Water, Acids & Bases, and Reactions (Notes Review)
Water: Structure and Polarity
Water is a molecule composed of Hydrogen and Oxygen: ext{H}_2 ext{O}
Oxygen is electronegative, meaning it attracts electrons more strongly (electrons are drawn toward O). Hydrogen atoms carry a partial positive charge, while the oxygen carries a partial negative charge.
Water is a polar molecule.
Because of polarity, water can form hydrogen bonds: each water molecule can participate in up to 4 hydrogen bonds.
Overall, water’s polar nature drives its interactions with other molecules and its solvent properties.
Cells are roughly 70–75% water by mass.
About 71% of Earth’s surface is covered by water.
Polar vs. nonpolar contrast: polar molecules (like water) interact well with water; nonpolar substances (like oil) do not dissolve readily in water (like dissolves like).
Hydrophilic substances interact readily with water; hydrophobic substances do not dissolve well in water.
Water as a Solvent and Properties Critical for Life
Water is the principal solvent in living things due to its high polarity.
Hydrophilic substances readily interact with water; hydrophobic substances interact poorly.
Water as solvent: great for polar substances and ionic compounds.
Nonpolar substances (like oil) do not dissolve well in water.
Capillary action: tendency of water to move against gravity, arising from cohesive forces (water–water) and adhesive forces (water–other substances). Capillary action is important in processes like water transport in trees.
Cohesive forces: attraction between water molecules; contribute to surface tension.
Adhesive forces: attraction of water molecules to other kinds of molecules; also supported by hydrogen bonds.
Surface tension: heightened by hydrogen bonding between water molecules.
Water helps maintain a stable temperature in organisms and environments.
Specific heat of water is high, so it absorbs/retains heat with relatively small temperature changes; this is due to hydrogen bonding.
High heat of vaporization: water requires a large amount of heat to convert from liquid to water vapor, aiding cooling of organisms and the ecosphere. It takes about 540\ ext{cal} to convert 1 g of ice to water vapor.
Ice density and freezing: solid water (ice) is less dense than liquid water, so ice floats. This is crucial for aquatic life and temperature cycling on Earth.
At 4°C, water begins to expand and become less dense as hydrogen bonds lock in place, leading to floating ice.
Floating ice helps prevent lakes and oceans from freezing solid, enabling temperature cycling on Earth.
Practical implication: the unique properties of water support life and climate stability.
Acids & Bases
Common acids include: lemons and oranges (citric acid), vinegar, urine.
Common bases include: soap, toothpaste, bleach.
Acids are proton donors; bases are proton acceptors.
Protons are hydrogen ions, denoted as ext{H}^+; hydrogen atom loses its electron to become a proton.
An acid is a substance that dissociates in solution to yield ext{H}^+; a base dissociates to yield hydroxide ions (OH⁻).
Acid dissociation (general): ext{HA}
ightleftharpoons ext{H}^+ + ext{A}^-Example: ext{HCl}
ightleftharpoons ext{H}^+ + ext{Cl}^-The hydrogen atom has atomic number 1; when it loses its electron, only the proton remains.
Thus, acids are proton donors; bases are proton acceptors.
Hydroxide ions are denoted as OH⁻.
A salt forms from acids and bases reacting together (neutralization), e.g., ext{HCl} + ext{NaOH}
ightarrow ext{NaCl} + ext{H}_2 ext{O}In aqueous solution, salts, acids, and bases can form ions (electrolytes) or may dissolve without forming ions (non-electrolytes).
Electrolytes conduct electricity when dissolved in water due to ion formation; non-electrolytes do not form ions.
Water Autolysis and pH Concepts
Pure water tends to slightly dissociate into hydrogen and hydroxide ions: ext{H}_2 ext{O}
ightleftharpoons ext{H}^+ + ext{OH}^-.In pure water, the concentrations of these ions are equal: [\text{H}^+] = [\text{OH}^-] = 10^{-7}\ ext{M}.
The product of the ion concentrations is constant: [\text{H}^+][\text{OH}^-] = 10^{-14}\ \text{M}^2 (at 25°C).
pH is a convenient shorthand for proton concentration: \text{pH} = -\log [\text{H}^+].
pH scale: acidic solutions have pH < 7; basic solutions have pH > 7; neutral is pH = 7.
For a basic solution with [\text{H}^+] = 10^{-10}\ \text{M}, the pH is: \text{pH} = -\log(10^{-10}) = 10.
Examples:
Acidic solution: elevated [\text{H}^+] and reduced [\text{OH}^-] (e.g., pH < 7).
Basic solution: reduced [\text{H}^+] and elevated [\text{OH}^-] (e.g., pH > 7).
For a pH value of 7, [\text{H}^+] = 10^{-7}\ ext{M} and [\text{OH}^-] = 10^{-7}\ ext{M}.
Buffers
Buffers minimize or resist changes in pH.
They typically involve weak acids and weak bases that can absorb added protons or hydroxide ions without large changes in pH.
Example: Carbon dioxide/bicarbonate system in water:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺
As H⁺ is added, equilibrium shifts to form more H₂CO₃; as OH⁻ is added, more H₂CO₃ dissociates to consume H⁺ and form CO₂/H₂O as needed.
Definitions: Solvent, Solute, Solution; Electrolytes vs Non-electrolytes; Mixtures
Solvent: the liquid in which a substance dissolves.
Solute: the dissolved substance.
Solution: solvent + solute.
Salts arise from acid-base reactions (neutralization).
Example: ext{HCl} + ext{NaOH}
ightarrow ext{NaCl} + ext{H}_2 ext{O}
Electrolytes: salts, acids, or bases that form ions in water and conduct electrical current.
Non-electrolytes: dissolve in water but do not form ions.
Mixtures: two or more elements/compounds that can be separated by physical means.
Types of mixtures:
Heterogeneous: not uniform throughout.
Homogeneous: uniform throughout.
Reactions Involving Water
Hydrolysis (hydro = water, lysis = split): A reaction in which water is added to break a bond between two molecules; relevant for breaking down large molecules into smaller ones.
Condensation (dehydration synthesis): Two molecules are joined together via the removal of water; relevant for assembly of molecules.
These reactions illustrate water’s essential role in both breakdown and synthesis of biological molecules.
Chapter 4: Organic Molecules (Note)
The provided transcript introduces Chapter 4: Organic Molecules but does not include the content. This section is a placeholder indicating that content follows in the full material.