1 2 The Chemical Context of Life

1.1 2.1-4 chem lol

†Go ask chemgod

1.2 2.5 Water

  • Structure and interactions:
    • Polar molecule: overall charge is unevenly distributed
    • Hydrogen bonding: the slightly positive hydrogen of one molecule is attracted to the slightly negative oxygen of a nearby molecule. Bonds form and break quickly in liquid water.
    • Four emergent properties arise

1.2.0.1 1. Cohesive behavior:

  • Cohesion: hydrogen bonds keep water molecules close together
  • Adhesion: water clings to other substances through hydrogen bonds
  • Cohesion gives water an unusually high surface tension

1.2.0.2 2. Moderation of temperature:

  • Water absorbs heat from warm air and releases heat to cooler air
  • Effective as a heat bank due to its high specific heat
  • Thermal energy: kinetic energy associated with random movement of atoms or molecules
    Temperature: average kinetic energy of the molecules
    Heat: thermal energy in transfer between bodies of matter
    Calorie: amount of heat needed to raise the temperature of 1 g of water by 1°C
    Joule: another unit of energy, one calorie equals 4.184 J
  • Specific heat: amount of heat needed for 1 g of a substance to change its temperature by 1°C
    • 1 cal/g\(\cdot\)°C for water
    • Water resists changing its temperature because much of the heat is first used to disrupt hydrogen bonds. When the temperature drops, newly formed hydrogen bonds release heat.
    • Can moderate temperatures near and in large bodies of water
  • Evaporative cooling: as a liquid evaporates, the surface of the liquid cools down
    • Heat of vaporization: the quantity of heat a liquid must absorb for 1 g of it to be converted to gas
    • Water also has a high heat of vaporization, hydrogen bonds must be broken
    • Contributes to the stability of temperature in bodies of water and prevents organisms from overheating

1.2.0.3 3. Floating of ice on liquid water:

  • When liquid water freezes, it expands:
    • As the temperature falls, molecules move too slowly to break hydrogen bonds
    • Hydrogen bonds lock molecules in a crystalline lattice and hold the molecules apart, becoming less dense
  • Floating ice insulates the liquid water below, preventing it from freezing and allowing life to exist

1.2.0.4 4. Versatility as a solvent:

  • Solution: a liquid that is a homogenous mixture of two or more substances
    Solvent: the dissolving agent
    Solute: the substance that is dissolved
    Aqueous solution: a solution with water as the solvent
  • Hydration shell: the sphere of water molecules around dissolved ions or polar molecules, separating and shielding them from one another
    • The charged regions of water molecules are attracted to the charged ions
    • Molecules of water surround polar solutes by forming hydrogen bonds
  • Hydrophilic: any substance that has a high affinity for water, does not necessarily have to dissolve
    Hydrophobic: substances with no affinity for water, nonionic or nonpolar
  • Solute concentration in aqueous solutions:
    • Molecular mass: the sum of the masses of all the atoms in a molecule
    • Mole: represents an exact number of objects. 6.02 \(\times\) 1023, Avogadro’s number, number of daltons in 1 g.
    • Molarity: the number of moles of solute per liter of solution

1.2.0.5 Acids and bases:

  • A hydrogen atom participating in a hydrogen bond may occasionally shift from one molecule to the other:
    • A hydrogen ion (H+) is transferred
    • Hydroxide ion (OH-): water molecule that loses a proton
    • Hydronium ion (H3O+): water molecule that gains a proton, conventionally represented by H+
  • Acid: a substance that increases the hydrogen ion concentration
    Base: a substance that reduces the hydrogen ion concentration, either by accepting hydrogen ions directly or by dissociating into hydroxide ions and forming water
  • Strong acids/bases dissociate completely, while weak acids/bases reversibly release and accept ions
  • pH: log scale for measuring hydrogen ion concentration. Values less than 7 are acidic, above 7 is basic.
  • Buffer: a substance that minimizes changes in the concentrations of H+ and OH-. Donates hydrogen ions when they are depleted and accepts ions when they are in excess.

