Chemical Reactions and Properties of Water

Law of Conservation of Matter in Chemical Reactions

The law of conservation of matter states that matter cannot be created or destroyed in a chemical reaction. This means that the number of atoms of each element must be the same in the reactants and the products of a chemical equation.

• Balancing Equations: To obey this law, chemical equations must be balanced so that the number of each type of atom on the reactant side equals the number on the product side.

• Example: For the reaction of hydrogen and oxygen to form water: There are 4 hydrogen atoms and 2 oxygen atoms on both sides of the equation.

Emergent Properties of Water

Water exhibits four key emergent properties due to hydrogen bonding, which are critical for life:

• Cohesion and Adhesion: Water molecules stick to each other (cohesion) and to other substances (adhesion), enabling processes like water transport in plants.

• Moderation of Temperature: Water has a high specific heat, allowing it to absorb or release large amounts of heat with little temperature change. This stabilizes environments and organisms.

• Expansion Upon Freezing: Water is less dense as a solid than as a liquid, so ice floats. This insulates aquatic environments in cold climates.

• Versatility as a Solvent: Water's polarity allows it to dissolve many substances, making it the "universal solvent" and facilitating biochemical reactions.

Role of Hydrogen Bonding: These properties arise because water molecules form hydrogen bonds with each other.

Acids, Bases, and pH

pH is a measure of hydrogen ion (H+) concentration in a solution. It is defined as:

• Acids: Substances with pH < 7, high [H+], low [OH-]

• Bases: Substances with pH > 7, low [H+], high [OH-]

• Neutral Water: pH = 7, [H+] = [OH-] = M at 25°C

Comparing pH Values of Solutions

• More Acidic: Lower pH value, higher [H+]

• More Basic: Higher pH value, higher [OH-]

• Quantitative Difference: Each pH unit represents a tenfold difference in [H+]. For example, a solution with pH 4 has 10 times more H+ than one with pH 5.

Buffers and pH Stability

Buffers are substances that minimize changes in pH by absorbing or releasing H+ or OH- ions. This is crucial in biological systems to maintain homeostasis and proper enzyme function.

• Example: The bicarbonate buffer system in blood helps maintain a stable pH.

Carbon Chemistry and Biological Diversity

Chemistry of Carbon and Biological Diversity

Carbon's unique ability to form four covalent bonds allows it to create a vast array of complex molecules, leading to the diversity of organic compounds essential for life.

• Tetravalence: Carbon can form four bonds, enabling branching and ring structures.

• Variety: Carbon forms single, double, and triple bonds, and bonds with many elements.

Variations in Hydrocarbon Skeletons

Hydrocarbon skeletons can differ in four main ways, contributing to molecular diversity:

• Length: Carbon chains vary in length (e.g., ethane vs. octane).

• Branching: Chains may be unbranched or branched (e.g., butane vs. isobutane).

• Double Bond Position: Double bonds can be in different locations (e.g., 1-butene vs. 2-butene).

• Rings: Carbon skeletons may form rings (e.g., cyclohexane).

Isomers

Isomers are compounds with the same molecular formula but different structures and properties.

• Structural Isomers: Differ in covalent arrangement of atoms.

• Cis-trans Isomers: Differ in spatial arrangement around double bonds.

• Enantiomers: Mirror images of each other, important in pharmaceuticals.

• Different Molecules: Have different molecular formulas.

• Same Molecule: Identical in both formula and structure.

Functional Groups in Organic Molecules

Functional groups are specific groups of atoms attached to carbon skeletons that confer particular chemical properties to organic molecules.

• Purpose: Determine the reactivity and function of organic molecules in biological systems.

• Examples: Hydroxyl, carbonyl, carboxyl, amino, phosphate.

Common Functional Groups

Macromolecules and Their Synthesis

Building Large Macromolecules

Cells build large biological molecules, or macromolecules, by joining smaller units called monomers through covalent bonds.

• Polymerization: The process of linking monomers to form polymers.

Dehydration and Hydrolysis Reactions

• Dehydration Reaction: Monomers are joined by removing a water molecule, forming a covalent bond.

• Hydrolysis Reaction: Polymers are broken down by adding a water molecule, breaking the covalent bond.

• Example: Formation and breakdown of starch in plants.

Classes and Subclasses of Macromolecules

There are four major classes of biological macromolecules, each with distinct properties and functions.

Protein Structure and Folding

Proteins have four levels of structure that determine their shape and function:

• Primary Structure: Sequence of amino acids in a polypeptide chain.

• Secondary Structure: Local folding into alpha-helices and beta-sheets, stabilized by hydrogen bonds.

• Tertiary Structure: Overall 3D shape of a single polypeptide, determined by interactions among R groups.

• Quaternary Structure: Association of multiple polypeptide chains into a functional protein.

• Relationship: The primary structure dictates higher-level structures and ultimately the protein's function.

Evolution of Lactose Tolerance

Lactose tolerance in some human populations is an example of recent human evolution. It results from genetic mutations that allow the enzyme lactase to be produced into adulthood, enabling the digestion of lactose, the sugar in milk.

• Significance: This adaptation is linked to the domestication of dairy animals and the consumption of milk products.