Lecture 2

Lecture Overview

• Topic: Water as the Biological Solvent, pH, and Buffers

• Referenced Text: Campbell Chapter 3

Importance of Water

• Water is essential for all living organisms.

• Most cells are composed of 70-95% water.

• The prevalence of water contributes significantly to Earth's habitability.

Properties of Water

Hydrogen Bonds

• Water molecules interact through hydrogen bonds, which occur due to polar covalent bonds.

  • Visual representation of bonds:

    • δ− (negative charge on oxygen) and δ+ (positive charge on hydrogen).

Emergent Properties of Water

• Four critical properties that enhance Earth’s suitability for life:

  • Cohesive/Adhesive Properties: Water molecules stick to each other (cohesion) and to other substances (adhesion).

  • Temperature Moderation: Water can absorb and retain heat, regulating temperature.

  • Expansion upon Freezing: Water expands and becomes less dense as it freezes, allowing ice to float.

  • Solvent Versatility: Water dissolves a wide range of substances, supporting metabolic processes.

Water Movement in Plants

• Water in conducting cells moves due to:

  • Cohesion: Water molecules stick together.

  • Adhesion: Water molecules stick to other materials.

Surface Tension

• Defined as the energy required to break the surface of a liquid.

• High surface tension in water is due to hydrogen bonds, allowing for phenomena like water striders walking on water.

Specific Heat

• Specific Heat: Amount of heat needed to raise the temperature of 1 g of water by 1°C, approximately 1 cal/g°C.

  • Compared to ethanol (0.55 cal/g°C).

  • Water’s high specific heat moderates temperature changes in environments.

Heat of Vaporization

• Heat of vaporization refers to the heat required to convert 1 g of water from liquid to gas.

• Evaporative cooling occurs as the fastest molecules escape as gas, cooling the remaining liquid.

Expansion Upon Freezing

• Water expands upon freezing due to the formation of stable hydrogen bonds in a crystalline structure (ice) similar to a hexagonal lattice.

• This open structure makes ice less dense than liquid water; hence, ice floats.

• Practical impacts include the potential for frozen water to break containers.

Water as the Solvent of Life

• Solution: Homogeneous mixture of substances where a solvent dissolves the solute.

• Aqueous Solutions: Solutions in which water acts as the solvent.

• Water, as a polar solvent, effectively dissolves other polar and ionic substances.

Dissolving Mechanism

• Molecules do not have to be ionic to dissolve in water; they can be polar as well.

Solute Concentration in Aqueous Solutions

• Molecular Mass: Sum of the masses of all atoms in a molecule calculated in daltons.

• Molarity (M): Number of moles of solute per liter of solution.

Example: Sucrose in Water

• Sucrose (C12H22O11) has a molecular mass of 342 daltons.

  • 1 mol = 342 g in 1 liter of water.

pH and Water Chemistry

• pH Scale: Measures the acidity/basicity of a solution based on [H+] and [OH-].

• Pure water has [H+] = [OH-] at 10^-7 M, defining neutrality (pH = 7).

Buffers

• Buffers: Substances that minimize changes in pH, essential for cellular function and metabolism.

• Buffers often consist of an acid-base pair that interact with H+. E.g., H2CO3 ↔ HCO3- + H+.

Ocean Acidification

• CO2 emissions lead to ocean acidification, impacting marine life and calcification processes.

• The reaction of CO2 with seawater to form carbonic acid decreases pH.

Calculations and Understanding pH

• The pH scale is logarithmic, where a change of 1 unit represents a tenfold change in [H+] concentration.

• Comparison questions can assess understanding of the relationship between different pH values.

Summary of Water Properties

• Water's polarity allows it to form hydrogen bonds, making it crucial for life.

• Key concepts covered include the role of H+ and OH- ions, the pH scale, and buffer systems.