Lecture 3 & 4: Chemistry of Water - Water's Properties

Lecture 3&4: The chemistry of water  The Molecule That Supports All of Life Water makes life possible on Earth Water is the only common substance to exist in the natural environment in all three physical states of matter Water’s unique emergent properties help make Earth suitable for...

Lecture 3&4: The chemistry of water  The Molecule That Supports All of Life Water makes life possible on Earth Water is the only common substance to exist in the natural environment in all three physical states of matter Water’s unique emergent properties help make Earth suitable for life The structure of the water molecule allows it to interact with other molecules © 2018 Pearson Education Ltd.  Polar covalent bonds in water molecules result in hydrogen bonding In the water molecule, the electrons of the polar covalent bonds spend more time near the oxygen than the hydrogen The water molecule is thus a polar molecule: The overall charge is unevenly distributed Polarity allows water molecules to form hydrogen bonds with each other © 2018 Pearson Education Ltd.  δ+ δ+ Polar covalent bond δ– δ– Region of partial Hydrogen bond δ+ negative charge δ– δ+ δ– δ+ δ– δ+  Four emergent properties of water contribute to Earth’s suitability for life Four of water’s properties that facilitate an environment for life are – 1. Cohesive behavior – 2. Ability to moderate temperature – 3. Expansion upon freezing – 4. Versatility as a solvent © 2018 Pearson Education Ltd.  Cohesion of Water Molecules Collectively, hydrogen bonds hold water molecules together, a phenomenon called cohesion Cohesion helps the transport of water against gravity in plants Adhesion is an attraction between different substances, for example, between water and plant cell walls © 2018 Pearson Education Ltd.  Evaporation pulls water upward. H2O Adhesion Two types of water-conducting cells Direction Cohesion of water 300 µm movement H2O H2O  Surface tension is a measure of how difficult it is to break the surface of a liquid Water has an unusually high surface tension due to hydrogen bonding between the molecules at the air-water interface and to the water below © 2018 Pearson Education Ltd.  2. Moderation of Temperature by Water Water absorbs heat from warmer air and releases stored heat to cooler air Water can absorb or release a large amount of heat with only a slight change in its own temperature © 2018 Pearson Education Ltd.  Temperature and Heat Kinetic energy is the energy of motion The kinetic energy associated with random motion of atoms or molecules is called thermal energy Temperature represents the average kinetic energy of the molecules in a body of matter Thermal energy in transfer from one body of matter to another is defined as heat © 2018 Pearson Education Ltd.  A calorie (cal) is the amount of heat required to raise the temperature of 1 g of water by 1ºC It is also the amount of heat released when 1 g of water cools by 1ºC The “Calories” on food packages are actually kilocalories (kcal); 1 kcal = 1,000 cal The joule (J) is another unit of energy; 1 J = 0.239 cal, or 1 cal = 4.184 J © 2018 Pearson Education Ltd.  Water’s 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 The specific heat of water is 1 cal/(g◦C ) Water resists changing its temperature because of its high specific heat Water has high specific heat capacity than any other liquid on earth https://www.engineeringtoolbox.com/specific-heat- fluids-d_151.html © 2018 Pearson Education Ltd.  Water’s high specific heat can be traced to hydrogen bonding – Heat is absorbed when hydrogen bonds break – Heat is released when hydrogen bonds form The high specific heat of water minimizes temperature fluctuations to within limits that permit life © 2018 Pearson Education Ltd.  Burbank San Bernardino Santa Barbara 23ºC 32ºC 38ºC Los Angeles Riverside 36ºC (Airport) 24ºC Santa Ana Palm Springs 20-24 (ºC) 29ºC 41ºC 25-29 Pacific Ocean 20ºC 30-34 >35 San Diego 22ºC 64 km What can you say about the temp of the cities?  Evaporative Cooling Evaporation (or vaporization) is transformation of a substance from liquid to gas Heat of vaporization is the heat a liquid must absorb for 1 g to be converted to gas As a liquid evaporates, its remaining surface cools, a process called evaporative cooling Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water © 2018 Pearson Education Ltd.  Floating of Ice on Liquid Water Ice floats in liquid water because hydrogen bonds in ice are more “ordered,” making ice less dense than water Water reaches its greatest density at 4ºC If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth Surface ice: easier to thaw, insulates deeper water  Hydrogen bond Liquid water: Hydrogen bonds break and re-form Ice: Hydrogen bonds are stable  Many scientists are worried that global warming is having a profound effect on icy environments around the globe The rate at which glaciers and Arctic sea ice are disappearing poses an extreme challenge to animals that depend on ice for their survival © 2018 Pearson Education Ltd.  Water: The Solvent of Life A solution is a liquid that is a completely homogeneous mixture of substances The solvent is the dissolving agent of a solution The solute is the substance that is dissolved An aqueous solution is one in which water is the solvent © 2018 Pearson Education Ltd.  