Chemical Bonds and Properties of Water in Biology

Chemical Bonds and Properties of Water

Introduction

This section explores the types of chemical bonds, with a focus on hydrogen bonds, and the unique properties of water that are essential for life. Understanding these concepts is foundational for studying biological molecules and cellular processes.

Chemical Bonds

Types of Chemical Bonds

• Covalent Bonds: Atoms share electrons to achieve stability. These are strong bonds found in molecules like H2O and CO2.

• Ionic Bonds: Electrons are transferred from one atom to another, resulting in oppositely charged ions that attract each other (e.g., NaCl).

• Hydrogen Bonds: A weak attraction between a hydrogen atom covalently bonded to an electronegative atom (like oxygen or nitrogen) and another electronegative atom. Critical in water and biological macromolecules.

• Van der Waals Interactions: Weak, transient attractions between molecules or parts of molecules that result from transient local partial charges.

Bond Polarity and Electronegativity

• Electronegativity: The ability of an atom to attract electrons in a covalent bond.

• Polar Covalent Bonds: Electrons are shared unequally, creating partial charges (e.g., in water molecules).

• Nonpolar Covalent Bonds: Electrons are shared equally between atoms (e.g., in O2 or CH4).

Hydrophobic and Hydrophilic Interactions

• Hydrophobic: Nonpolar molecules that do not interact favorably with water (e.g., oils).

• Hydrophilic: Polar or charged molecules that interact well with water.

Properties of Water Necessary for Life

The Structure and Polarity of Water

• Water (H2O) is a polar molecule with a bent shape, resulting in a partial negative charge near the oxygen and partial positive charges near the hydrogens.

• This polarity allows water molecules to form hydrogen bonds with each other and with other polar substances.

Four Key Properties of Water

1. Cohesion and Adhesion:

   • Cohesion: Water molecules stick to each other due to hydrogen bonding, resulting in high surface tension.

   • Adhesion: Water molecules stick to other polar or charged surfaces, aiding in processes like capillary action in plants.

2. Moderation of Temperature:

   • Water has a high specific heat, meaning it can absorb or release large amounts of heat with only a slight change in its own temperature.

   • This property helps stabilize temperatures in organisms and environments.

   • Equation: $q = m c \Delta T$ (where $q$ is heat, $m$ is mass, $c$ is specific heat, $\Delta T$ is temperature change)

3. Expansion Upon Freezing:

   • Water is less dense as a solid than as a liquid because hydrogen bonds stabilize and keep molecules further apart in ice.

   • This allows ice to float, insulating aquatic life in winter.

4. Versatility as a Solvent:

   • Water's polarity allows it to dissolve many substances, making it the "universal solvent" for biochemical reactions.

   • Solution: A homogeneous mixture of solute dissolved in solvent (water).

   • Aqueous Solution: A solution where water is the solvent.

Hydrogen Bonds in Water

• Each water molecule can form up to four hydrogen bonds with neighboring water molecules.

• Hydrogen bonding is responsible for water's unique properties, including high boiling point, surface tension, and solvent abilities.

Solute Concentration in Aqueous Solutions

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

$\text{Molarity (M)} = \frac{\text{moles of solute}}{\text{liters of solution}}$

Chemical Reactions in Water

• Most biochemical reactions occur in aqueous solutions.

• Chemical reactions involve the making and breaking of chemical bonds, leading to changes in the composition of matter.

• Reactants: Starting materials in a reaction.

• Products: Substances formed as a result of a reaction.

$\text{Reactants} \rightarrow \text{Products}$

Vocabulary

• Intermolecular bond

• Intramolecular bond

• Hydrogen bond

• Cohesion

• Adhesion

• Surface tension

• Solute

• Solvent

• Aqueous solution

• Molarity

• Hydrophilic

• Hydrophobic

• Reactant

• Product

• Chemical equilibrium

Study Guide

1. Contrast inter vs intra-molecular interactions. Answer: Intramolecular bonds: Bonds within a molecule (covalent and ionic bonds that hold atoms together). Intermolecular bonds: Attractions between molecules (hydrogen bonds and van der Waals interactions). Hydrogen bonds: Weak attraction between hydrogen bonded to an electronegative atom (O or N) and another electronegative atom. Van der Waals interactions: Very weak, temporary attractions caused by transient partial charges.

2. Name and describe the various types of inter-molecular interactions. Answer: Hydrogen bonds: Weak attraction between hydrogen bonded to an electronegative atom (O or N) and another electronegative atom. Van der Waals interactions: Very weak, temporary attractions caused by transient partial charges.

3. Name the weaker chemical bonds. How do they help stabilize molecules if they are held together weakly? Answer: Hydrogen bonds and van der Waals interactions are weak individually. They stabilize molecules because many weak bonds together provide stability, especially in water and biological macromolecules.

4. Describe the hydrophobic effect. Use terms like polar, nonpolar molecules, aggregation, and insoluble. Answer: They are polar molecules with hydrogen bonded to electronegative atoms (such as oxygen or nitrogen), allowing partial positive and negative charges that form hydrogen bonds.

5. What are the properties of molecules capable of forming hydrogen bonds with each other? Answer: They are polar molecules with hydrogen bonded to electronegative atoms (such as oxygen or nitrogen), allowing partial positive and negative charges that form hydrogen bonds.

6. List and explain the four properties of water that emergeas a result of its ability to form hydrogen bonds. Answer:

   1. Cohesion: water sticks to water (surface tension) Adhesion: water sticks to other polar surfaces (capillary action)

   2. Moderation of Temperature

      High specific heat lets water absorb/release heat with little temperature change

   3. Expansion Upon Freezing

      Ice is less dense because hydrogen bonds hold molecules farther apart

   4. Versatility as a Solvent

      Water’s polarity allows it to dissolve many substances (aqueous solutions) 

7. Try to explain to someone why water expands when it freezes. Answer: Hydrogen bonds stabilize water molecules in ice, keeping them farther apart, making solid water less dense than liquid water.

8. Distinguish between the following sets of terms: hydrophobic and hydrophilic substances; a solute, a solvent, and a solution. Answer:

   1. Hydrophobic vs Hydrophilic

      • Hydrophobic: nonpolar molecules that do not interact well with water

      • Hydrophilic: polar or charged molecules that interact well with water

   2. Solute, Solvent, Solution

      • Solute: substance being dissolved

      • Solvent: substance doing the dissolving (water)

      • Solution: homogeneous mixture of solute in solvent

9. How do the number of H-bonds between water molecules determine the state of water? Answer: More hydrogen bonds form in ice, holding molecules farther apart. Fewer hydrogen bonds exist in liquid water, allowing molecules to move more freely.

10. Identify the reactants and the products in a chemical reaction. Answer:

    Reactants: starting materials

    Products: substances formed after the reaction

Table: Types of Chemical Bonds

Bond Type	Description	Relative Strength	Example
Covalent	Atoms share electrons	Strong	H2O, CH4
Ionic	Electrons transferred; ions attract	Strong (in dry conditions)	NaCl
Hydrogen	Attraction between H and electronegative atom	Weak (individually)	Between water molecules
Van der Waals	Transient, weak attractions	Very weak	Between nonpolar molecules