Chemistry (university) glossary
92 key terms from Chemistry (university), A to Z. Each is explained in one plain sentence and links to its free lesson.
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A
- Acid-base titrationAcids, bases and electrochemistry
- In a titration, a solution of known concentration (the titrant) is added slowly to a measured sample until the reaction is just complete.
- Activation energy and the Arrhenius equationChemical kinetics
- Activation energy (Ea) is the minimum energy needed for a collision to react.
- Atomic and ionic radiusAtomic structure and periodic trends
- Atomic radius shrinks across a period because Zeff grows, and grows down a group because new shells are added.
- Atomic emission spectrumAtomic structure and periodic trends
- When atoms are heated, they give off light at only a few exact colours.
- Atomic orbitalAtomic structure and periodic trends
- An orbital is a region around the nucleus where an electron is most likely to be found.
B
- Balancing redox equationsStoichiometry and reactions in solution
- The half-reaction method splits a redox reaction into an oxidation and a reduction half.
- Bond enthalpyThermochemistry and thermodynamics
- Bond enthalpy is the energy needed to break one mole of a particular bond in the gas phase.
- Bond polarity and dipole momentChemical bonding and molecular shape
- A bond is polar when electrons are shared unequally, giving one end a partial negative charge (δ−) and the other a partial positive charge (δ+).
- Brønsted-Lowry acids and basesAcids, bases and electrochemistry
- A Brønsted-Lowry acid gives away a proton (H⁺).
- Buffer solutionAcids, bases and electrochemistry
- A buffer resists pH changes.
C
- CalorimetryThermochemistry and thermodynamics
- Calorimetry measures heat by watching a temperature change.
- Carbocation stabilityOrganic reactions and mechanisms
- A carbocation is a carbon with three bonds and a positive charge.
- CatalystChemical kinetics
- A catalyst speeds up a reaction without being used up.
- Chirality and stereocentresOrganic structure and stereochemistry
- A molecule is chiral if it cannot be placed on top of its mirror image.
- Colligative propertiesIntermolecular forces, gases and solutions
- Colligative properties depend on the number of dissolved particles, not on what they are.
- Collision theoryChemical kinetics
- For two molecules to react, they must collide with enough energy and with the right orientation.
- Common ion effectChemical equilibrium and solubility
- Adding an ion that is already part of a dissolving equilibrium pushes the equilibrium back toward the solid.
- Conformations and torsional strainOrganic structure and stereochemistry
- Conformations are the different shapes a molecule makes by rotating around single bonds.
- Constitutional isomersOrganic structure and stereochemistry
- Constitutional (structural) isomers have the same molecular formula but different atom connections.
- Cyclohexane chairOrganic structure and stereochemistry
- Cyclohexane is not flat.
D
- Dalton's law of partial pressuresIntermolecular forces, gases and solutions
- In a mixture of gases, each gas adds its own pressure as if it were alone.
- Dynamic equilibriumChemical equilibrium and solubility
- A reversible reaction reaches equilibrium when the forward and reverse rates are equal.
E
- E/Z isomerismOrganic structure and stereochemistry
- Rotation about a C=C double bond is blocked, so groups on each side stay fixed.
- E2 eliminationOrganic reactions and mechanisms
- In E2 elimination, a base removes a hydrogen while the leaving group departs from the next carbon, forming a double bond in one step.
- Effective nuclear chargeAtomic structure and periodic trends
- Effective nuclear charge (Zeff) is the pull an outer electron actually feels from the nucleus.
- ElectrolysisAcids, bases and electrochemistry
- Electrolysis uses electricity to force a non-spontaneous reaction.
- Electron configurationAtomic structure and periodic trends
- The electron configuration lists which orbitals an atom's electrons fill.
- ElectronegativityChemical bonding and molecular shape
- Electronegativity is how strongly an atom pulls shared electrons toward itself in a bond.
- Electrophilic addition to alkenesOrganic reactions and mechanisms
- The pi bond of an alkene is electron-rich and attacks electrophiles such as H–Br.
- Electrophilic aromatic substitutionOrganic reactions and mechanisms
- Benzene's ring is very stable because its electrons are spread around the ring (aromatic).
- Empirical and molecular formulaStoichiometry and reactions in solution
- The empirical formula is the simplest whole-number ratio of atoms in a compound.
