BIOL 2112 Chapter Notes - Chapter 3: Electronegativity, Ionic Compound, Sodium Chloride
10 May 2018
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Lecture 2 – Properties of Water
1) Describe the two types of weak chemical bonds and why they are important in living
systems. The reversibility of weak chemical bonds can be an advantage: Two molecules
can come together, affect one another in some way, and then separate. Hydrogen bonds
occur when a hydrogen atom is covalently bonded to an electronegative atom and the
hydrogen has a partially positive charge that allows it to be attracted to a different
electronegative atom nearby. In living cells, the electronegative partners are usually
oxygen or nitrogen atoms. Van der Waals interactions can occur when a molecule with
nonpolar covalent bonds has positively and negatively charged regions. This occurs
because electrons are not always evenly distributed; at any instant, they may accumulate
by chance in one part of a molecule or another. The results are ever-changing regions of
positive and negative charge that enable all atoms and molecules to stick to one another.
These interactions are individually weak and only occur when atoms and molecules are
very close together. 2) What are the three elements that engage in hydrogen bonding in
living systems? Hydrogen, Oxygen, Nitrogen 3) Describe how the shape of a molecule is
established at two levels; the atomic and the molecular. The shape of a molecule is
determined by the shape of its component atoms. The shapes of the atoms are
deteried y the positios of the atos’ oritals. Whe a ato fors ovalet ods,
the orbitals in its valence shell undergo rearrangement. 4) What is the formal definition
of chemical equilibrium? Chemical equilibrium is when the forward and reverse reactions
occur at the same rate, and the relative concentrations of products and reactants stop
changing. At this point, reactions are still going on in both directions but with no net
effect on the concentrations of reactants and products. 5) Describe the molecular basis of
each of the four emergent properties of water. Cohesion is a phenomenon when water
molecules are linked by multiple hydrogen bonds collectively. Adhesion is the clinging of
one substance to another. Surface tension is the measure of how difficult it is to stretch or
break the surface of a liquid. Water moderates air by absorbing heat from air that is
warmer and releasing its stored heat to air that is cooler. When the temperature of water
drops slightly, many additional hydrogen bonds form, releasing a considerable amount of
energy in the form of heat. A large body of water can absorb and store a huge amount of
heat from the sun in the daytime and during the summer while warming up only a few
degrees. At night and during winter, the gradually cooling of water can warm the air. This
capability of water serves to moderate air temperatures in coastal areas. The high specific
heat of water also tends to stabilize ocean temperatures, creating a favorable
environment for marine life. Water that covers most of Earth keeps the temperature
fluctuations on land and in water within the limits that permit life. As a liquid evaporates,
the surface of the liquid that remains behind cools down – evaporative cooling. This
contributes to the stability of temperature in lakes and ponds and also provides a
mechanism that prevents terrestrial organisms from overheating. Water is one of the few
substances that are less dense as a solid than as a liquid. Ice floats on liquid water. When
water solidifies, it expands. In ice, each molecule is hydrogen-bonded to 4 neighbor
molecules in a three-dimensional crystal. Because the crystal is spacious, ice has fewer
molecules than an equal volume of liquid water. Floating ice becomes a barrier that
insulates liquid water below from the colder air. Besides insulating the water below, ice
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Document Summary
Lecture 2 properties of water: describe the two types of weak chemical bonds and why they are important in living systems. The reversibility of weak chemical bonds can be an advantage: two molecules can come together, affect one another in some way, and then separate. Hydrogen bonds occur when a hydrogen atom is covalently bonded to an electronegative atom and the hydrogen has a partially positive charge that allows it to be attracted to a different electronegative atom nearby. In living cells, the electronegative partners are usually oxygen or nitrogen atoms. Van der waals interactions can occur when a molecule with nonpolar covalent bonds has positively and negatively charged regions. This occurs because electrons are not always evenly distributed; at any instant, they may accumulate by chance in one part of a molecule or another. The results are ever-changing regions of positive and negative charge that enable all atoms and molecules to stick to one another.
