2 The Chemical Context of Life
Water: The Solvent of Life A solution is a liquid that is a homogeneous mixture of substances A 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 © 2016 Pearson Education, Inc.
Water is a versatile solvent due to its polarity, which allows it to form hydrogen bonds easily When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell © 2016 Pearson Education, Inc.
Na Cl Cl Na Figure 2.21 Figure 2.21 Table salt dissolving in water © 2016 Pearson Education, Inc.
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 © 2016 Pearson Education, Inc.
- - Figure 2.22 Figure 2.22 A water-soluble protein © 2016 Pearson Education, Inc.
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 covalent bonds © 2016 Pearson Education, Inc.
Solute Concentration in Aqueous Solutions Most chemical reactions in organisms involve solutes dissolved in water Chemical reactions depend on the concentration of solutes, or the number of molecules in a volume of an aqueous solution © 2016 Pearson Education, Inc.
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 © 2016 Pearson Education, Inc.
Acids and Bases Sometimes a hydrogen ion (H) is transferred from one water molecule to another, leaving behind a hydroxide ion (OH−) The proton (H) binds to the other water molecule, forming a hydronium ion (H3O) By convention, H is used to represent the hydronium ion © 2016 Pearson Education, Inc.
H H O O O H H O H H H H 2 H2O Hydronium ion (H3O) Hydroxide Figure 2.UN03 H H O O O H H O H H H H 2 H2O Hydronium ion (H3O) Hydroxide ion (OH) Figure 2.UN03 In-text figure, water dissociation, p. 37 © 2016 Pearson Education, Inc.
H and OH− are very reactive Though water dissociation is rare and reversible, it is important in the chemistry of life H and OH− are very reactive Solutes called acids and bases disrupt the balance between H and OH− in pure water Acids increase the H concentration in water, while bases reduce the concentration of H © 2016 Pearson Education, Inc.
This is a reversible reaction, as shown by the double arrows: A strong acid like hydrochloric acid, HCl, dissociates completely into H and Cl− in water: HCl → H + Cl− Ammonia, NH3, acts as a relatively weak base when it attracts a hydrogen ion from the solution and forms ammonium, NH4 This is a reversible reaction, as shown by the double arrows: © 2016 Pearson Education, Inc.
The hydroxide ions then combine with hydrogen ions to form water Sodium hydroxide, NaOH, acts as a strong base indirectly by dissociating completely to form hydroxide ions: NaOH → Na OH− The hydroxide ions then combine with hydrogen ions to form water © 2016 Pearson Education, Inc.
Weak acids act reversibly and accept back hydrogen ions Carbonic acid, H2CO3, acts as a weak acid: © 2016 Pearson Education, Inc.
The pH Scale In any aqueous solution at 25C, the product of H and OH− is constant and can be written as [H+][OH] = 1014 The pH of a solution is defined as the negative logarithm of H concentration, written as pH = log [H+] For a neutral aqueous solution, [H] is 107 M, so log [H] (7) 7 © 2016 Pearson Education, Inc.
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 © 2016 Pearson Education, Inc.
2 Gastric juice, lemon juice Figure 2.23 pH Scale 1 Battery acid 2 Gastric juice, lemon juice H H H OH- H 3 Vinegar, wine, cola OH- H Increasingly Acidic [H] [OH_] H H H Acidic solution 4 Tomato juice Beer 5 Black coffee Rainwater 6 Urine Saliva OH- OH- Neutral [H] [OH_] OH- 7 Pure water Human blood, tears H H OH- OH- H H H 8 Seawater Inside of small intestine Neutral solution 9 Figure 2.23 The pH scale and pH values of some aqueous solutions 10 Increasingly Basic [H] [OH_] Milk of magnesia OH- OH- 11 OH- H OH- Household ammonia OH- OH- H OH- 12 Basic solution Household bleach 13 Oven cleaner 14 © 2016 Pearson Education, Inc.