- Enantiomers, diastereomers and meso compoundsOrganic structure and stereochemistry
- Enantiomers are non-superimposable mirror images.
- EnthalpyThermochemistry and thermodynamics
- Enthalpy change (ΔH) is the heat flow at constant pressure, which is how most lab reactions run. ΔH is negative for exothermic reactions (heat released) and positive for endothermic reactions (heat absorbed).
- EntropyThermochemistry and thermodynamics
- Entropy (S) measures how many ways energy and particles can be arranged, often called disorder.
- Equilibrium constantChemical equilibrium and solubility
- For aA + bB ⇌ cC + dD, the equilibrium constant is K = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ).
F
- First law of thermodynamicsThermochemistry and thermodynamics
- Energy cannot be created or destroyed, only moved.
- First-order integrated rate lawChemical kinetics
- For a first-order reaction, ln[A] = ln[A]₀ − kt.
- Formal chargeChemical bonding and molecular shape
- Formal charge is a bookkeeping number for an atom in a Lewis structure: valence electrons minus lone-pair electrons minus number of bonds.
- Free energy and the equilibrium constantChemical equilibrium and solubility
- The standard free energy change and K are linked: ΔG° = −RT ln K.
- Functional groupOrganic structure and stereochemistry
- A functional group is a small group of atoms in an organic molecule that decides how it reacts.
G
- Galvanic cellAcids, bases and electrochemistry
- A galvanic (voltaic) cell turns a spontaneous redox reaction into electric current.
- Gibbs free energyThermochemistry and thermodynamics
- Gibbs free energy combines enthalpy and entropy: ΔG = ΔH − TΔS.
H
- Half-lifeChemical kinetics
- The half-life (t½) is the time for a reactant's concentration to fall to half.
- Hess's lawThermochemistry and thermodynamics
- Hess's law says the total enthalpy change of a reaction is the same whether it happens in one step or many.
- HybridizationChemical bonding and molecular shape
- Hybridization mixes atomic orbitals on one atom to make new, equal orbitals that match the observed shape.
- Hydrogen bondingIntermolecular forces, gases and solutions
- A hydrogen bond is a strong attraction between a hydrogen atom bonded to N, O or F and a lone pair on another N, O or F.
I
- ICE tableChemical equilibrium and solubility
- An ICE table tracks Initial, Change and Equilibrium amounts to find an unknown equilibrium concentration.
- Ideal gas lawIntermolecular forces, gases and solutions
- The ideal gas law is PV = nRT.
- Intermolecular forcesIntermolecular forces, gases and solutions
- Intermolecular forces are attractions between separate molecules.
- Ionic bonding and lattice energyChemical bonding and molecular shape
- In an ionic bond, one atom hands electrons to another, making positive and negative ions that attract.
- Ionization energyAtomic structure and periodic trends
- Ionization energy is the energy needed to pull one electron off a gas-phase atom.
- IUPAC naming of alkanesOrganic structure and stereochemistry
- Find the longest carbon chain for the parent name (meth, eth, prop, but, pent, hex...).
K
- Kinetic molecular theoryIntermolecular forces, gases and solutions
- Kinetic molecular theory models a gas as tiny particles in constant random motion with no attraction between them.
- Kp and KcChemical equilibrium and solubility
- For gas reactions, K can be written with partial pressures (Kp) or concentrations (Kc).
L
- Le Chatelier's principleChemical equilibrium and solubility
- If a system at equilibrium is disturbed, it shifts to partly cancel the change.
- Lewis structureChemical bonding and molecular shape
- A Lewis structure shows every valence electron as bonds (shared pairs) or lone pairs.
- Limiting reagentStoichiometry and reactions in solution
- The limiting reagent is the reactant that runs out first and so decides how much product forms.
M
- Method of initial ratesChemical kinetics
- To find orders, run several experiments with different starting concentrations and measure the initial rate.
- Molarity and dilutionStoichiometry and reactions in solution
- Molarity (M) is moles of solute per litre of solution.
- Mole and molar massStoichiometry and reactions in solution
- A mole is 6.022×10²³ particles (Avogadro's number).
- Mole ratio from a balanced equationStoichiometry and reactions in solution
- The coefficients in a balanced equation give the ratio of moles reacting.
- Molecular orbital theoryChemical bonding and molecular shape
- In molecular orbital (MO) theory, atomic orbitals combine to make orbitals spread over the whole molecule.
