Chemistry A-level OCR keywords

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The amount of any substance containing as many elementary particles as there are carbon atoms in exactly 12 g of the carbon-12 isotope which is 6.023 x 10^23.
molar mass
the mass of one mole of a substance, in units of g mol^-1
Molar gas volume
The volume per mole of gas molecules at a stated temperature and pressure
Empirical formula
The simplest whole number ratio of atoms of each element present in a compound
Water of crystallisation
Water molecules that are bonded into a crystalline structure of a compound
Amount of substance
The quantity whose unit of the mole, used for counting any species such as atoms ions or molecules.
without water
Avagadro's constant
The number of atoms per mole of the carbon-12 isotope. 6.023 x 10^23
ionic bonding
The electrostatic attraction between oppositely charged ions
Covalent bonding
The strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
Dative covalent bond/ coordinate bond
A shared pair of electrons in which the bonded pair has been provided by one of the bonding atoms only.
A separation in electrical charge so that one atom of a polar covalent bond, or one end of a polar molecule, has a small positive charge and the other has a small negative charge
London forces
Attractive forces between induced dipoles in different molecules; also known as induced dipole-dipole interactions
Electronic configuration
A shorthand representation that shows how electrons occupy sub-shells in an atom
A measure of the attraction of a bonded atom for the pair of electrons in a covalent bond
Endothermic reaction
A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat being taken in from the surroundings (+ve).
The heat content that is stored in a chemical system
Enthalpy change
The difference between the enthalpy of the products and the enthalpy of the reactants
Exothermic reaction
A reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings (-ve).
First ionisation energy
The energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions
Displayed formula
A formula showing the relative positioning of all the atoms in a molecule and the bonds between them.
General formula
The simplest algebraic formula of a member of a homologous series
A crystalline compound containing water molecules
Hydrogen bond
A strong dipole-dipole attraction between an electron-deficient hydrogen atom of -NH, -OH or HF on one molecule and a lone pair of electrons on a highly electronegative atom containing N, O or F on a different molecule.
Intermolecular force
forces of attraction between molecules
a positively or negatively charged atom
Atoms of the same element that have different numbers of neutrons and different masses.
Lone pair
An outer shell pair of electrons that is not involved in chemical bonding
Metallic bond
The electrostatic attraction between positive metal ions and delocalised electrons.
molecular formula
A formula that shows the number and type of atoms of each element present in a molecule
Permanent dipole
A small charge difference that does not change across a bond, with positive and negative partial changes on the bonded atoms: the result of a bonded atoms having different electronegativities
Permanent dipole-dipole interaction
An attractive force between permanent dipoles in neighbouring polar molecules.
Polar covalent bond
A bond with a permanent dipole, having partial charges on the bonded atoms.
Polar molecule
A molecule with an overall dipole, having taken into account any dipoles across bonds and the shape of the molecule
Relative atomic mass
The weighted mean mass of an atom of an element compared with one-twelfth of the mass of an atom of carbon-12.
Relative formula mass
The weighted mean mass of the formula unit of a compound compared with one-twelfth of the mass of an atom of carbon-12.
Relative isotopic mass
The mass of an atom of an isotope compared with one-twelfth of the mass of an atom of carbon-12.
Relative molecular mass
The weighted mean mass of a molecule of a compound compared with one-twelfth of the mass of an atom of carbon-12.
the repulsion between electrons in different inner shells . It reduces the net attractive force between the positive nucleus on the outer shell electrons
Everything that is not the chemical system
The chemicals involved in the reaction
The ratio of the amount, in moles, of each substance in a chemical reaction
Standard state
The physical state of a substance under standard conditions of 100kPa and a stated temp (usually 298K)
Standard solution
a solution of known concentration
Standard conditions
298K,101kPa pressure (1 atm), 1 moldm^-3 when using solutions.
Standard enthalpy change of combustion
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states.
Standard enthalpy change of formation
the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.
Standard enthalpy change of reaction
The enthalpy change associated with any chemical reaction as shown by a chemical equation.
Hess’ Law
The overall enthalpy change for a chemical reaction is the same regardless of the number of steps in the reaction
Average bond enthalpy
The amount of energy required to break a bond.