OCR Chemistry Paper 1 Definitions

Isotopes

Atoms of the same element with different numbers of neutrons

Relative Atomic Mass

The average mass of one atom of an element relative to 1/12th of the mass of one atom of carbon-12

Relative Isotopic Mass

The mass of one atom of an isotope relative to 1/12th mass of one atom of carbon-12

Alkali

A substance that releases OH- ions in aqueous solution

Acid

A substance that releases H+ ions in aqueous solution

Strong Acid

An acid that fully dissociates in water

Weak Acid

An acid that partly dissociates in water

Orbital

A region that can hold up to two electrons

Ionic Bond

The electrostatic attraction between positive and negative ions

Metallic Bond

The attraction between positive ions and delocalized electrons

Covalent Bond

A shared pair of electrons

Electronegativity

The ability of an atom to attract the electrons in a covalent bond

Periodicity

A repeating pattern of properties which is shown across different periods

First Ionisation Energy

The energy needed to remove one electron from each atom in one mole of gaseous atoms

Disproportionation Reaction

A reaction in which the same element is both oxidized and reduced

Activation Energy

The minimum amount of energy needed for a reaction to take place

Enthalpy Change of Formation

The enthalpy change when one mole of a substance is formed from its elements in their standard states

Enthalpy Change of Combustion

The enthalpy change when one mole of a substance is completely burned in excess oxygen

Enthalpy Change of Neutralisation

The enthalpy change when an acid is neutralized by an alkali to form one mole of water

Average Bond Enthalpy

The energy required to break one mole of bonds in gaseous molecules

Catalyst

A substance that increases the rate of reaction without being used up

Homogenous Catalyst

A catalyst in the same state as the reactants

Heterogeneous Catalyst

A catalyst which is in a different state to the reactants

Bronsted-Lowry Acid

A proton donor

Bronsted-Lowry Base

A proton acceptor

Buffer

A solution which resists changes in pH when small amounts of acid and alkali are added to it

Lattice Enthalpy

The enthalpy change when one mole of an ionic compound is formed from its gaseous ions

Enthalpy Change of Solution

The enthalpy change when one mole of a substance is dissolved in water

Enthalpy Change of Hydration

The enthalpy change when one mole of aqueous ions is formed from one mole of gaseous ions

Oxidation

Increase in oxidation number or loss of electrons

Oxidising Agent

A substance which is itself reduced/gains electrons

Reduction

Decrease in oxidation number or the gain of electrons

Reducing Agent

A substance which is itself oxidized/loses electrons

Transition Element

An element which forms at least one ion with a partly filled d subshell

Complex Ions

Consist of a metal ion which is bonded to a number of ligands by dative covalent (coordinate) bonds

Ligand

A species which can donate an electron pair to a metal ion to form a dative covalent bond

Bidentate Ligand

A species which donates two electron pairs to a metal ion to form two dative covalent bonds