Water and Life: Chemical Bonds, Properties of Water, and pH

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Lecture 3: Water and Life

Overview

This lecture covers the fundamental chemical bonds that allow atoms to form molecules, the unique properties of water resulting from its molecular structure, and the concepts of acids, bases, and pH. Understanding these topics is essential for grasping the chemical basis of life and the behavior of biological molecules in aqueous environments.

Chemical Bonds and Molecular Formation

Types of Chemical Bonds

Atoms combine to form molecules through various types of chemical bonds, which are attractive forces that link atoms together. The four major types of bonds/interactions are:

Covalent bond: Atoms share electrons; strongest bond type in biological molecules. Bond energy: 50–110 kcal/mol
Ionic bond: Attraction between oppositely charged ions. Bond energy: 3–7 kcal/mol
Hydrogen bond: Weak attraction between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom. Bond energy: 3–7 kcal/mol
Van der Waals interaction: Weak, transient attractions due to temporary dipoles. Bond energy: ~1 kcal/mol

Example: Water molecules are held together by hydrogen bonds, which are crucial for water's unique properties.

Nonpolar and Polar Covalent Bonds

Covalent bonds form when atoms share electrons to fill their valence shells. The sharing can be equal or unequal:

Nonpolar covalent bond: Electrons are shared equally between atoms (e.g., H2, O2, CH4).
Polar covalent bond: Electrons are shared unequally, resulting in partial charges (e.g., H2O).

Example: In a water molecule (H2O), oxygen is more electronegative than hydrogen, so electrons are shared unequally, making water a polar molecule.

Structure and Properties of Water

Water's Molecular Structure

Water (H2O) is a polar molecule due to the high electronegativity of oxygen. The unequal sharing of electrons creates a partial negative charge near oxygen and a partial positive charge near hydrogen.

Hydrogen bonds: Form between the partially positive hydrogen of one water molecule and the partially negative oxygen of another.
Polarity: Responsible for water's ability to dissolve many substances and its cohesive behavior.

Unique Properties of Water

Hydrogen bonding gives water several important properties essential for life:

Cohesion: Water molecules stick together, enabling surface tension and transport in plants.
Adhesion: Water molecules stick to other substances, aiding capillary action.
High specific heat: Water can absorb or release large amounts of heat with little temperature change, moderating Earth's climate.
Expansion upon freezing: Ice is less dense than liquid water due to hydrogen bond arrangement, allowing ice to float.
Versatility as a solvent: Water dissolves many ionic and polar substances, making it the 'solvent of life.'

Example: Water's high specific heat helps organisms maintain stable internal temperatures.

Hydrophilic and Hydrophobic Substances

Water interacts differently with various substances:

Hydrophilic: Substances that dissolve well in water (ionic compounds, polar molecules).
Hydrophobic: Substances that do not dissolve in water (nonpolar molecules).

Type	Examples
Hydrophilic	NaCl, glucose
Hydrophobic	Oils, fats, CH4

Acids, Bases, and pH

Definitions and Concepts

Acids and bases are substances that alter the concentration of hydrogen ions (H+) in a solution:

Acid: Increases the concentration of H+ ions.
Base: Reduces the concentration of H+ ions, often by increasing OH- ions.

In pure water at 25°C:

[H+] = [OH-] = M
The product of [H+] and [OH-] is constant:

pH Scale

The pH scale measures the concentration of hydrogen ions in a solution:

Formula:
Neutral solution: pH = 7
Acidic solution: pH < 7
Basic solution: pH > 7

Example: If [H+] = M, then pH = 7.

Molar Mass and Molarity

Molar mass is the sum of the atomic masses of all atoms in a molecule, expressed in grams per mole. Molarity (M) is the number of moles of solute per liter of solution.

Mole: particles (Avogadro's number)
Molarity:
Example: NaCl (molecular mass = 58 g/mol); 1 M NaCl solution contains 58 g NaCl per liter.

Table: Types of Chemical Bonds and Their Energies

Bond Type	Bond Energy (kcal/mol)
Covalent bond	50–110
Ionic bond	3–7
Hydrogen bond	3–7
Van der Waals interaction	1

Table: Hydrophilic vs. Hydrophobic Substances

Property	Hydrophilic	Hydrophobic
Examples	NaCl, glucose	Oils, fats, CH4
Interaction with Water	Dissolves easily	Does not dissolve

Summary

Chemical bonds are essential for molecular formation and biological function.
Water's unique properties arise from its polar structure and hydrogen bonding.
Acids, bases, and pH are critical for maintaining biological homeostasis.
Molar mass and molarity are key concepts for quantifying substances in solution.