Chemistry Glossary — Key Terms and Definitions

graph TD
    R[Reactants] -->|energy| P[Products]
    R -->|catalyst| P

Accuracy: How close a measurement is to the true or accepted value. Related to Precision.

Atom: The smallest unit of an element that retains the chemical properties of that element. Consists of a nucleus (protons + neutrons) and electrons.

Avogadro’s Number: The number of particles in one mole: 6.022 × 10²³. Relates atomic mass to molar mass.

Bond Energy: The energy required to break a chemical bond. Measured in kJ/mol. Higher bond energy means a stronger bond.

Catalyst: A substance that increases the rate of a chemical reaction without being consumed. Lowers the activation energy.

Chemical Equation: A symbolic representation of a chemical reaction. Shows reactants on the left and products on the right.

Chemical Bond: An attractive force holding atoms together in a molecule or compound. Types include ionic, covalent, and metallic bonds.

Chemical Reaction: A process where substances are transformed into different substances through the breaking and forming of chemical bonds.

Concentration: The amount of solute in a given volume of solution. Common units: molarity (mol/L), mass percent, ppm.

Electron Configuration: The distribution of electrons in an atom’s orbitals. Follows the Aufbau principle, Pauli exclusion, and Hund’s rule.

Element: A pure substance consisting of only one type of atom, defined by its atomic number (number of protons).

Empirical Formula: The simplest whole-number ratio of atoms in a compound. For glucose (C₆H₁₂O₆), the empirical formula is CH₂O.

Hypothesis: A testable explanation for an observation. Forms the basis of scientific experiments. Related to Theory.

Molar Mass: The mass of one mole of a substance, equal to the atomic or molecular mass in g/mol.

Mole: A unit of amount of substance containing 6.022 × 10²³ particles. Bridges atomic and macroscopic scales.

Periodic Table: A tabular arrangement of elements by atomic number, electron configuration, and chemical properties.

Scientific Method: A systematic approach to understanding nature: observation → hypothesis → experiment → analysis → conclusion.

Significant Figures: Digits in a measurement that carry meaning about its precision. Rules govern how many digits to report.

Solute: The substance dissolved in a solvent to form a solution. Related: Solvent, Solution.

Solution: A homogeneous mixture of two or more substances. The solute is dissolved in the solvent.

Solvent: The substance that dissolves the solute. In aqueous solutions, water is the solvent.

Standard State: The reference state for thermodynamic quantities: 1 atm pressure and specified temperature (usually 25°C).

Theory: A well-substantiated explanation of natural phenomena supported by extensive evidence. More robust than a Hypothesis.

Activation Energy: The minimum energy required for a chemical reaction to occur. Denoted Eₐ. Higher Eₐ means slower reaction.

Arrhenius Equation: Relates reaction rate to temperature: k = Ae^(-Eₐ/RT), where k is rate constant, A is pre-exponential factor.

Boyle’s Law: At constant temperature, the pressure of a gas is inversely proportional to its volume: PV = constant.

Charles’s Law: At constant pressure, the volume of a gas is directly proportional to its temperature: V/T = constant.

Collision Theory: Chemical reactions occur when particles collide with sufficient energy and proper orientation.

Dynamic Equilibrium: A state where forward and reverse reaction rates are equal. Concentrations remain constant but reactions continue.

Electrochemistry: The study of chemical reactions involving electron transfer. Includes galvanic and electrolytic cells.

Endothermic: A reaction that absorbs heat from the surroundings. ΔH is negative for the surroundings, positive for the system.

Entropy: A measure of disorder or randomness. Increases in spontaneous processes. Related: Second Law of Thermodynamics.

Exothermic: A reaction that releases heat to the surroundings. ΔH is negative for the system.

Gibbs Free Energy: The energy available to do work: ΔG = ΔH - TΔS. Negative ΔG indicates a spontaneous process.

Hess’s Law: The total enthalpy change of a reaction is independent of the path taken. Allows calculation of ΔH from known values.

Ideal Gas Law: PV = nRT. Relates pressure, volume, temperature, and amount of gas. R = 8.314 J/(mol·K).

Kinetics: The study of reaction rates and mechanisms. Factors affecting rate: concentration, temperature, catalysts, surface area.

Le Chatelier’s Principle: If a system at equilibrium is disturbed, it shifts to counteract the change and restore equilibrium.

Nernst Equation: Relates cell potential to standard potential and concentrations: E = E° - (RT/nF)lnQ.

Oxidation: Loss of electrons by a substance. Oxidation states increase. Always accompanied by Reduction in a redox reaction.

Reduction: Gain of electrons by a substance. Oxidation states decrease. Always accompanied by Oxidation.

Redox Reaction: A reaction involving electron transfer between species. One species is oxidized, another is reduced.

Second Law of Thermodynamics: The total entropy of an isolated system always increases for spontaneous processes.

Thermodynamics: The study of energy and its transformations. Deals with heat, work, and the feasibility of processes.

