acid precipitation is rain, fog, or snow with a pH lower than 5.2 Acid precipitation damages life in lakes and streams and changes soil chemistry on land None of this is conducive to life  Hydrogen Bonds and Life Hydrogen bonds at core of life-sustaining properties of water Density Specific heat & high heat of vaporisation Cohesion and Adhesion Surface tension Solvent for polar/ionic substances Dissociation - pH