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Chemical Equilibrium

Relationship between K and Rate Constants

The equilibrium constant of an elementary reaction equals the ratio of its forward and reverse rate constants.

K = k_forward / k_reverse Various
Acids, Bases & Salts

Relationship between Ka and Kb

The reciprocal link between the strengths of an acid and its conjugate base.

Ka × Kb = Kw Dimensionless
Chemical Equilibrium

Relationship between Kp and Kc

The conversion linking the pressure and concentration forms of the equilibrium constant for a gaseous reaction.

Kp = Kc(RT)^Δn Dimensionless
Foundations of Chemistry

Relative Atomic Mass

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) Dimensionless
Solutions & Colligative Properties

Relative Lowering of Vapour Pressure

The fractional reduction in a solvent's vapour pressure caused by dissolving a non-volatile solute.

(p° − p) / p° = x_solute Dimensionless
Chemical Bonding & Molecular Structure

Resonance

The description of a molecule whose true structure is a weighted average of two or more valid Lewis structures.

A ↔ B ↔ C Dimensionless
Organic Chemistry

Resonance Effect

The delocalisation of pi or lone pair electrons through a conjugated system, altering electron density at distant atoms.

−M and +M effects Dimensionless
Chemical Bonding & Molecular Structure

Resonance Energy

The extra stability a molecule gains from electron delocalisation, measured against the most stable single Lewis structure.

ΔE_res = E(hybrid) − E(best structure) kJ/mol
Analytical Chemistry & Spectroscopy

Retention Factor

The ratio of the distance travelled by a component to the distance travelled by the solvent front.

Rf = d_spot / d_solvent Dimensionless
Solutions & Colligative Properties

Reverse Osmosis

The forced flow of solvent from a concentrated to a dilute solution by applying pressure greater than the osmotic pressure.

p_applied > π atm
Thermochemistry & Thermodynamics

Reversible Process

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 J
Stoichiometry & Chemical Reactions

Reversible Reaction

A reaction that can proceed in both the forward and reverse directions under the same conditions.

A + B ⇌ C + D Dimensionless
Biochemistry & Polymer Chemistry

RNA

A generally single-stranded nucleic acid that carries out the expression of genetic information.

Dimensionless
Stoichiometry & Chemical Reactions

Roasting

The heating of a sulfide ore strongly in a plentiful supply of air to convert it into the oxide.

2MS + 3O₂ → 2MO + 2SO₂ °C
States of Matter & Gas Laws

Root Mean Square Speed

The square root of the average of the squared molecular speeds in a gas sample.

c_rms = √(3RT / M) m/s
Electrochemistry & Redox

Rusting

The corrosion of iron in the presence of both oxygen and water to form hydrated iron(III) oxide.

4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O Dimensionless
Atomic Structure & Periodicity

Rutherford Model

The nuclear model of the atom, in which a tiny dense positive nucleus is surrounded by orbiting electrons in mostly empty space.

Dimensionless
Atomic Structure & Periodicity

Rydberg Formula

The empirical equation predicting the wavelengths of the spectral lines emitted by a hydrogen-like atom.

1/λ = R_H(1/n₁² − 1/n₂²) m⁻¹
Atomic Structure & Periodicity

s-Block Elements

The elements of groups 1 and 2, plus helium, whose outermost electrons occupy an s orbital.

ns¹ or ns² Dimensionless
Electrochemistry & Redox

Sacrificial Anode

A block of reactive metal attached to a structure so that it corrodes preferentially and protects the structure.

Dimensionless
Acids, Bases & Salts

Salt

An ionic compound formed when the hydrogen of an acid is replaced by a metal or other cation.

acid + base → salt + water Dimensionless
Electrochemistry & Redox

Salt Bridge

A conducting connection between the two half-cells of a galvanic cell that completes the circuit and maintains electrical neutrality.

Dimensionless
Acids, Bases & Salts

Salt Hydrolysis

The reaction of a salt's ions with water to produce an acidic or alkaline solution.

A⁻ + H₂O ⇌ HA + OH⁻ mol/L
Analytical Chemistry & Spectroscopy

Sampling

The process of selecting a small portion of material that genuinely represents the bulk being analysed.

g