Alcohol: An organic compound containing a hydroxyl (-OH) group bonded to a carbon atom. General formula: R-OH.

Aldehyde: An organic compound containing a formyl group (-CHO). More reactive than ketones due to the exposed carbonyl.

Alkane: A saturated hydrocarbon with only single bonds. General formula: CₙH₂ₙ₊₂. Includes methane, ethane, propane.

Alkene: An unsaturated hydrocarbon containing at least one carbon-carbon double bond. General formula: CₙH₂ₙ.

Alkyne: An unsaturated hydrocarbon containing at least one carbon-carbon triple bond. General formula: CₙH₂ₙ₋₂.

Aromatic Compound: A cyclic, planar compound with delocalized π electrons following Hückel’s rule (4n+2 π electrons). Example: benzene.

Carbonyl Group: A functional group containing a carbon-oxygen double bond (C=O). Found in aldehydes, ketones, carboxylic acids.

Carboxylic Acid: An organic compound containing a carboxyl group (-COOH). Weak acids that partially dissociate in water.

Chirality: The property of a molecule that makes it non-superimposable on its mirror image. Chiral molecules have enantiomers.

Ester: An organic compound formed from the reaction of a carboxylic acid and an alcohol. General formula: R-COO-R’.

Functional Group: A specific group of atoms within a molecule that determines its chemical properties and reactivity.

Hydrocarbon: An organic compound containing only carbon and hydrogen. Includes alkanes, alkenes, alkynes, and aromatics.

Isomer: Molecules with the same molecular formula but different structural arrangements. Types: structural, geometric, optical.

Ketone: An organic compound containing a carbonyl group (C=O) bonded to two carbon atoms. General formula: R-CO-R’.

Molecular Orbital Theory: Describes bonding as the combination of atomic orbitals to form molecular orbitals that span the entire molecule.

Nucleophilic Substitution: A reaction where a nucleophile replaces a leaving group. SN1 (unimolecular) and SN2 (bimolecular) mechanisms.

Polymer: A large molecule made of repeating structural units (monomers). Examples include polyethylene, nylon, and DNA.

Reaction Mechanism: The step-by-step sequence of elementary reactions by which an overall chemical change occurs.

Saturated Compound: An organic compound with only single bonds between carbon atoms. Contains the maximum number of hydrogen atoms.

Stereochemistry: The study of the three-dimensional arrangement of atoms in molecules and its effect on chemical reactions.

Unsaturated Compound: An organic compound containing at least one double or triple bond between carbon atoms.

Acid: A substance that donates protons (H⁺) in aqueous solution (Brønsted-Lowry) or accepts electron pairs (Lewis).

Acid-Base Reaction: A reaction involving proton transfer between acid and base. Produces water and a salt in neutralization.

Base: A substance that accepts protons (H⁺) in aqueous solution (Brønsted-Lowry) or donates electron pairs (Lewis).

Buffer: A solution that resists changes in pH when small amounts of acid or base are added. Made of a weak acid and its conjugate base.

Chelate: A complex formed when a polydentate ligand forms two or more coordinate bonds with a metal ion.

Complex Ion: An ion consisting of a central metal atom or ion bonded to surrounding ligands. Example: [Cu(NH₃)₄]²⁺.

Conjugate Acid-Base Pair: Two species that differ by one proton. The conjugate acid donates the proton; the conjugate base accepts it.

Coordination Compound: A compound containing a central metal ion bonded to ligands. The number of ligands is the coordination number.

Crystal Field Theory: Describes the electronic structure of transition metal complexes. Ligands split the d-orbitals into different energy levels.

Ligand: A molecule or ion that donates an electron pair to a central metal ion to form a coordinate bond.

pH: A measure of acidity: pH = -log[H⁺]. pH < 7 is acidic; pH = 7 is neutral; pH > 7 is basic.

pKa: The negative logarithm of the acid dissociation constant: pKa = -log(Ka). Lower pKa means a stronger acid.

Transition Metal: A metal in the d-block of the periodic table. Often forms colored compounds, acts as a catalyst, and has variable oxidation states.

Amino Acid: The building block of proteins. Contains an amino group (-NH₂), a carboxyl group (-COOH), and a variable R group.

Carbohydrate: Organic compounds with the general formula Cₙ(H₂O)ₙ. Includes sugars, starches, and cellulose.

DNA (Deoxyribonucleic Acid): A nucleic acid that stores genetic information. Structure: double helix of nucleotide base pairs.

Enzyme: A biological catalyst, usually a protein, that speeds up specific biochemical reactions. Highly specific and regulated.

Lipid: A diverse group of organic compounds insoluble in water but soluble in organic solvents. Includes fats, oils, and waxes.

Nucleic Acid: A polymer of nucleotides that stores and transmits genetic information. Types: DNA and RNA.

Protein: A polymer of amino acids linked by peptide bonds. Performs structural, enzymatic, transport, and regulatory functions.

RNA (Ribonucleic Acid): A nucleic acid involved in protein synthesis. Types: mRNA, tRNA, rRNA.

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