Y12 Chem Definitions

Atomic Number

the number of protons in the nucleus of an atom .

Isotopes

atoms of the same element with the same number of protons and electrons but different numbers of neutrons . Isotopes of an element have different masses .

Relative Atomic Mass

the weighted mean mass of an atom compared with 1 / 12th mass of an atom of carbon - 12 .

Relative Isotopic Mass

the mass of an atom of an isotope compared with 1 / 12th mass of an atom of carbon - 12 .

Relative Molecular Mass ( M₁ )

the mass of a simple molecule .

Empirical Formula

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

Ideal Gas

a gas which has molecules that occupy negligible space with no interactions between them . The ideal gas equation is : pV = nRT .

Mole ( mol )

the amount of any substance containing as many particles as there are carbon atoms in exactly 12g of carbon - 12 isotope .

Molecular Formula

the number and type of atoms of each element in a molecule .

Base

a substance that can accept H * ions from another substance .

Titration

a technique used to determine the amount of one solution of a known concentration required to completely react with a known volume of another solution of unknown concentration .

Oxidation

loss of electrons / increase in oxidation number .

Oxidation Number

a number that represents the number of electrons lost or gained by an atom of an element . A positive oxidation number indicates the loss of electrons . Roman numerals are typically used to indicate the oxidation number of elements that may have different oxidation states ( e.g. iron ( II ) and iron ( III ) )

Redox Reaction

a reaction in which one element is oxidised and another is reduced .

Covalent bond

a strong bond formed between 2 atoms due to the electrostatic attraction between a shared pair of electrons and the atomic nuclei .

Dative Covalent ( Coordinate ) bond

a type of covalent bond in which both of the electrons in the shared pair come from one atom .

Electronegativity

the ability of an atom to attract bonding electrons in a covalent bond . This is often quantified using Pauling's electronegativity values . Electronegativity increases towards F in the periodic table .

Hydrogen Bonding

a type of intermolecular bonding that occurs between molecules containing N , O or F and a H atom of -NH , -OH or HF . A lone pair on the electronegative atom ( N , O or F ) allows the formation of a hydrogen bond .

Ionic Compounds

compounds made up of oppositely charged ions . These compounds generally have high melting and boiling points . Typically , ionic compounds are soluble and can conduct electricity when liquid or aqueous ( but not when solid ) .

London ( Dispersion ) Forces

induced dipole - dipole interactions caused when the random movement of electrons creates a temporary dipole in one molecule which then induces a dipole in a neighbouring molecule .

Non - linear

the shape of a molecule in which the central atom has 2 bonding pairs and 2 lone pairs .

Octahedral

the shape of a molecule in which the central atom has 6 bonding pairs .

Permanent Dipole

a permanent uneven distribution of charge .

Polar Molecule

a molecule that contains polar bonds with dipoles that don't cancel out due to their direction ( must be unsymmetrical ) .

Tetrahedral

the shape of a molecule in which the central atom has 4 bonding pairs .

Trigonal Planar

the shape of a molecule in which the central atom has 3 bonding pairs .

Trigonal bipyramidal

the shape of a molecule in which the central atom has 5 bonding pairs .

Displayed Formula

the relative positions of atoms and the bonds between them .

Functional Group

a group of atoms responsible for the characteristic reactions of a compound .

Heterolytic Fission

when a covalent bond breaks , one bonding atom receives both electrons from the bonded pair .

Homolytic Fission

when a covalent bond breaks , each bonding atom receives one electron from the bonding pair , forming 2 radicals .

Saturated

an organic compound which only contains C - C single bonds .

Skeletal Formula

the simplified organic formula , shown by removing hydrogen atoms from alkyl chains , leaving the carbon skeleton and the functional groups . E.g. butan - 2 - ol

Structural Formula

the minimal detail that shows the arrangement of atoms in a molecule . E.g. butane : CH , ( CH₂ ) ₂CH₂ .

Structural Isomers

compounds with the same molecular formula but different structural formulae .

Unsaturated

an organic compound which contains at least one C = C double bond , a C = C triple bond or an aromatic ring .

Alkane

a homologous series with the general formula C₂H₂n + 2 +

Bond Enthalpy

the energy required to break one mole of gaseous bonds . Actual bond enthalpies may differ from the average as the average bond enthalpy considers a particular bond in a range of molecules .

