OCR A Level Chemistry Definitions

Activation Energy

The minimum energy required to start a reaction by breaking bonds in the reactants

Addition Polymer

A very long molecular chain formed by repeated addition reactions of many unsaturated alkene molecules

Addition Reaction

A reaction in which a reactant is added to an unsaturated molecule to make a saturated molecule

Acid Dissociation Constant

Ka\= [H+(aq)][A-(aq)]/[HA(aq)]

Aliphatic Hydrocarbon

A hydrocarbon where carbon atoms are joined together in straight or branched chains

Alkali

A base that dissolves in water forming OH- ions

Alkanes

The homologous series with general formula CnH2n+2

Alkylammonium salt

A compound where the hydrogen's on an ammonium ion have been substituted by alkyl chains

Alkylation

Addition of hydrocarbon chains to an organic compound

Alkyl group

An alkane with a hydrogen atom removed e.g. CH3, C2H5

Amide

A compound with a functional group made of an alcyl group, which is directly attached to an amine

Amphoteric chemicals

Chemicals that can react with both acids and bases

Anion

A negatively charged ion

Anhydrous

A substance that contains no water molecules

Arrhenius plot

A graph of lnK \= lnA - Ea/R x 1/T

Atomic Orbital

A region of space where you will likely find electrons. Each orbital can hold 2 electrons, with opposite spins

Atomic number

The number of protons in the nucleus of an atom

Average bond enthalpy

The mean energy needed for 1 mole of a given type of gaseous bonds to undergo homolytic fission

Avogadro constant

The number of atoms per mole of the carbon-12 isotope (6.02x10^23)

Base

A chemical that will react with an acid

Benzene

A naturally occurring aromatic compound, which is very stable planar ring structure with delocalized electrons

Benzene Derivative

A benzene ring that has undergone a substitution reaction

Boltzman Distribution

The distribution of energies of molecules at a particular temperature

Bronsted-Lowry acid

A proton donor

Bronsted- Lowry base

A proton acceptor

Buffer solution

A mixture that minimizes pH changes on addition of small amounts of acid or base

Carbocation

An organic ion in which a carbon atom has a positive charge

Catalyst

A substance that increases the rate of a reaction by providing an alternative route to start the reaction with lower activation energy, without being used up in the process

Cation

A positively charged ion

Chemical Shift

The scale that compares the frequency of NMR absorption with the frequency of the reference peak of TMS

Chiral Carbon

A carbon with 4 different groups attached to it

Chromatogram

A visible record showing the result of separation of the components of a mixture by chromatography

Cis-trans isomerism

A type of E/Z isomerism where each carbon of the C\=C double bond carries the same atom or group

Complex ion

A transition metal ion bonded to one or more ligands by dative covalent bonds

Condensation polymer

The chemical reaction to form a long chain molecule by elimination of a small molecule e.g. water

Conjugate acid-base pair

Two species that transform into each other by the gain or loss of a proton

Coordination number

The total number of dative covalent bonds formed between a central metal ion and its ligands

Covalent bond

A bond formed by a shared pair of electrons between nuclei

Curly arrow

A symbol used in reaction mechanisms to show the movement of an electron pair

Dative covalent bond

A bond formed by a shared pair of electrons that has been provided by one of the bonding atoms only (AKA coordinate bond)

Degradable polymer

A polymer that breaks down into smaller fragments when exposed to light, heat or moisture

Delocalised electrons

Electrons that are shared between more than 2 atoms

Deuterium

An isotope of hydrogen and does not produce a signal in proton NMR

Directing effect

How a functional group attached directly to an aromatic ring affects which carbon atoms are more likely to undergo substitution

Displayed formula

A formula which shows the relative positions of atoms and the bonds between them

Disproportionation

The oxidation and reduction of the same element in a redox reaction

Distillation

A technique used to separate miscible liquids or solutions

Dynamic Equilibrium

The equilibrium that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction and all the chemicals have their concentrations maintained

Electron configuration

The arrangement of electrons in an atom or ion

Electrophile

An electron pair acceptor

Electrophilic substitution

A substitution reaction where an electrophile is attracted to an electron rich atom or part of a molecule and a new covalent bond is formed by the electrophile accepting a pair of electrons