Na+ – + + – – + – – Na+ – + + CI– CI– + – – + – + – –  Water is a versatile solvent due to its polarity When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell © 2018 Pearson Education Ltd.  Water can also dissolve compounds made of nonionic polar molecules Even large polar molecules such as proteins can dissolve in water if they have ionic and polar regions © 2018 Pearson Education Ltd. Figure 3.9 δ+ δ– δ– δ+  Hydrophilic and Hydrophobic Substances A hydrophilic substance is one that has an affinity for water A hydrophobic substance is one that does not have an affinity for water Oil molecules are hydrophobic because they have relatively nonpolar bonds Hydrophobic molecules related to oils are the major ingredients of cell membranes © 2018 Pearson Education Ltd.  Solute Concentration in Aqueous Solutions Most chemical reactions in organisms involve solutes dissolved in water When carrying out experiments, we use mass to calculate the number of solute molecules in an aqueous solution © 2018 Pearson Education Ltd.  Molecular mass is the sum of all masses of all atoms in a molecule Numbers of molecules are usually measured in moles, where 1 mole (mol) = 6.02  1023 molecules Avogadro’s number and the unit dalton were defined such that 6.02  1023 daltons = 1 g Molarity (M) is the number of moles of solute per liter of solution © 2018 Pearson Education Ltd.  Possible Evolution of Life on Other Planets Biologists seeking life on other planets have concentrated their search on planets that might have water More than 800 planets have been found outside our solar system; there is evidence that a few of them have water vapor In our solar system, Mars has been found to have water © 2018 Pearson Education Ltd. Dark streaks  Acidic and basic conditions affect living organisms A hydrogen atom in a hydrogen bond between two water molecules can shift from one to the other – The hydrogen atom leaves its electron behind and is transferred as a proton, or hydrogen ion (H+) – The molecule that lost the proton is now a hydroxide ion (OH–) – The molecule with the extra proton is now a hydronium ion (H3O+), though it is often represented as H+ © 2018 Pearson Education Ltd.  Water is in a state of dynamic equilibrium in which water molecules dissociate at the same rate at which they are being reformed © 2018 Pearson Education Ltd.  + – H H O H O O H O H H H H 2 H2O Hydronium Hydroxide ion (H3O+) ion (OH–)  Though statistically rare, the dissociation of water molecules has a great effect on organisms Changes in concentrations of H+ and OH– can drastically affect the chemistry of a cell © 2018 Pearson Education Ltd.  Concentrations of H+ and OH– are equal in pure water Adding certain solutes, called acids and bases, modifies the concentrations of H+ and OH– Biologists use the pH scale to describe whether a solution is acidic or basic (the opposite of acidic) © 2018 Pearson Education Ltd.  Acids and Bases An acid is a substance that increases the H+ concentration of a solution A base is a substance that reduces the H+ concentration of a solution Strong acids and bases dissociate completely in water Weak acids and bases reversibly release and accept back hydrogen ions, but can still shift the balance of H+ and OH– away from neutrality © 2018 Pearson Education Ltd.  The pH Scale In any aqueous solution at 25ºC, the product of H+ and OH– is constant and can be written as [H+][OH–] = 10–14 The pH of a solution is defined by the negative logarithm of H+ concentration, written as pH = –log [H+] For a neutral aqueous solution, [H+] is 10–7, so pH = –(–7) = 7 © 2018 Pearson Education Ltd.  Acidic solutions have pH values less than 7 Basic solutions have pH values greater than 7 Most biological fluids have pH values in the range of 6 to 8 © 2018 Pearson Education Ltd.  pH Scale 0 1 Battery acid Increasingly Acidic 2 Gastric juice (in stomach), lemon juice [H+] > [OH–] 3 Vinegar, wine, cola Acidic 4 Tomato juice solution Beer 5 Black coffee Rainwater 6 Urine Saliva Neutral 7 Pure water [H+] = [OH–] Human blood, tears 8 Seawater Neutral Inside small intestine solution Increasingly Basic 9 [H+] < [OH–] 10 Milk of magnesia 11 Household ammonia 12 Basic solution Household 13 bleach Oven cleaner 14  Buffers The internal pH of most living cells is close to 7 Buffers are substances that minimize changes in concentrations of H+ and OH– in a solution Most buffer solutions contain a weak acid and its corresponding base, which combine reversibly with H+ ions E.g. Bicarbonate buffer system: © 2018 Pearson Education Ltd.  Acidification: A Threat to Our Oceans Human activities such as burning fossil fuels threaten water quality CO2 is the main product of fossil fuel combustion About 25% of human-generated CO2 is absorbed by the oceans CO2 dissolved in seawater forms carbonic acid; this process is called ocean acidification © 2018 Pearson Education Ltd. Figure 3.12 CO2 CO2 + H2O H2CO3 H2CO3 H+ + HCO3– H+ + CO32– HCO3– CO32– + Ca2+ CaCO3  As seawater acidifies, H+ ions combine with carbonate ions to produce bicarbonate Carbonate is required for calcification (production of calcium carbonate) by many marine organisms, including reef-building corals We have made progress in learning about the delicate chemical balances in oceans, lakes, and rivers © 2018 Pearson Education Ltd.  δ– δ+ δ+ δ– δ– δ+ δ+ δ–  0 Acidic + – [H ] > [OH ] + Acids donate H in aqueous solutions. Neutral + – [H ] = [OH ] 7 – Bases donate OH + or accept H in aqueous solutions. Basic + – [H ] < [OH ] 14