Leaving Cert Chemistry Definitions

Metal

Any element that loses electrons to form a cation.

Atomic Number

Number of protons in the nucleus.

Mass Number

The number of protons plus neutrons in the nucleus.

Isotopes

Same atomic number but different mass number due to the different number of neutrons in the nucleus.

Relative Atomic Mass Number

Average mass of an atom of an element compared to 1/12th the mass of the carbon-12 isotope, taking relative abundances of the naturally occuring isotopes into account.

Relative Molecular Mass Number

Average mass of a molecule of a compound compared to 1/12th the mass of the carbon-12 isotope, taking relative abundances of the naturally occuring isotopes into account.

Radioactivity

The spontaneous disintergration of the nucleus with the emission of radiation.

Half-Life

The time taken for half the nuclei to decay in a given sample.

Emission Spectrum

Coloured lines on a dark background.

Absorption Spectrum

Dark lines on a coloured background.

Heisenberg's Uncertainty Principle

It is impossible to know both the position and velocity of an electron at the same time.

Energy Level

The discrete amount of energy an eletron has when it is in an atom.

Energy Sublevel

Group of atomic orbitals with the same energy within an atom.

Atomic Orbital

Region in space around the nucleus of an atom where there is a high probability of finding an electron.

Aufbau Principle

Electrons occupy the lowest available energy level.

Hunds Rule of Max Multiplicity

When two or more orbitals of equal energy are available, electrons fill them singly before filling in pairs.

Pauli Exclusion Principle

No more than two electrons can occupy an orbital and they must have opposite spin.

Transition Metal

Forms at least one ion with a partially filled d-sublevel.

Compound

Substance made of two or more elements chemically combined.

Element

Substance which can not be reduced to simpler substances by chemical means.

Octet Rule

When atoms bond they tend to want an eight electron arrangement in outer shell.

Valency

Number of H atoms with which each atom of the element combines.

Valence Shell Electron Pair Repulsion Theory

The outermost pairs of electrons in a molecule try to get as far away from each other as possible so as to minimise the repulsion between these electrons.

Bonding Pairs

Pairs of electrons in the outermost shell that are taking part in bonding.

Lone Pairs

Pairs of electrons in the outermost shell that do not take part in bonding.

Oxidation

When an element loses electrons, with an increase in oxidation number.

Reduction

When an element gains electrons, with a decrease in oxidation number.

Oxidation Number

The charge an element has or appears to have when it is in a compound when certain rules are applied.

Atomic Covalent Radius

Half the distance between the centres of singly bonded atoms of the same element.

First Ionisation Energy

Minimum amount of energy required to remove the first most loosely bound electron from a mole of isolated atoms of an element in its neutral gaseous ground state.

Electronegativity

The relative power of attraction an atom of an element has for the shared pairs of electrons in a covalent bond.

Cations

An atom which has lost electrons, resulting in a positive charge.

Anions

An atom which has gained electrons, resulting in a negative charge.

Ionic Bonding

An ionic bond is formed due to the electrostatic attraction between oppositely charged ions caused by the complete transfer of electrons from one atom to another.

Covalent Bonding

Occurs when electrons are shared between atoms.

Sigma Bonding

When orbitals overlap 'head-on'.

Pi Bonding

When orbitals overlap 'side-ways'.

Pure Covalent Bond

Occurs when electrons are shared equally between atoms.

Polar Covalent Bond

Occurs when electrons are shared unequally between atoms.

Dative Covalent Bond

One atom supplies the electrons needed to make a covalent bond.

Polar Molecule

Slight positive and slight negative poles of a molecule separated by a distance - they do not coincide.

Hydrogen Bonding

Force of attraction that occurs between molecules when hydrogen is bonded to a smaller more electronegative element such as O, F or N.