Elimination reaction

An organic chemical reaction in which one reactant forms 2 products, usually a small molecule is released

Empirical formula

The simplest whole number ratio of atoms of each element present in a compound

Enantiomer

An optical isomer

Endothermic

A reaction in which the enthalpy of products is greater than the enthalpy of reactants, so heat is taken in from the surroundings (positive delta H)

End point

The point in a titration at which there are equal concentrations of the weak acid and conjugate base forms of the indicator

Enthalpy cycle

A diagram showing alternative routes between reactants and products

Enthalpy profile diagram

A diagram of a reaction that allows you to compare the enthalpy of the reactants with the enthalpy of the products

Enthalpy, H

The heat content that is stored in a chemical system

Entropy, S

A measure of disorder in a system

Equivalence point

The point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution. This matches the stoichiometry of the reaction taking place

Equivalent carbon atoms

Carbon atoms bonded to the same atoms that therefore experience the same magnetic field in the NMR

Equivalent protons

Hydrogen atoms bonded to the same atoms that therefore experience the same magnetic field in the NMR

Esterification

The chemical reaction which forms an ester

Exothermic

A reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, so heat is lost (negative delta H)

E/Z isomerism

A type of stereoisomerism that is caused by the restriction of rotation around the double bond. 2 different groups are attached to each carbon atom of the C\=C bond

First electron affinity

The enthalpy change when 1 mole of gaseous 1- ions are formed from gaseous atoms

First ionization energy

The energy change when 1 mole of electrons is removed from 1 mole of gaseous atoms to from 1 mole of gaseous 1+ ions

Free energy change, delta G

Delta G\= delta H-T x delta S. The reaction can take place when delta G

Friedel Crafts reaction

A substitution reaction where hydrogen is exchanged for an alkyl or alcyl chain. They allow electrophilic substitution to occur on an aromatic ring

Functional group

A group of atoms that is responsible for the characteristic chemical reactions of a compound

General formula

The simplest algebraic formula for a homologous series

Geometric/stereo isomers

Molecules that have the same structural formula but a different arrangement in space

Giant ionic lattice

3D structure of atoms that are all bonded together by strong covalent bonds

Giant metallic lattice

3D structure of positive ions and delocalized electrons, bonded together by strong metallic bonds

Half-life

The time taken for the concentration of a reactant to reduce by half

Hess' Law

States that the enthalpy change in a chemical reaction is independent of the route it takes

Heterogeneous equilibrium

An equilibrium in which species making up the reactants and products are in different physical states

Homogenous equilibrium

An equilibrium in which all species making up the reactants and products are the same physical state

Homologous series

A series of organic compounds that have the same functional group with successive members differing by CH2

Hydrolysis

A chemical reaction where water is a reactant in a decomposition reaction

Intermediate

A species formed in one step of a multi-step reaction that is used up in a subsequent step, and is not seen in the overall equation

Ionic bonding

The electrostatic attraction between oppositely charged ions

Kw

\[H+(aq)][OH-(aq)]. At 25c Kw\=1.00x10^-14

Isoelectric point

The pH value at which the amino acid exists as a zwitterion

Lattice enthalpy

The enthalpy change when one mole of an ionic lattice is formed from its gaseous ions under standard conditions

Le Chatelier's principle

States that when a system in dynamic equilibrium is subjected to change, the position of equilibrium will shift to minimize the change

Lewis acid

An electron pair acceptor

Ligand

A molecule or ion that can donate a pair of electrons to the transition metal ion to form a coordinate bond

Ligand substitution

A reaction in which one ligand in a complex ion is replaced by another ligand

Lone pair

An outer shell pair of electrons that is not involved in chemical bonding

Mass number

The number of protons and neutrons in a nucleus

Mobile phase

The phase that moves in chromatography

Moles

n\= MxMr

Mole fraction

A measure of how much of a given substance is present in a reaction mixture

Molecular formula

Shows the numbers and type of the atoms of each element in a compound

Molecular ion

The positive ion formed in mass spectrometry when a molecule loses an electron

Monomers

Small molecules used to make polymers

Neutralisation

A chemical reaction in which an acid and a base react together to produce a salt and water

Nitrile

An organic chemical with a -CN group