N
- Nernst equationAcids, bases and electrochemistry
- The Nernst equation gives the cell potential when concentrations are not standard: E = E° − (RT/nF) ln Q.
- Nucleophiles, electrophiles and curved arrowsOrganic reactions and mechanisms
- A nucleophile is electron-rich and donates a pair of electrons.
- Nucleophilic acyl substitutionOrganic reactions and mechanisms
- A carboxylic acid derivative has a leaving group on a carbonyl carbon.
- Nucleophilic addition to carbonylsOrganic reactions and mechanisms
- The carbon of a C=O group is partly positive, so nucleophiles attack it, pushing the pi electrons onto oxygen.
O
- Oxidation of alcoholsOrganic reactions and mechanisms
- Oxidising an alcohol removes hydrogen and adds a C=O.
- Oxidation state and redoxStoichiometry and reactions in solution
- Redox reactions move electrons.
P
- Pauli exclusion principle and Hund's ruleAtomic structure and periodic trends
- Pauli: no two electrons in an atom can have all four quantum numbers the same, so an orbital holds at most two, with opposite spins.
- Percent yieldStoichiometry and reactions in solution
- Theoretical yield is the maximum product the equation predicts.
- pH and KwAcids, bases and electrochemistry
- pH = −log[H⁺].
- Photon energyAtomic structure and periodic trends
- Light comes in tiny packets called photons.
- Precipitation reaction and net ionic equationStoichiometry and reactions in solution
- In a precipitation reaction, two soluble salts react and an insoluble solid, the precipitate, forms.
Q
- Quantum numbersAtomic structure and periodic trends
- Four numbers describe each electron in an atom. n is the shell (size and energy), l is the subshell shape, mₗ is the orbital direction and mₛ is the spin, +½ or −½.
R
- R/S configurationOrganic structure and stereochemistry
- The Cahn-Ingold-Prelog (CIP) rules label a stereocentre R or S.
- Rate law and reaction orderChemical kinetics
- A rate law links rate to concentrations: rate = k[A]ᵐ[B]ⁿ.
- Reaction mechanismChemical kinetics
- A mechanism is the series of simple steps (elementary steps) that make up an overall reaction.
- Reaction quotientChemical equilibrium and solubility
- The reaction quotient Q has the same form as K but uses the concentrations right now.
- Reaction rateChemical kinetics
- Reaction rate is how fast a reactant is used up or a product appears: change in concentration divided by time.
- Real gases and van der Waals equationIntermolecular forces, gases and solutions
- Real gases differ from ideal gases at high pressure and low temperature because their molecules take up space and attract each other.
- ResonanceChemical bonding and molecular shape
- Sometimes one Lewis structure cannot show a molecule correctly.
S
- Second law of thermodynamicsThermochemistry and thermodynamics
- In any spontaneous process, the total entropy of the system plus its surroundings increases: ΔS(universe) > 0.
- SN1 reactionOrganic reactions and mechanisms
- SN1 is substitution, nucleophilic, unimolecular.
- SN2 reactionOrganic reactions and mechanisms
- SN2 is substitution, nucleophilic, bimolecular.
- Solubility and 'like dissolves like'Intermolecular forces, gases and solutions
- A substance dissolves when attractions between solute and solvent are about as strong as the attractions it must break.
- Solubility productChemical equilibrium and solubility
- For a slightly soluble salt, Ksp is the equilibrium constant for its dissolving.
- Standard cell potentialAcids, bases and electrochemistry
- The standard cell potential is E°cell = E°cathode − E°anode, using standard reduction potentials in volts.
- Standard enthalpy of formationThermochemistry and thermodynamics
- The standard enthalpy of formation (ΔHf°) is the heat change when one mole of a compound forms from its elements in their standard states.
V
- Vapour pressureIntermolecular forces, gases and solutions
- Vapour pressure is the pressure of vapour in balance with its liquid in a closed container.
- VSEPR theoryChemical bonding and molecular shape
- VSEPR stands for valence shell electron pair repulsion.
W
- Wave-particle dualityAtomic structure and periodic trends
- Light behaves like a wave and like a stream of particles.
- Weak acids and KaAcids, bases and electrochemistry
- A weak acid only partly ionizes: HA ⇌ H⁺ + A⁻, with Ka = [H⁺][A⁻]/[HA].