H+ OH OH OH OH H+ H+ H+ OH OH OH H+ OH H+ H+ OH OH H+ H+ OH Figure 2.23-1 H+ OH OH OH OH H+ H+ H+ OH OH OH H+ OH H+ H+ OH OH H+ H+ OH OH OH H+ OH H+ H+ H+ H+ H+ Figure 2.23-1 The pH scale and pH values of some aqueous solutions (part 1: ions) Basic solution Neutral solution Acidic solution © 2016 Pearson Education, Inc.
Buffers The internal pH of most living cells must remain close to pH 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 © 2016 Pearson Education, Inc.
Carbonic acid is a buffer that contributes to pH stability in human blood: Response to a rise in PH H+ donor Response to H+ acceptor Hydrogen (acid) a drop in pH (base) ion © 2016 Pearson Education, Inc.
Acidification: A Threat to Our Oceans Human activities such as burning fossil fuels threaten water quality CO2 is a product of fossil fuel combustion About 25% of human-generated CO2 is absorbed by the oceans CO2 dissolved in seawater forms carbonic acid; this causes ocean acidification © 2016 Pearson Education, Inc.
As seawater acidifies, hydrogen ions combine with carbonate ions to form bicarbonate ions (HCO3–) It is predicted that carbonate ion concentrations will decline by 40% by the year 2100 This is a concern because organisms that build coral reefs or shells require carbonate ions © 2016 Pearson Education, Inc.
CO2 CO2 H2O H2CO3 H2CO3 H HCO3 H CO32 HCO3 Figure 2.24 CO2 CO2 H2O H2CO3 H2CO3 H HCO3 H CO32 HCO3 Figure 2.24 Atmospheric CO2 from human activities and its fate in the ocean CO32 Ca2 CaCO3 © 2016 Pearson Education, Inc.
Figure 2.UN01 In-text figure, Van der Waals interactions, p. 30 © 2016 Pearson Education, Inc.
[mmol CaCO3/(m2 day)] Calcification rate Figure 2.UN04-1 20 [mmol CaCO3/(m2 day)] Calcification rate 10 Figure 2.UN04-1 Scientific skills: interpreting a scatter plot with a regression line (part 1) 220 240 260 280 [CO32] (mol/kg of seawater) Data from C. Langdon et al., Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef, Global Biogeochemical Cycles 14:639–654 (2000). © 2016 Pearson Education, Inc.
Figure 2.UN04-2 Figure 2.UN04-2 Scientific skills: interpreting a scatter plot with a regression line (part 2) © 2016 Pearson Education, Inc.
Nucleus Protons ( charge) determine element Electrons ( charge) Figure 2.UN05 Nucleus Protons ( charge) determine element Electrons ( charge) form negative cloud and determine chemical behavior Neutrons (no charge) determine isotope Figure 2.UN05 Summary of key concepts: atom components Atom © 2016 Pearson Education, Inc.
+ H O + H + + Figure 2.UN06 Figure 2.UN06 Summary of key concepts: hydrogen bonds + © 2016 Pearson Education, Inc.
Ice: stable hydro- Liquid water: gen bonds transient hydrogen bonds Figure 2.UN07 Figure 2.UN07 Summary of key concepts: ice and liquid water Ice: stable hydro- gen bonds Liquid water: transient hydrogen bonds © 2016 Pearson Education, Inc.
Acidic [H] [OH] Acids donate H in aqueous solutions. Neutral Figure 2.UN08 Acidic [H] [OH] Acids donate H in aqueous solutions. Neutral [H] [OH] 7 Figure 2.UN08 Summary of key concepts: pH scale Bases donate OH or accept H in aqueous solutions. Basic [H] [OH] 14 © 2016 Pearson Education, Inc.
Figure 2.UN09 Figure 2.UN09 Test your understanding, question 11 (female luna moth, Actias luna) © 2016 Pearson Education, Inc.
Figure 2.UN10 Test your understanding, question 14 (how cats drink) © 2016 Pearson Education, Inc.