Complete Combustion

when a compound is burnt in a plentiful supply of oxygen . When alkanes are completely combusted , the only products are water and carbon dioxide .

Hydrocarbon

a compound which contains hydrogen and carbon atoms only .

Incomplete Combustion

when a compound is burnt in a limited supply of oxygen . When alkanes are incompletely combusted , water , carbon monoxide , particulates and some carbon dioxide may be produced .

Propagation

the intermediate steps in a radical substitution mechanism where a radical reacts with another species .

Radical Substitution

a type of substitution reaction in which a radical replaces another atom / group of atoms in a compound . Alkanes can undergo radical substitution reactions with chlorine or bromine in the presence of UV light , forming a mixture of organic products ( further substitution or reactions at different positions may occur ) .

Alkyl

a group with the general formula CnH₂n + 1 .

Addition Polymerisation

the formation of a long chain molecule when many monomers join together ( the polymer is the only product ) .

Catalyst

a substance which increases the rate of a reaction without being used up .

Cis - Trans Isomerism

a type of E / Z isomerism in which the two substituent groups attached to the cabon atoms are the same . According to the CIP priority rules , cis isomers have the highest priority substituents on the same side of the double bond ( both above or below ) while trans isomers have them on the opposite sides ( one above and one below ) .

Electrophile

an electron pair acceptor .

Electrophilic addition

a reaction in which a π bond is broken and 2 new o bonds form as a result of the addition of an electron pair acceptor ( electrophile ) .

E / Z Isomerism

a type of stereoisomerism caused by the restricted rotation of TT bonds . Two different groups must be attached to each carbon atom of the C = C group . According to the CIP priority rules , Z isomers have the highest priority substituents on the same side of the double bond ( both above or below ) while E isomers have them on the opposite sides ( one above and one below ) .

Stereoisomers

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

Alcohol

an organic compound containing the OH functional group . The polarity of the OH functional group means that alcohols can form hydrogen bonds and are soluble in water ( when the alkyl chain is short ) . In addition to this , alcohols have a relatively low volatility compared with alkanes .

Alkene

an organic compound containing a C = C double bond . Alkenes can be formed from alcohols via the elimination of H₂O using an acid catalyst and heat .

Carboxylic Acid

an organic compound containing the -COOH functional group . Carboxylic acids can be formed from the oxidation of primary alcohols using Cr₂O₂2 / H * and reflux .

Elimination

a type of reaction in which 2 atoms / groups of atoms are removed from a molecule .

Ketone

an organic compound containing the C = O functional group in the middle of an alkyl chain . Ketones can be formed by the oxidation of a secondary alcohol .

Oxidising Agent

a substance that can oxidise another species while being reduced .

Primary Alcohol

an alcohol in which the OH is attached to a primary carbon atom ( i.e. RCH₂OH ) . Primary alcohols can be oxidised to form either an aldehyde or a carboxylic acid , depending on the conditions .

Secondary Alcohol

an alcohol in which the OH is attached to a secondary carbon atom ( i.e. R₂CHOH ) . Secondary alcohols can be oxidised under reflux with Cr₂O , ² / H * to form a ketone .

Tertiary Alcohol

an alcohol in which the OH is attached to a tertiary carbon atom ( i.e. R , COH ) . Tertiary alcohols cannot be oxidised .

CFC

A class of compound made up of carbon , fluorine and chlorine , used as refrigerants and aerosol propellants . CFCs can be broken down by UV light in the upper atmosphere to form chlorine radicals which catalyse the breakdown of ozone .

Nucleophile

an electron pair donor .

Nucleophilic Substitution

a reaction in which an electron pair donor attacks an electrophilic atom ( an atom with a partial or full positive charge ) to replace an atom / group of atoms .

Reducing Agent

a substance that can reduce another substance .

Distillation

a technique in which a liquid is heated then the vapour is cooled and collected in a separate flask to the reaction mixture . During distillation , the condenser must be positioned horizontally .

Reflux

the continual boiling and condensing of a reaction mixture . This is to ensure that the reaction goes to completion . During reflux , the condenser must be positioned vertically .

