Be able to Explain how hydrogen bonds affect the structure and behavior of water Define hydrophobic and hydrophilic Calculate quantities concentrations of atoms and molecules as well as Solution concentrations pH Convert units Define acid base weak acid and weak base Explain how buffers work Water The Molecule That Supports All of Life Water is the biological medium here on Earth All living organisms require water The polarity of water molecules results in hydrogen bonding FIGURE 2 13 The polarity of water The polarity of water is due to the differing electronegativities of hydrogen and oxygen As a consequence hydrogen bonds are formed when the slightly negative oxygen on one water molecule is attracted to the slightly positive hydrogen of another water molecule credit Rao A Fletcher S Ryan K Tag A and Hawkins A Department of Biology Texas A M University This OpenStax ancillary resource is Rice University under a CC BY 4 0 International license it may be reproduced or modified but must be attributed to OpenStax Rice University and any changes must be noted Any images credited to other sources are similarly available for reproduction but must be attributed to their sources Hydrophilic and Hydrophobic Substances A hydrophilic substance Has an affinity for water water loving Ions polar molecules which contain O H or N H polar covalent bonds A hydrophobic substance water fearing Does not have an affinity for water Noncharged and nonpolar molecules such as fats and oils Structures with polar bonds are hydrophilic water loving Structures with non polar bonds are hydrophobic water fearing Hydrophilic Surface Hydrophobic Surface FIGURE 2 14 Hydrogen bonding makes ice less dense than liquid water The a lattice structure of ice makes it less dense than the freely flowing molecules of liquid water enabling it to b float on water credit a modification of work by Jane Whitney image created using Visual Molecular Dynamics VMD software 1 credit b modification of work by Carlos Ponte This OpenStax ancillary resource is Rice University under a CC BY 4 0 International license it may be reproduced or modified but must be attributed to OpenStax Rice University and any changes must be noted Any images credited to other sources are similarly available for reproduction but must be attributed to their sources 4 emergent properties of water 1 High Heat Capacity 2 Heat of Vaporization 3 Cohesive and Adhesive Properties 4 Versatility as a solvent 1 Water s High Specific Heat specific heat of a substance amount of heat that must be absorbed or lost for 1 gram of that substance to change its temperature by 1 C Heat is absorbed when hydrogen bonds break Heat is released when hydrogen bonds form Moderation of Temperature Water moderates air temperature By absorbing heat from air that is warmer and releasing the stored heat to air that is cooler allows water to minimize temperature fluctuations to within limits that permit life in many environments Heat and Temperature Kinetic energy energy of motion Heat thermal energy a measure of the total amount of kinetic energy due to molecular motion can be transferred between molecules Temperature measure of the average kinetic energy Heat and Temperature Temperature is measured on the Celsius scale C At sea level water freezes at 0 C and boils at 100 C Human body temperature averages 37 C Heat and Temperature One convenient unit of heat energy is the calorie cal One calorie is the amount of heat energy necessary to raise the temperature of one gram of water by 1 C A kilocalorie kcal is 1000 calories food Calories equal a kcal 2 Water s Heat of Vaporization Heat of vaporization quantity of heat a liquid must absorb for 1 gram to be converted from a liquid to a gas water requires 586 cal Evaporation Is the transformation of a substance from a liquid to a gas Evaporative cooling Allows water to cool a surface ex sweating 3 Cohesion of Water Molecules Cohesion bonding of a high percentage of the molecules to neighboring molecules due to hydrogen bonding Surface tension measure of how hard it is to break the surface of a liquid related to cohesion Adhesion attraction between water molecules and other molecules FIGURE 2 17 water clings to walls of vessels or tubes The adhesive forces exerted by the glass internal surface exceeding the cohesive forces between the water molecules themselves causes capillary action in a glass tube credit modification of work by Pearson Scott Foresman donated to the Wikimedia Foundation 4 Water s Versatility as a Solvent due to its polarity can form aqueous solutions of a large variety of charged and polar solutes Salts Amino acids Sugars The Solvent of Life Water s molecular properties make it an excellent solvent The different regions of the polar water molecule can interact with ionic compounds and dissolve them FIGURE 2 15 Negative oxygen regions of polar water molecules are attracted to sodium cations Na Positive hydrogen regions of water molecules cling to chloride anions Cl Na Cl Na Cl When we mix table salt NaCl in water it forms spheres of hydration around the ions The Solvent of Life Water can also interact with large polar molecules such as proteins This oxygen is attracted to a slight positive charge on the lysozyme molecule This oxygen is attracted to a slight negative charge on the lysozyme molecule a Lysozyme molecule in a b Lysozyme molecule purple in an c Ionic and polar regions on the protein s Surface nonaqueous environment aqueous environment such as tears attract water molecules or saliva Solute Concentration in Aqueous Solutions Chemical reactions are affected by the concentrations of reactants and products Since most biochemical reactions occur in water It is important to learn to calculate the concentration of solutes in an aqueous solution A mole Represents an exact number of molecules similar to a dozen 12 a mole is 6 023 x 1023 molecules Avogadro s number The mass of 1 mole of a substance is determined by it s atomic or molecular mass Example Carbon has an atomic mass of 12 Daltons 1 mole of carbon is 12 g 12 g of carbon contains 6 023 x 1023 atoms For example What is the mass of 1 mole of table sugar sucrose C12H22O11 The molecular weight of a molecule of sucrose is 342 Daltons the sum of the masses of all the atoms in the molecule The mass of a carbon atom is 12 Daltons hydrogen 1 Dalton and oxygen 16 Daltons 12x12 1x22 16x11 342 Da 6 023 X 1023 molecules 342 g sucrose Sucrose is 342g mol What is the molecular
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