A2 Chemistry definitions

Carbonyl group

C=O

General formula for aldehydes and ketones

CₙH₂ₙO

Nucleophile

A nucleophile is an ion or molecule with a lone pair of electrons that attacks regions of low electron density

In situ

made in the reaction

Optical isomers

Optical isomers are molecules that exist as non-superimposable mirror images

Chiral centre

A chiral centre is when an atom has 4 different atoms or groups attached to the central carbon atom

Optically active

A sample that rotates the plane of plane-polarized light

Racemic mixture

A 50:50 mixture of optical isomers

Bronsted-Lowry acid

A proton donor

Bronsted-Lowry base

A proton acceptor

Strong acid

A strong acid is fully dissociated into its ions in solution

Weak acid

A weak acid is partially dissociated into its ions in solution

Monobasic acid

Monobasic acids donate one proton per molecule of acid

Dibasic acid

Dibasic acids donate two protons per molecule of acid

Strong base

A strong base is fully dissociated into its ions in solution

Weak base

A weak base is partially dissociated into its ions in solution

soluble bases

alkalis

Amphoteric

A substance that can act as both a Bronsted-Lowry acid and base

Buffer

A buffer is a solution that resists changes in pH on addition of small amounts of acid or alkali

Rate of reaction

The change in concentration of a reactant or product with respect to time

Units for rate

mol dm⁻³ s⁻¹

rate equation

rate= k[A]x[B]y

Order of reaction

The power to which the concentration of a reactant is raised in the rate equation

Rate constant

The proportionality constant that links the rate of reaction to the concentrations in the rate equation

Overall order of the reaction

The sum of all the orders in the rate equation

Rate determining step

The slowest step in a mechanism for a reaction

Feasible reaction

∆G≤0

Entropy

Entropy is a measure of disorder or randomness in a system

Standard enthalpy of formation

The enthalpy change when one mole of a compound is formed from its elements under standard conditions

First ionisation energy

The energy required to convert one mole of gaseous atoms into gaseous ions with a single positive charge

Second ionisation energy

The energy required to convert one mole of gaseous ions with a single positive charge into gaseous ions with a double positive charge

First electron affinity

The enthalpy change when one mole of gaseous atoms is converted into gaseous ions with a single negative charge

Second electron affinity

The enthalpy change when one mole of gaseous ions with a single negative charge are converted into gaseous ions with a double negative charge

Enthalpy of atomisation

The enthalpy change when one mole of gaseous atoms are formed from the element in its standard state

Bond dissociation enthalpy

The energy required to break one mole of a covalent bond under standard conditions

Standard lattice enthalpy

The enthalpy change when one mole of an ionic compound is converted to gaseous ions