# All Chemistry Definitions to Learn for Leaving Certificate

## The Atom

• Dalton’s Atomic Theory: All matter is made up of indivisible atoms which cannot be created or destroyed.
• William Crookes discovered the Electron. Cathode ray tube.
• JJ Thomson determined that electrons were negative. Found E/M ratio.
• Robert Millikan’s Oil drop experiment discovered size of electron’s charge.
• Plum Pudding Model: Solid positive sphere with electrons embedded.
• Rutherford discovered the Nucleus. Fired alpha particles at gold foil. Proton.
• Chadwick discovered the neutron by firing alpha particles at beryllium.
• Energy level is the fixed energy value that an electron in an atom may have.
• Heisenberg’s Uncertainty Principle states that it is impossible to measure at the same time both the velocity and the position of an electron.
• Orbital is a region of space in which there is a high probability of finding an electron.

## Periodic Table

• Element is a substance which cannot be split into simpler substances by chemical means.
• Dobereiner’s Triads are groups of three elements of similar chemical properties in which the atomic weight of the middle element is approximately equal to the average of the other two.
• Newland’s Octaves are groups of elements arranged in order of increasing atomic weight, in which the first and eight element of each group have similar properties.
• Mendeleev’s Periodic Law: When elements are arranged in order of increasing atomic weight, the properties of the elements vary periodically.
• Atomic Number of an atom is the number of protons in the nucleus of that atom.
• Modern Periodic Law: When elements are arranged in order of increasing atomic number, the properties of the elements vary periodically.
• Mass number of an element is the sum of the number of protons and neutrons in the nucleus of an atom of that element.
• Isotopes are atoms of the same element, with the same atomic number, but different mass numbers due to the different number of neutrons in the nucleus.
• Relative Atomic Mass is defined as: The average of the mass numbers of the isotopes of the element, as they occur naturally, taking their abundances into account. Expressed on a scale in which the atoms of the carbon 12 isotope have a mass of exactly 12 units.
• Mass Spectrometer: Vaporisation, Ionisation, Acceleration, Separation, Detection.
• Aufbau Principle states that when building up the electronic configuration of an atom in its ground state, the electrons occupy the lowest available energy level.
• Hund’s Rule of Maximum Multiplicity states that when two or more orbitals of equal energy are available, the electrons occupy them singly before filling them in pairs.
• Pauli Exclusion Principle states that no more than two electrons may occupy an orbital and they must have opposite spin.
• Atomic radius of an atom is defined as half the distance between the nuclei of two atoms of the same element that are joined together by a single covalent bond.
• First ionisation energy of an atom is the energy required to completely remove the most loosely bound electron from a neutral gaseous atom.

## Chemical Bonding

• Compound is a substance that is made up of two or more different elements combined together chemically.
• Octet Rule states that when bonding occurs, atoms tend to reach an electron arrangement with eight electrons in the outermost shell.
• Ion is a charged atom or group of atoms.
• Ionic bond is the force of atttraction between oppositely charged ions in a compound.
• Transition metal is one that forms at least one ion with a partially filled d sublevel.
• Molecule is a group of atoms joined together. It is the smallest particle of an element or compound that can exist independently.
• Valency of an element is defined as the number of atoms of hydrogen or any other monovalent element with which each atom of the element combines.
• Electronegativity is the relative attraction that an atom in a molecule has for the shared pair of electrons in a covalent bond.
• Van der Waals forces are weak attractive forces between molecules resulting from the formation of temporary dipoles.
• Dipole – Dipole forces are forces of attraction between the negative pole of one molecule and the positive pole of another.
• Hydrogen bonds are particular types of dipole-dipole attractions between molecules in which the hydrogen atoms are bonded to nitrogen, oxygen or fluorine. The hydrogen atom carries a partial positive charge and is attracted to the electronegative atom in another molecule. Thus, it acts as a bridge between two electronegative atoms.

## Chemical Equations – Tests for anions

• Law of Conservation of Mass states that the total mass of the products of a chemical reaction is the same as the total mass of the reactants.
• Law of Conservation of Matter states that in any chemical reaction, matter is neither created nor destroyed, but merely changes from one form into another.

• Radioactivity is the spontaneous breaking up of unstable nuclei with the emission of one or more types of radiation.
• Half Life of an element is the time taken for half of the nuclei in any given sample to decay.

## Mole Concept

• One mole of a substance is the amount of that substance that contains 6 x 10^23 particles of that substance. Mass = Rel.Molecular Mass x Number of moles
• Relative Molecular Mass of a molecule is the sum of the relative atomic masses of all the atoms in a molecule of the compound.

## Properties of Gases

• A Gas is defined as a substance that has no well-defined boundaries but diffuses rapidly to fill any container in which it is placed.
• Standard Temperature = 273K
• Boyle’s Law states that, at constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure.
• Charles’ Law states that, at constant pressure, the volume of a fixed mass of gas is directly proportional to its temperature measured on theKelvin scale.
• Gay-Lussac’s Law of Combining Volumes: In a reaction between gases, the volumes of the reacting gases and the volumes of any gaseous products are in the ratio of small whole numbers provided the volumes are measured at the same temperature and pressure.
• Avagadro’s Law states that equal volumes of gases contain equal numbers of molecules, under the same conditions of pressure and temperature.
• Ideal Gas is one that obeys all the assumptions of the kinetic theory of gases under all conditions of temperature and pressure.
• Real gases vs Ideal Gases: Forces of attraction/repulsion between the molecules. Volume of molecules is not negligible.

