Acids, Bases, and Proteins: Study Guide for GOB Chemistry

Acids and Bases

Definitions

Acids and bases are fundamental concepts in chemistry, especially in biological and environmental systems. Understanding their definitions and properties is essential for predicting chemical behavior.

• Acids: Proton donors (release H+ in water).

• Bases: Proton acceptors (accept H+ or release OH- in water).

Recognizing Acids and Bases in Reactions

• Strong Acids and Bases: Completely ionize in water.

  • Strong Acids: HCl, HBr, HI, HNO3, H2SO4, HClO4

  • Strong Bases: LiOH, NaOH, KOH, Ca(OH)2

• Weak Acids and Bases: Only partially ionize in water; equilibrium constants describe their behavior.

Neutralization and Salt Formation

Acids and bases react to give H2O and a salt. Always balance the equation.

• Example:

  • $\mathrm{HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)}$

  • $\mathrm{H_2SO_4(aq) + Ca(OH)_2(aq) \rightarrow 2H_2O(l) + CaSO_4(s)}$

Equilibrium (When Reactions Are Partially Reversible)

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, and concentrations of reactants and products remain constant.

• Equilibrium Constant ($K_{eq}$): Describes the ratio of product to reactant concentrations at equilibrium.

• Expression:

  • $K_{eq} = \frac{[\text{Products}]}{[\text{Reactants}]}$ (excluding solids and pure liquids)

• Le Châtelier’s Principle: If a system at equilibrium is disturbed, it will shift to counteract the disturbance.

  • Add reactants or remove products: shifts toward products.

  • Add products or remove reactants: shifts toward reactants.

  • Heat and pressure changes can also shift equilibrium.

Conjugate Acids and Bases

Acids and bases exist in pairs called conjugates. When an acid donates a proton, it forms its conjugate base; when a base accepts a proton, it forms its conjugate acid.

• Example: $\mathrm{NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-}$

• NH3 is a base; NH4+ is its conjugate acid.

pH and pOH Calculations

pH is a measure of the hydrogen ion concentration in a solution. It is calculated as:

• $\mathrm{pH = -\log[H^+]}$

• $\mathrm{pOH = -\log[OH^-]}$

• $\mathrm{pH + pOH = 14}$ (at 25°C)

Use these equations to determine acidity/basicity of solutions.

Proteins

Amino Acids: Structure and Categories

Amino acids are the building blocks of proteins. Each amino acid contains an amino group, a carboxyl group, a hydrogen atom, and a unique side chain (R group) attached to a central carbon.

• General Structure: $\mathrm{H_2N-CHR-COOH}$

• There are 20 standard amino acids, each with a different R group.

• Amino acids can be classified as nonpolar, polar, acidic, or basic based on their side chains.

Isomerism in Amino Acids

• Amino acids exist as L- and D- isomers; only L-amino acids are used in proteins.

Peptide Bonds and Protein Structure

• Peptide Bond: Formed by condensation (dehydration) reaction between the amino group of one amino acid and the carboxyl group of another.

  • $\mathrm{Amino~acid_1 + Amino~acid_2 \rightarrow Dipeptide + H_2O}$

• Hydrogen bonds formed through peptide bonds are crucial for higher-level protein structures.

Levels of Protein Structure

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

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

  • Alpha-helix: Coiled structure.

  • Beta-sheet: Flat, extended structure.

• Tertiary Structure: Overall 3D shape stabilized by interactions such as hydrogen bonds, hydrophobic interactions, ionic bonds, and disulfide bridges.

• Quaternary Structure: Assembly of multiple polypeptide chains into a functional protein (e.g., hemoglobin).

Denaturation of Proteins

Denaturation is the process by which proteins lose their native structure due to external stress (e.g., heat, pH changes), resulting in loss of function.

• Denaturation disrupts secondary, tertiary, and quaternary structures but not the primary structure.

Protein Functions

• Enzymes: Catalyze biochemical reactions.

• Antibodies: Recognize and bind to foreign molecules.

• Structural proteins: Provide support (e.g., collagen in skin, keratin in hair).

Enzyme Activity

• Enzymes lower the activation energy of reactions, increasing reaction rates.

• Enzyme specificity is due to the unique shape of the active site.

• Proximity and orientation of reactants in the active site facilitate reactions.