Chemistry Definitions

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Hydrocarbons

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organic compounds that contain hydrogen and carbon only

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Saturated hydrocarbons

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hydrocarbon with single bonds only

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unsaturated hydrocarbons

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hydrocarbons containing carbon to carbon multiple bonds

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aliphatic hydrocarbons

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hydrocarbons with carbon atoms joined together in straight or branched chains

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alicyclic hydrocarbons

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hydrocarbons with carbon atoms joined together in a ring structure

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functional group

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part of the organic molecule responsible for its chemical reactions

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homologous series

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

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alkanes

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homologous series with the general formula C_NH_2N+2

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nomenclature

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system of naming compounds

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alkyl group

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alkane with a hydrogen atom removed, e.g CH_3 or C_2H_5. alkyl groups are often shown as R

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general formula

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the simplest algebraic formula of a member of a homologous series. e.g general formula of alkanes is C_NH_2N+2

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displayed formula

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shows the relative positioning of all the atoms in a molecule and the bonds between them

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structural formula

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shows the minimal detail for the arrangement of atoms in a molecule

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skeletal formula

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simplified organic formula with hydrogen atoms removed from alkyl chains leaving just a carbon skeleton and associated functional groups

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structural isomers

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molecules with the same molecular formula but with different structural arrangements of atoms

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stereoisomers

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compounds with the same structural formula but with a different arrangement of the atoms in space

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E/Z isomerism

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type of stereoisomerism in which different groups attached to each carbon of a C=C bond may be arranged differently in space because of the restricted rotation of the C+C bond

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cis-trans isomerism

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special type of E/Z isomerism in which there is a non hydrogen group and hydrogen on each C of a C=C double bond. the cis isomer (Z) has the Hatoms on each carbon on the same side; the trans isomer (E) has the hydrogen on each carbon on different sides

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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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heterolytic fission

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the breaking of a covalent bond with both of the bonded electrons going to one of the atoms forming a cation (+) and an anion (-)

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radical

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a species with an unpaired electron

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nucleophile

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an atom (or group of atoms) that is attracted to an electron deficient centre or atom where it donates a pair of electrons to form a new 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 of atom where it accepts a pair of electrons to form a new covalent bond

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addition reaction

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

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substitution reaction

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a reaction in which an atom or group of atoms is replaced with an atom or group of atoms

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elimination reaction

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refers to the removal of a molecule from a saturated molecule to make an unsaturated molecule

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fractional distillation

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the separation of the components in a mixture into fractions which differ in boiling point (and hence chemical composition) by means of distillation, typically using a fractionating column

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cracking

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refers to the breaking down of long chained hydrocarbons to form a mixture of shorter chained alkanes and alkenes

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catalyst

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a substance that increases the rate if a chemical reaction without being used up in the process

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radical substitution

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type of substitution reaction in which a radical replaces a different atom or group of atoms

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mechanism

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sequence of steps showing the path taken by electrons in a reaction

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initiation

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the first step in a radical substitution in which the free radicals are generated by UV radiation

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propagation

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the two repeated steps in radical substitution that builds up the products in a chain reaction

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termination

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the final step in a radical substitution when two radicals combine to form a molecule

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pi bond

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the reactive part of a double bond formed above and below the plane of the bonded atoms by sideways overlap of p-orbitals

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electrophilic addition

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type of addition reaction in which an electrophile is attracted to an electron rich centre or atom where it accepts a pair of electrons to form a new covalent bond

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carbocation

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organic ion in which a carbon atom has a positive charge

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curly arrow

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symbol used in reaction mechanisms to show the movement of an electron pair in the breaking or formation of a covalent bond

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polymer

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long molecular chain built up from monomer units

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monomer

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a small molecule that combines with many other monomers to form a polymer

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addition polymerisation

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the process in which unsaturated alkene molecules (monomers) add on to a growing polymer chain one at a time to form a very long saturated molecular chain (addition polymer)

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repeat unit

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specific arrangement of atoms that occurs in the structure over and over again.

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biodegradable materials

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a material that is broken down naturally in the environment by living organisms

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volatility

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the ease that a liquid turns into a gas. volatility increases ad boiling point decreases

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reflux

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the continual boiling and condensing of a reaction mixture to ensure that the reaction takes place without the contents of the flask boiling dry

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esterification

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the reaction of on alchohol with a carboxylic acid to produce an ester and water

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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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hydrolysis

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a reaction with water or aqueous hydroxide ions that breaks a chemical compound into two compounds

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nucleophilic substitution

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a type of substitution reaction in which a nucleophile is attracted to an electron deficient centre or atom where it donates a pair of electrons to form a new covalent bond

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percentage yield

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actual moles / theoretical moles * 100

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atom economy

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desired molecular mass / total molecular mass * 100

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limiting reagent

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the substance in a chemical reaction that runs out first

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molecular ion M+

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

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fragmentation

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the process in mass spectrometry that causes a positive ion to split into pieces, one of which is a positive fragment ion

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enthalpy

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the heat content that is stored in a chemical system

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exothermic

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a reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings

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endothermic

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a reaction in which the enthalpy og the products is greater than the enthalpy of the reactants, resulting in heat being taken in from the surroundings

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enthalpy profile diagram

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a diagram for a reaction to compare the enthalpy of the reactants with the enthalpy of the products

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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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standard conditions

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pressure of 1 kPa (1 atm), 298 K (25 deg. cel.) and a concentration of 1 moldm^-3 (for reactions with aqueous solutions)

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standard state

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the physical state of a substance under the standard conditions of 100 kPa (1atm), 298 K

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standard enthalpy change of reaction

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the enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard state

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standard enthalpy of combustion

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the enthalpy change that takes place when one mole of a substance completely reacts with oxygen under standard conditions, all reactants and products being in their standard state

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standard enthalpy of formation

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the enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions. enthalpy change of formation of an element is 0 kJmol^-1

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specific heat capacity

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the energy required to raise the temperature of 1g of a substance by 1 deg. cel.

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bond enthalpy

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the enthalpy change that takes place when breaking by homolytic fission 1 mol of a given bond in the molecules of a gaseous species

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hess' law

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states that, if a reaction can take place by more than 1 route and the initial and final conditions are the same, the total enthalpy change is the same for each route

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enthalpy cycle

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a diagram showing alternate routes between reactants and products, which allows the indirect determination of an enthalpy change from other known enthalpy changes using hess' law

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rate of reaction

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the change in concentration of a reactant or product in a given time

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boltzmann distribution

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the distribution of energies of molecules at a particular temperature, often shown as a graph

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le chatelier's principle

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

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the greenhouse effect

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the process in which the absorption and subsequent emission of infrared radiation by atmospheric gases warms the lower atmosphere and the planet's surface

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troposhere

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the lowest layer of the earth's atmosphere extending from the earth's surface up to about 7km (above the poles) and to above 20km (above the tropics)

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stratosphere

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the second layer of the earth's atmosphere, containing the ozone layer, about 10km to 50km above the earth's surface

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adsorption

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the process that occurs when a gas, liquid or solute is held to the surface of a solid, or more rarely, a liquid