1.1 Structure of Water and Hydrogen Bonding
Keywords
| English Term | 中文翻译 | Definition & Explanation |
|---|---|---|
| Polarity | 极性 | A separation of electric charge leading to a molecule having a partial positive end and a partial negative end. |
| Hydrogen Bonding | 氢键 | A weak chemical bond formed when the slightly positive hydrogen atom of one molecule is attracted to the slightly negative atom of another. |
| Electronegativity | 电负性 | A measure of the tendency of an atom to attract a bonding pair of electrons. |
| Cohesion | 内聚力 | The linking together of like molecules, often by hydrogen bonds, which holds a substance together. |
| Adhesion | 附着力 | The clinging of one substance to another, such as water molecules attaching to plant cell walls. |
| Surface Tension | 表面张力 | A measure of how difficult it is to stretch or break the surface of a liquid, caused by the cohesive forces between molecules. |
| Specific Heat | 比热容 | The amount of heat that must be absorbed or lost for 1 g of a substance to change its temperature by 1°C. |
| Evaporative Cooling | 蒸发冷却 | The process in which the surface of an object becomes cooler during evaporation, as the molecules with the highest kinetic energy leave. |
| Transpiration | 蒸腾作用 | The process of water movement through a plant and its evaporation from aerial parts, such as leaves and stems. |
1. The Polarity of Water
Water is a polar molecule because of the unequal sharing of electrons between oxygen and hydrogen.
Core Concept: Polarity and Electronegativity
- Electronegativity: Oxygen is significantly more electronegative than Hydrogen. This means the shared electrons are pulled closer to the Oxygen nucleus.
- Polar Covalent Bonds: Within one water molecule, the O and H atoms are held together by polar covalent bonds.
- Charge Distribution:
- Oxygen atom carries a partial negative charge (δ−).
- Hydrogen atoms carry a partial positive charge (δ+).
- Here, δ (read as "delta") means "slightly," so δ+ means slightly positive, and δ− means slightly negative.
Polar covalent bonds and partial charges in \( \ce{H2O} \).
2. Hydrogen Bonding
Hydrogen bonds are intermolecular forces (between different molecules) that form when the δ+ hydrogen of one water molecule is attracted to the δ− oxygen of another.
Properties of Hydrogen Bonds
- Strength: Individually, they are weak and transitory. However, in large numbers, they provide significant structural integrity to liquid water and biological molecules (like DNA).
- Requirement: They only form between a hydrogen atom already bonded to a highly electronegative atom (O, N, or F) and another electronegative atom.
Dotted lines represent the hydrogen bonds connecting water molecules.
3. Emergent Properties & Biological Significance
A. Cohesion, Adhesion, and Surface Tension
- Cohesion: Attraction between like molecules (water-water). It causes Surface Tension, which allows the surface of water to resist rupture.
- Adhesion: Attraction between unlike molecules (water-polar surfaces).
Biological Application: Transpiration
In plants, water travels from roots to leaves against gravity through the Xylem. 1. Evaporation at the leaf creates a pulling force. 2. Cohesion holds the water column together like a chain. 3. Adhesion allows water to stick to the cellulose walls of the xylem to prevent the column from falling back down.
Cohesion and Adhesion working together in plant transport.
B. Thermal Properties
- High Specific Heat: Water requires a large amount of heat to change its temperature because H-bonds must be broken before the molecules can move faster.
- Significance: Buffers the temperature of oceans and the internal environment of organisms (Homeostasis).
- High Heat of Vaporization: Turning liquid water into gas requires significant energy.
- Significance: Evaporative Cooling. As sweat evaporates, it removes heat from the body, preventing overheating.
C. Expansion Upon Freezing
- Unlike most substances, water is less dense as a solid than as a liquid. H-bonds push molecules into a stable crystalline lattice.
- Significance: Ice floats, insulating the water below and allowing aquatic life to survive winter.
Quiz
Source: Campbell Biology Practice Test - Chapter 3