Infrared Spectroscopy

a technique used to identify particular bonds and functional groups within a molecule . This technique has been used to link global warming with increased energy usage as atmospheric gases containing C = O , O - H and C - H bonds ( such as CO₂ , H₂O and CH₂ ) show distinct peaks on IR spectra . IR spectroscopy is used to monitor gases causing air pollution ( CO and NO from car emissions ) and to measure alcohol in the breath in modern breathalysers .

Mass spectrometry

a technique used to identify compounds and determine relative molecular mass .

Giant Covalent Lattice

a network of atoms bonded by strong covalent bonds ( e.g. carbon ( diamond , graphite and graphene ) and silicon ) . Giant covalent lattices typically insoluble with a high melting and boiling point due to the presence of strong covalent bonds . They are also poor electrical conductors as they don't contain mobile charged particles .

Metallic Bonding

strong electrostatic attraction between cations and delocalised electrons .

Periodicity

a repeating trend in physical and chemical properties across the periods of the periodic table .

Displacement Reaction

a reaction in which one atom is replaced by another . Halogens can undergo displacement reactions as their reactivity decreases down the group . The more reactive halogen will displace the less reactive halogen from a solution of its salt .

Activation Energy

the minimum energy required for a reaction to take place .

Endothermic

a reaction which takes in energy ( AH is positive ) . More energy is required to break bonds than is released by making bonds .

Enthalpy Change ( AH )

the change in the heat content of a system during a reaction . This can be determined from experimental results using q = mcAT ( where q is the heat change of the surroundings , m is the mass of the surroundings , c is the specific heat capacity and AT is the change in temperature ) .

Enthalpy Change of Combustion ( AH )

the enthalpy change that takes place when one mole of a substance is completely combusted .

Enthalpy Change of Formation ( A , H ) :

the enthalpy change that takes place when one mole of a compound is formed from its elements .

Enthalpy Change of Neutralisation ( AneutH )

the enthalpy change that takes place when one mole of water is formed from a neutralisation reaction .

Enthalpy Change of Reaction ( A , H )

the enthalpy change that is associated with a particular chemical equation .

Exothermic

a reaction which gives out energy ( AH is negative ) . More energy is released by bond making than is used in bond breaking .

Hess ' Law

the enthalpy change of a reaction is independent of the route it takes .

Boltzmann Distribution

a graph showing the distribution of the energies of molecules in relation to the activation energy . Increasing temperature will increase the proportion of molecules with energy above the activation energy . A catalyst lowers the activation energy meaning more molecules will have sufficient energy to react .

Collision Theory

the theory which states that molecules must collide with sufficient energy at the correct orientation for a reaction to occur .

Heterogeneous Catalyst

a catalyst that is in a different state to the reactants ( e.g. a solid catalyst with gaseous reactants ) .

Homogeneous catalyst

a catalyst which is in the same state as the reactants .

Rate of Reaction

a measure of how quickly a reactant is used up / a product is formed . Rate can be determined by measuring concentration , the volume of gas produced or the mass lost over time .

Dynamic Equilibrium

a closed system in which the rates of the forward and reverse reactions are equivalent . The concentrations of reactants and products don't change .

Le Chatelier's Principle

when a system in dynamic equilibrium is subject to change , the position of equilibrium will shift to minimise the change . This principle is used to determine the effect of changing pressure , temperature or concentration on the position of equilibrium

Mass number

the total number of protons and neutrons in the nucleus of an atom

Concentration

A measurement of how much solute exists within a certain volume of solvent

Balanced symbol equation

a symbol equation in which there are equal numbers of each type of atom on either side of the equation

Ionic equation

an equation in which substances that primarily exist as ions in solution are shown as ions

State symbol

solid (s), liquid (l), gas (g), and aqueous (aq)

Acid

A substance that increases the hydrogen ion concentration of a solution.

Percentage yield

actual yield/theoretical yield x 100

Atom economy

(Molecular mass of desired products/ Molecular mass of all products) x 100

Orbital

arrangement of electrons within a specific region around the nucleus

Ionisation energy

the energy required to remove an electron from an atom

Simple molecular

A three-dimensional structure of molecules, bonded together by weak intermolecular forces.

Reduction

gain of electrons