Ocr As Chemistry A Definitions

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Activation energy

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The minimum energy required to start a reaction by the breaking of bonds.

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Average bond enthalpy

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The average enthalpy change that take place when breaking by homolytic fission one mole of a given type of bond in the molecules of a gaseous species.

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Binary compound

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A compound containing two elements only

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Catalyst

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A substance that increases the rate of a chemical reaction without being used up in the process. Provides an alternative route for the reaction with lower activation energy

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Collision theory

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Two reaction particles must collide for a reaction to occur. Must be in the correct orientation and have sufficient energy to overcome the activation energy of the reaction

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Dative covalent bond

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A shared pair of electrons in which the bonded pair has been provided by one of the bonding atoms only

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dehydration

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an elimination reaction in which water is removed from a saturated molecule to make an unsaturated molecule

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Delocalised electrons

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Electrons that are shared between more than two atoms

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Disproportionation

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A redox reaction in which the same element is both oxidised and reduced

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Dynamic equilibrium

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Equilibrium that exists in a closed system when the rate of of the forward reaction is equal to the rate of the reverse reaction and concentrations don't change

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Electronegativity

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A measure of the attraction of a bonded atom for the pair of electrons in a covalent bond

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Electrophile

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An atom or group of atoms that is attracted to an electron-rich centre, where it accepts a pair of electrons

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Electrophilic addition

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An addition reaction in which the first step is attack by an electrophile on a region of high electron density.

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Empirical formula

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A formula that shows the simplest whole number ratio of atoms of each element present in a compound

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First ionisation energy

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The energy required to remove one electron from each atom in one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions.

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Heterogenous catalysis

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A reaction in which the catalyst (solid) has a different physical state from the reactants (gas)

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Heterolytic fission

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The breaking of a covalent bond with both of the bonded electrons going to each atom, forming a cation and anion

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Homogenous catalysis

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A reaction in which the catalyst and reactants are in the same physical state. Mostly in aqueous or gaseous

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Homolytic fission

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The breaking of a covalent bond with one of the bonded electrons going to each atom, forming two radicals.

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Homologous series

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A series of organic compounds with the same functional group but with each successive member differing by CH2.

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Nucleophile

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An atom or group of atoms that is attracted to an electron-deficient carbon atom where it donates a pair of electrons to form a new covalent bond.

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Nucleophilic substitution

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A reaction in which a nucleophile is attracted to an electron-deficient carbon atom and replaced an atom or group of atoms on it.

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partial dissociation

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The splitting of some of a species in solution into aqueous ions

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Radical

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A species with an unpaired electron

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Relative atomic mass Ar

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The weighted mean mass of an atom of an element compared with one-twelfth of the mass of an atom of carbon-12

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Relative formula mass

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The weighted mean mass of the formula unit of a compound compared with one-twelfth of the mass of an atom of carbon-12

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Relative isotopic mass

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Mass of an isotope compared with one-twelfth of the mass of an atom of carbon-12

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Relative molecular mass, Mr

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The weighted mean mass of a molecule of a compound compared with one twelfth of the mass of an atom of carbon-12.

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Standard enthalpy of combustion

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The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions with all reactants and products in their standard states.

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Standard enthalpy change of formation

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The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard states.

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Standard enthalpy change of neutralisation

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The enthalpy change that accompanies the reaction of an acid by a base to form one mole of H2O under standard conditions with all the reactants and products in their standard states