Chapter 3: The Chemistry of Water PDF

Chapter 3 The Chemistry of Water Lecture Presentations by Nicole Tunbridge and © 2021 Pearson Education Ltd. Kathleen Fitzpatrick © 2021 P...

Chapter 3 The Chemistry of Water Lecture Presentations by Nicole Tunbridge and © 2021 Pearson Education Ltd. Kathleen Fitzpatrick © 2021 Pearson Education Ltd. CONCEPT 3.1: 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 © 2021 Pearson Education Ltd. Figure 3.2 © 2021 Pearson Education Ltd. CONCEPT 3.2: Four emergent properties of water contribute to Earth’s suitability for life Four of water’s properties that facilitate an environment for life are – cohesive behavior – ability to moderate temperature – expansion upon freezing – versatility as a solvent © 2021 Pearson Education Ltd.  Cohesion of Water Molecules Collectively, hydrogen bonds hold water molecules together, a phenomenon called cohesion Cohesion results in high surface tension (tendency of liquid surfaces at rest to shrink into the minimum surface area possible), a measure of how difficult it is to stretch or break the surface of a liquid Cohesion contributes to the transport of water and dissolved nutrients against gravity in plants © 2021 Pearson Education Ltd.  Adhesion is an attraction between different substances, for example, between water and plant cell walls This helps to counter the downward pull of gravity https://youtu.be/IxXDaQV7w e8 Cohesion & Adhesion -Demonstration © 2021 Pearson Education Ltd. Figure 3.4 © 2021 Pearson Education Ltd. Animation: Water Transport in Plants © 2021 Pearson Education Ltd. 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 © 2021 Pearson Education Ltd. Temperature and Heat Kinetic energy is the energy of motion; Kinetic energy associated with random motion of atoms or molecules is called thermal energy Heat is the measure of total kinetic energy due to its molecular motion. 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 © 2021 Pearson Education Ltd. Water’s High Specific Heat 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 1ºC The specific heat of water is 1 cal or 4.184 j/g/ºC. Water’s high specific heat can be traced to hydrogen bonding – During heating, heat is absorbed for hydrogen bonds to break – After heating stopped, hydrogen bonds begin to reform to release heat https://youtu.be/UtyyUHJGsUY © 2021 Pearson Education Ltd.  The high specific heat of water minimizes temperature fluctuations to within limits that permit life A large body of water can absorb and store a huge amount of heat from the sun in daytime and during summer while warming up only a few degrees At night and during the winter the gradually cooling water can warm the air This serves to moderate air temperature in coastal areas © 2021 Pearson Education Ltd. 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, through a process called evaporative cooling Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water © 2021 Pearson Education Ltd. Floating of Ice on Liquid Water Water is less dense as a solid than as a liquid At 0ºC, water molecules are locked into a crystalline lattice The hydrogen bonds keep the molecules far enough apart to make the ice ~10% less dense than liquid water Water reaches its greatest density at 4ºC If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth https://youtu.be/UukRgqzk-KE?si=AcHm54eET95y9Kwu © 2021 Pearson Education Ltd. Figure 3.1b A polar molecule is a chemical species in which the distribution of electrons between the covalently bonded atoms is not even © 2021 Pearson Education Ltd.  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 © 2021 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 © 2021 Pearson Education Ltd.  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 © 2021 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 © 2021 Pearson Education Ltd. 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 © 2021 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 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 Molarity (M) is the number of moles of solute per liter of solution © 2021 Pearson Education Ltd.  CONCEPT 3.3: Acidic and basic conditions affect living organisms Acids and Bases An acid is a substance that increases the H+ concentration of a solution – Proton donor A base is a substance that can accept the H+ ions in a solution – Proton acceptor © 2021 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 © 2021 Pearson Education Ltd. The pH Scale pH scale describes whether a solution is acidic or basic (the opposite of acidic) 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 In any aqueous solution at 25ºC, the product of H+ and OH– is constant and can be written as [H+][OH–] = 10–14 © 2021 Pearson Education Ltd. Water molecules: 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+ © 2021 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 H+ and OH– are very reactive Changes in their concentrations can drastically affect the chemistry of a cell Concentrations of H+ and OH– are equal in pure water © 2021 Pearson Education Ltd. Buffers The internal pH of most living cells is close to 7 Even a slight change in pH can be harmful 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 © 2021 Pearson Education Ltd.