Browse All 960 Chemistry Terms A–Z

Relationship between K and Rate Constants Chemical Equilibrium Various The equilibrium constant of an elementary reaction equals the ratio of its forward and reverse rate constants. K = k_forward / k_reverse

Relationship between Ka and Kb Acids, Bases & Salts Dimensionless The reciprocal link between the strengths of an acid and its conjugate base. Ka × Kb = Kw

Relationship between Kp and Kc Chemical Equilibrium Dimensionless The conversion linking the pressure and concentration forms of the equilibrium constant for a gaseous reaction. Kp = Kc(RT)^Δn

Relative Atomic Mass Foundations of Chemistry Dimensionless The weighted average mass of the atoms of an element, expressed relative to one twelfth the mass of a carbon-12 atom. A_r = Σ(isotope abundance × isotope mass)

Relative Lowering of Vapour Pressure Solutions & Colligative Properties Dimensionless The fractional reduction in a solvent's vapour pressure caused by dissolving a non-volatile solute. (p° − p) / p° = x_solute

Resonance Chemical Bonding & Molecular Structure Dimensionless The description of a molecule whose true structure is a weighted average of two or more valid Lewis structures. A ↔ B ↔ C

Resonance Effect Organic Chemistry Dimensionless The delocalisation of pi or lone pair electrons through a conjugated system, altering electron density at distant atoms. −M and +M effects

Resonance Energy Chemical Bonding & Molecular Structure kJ/mol The extra stability a molecule gains from electron delocalisation, measured against the most stable single Lewis structure. ΔE_res = E(hybrid) − E(best structure)

Retention Factor Analytical Chemistry & Spectroscopy Dimensionless The ratio of the distance travelled by a component to the distance travelled by the solvent front. Rf = d_spot / d_solvent

Reverse Osmosis Solutions & Colligative Properties atm The forced flow of solvent from a concentrated to a dilute solution by applying pressure greater than the osmotic pressure. p_applied > π

Reversible Process Thermochemistry & Thermodynamics J An idealised process carried out through a continuous series of equilibrium states, so it can be reversed by an infinitesimal change. w_rev = −∫p dV

Reversible Reaction Stoichiometry & Chemical Reactions Dimensionless A reaction that can proceed in both the forward and reverse directions under the same conditions. A + B ⇌ C + D

RNA Biochemistry & Polymer Chemistry Dimensionless A generally single-stranded nucleic acid that carries out the expression of genetic information.

Roasting Stoichiometry & Chemical Reactions °C The heating of a sulfide ore strongly in a plentiful supply of air to convert it into the oxide. 2MS + 3O₂ → 2MO + 2SO₂

Root Mean Square Speed States of Matter & Gas Laws m/s The square root of the average of the squared molecular speeds in a gas sample. c_rms = √(3RT / M)

Rusting Electrochemistry & Redox Dimensionless The corrosion of iron in the presence of both oxygen and water to form hydrated iron(III) oxide. 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O

Rutherford Model Atomic Structure & Periodicity Dimensionless The nuclear model of the atom, in which a tiny dense positive nucleus is surrounded by orbiting electrons in mostly empty space.

Rydberg Formula Atomic Structure & Periodicity m⁻¹ The empirical equation predicting the wavelengths of the spectral lines emitted by a hydrogen-like atom. 1/λ = R_H(1/n₁² − 1/n₂²)

s-Block Elements Atomic Structure & Periodicity Dimensionless The elements of groups 1 and 2, plus helium, whose outermost electrons occupy an s orbital. ns¹ or ns²

Sacrificial Anode Electrochemistry & Redox Dimensionless A block of reactive metal attached to a structure so that it corrodes preferentially and protects the structure.

Salt Acids, Bases & Salts Dimensionless An ionic compound formed when the hydrogen of an acid is replaced by a metal or other cation. acid + base → salt + water

Salt Bridge Electrochemistry & Redox Dimensionless A conducting connection between the two half-cells of a galvanic cell that completes the circuit and maintains electrical neutrality.

Salt Hydrolysis Acids, Bases & Salts mol/L The reaction of a salt's ions with water to produce an acidic or alkaline solution. A⁻ + H₂O ⇌ HA + OH⁻

Sampling Analytical Chemistry & Spectroscopy g The process of selecting a small portion of material that genuinely represents the bulk being analysed.

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