NSSCAS Chemistry (8224) flashcards
NSSCAS Chemistry (8224) · Build recall with a focused review session.
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- Atomic number
- The number of protons in the nucleus of an atom; it uniquely identifies the element and equals the number of electrons in a neutral atom.
- Isotopes
- Atoms of the same element with the same number of protons but different numbers of neutrons, hence different mass numbers but identical chemical properties.
- First ionisation energy
- The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions.
- Ionic bonding
- The electrostatic attraction between oppositely charged ions in a giant ionic lattice, formed by transfer of electrons from metal to non-metal atoms.
- VSEPR theory
- Valence shell electron pair repulsion theory: electron domains around a central atom repel to be as far apart as possible, determining molecular shape and bond angles.
- Hydrogen bonding
- A strong intermolecular force between a hydrogen atom covalently bonded to N, O, or F and a lone pair on N, O, or F of another molecule.
- Mole
- The amount of substance containing the Avogadro number (6.022 × 10²³) of particles; one mole of any gas occupies 24 dm³ at r.t.p.
- Empirical formula
- The simplest whole-number ratio of atoms of each element in a compound; derived from percentage composition by dividing by atomic masses.
- Limiting reagent
- The reactant that is completely consumed first in a chemical reaction, determining the maximum amount of product that can be formed.
- Hess's law
- The enthalpy change of a reaction is independent of the route taken, provided initial and final conditions are the same; used to calculate unknown enthalpy changes.
- Standard enthalpy of formation
- The enthalpy change when one mole of a compound is formed from its elements in their standard states at 298 K and 100 kPa.
- Bond enthalpy
- The average enthalpy change when one mole of a particular covalent bond is broken in the gaseous state; used to estimate reaction enthalpy changes.
- Activation energy
- The minimum energy that colliding particles must possess for a reaction to occur; a catalyst lowers it by providing an alternative reaction pathway.
- Dynamic equilibrium
- A state in a reversible reaction where the forward and reverse rates are equal, so concentrations of reactants and products remain constant.
- Le Chatelier's principle
- When a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, the equilibrium shifts to oppose the change.
- pH
- A logarithmic measure of hydrogen ion concentration: pH = −log₁₀[H⁺]; values below 7 are acidic, above 7 are basic at 298 K.
- Buffer solution
- A solution containing a weak acid and its conjugate base (or weak base and conjugate acid) that resists pH change when small amounts of acid or base are added.
- Salt hydrolysis
- The reaction of ions from a dissolved salt with water, producing acidic or basic solutions depending on the strengths of the parent acid and base.
- Oxidation
- The loss of electrons by a species, accompanied by an increase in oxidation state; the oxidised species acts as a reducing agent.
- Standard electrode potential
- The electromotive force of a cell with the standard hydrogen electrode on the left and the half-cell under standard conditions on the right, measured in volts.
- Electrolysis
- The decomposition of an ionic compound by passing a direct electric current through it in molten or aqueous solution, producing oxidation at the anode and reduction at the cathode.
- Disproportionation
- A redox reaction in which the same element is simultaneously oxidised and reduced, forming two different products from one reactant.
- Group 2 trends
- Reactivity increases down Group 2 as ionisation energy decreases; hydroxide solubility increases and sulfate solubility decreases down the group.
- Halogen displacement
- A more reactive halogen displaces a less reactive halide from its aqueous salt solution, demonstrating decreasing oxidising power down Group 17.
- Transition metal properties
- Characteristic features include variable oxidation states, coloured compounds, catalytic activity, and formation of complex ions with ligands.
- Homologous series
- A family of organic compounds with the same general formula and functional group, showing gradual change in physical properties and similar chemical reactions.
- Electrophilic addition
- A reaction in which an electrophile attacks the electron-rich double bond of an alkene, forming a carbocation intermediate and then a saturated product.
- Tollens' reagent
- Ammoniacal silver nitrate solution used to test for aldehydes; aldehydes reduce it to a silver mirror, while ketones give no reaction.
- Esterification
- The reaction of a carboxylic acid with an alcohol in the presence of a strong acid catalyst to form an ester and water; it is reversible.
- Chromatography Rf value
- The retention factor: distance travelled by the spot divided by distance travelled by the solvent front; used to identify substances in a mixture.
- Concordant titres
- Titration results that agree within a small range (typically 0.1 cm³); they are averaged to improve accuracy and reduce random error.
- Systematic error
- A consistent error that affects all measurements in the same direction, often due to faulty apparatus or calibration, reducing accuracy of results.