Arrhenius Acid

Produces H+ ions in water

Arrhenius Base

Produces OH- ions in water

Alkali

Base that is soluble in water

Monobasic Acid

Produces one H+ ion in solution

Dibasic Acid

Produces two H+ ions in solution

Tribasic Acid

Produces three H+ ions in solution

Bronsted-Lowry Acid

Proton donor

Bronsted-Lowry Base

Proton acceptor

Amphoteric

Substance that can act as both an acid and a base

Conjugate Pairs

Two substances that differ by one proton

Conjugate Acid

Base plus a H+ ion

Conjugate Base

Acid minus a H+ ion

Dissociation

When acids are placed in solutions they split up into their ions, hydrogen ions and anions.

Salt

Formed when the H+ of the acid is replaced by a metal ion or NH4+

Strong Acid

An acid that fully dissociates in solution and is a good proton donor.

Weak Acid

An acid that only slightly dissociates in solution and is a poor proton donor.

Strong Base

Good acceptor of protons

Weak Base

Poor acceptor of protons

Mole

Amount of a substance which contains the Avogardo's constant number of particles.

Avogardo's Constant

The number of carbon atoms in exactly 12 grams of the C-12 isotope = 6 x 10 to power of 23

Standard Solution

Solution of known concentration

Primary Standard

Substance that can be made up to a standard solution directly as it is 100% pure, stable in air, has a high molar mass for accuracy in weighing and dissolves easily in water.

Secondary Standard

Substance that cannot be made up to a standard solution directly as it is not 100% pure, stable in air, has a low molecular mass and does not dissolve readily in water.

Standardise

Find out the concentration of a solution by titration or colorimetry.

Saturated

Only single bonds around carbon atoms.

Unsaturated

Multiple bond between two carbon atoms

Hydrocarbon

Compound that consists of the elements carbon and hydrogen

Aliphatic Hydrocarbons

Straight or branched-chained hydrocarbon molecules

Aromatic Hydrocarbons

Contains a ring in their structure.

Structural Isomers

Molecules with the same molecular formula but different strucutral formulas.

Homologous Series

Group of compounds whose members contain the same functional group and succesive members differ by CH2.

Functional Group

Group of atoms upon which the characteristic properties of a homologous series depends.

Octane Number

Measure of the tendency of a fuel to resist auto-ignition

Reforming

Involves changing straight-chained hydrocarbons into branched chained hydrocarbons or cyclic hydrocarbons

Cracking

Changing long-chained hydrocarbons into short chained hydrocarbons

Auto-Ignition

The early explosion of a petrol-air mixture caused by the increasing pressure in the engine.

Chloroalkanes

Alkane where one or more of the hydrogen atoms have been replaced by chlorine atoms.

Alcohols

Alkane where one of the hydrogens is replaced with an OH group

Primary Alcohol

One carbon directly attached to the carbon atom that has the OH group

Secondary Alcohol

Two carbons directly attaced to the carbon atom that has the OH group

Heat of Reaction

Heat change that occurs when a reaction takes place according to a given balanced chemical equation.

Exothermic Reaction

Heat is given out

Endothermic Reaction

Heat is taken in

Heat of Combustion

Heat change that occurs when 1 mole of a substance is burned in an excess of oxygen.

Kilogram Calorific Value

Heat produced when 1 kg of the fuel is burned in an excess of oxygen.

Heat of Formation

Heat change that occurs when 1 mole of a substance is formed from its elements in their standard states.

Hess' Law

If a chemical reaction takes place in several stages, the sum of the individual stages equals the heat change if the reaction had been carried out in one single stage.

Law of Conservation of Energy

Energy cannot be created or destroyed but can only be converted from one form of energy into another.

Rate of Reaction

Change in concentration per unit time of any one reactant or product.

Catalyst

Substance that alters the rate of a chemical reaction but is not used up during the reaction.

Heterogeneous Catalysis

When the reactants and the catalyst are in different phases.

Homogeneous Catalysis

When the reactants and the catalyst are in the same phase

Autocatalysis

When the catalyst is a product of the reaction

Activation Energy

Minimum amount of energy that colliding particles must have in order for a reaction to occur.

Avogadro's Law

Equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules.

Molar Mass

Atomic mass expressed in grams.

Molar Volume

At stp the molar volume of any gas occupies a volume of 22.5 l.

Limiting Reactant

Reactant that is fully consumed when a reaction goes to completion