1.2.0.6 Concept check

Concept check

  1. Describe how properties of water contribute to the upward movement of water in a tree.

    Hydrogen bonds hold neighboring water molecules together. This cohesion helps the chain of water molecules move upward against gravity in water-conducting cells as water evaporates from the leaves. Adhesion between water molecules and the walls of the water-conducting cells also helps counter gravity.

  2. How can the freezing of water crack boulders?

    As water freezes, it expands because water molecules move farther apart in forming ice crystals. When there is water in a crevice of a boulder, expansion due to freezing may crack the boulder.

  3. The concentration of the appetite-regulating hormone ghrelin is about 1.3 \(\times\) 10–10 M in a fasting person. How many molecules of ghrelin are in 1 L of blood?

    A liter of blood would contain 7.8 \(\times\) 1013 molecules of ghrelin (1.3 \(\times\) 10–10 moles per liter \(\times\) 6.02 \(\times\) 1023 molecules per mole).

  4. Compared with a basic solution at pH 9, the same volume of an acidic solution at pH 4 has ___ times as many hydrogen ions (H+).

    105

  5. What would be the effect on the properties of the water molecule if oxygen and hydrogen had equal electronegativity?

    The covalent bonds of water molecules would not be polar, and water molecules would not form hydrogen bonds with each other.

1.3 Test your understanding

Test your understanding

  1. The reactivity of an atom arises from
    1. the average distance of the outermost electron shell from the nucleus. ✗
    2. the existence of unpaired electrons in the valence shell. ✓
    3. the sum of the potential energies of all the electron shells. ✗
    4. the potential energy of the valence shell. ✗
    5. the energy differences between the electron shells. ✗
  2. Which of the following statements correctly describes any chemical reaction that has reached equilibrium?
    1. The concentrations of products and reactants are equal. ✗
    2. The reaction is now irreversible. ✗
    3. Both forward and reverse reactions have halted. ✗
    4. The rates of the forward and reverse reactions are equal. ✓
    5. No reactants remain. ✗
  3. Many mammals control their body temperature by sweating. Which property of water is most directly responsible for the ability of sweat to lower body temperature?
    1. water’s change in density when it condenses ✗
    2. water’s ability to dissolve molecules in the air ✗
    3. the release of heat by the formation of hydrogen bonds ✗
    4. the absorption of heat by the breaking of hydrogen bonds ✓
    5. water’s high surface tension ✗
  4. We can be sure that a mole of table sugar and a mole of vitamin C are equal in their
    1. mass in daltons. ✗
    2. mass in grams. ✓
    3. volume. ✗
    4. number of atoms. ✗
    5. number of molecules. ✗
  5. Measurements show that the pH of a particular lake is 4.0. What is the hydrogen ion concentration of the lake?
    1. 4.0 M ✗
    2. 10-10 M ✗
    3. 10-4 M ✓
    4. 104 M ✗
    5. 4% ✗
  6. The atomic number of sulfur is 16. Sulfur combines with hydrogen by covalent bonding to form a compound, hydrogen sulfide. Based on the number of valence electrons in a sulfur atom, predict the molecular formula of the compound.
    1. HS ✗
    2. HS2 ✗
    3. H2S ✓
    4. H3S2 ✗
    5. H4S ✗
  7. What coefficients must be placed in the following blanks so that all atoms are accounted for in the products? C6H12O6 → ___ C2H6O + ___ CO 2
    1. 1; 2 ✗
    2. 3; 1 ✗
    3. 1; 3 ✗
    4. 1; 1 ✗
    5. 2; 2 ✓
  8. A slice of pizza has 500 kcal. If we could burn the pizza and use all the heat to warm a 50-L container of cold water, what would be the approximate increase in the temperature of the water?
    1. 50°C ✗
    2. 5°C ✗
    3. 1°C ✗
    4. 100°C ✗
    5. 10°C ✓