Lecture 3: Water is Critical to Life PDF

Lecture 3 Water is critical to life Learning outcomes: Why is water essential to life? Define the properties of water Tetrahedral structure Polar molecule High specific heat and heat of vaporization Cohesion, surface tension, and adhesion Why is water...

Lecture 3 Water is critical to life Learning outcomes: Why is water essential to life? Define the properties of water Tetrahedral structure Polar molecule High specific heat and heat of vaporization Cohesion, surface tension, and adhesion Why is water an unusual solvent? What is a mole of substance? What is Avogadro’s number? How to prepare a solution of given molarity. Explain the principle of pH Explain why water is considered a weak acid and a base What are buffers and how they maintain homeostasis Water is essential to life Make up large proportion of living tissues Is a medium for chemical reactions 95% Chemical properties of water Small molecules of tetrahedral structure 20% Water is a polar molecule & forms hydrogen bonds Water is a solvent (dissolve many substances) High specific heat High heat of vaporization Water is a weak acid and weak base (serves in buffering) 85% Water molecule has tetrahedral shape How many hydrogen bonds can a molecule of water form? Explain the following properties of water: Why ice floats? Why lakes do not freeze during winter? Why is temperature on Earth relatively stable between a night and a day? It takes a lot of energy to change the physical state of water Water has a high specific heat Material Specific heat (kJ/kg°C) Specific heat = the amount of heat energy Alcohol 2.3 required to raise the temperature of 1 gram Gasoline 2.22 Glycerine 2.43 of a substance by 1°C. Water 4.18 Olive oil 1.97 Water has a high heat of vaporization Iron 0.45 Lots of heat is required to change water from its liquid state to gas. Aquatic environments are relatively stable Oceans help to minimize variations in Implications to life? atmospheric temperature across Earth Sweating contributes to thermoregulation Cohesion, surface tension & adhesion Water is a dynamic fluid A single molecule may break and reform hydrogen bonds about a trillion (1012) times every minute A water molecule forms on average 3.4 hydrogen bonds with other water molecules Cohesion Surface tension Adhesion Water is a solvent A mole is the amount of a substance (in grams) that is numerically equal to its molecular weight, thus the number of molecules in 1 mole is constant in all substances. # molecules (1 mole of salt (NaCl) = # molecules (1 mole glucose (C6H12O6) Avogadro’s number = 6.02 x 1023 molecules/mole A 1 molar (1 M or 1mol) solution contains 1 mole of substance dissolved in water to make 1 liter. Concept check: How many molecules are in 10 mM solution? 10 mM = 10-2 M 6.02 X 1023 x 10-2 = 6.02 x 1021 molecules/liter Aquaeous solutions may be acidic Acid = a substance that releases H+; H+ ions can attach to other molecules and change their properties HCl ® H+ + Cl– Biological acids have a carboxyl group: —COOH ® —COO– + H+ Weak acids: Not all the acid molecules dissociate into ions (e.g., acetic acid [CH3COOH]). Strong acids, such as hydrochloric acid (HCl), dissociate completely. Aquaeous solutions may be basic Base = a substance that accepts H+ in solution NaOH is a strong base (it accepts H+). In water: NaOH ® Na+ + OH– OH– + H+ ® H2O Weak bases: Bicarbonate ion: HCO3– + H+ ® H2CO3 Ammonia: NH3 + H+ ® NH4+ Compounds with amino groups: —NH2 + H+ ® —NH3+ Acid–base reactions may be reversible. Acetic acid: Only 4 / 100 molecules dissociate CH3COOH ® CH3COO– + H+ Ionization of strong acids and bases is irreversible. Ionization of weak acids and bases is somewhat reversible. Water has a slight tendency to ionize, thus water acts as a weak acid and a base: Water acts as both a weak acid and a weak base. Ionization of water is important to all organisms because of the abundance of water in living systems and the reactive nature of the H+ ions. Measuring acidity & alkalinity pH: Negative log of the molar concentration of H+ ions. pH = –log[H+]  Maintaining homeostasis pH influences rates of biological reactions and can change the 3-D structure of biological molecules, which impacts function. A buffer is a weak acid and its corresponding base. ® HCO3– + H+ ® H2CO3 Buffers illustrate the law of mass action: addition of reactant on one side of a reversible system drives the system in the direction that uses up that compound. Addition of an acid drives the reaction in one direction; addition of a base drives the reaction in the other direction.