## Acids & Bases, pH

• Arrhenius definitions:
• Acid: Substance that dissociates in water to produce H+ ions.
• Base: Substance that dissociates in water to produce OH-ions.
• Brønsted Lowry definitions: Acid is a proton donor. Base is a proton acceptor.
• Conjugate Acid: Acid will change to a conjugate base when it donates a proton.
• Conjugate Base: Base will change to a conjugate acid when it accepts a proton. The stronger the acid, the weaker the conjugate base.
• Conjugate Acid-Base Pair is any pair consisting of an acid and a base which differ by one proton.
• Neutralisation is the reaction between an acid and a base to form a salt and water.

## Volumetric Analysis

• Concentration of a solution is the amount of solute that is dissolved ina given volume of solution.
• Primary standard is a substance which can be obtained in a stable, pure and soluble solid form so that it can be weighed out and dissolved in water to give a solution of accurately known concentration.

## Oxidation & Reduction

• OIL RIG -Oxidation is loss of electrons, Reduction is gain of electrons.
• Oxidising agent is a substance that brings about oxidation in other substances.
• Reducing agent is a substance that brings about reduction in other substances.
• Oxidation number is the charge that an atom has or appears to have when electrons are distributed according to certain rules.
• Oxidation = Increase in ON
• Reduction = Decrease in ON

## Rates of Reaction

• Rate of Reaction is defined as the change in concentration per unit time of any one reactant or product.
• Catalyst is a substance that alters the rate of chemical reactio but is not used up in the reaction.
• Activation energy is the minimum energy which colliding particles must have for a reaction to occur.

## Chemical Equilibrium

• Chemical Equilibrium is a state of dynamic balance where the rate of the forward reaction equals the rate of the reverse reaction.
• Le Chatelier’s Principle: If a stress is applied to a system at equilibrium, the system readjusts to relieve the stress applied.

## Water

• Hard water is water that will not easily form a lather with soap. Hardness in water is caused by the presence of $Ca^{2+}$ or $Mg^{2+}$ ions.
• Water Treatment: Screening – Flocculation – Sedimentation – Filtration – Chlorination – Flouridation – pH adjustment
• Flocculant is a chemical added to water to coagulate suspended particles.
• B.O.D -Biochemical Oxygen demand is defined as the amount of dissolved oxygen consumed by bilogical action when a sample of water is kept at 20 degrees in the dark for 5 days.
• Eutrophication is the enrichment of water with nutrients, which leads to excessive growth of algae.

## Electrochemistry

• Electrolysis is the use of electricity to bring about a chemical reaction.
• Electroplating is a process where electrolysis is used to put a layer of one metal on the surface of another.
• The electrochemical series is a list of elements in order of their standard electrode potentials.

## Organic Chemistry

• Hydrocarbon is a compound that contains only hydrogen and carbon.
• Homologous Series is defines as a series of compounds of uniform chemical type, showing gradations in physical properties, having a general formula for its members, each member having a similar method of preparation, each member differing from the previous by a CH2 unit.
• Structural isomers are compounds with the same molecular formula but different structural formulae.
• Aliphatic compound is an organic compound that consists of open chains of carbon atoms and closed chain compounds (rings) that resemble them in chemical properties.
• Aromatic compounds are compounds that contain a benzene ring in their structure.
• Octane Number of a fuel is a measure of its tendency to resist knocking.
• Catalytic Cracking is the breaking down of long-chain hydrocarbon molecules into short chain molecules for which there is greater demand.
• Heat of Reaction is the heat change when the number of moles of reactants indicated in the balanced equation for the reaction react completely.
• Heat of Combustion of a substance is the heat change when one mole of the substance is burned completely in excess oxygen.
• Kilogram Calorific Value of a a fuel is the heat energy produced when one kg of the fuel is burned completely in oxygen.
• Bond energy is the energy required to break one mole of covalent bonds and to separate the neutral atoms completely from each other.
• Heat of Neutralisation is the heat change when one mole of H+ ions from an acid reacts with one mole of OHions from a base.
• Hess’s Law states that if a chemical reaction takes place in a number of stages, the sum of the heat changes in the separate stages is equal to the heat change if the reaction is carried out in one stage.
• Law of Conservation of Energy states that energy cannot be created or destroyed, but can only be converted from one form of energy into another.
• Functional group is an atom or group of atoms which is responsible for the characteristic properties of a series of organic compounds.
• Substitution reaction is a chemical reaction in which an atom or group of atoms in a molecule is replaced by another atom or group of atoms.
• Mechanism of a reaction is the detailed step by step description of how the overall reaction occurs.
• Addition reaction is one in which two substances react together to form a single substance.
• Polymers are long chain molecules made by joining together many small chain molecules.
• Organic Synthesis is the process of making organic compounds from simpler starting materials.
• Chromatography is a separation technique in which a mobile phase carrying a mixture moves in contact with a selectively adsorbent stationary phase.
• Reflux is literally a process in which the reactants are boiled in a container usually a round bottom flask. The vapour that is formed in the boiling process remains in the container (e.g. Round bottom flask) because of the cooking process involving the condenser attached to the round bottom flask. It helps to keeps the product formed in the container without escaping. The product that is in the vapour state returns to it’s liquid form.
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