Chemistry A Level Definitions (Edexcel) Questions and Answers

Chemistry A Level Definitions (Edexcel) Questions and Answers Mass number Total number of protons and neutrons in the nucleus Atomic number Number of protons in the nucleus Isotope Atoms which have the same number of protons but different number of neutrons Relative Atomic Mass (Ar) The weighted mean mass of an atom of an element compared to 1/12th of the mass of an atom of carbon-12 Relative isotopic mass The mass of an atom of an isotope compared with 1/12th of the mass of an atom of carbon-12 Relative molecular mass The average mass of a molecule compared to 1/12th of the mass of an atom of carbon-12 Shape of s-orbitals spherical Shape of p-orbitals 3 dumbbells Electromagnetic radiation Energy that is transmitted as waves with a spectrum of different frequencies First ionisation energy The energy required to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions Second ionisation energy The energy required to remove 1 electron form each ion in 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions Equation for nth ionisation energy X(n-1)+ --- Xn+ + e- Dmitri Mendeleev The person who put the elements in a meaningful order in 1869 Element A group of atoms which all have the same number of protons Ion An atom which has different numbers of protons and electrons and hence has a positive or negative charge Orbital A region within an atom that can hold up to 2 electrons with opposite spins Relative Formula Mass The mass of a formula unit of a giant structure relative to 1/12th of the mass of an atom of carbon-12 Molecular Formula Actual number of atoms of each element in the molecule Empirical Formula Simplest whole number ratio of atoms of each element in the compound Disproportionation Occurs when an element is simultaneously oxidised and reduced to form two different products Oxidising Agent A compound which oxidises another compound by gaining electrons and hence itself is reduced Reducing Agent A compound which reduces another compound by losing electrons and hence itself is oxidised Oxidation Loss of Electrons Reduction Gain of Electrons Ionic bonding The strongest electrostatic attraction between oppositely charged ions Metallic Bonding A strong electrostatic attraction between cations and delocalised electrons Covalent Bond A strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms Dative Covalent Bond A covalent bond where both electrons are donated by one of the atoms London Forces The weak attractive force between an instantaneous dipole and an induced dipole in a neighbouring molecule Permanent dipole-permanent dipole forces The attractive force between permanent dipoles in neighbouring molecules Hydrogen Bond A strong dipole-dipole attraction between an electron deficient hydrogen atom and a lone pair of electrons on a highly electronegative atom in a neighbouring molecule Electronegativity The power of an atom to attract the electrons in a covalent bond Polar molecule The electron density is shifted more towards one end than another Permanent dipole A small charge difference across a bond that results from a difference in electronegativities of the bonded atoms Enthalpy Change The heat energy change of a reaction measured at a constant pressure Standard enthalpy change of reaction The enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation under standard conditions Standard enthalpy change of formation The enthalpy change when one mole of a compound in formed from its constituent elements in their standard states under standard conditions Standard Enthalpy change of combustions The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions Standard enthalpy change of neutralisation The enthalpy change when an acid reacts with an alkali to form one mole of water under standard conditions Standard conditions A pressure of 100kPa and a specified temperature Standard states The physical states of substances under standard conditions Bond Enthalpy The energy required to break one mole of the specified bond in gaseous molecules Exothermic reactions Give off energy (delta H is negative) Endothermic reactions Take in energy (delta H is positive) Hess' Law The total enthalpy change for a reaction is always the same no matter which chemical route is taken Rate of Reaction The change in concentration of a reactant or product over time Catalyst A chemical that speeds up a reaction without being chemically changed or used up by the overall reaction Homogeneous catalyst A catalyst which has the same physical state as the reactants Heterogeneous catalyst A catalyst which has a different physical state as the reactants Dynamic Equilibrium Exists in a closed system when the forward and reverse reactions are both occurring at the same rate so there is no overall change in concentrations of the reactants or products Le Chatelier's Principle The equilibrium position will shift to minimise the change when a system is in dynamic equilibrium is subjected to a change Periodicity A repeating pattern of trends in physical and chemical properties which occurs different periods in the periodic table Functional Group A group of atoms responsible for the characteristic reactions of a compound Homologous series A series of organic compounds with the same functional group where each successive member differs by CH2 Hydrocarbon A compound containing only hydrogen and carbon atoms Alkyl group A group derived from an alkane with one hydrogen atom missing (CnH2n+1) Aromatic/Arene A compound containing a benzene ring Saturated A hydrocarbon with single C-C bonds only Unsaturated A hydrocarbon containing double or triple C-C bonds Stereoisomers Compounds with the same structural formula but with a different arrangement of the atoms in space Structural Isomers Compounds with the same molecular formula but with different structural formulae Sigma bonds Single bonds formed by the overlap of orbitals between atoms Pi bonds A Double bond is formed by the overlap of p-orbitals between 2 atoms if there is already a sigma bond Electrophile Electron Pair acceptor Nucleophile Electron pair doner Homolytic fission The breaking of a covalent bond when 1 electron goes to each atom forming 2 radicals Heterolytic fission The breaking of a covalent bond where both electrons go to one of the atoms forming a cation and an anion Radical A species with an unpaired electron Bronsted-Lowry acid Proton donor Bronsted-Lowry base Proton acceptor Buffer A solution which minimises the change in pH upon addition of a small amount of acid or base Lattice energy ∆leH° The enthalpy change when one mole of an ionic solid is formed from its gaseous ions under standard conditions Enthalpy of Atomisation ∆atH° The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state Enthalpy of solution ∆solH° The enthalpy change when one mole of a compound is dissolved in water Enthalpy of Hydration ∆hydH° The enthalpy change when one mole of gaseous ions id dissolved in water First electron affinity The enthalpy change when each atom in 1 mole of gaseous atoms gains an electron to form 1 mole of gaseous 1- ions Second electron affinity The enthalpy change when each atom in 1 mole of gaseous 1- ions gains an electron to form 1 mole of gaseous 2- ions Entropy (S) The measure of the degree of disorder in a system Storage cell An electrochemical cell which produces a voltage/electricity from the reaction between the electrodes and electrolytes stored inside it Fuel cell An electrochemical cell which produces a voltage/electricity from the reaction between a hydrogen-rich fuel and oxygen Rate of reaction The change in amount of reactant or product per unit time Rate-Determining step The slowest step in a reaction Half-life The half-life of a reaction is the time it takes for the amount of reactant to halve Standard electrode potential The standard electrode (redox) potential of a half cell is the voltage measure when the half cell is connected to a standard hydrogen half cell under standard conditions Transition Metal An element which can form one or more stable ions with a partially filled d-orbital Conjugate acid-base pair 2 species that are linked by the transfer of a proton Strong Acid-Strong Base Weak Acid-Strong Base Strong Acid-Weak Base Weak Acid-Weak Base Complex Ion A central metal ion surrounded by dative covalently bonded ligands Ligands A molecule or ion that donates a pair of electrons to a metal atom or ion to form a dative covalent bond Coordination Number The total number of coordinate bonds formed between a central metal atom or ion and its ligands Monodentate Ligand A ligand which donates one lone pair of electrons to the central metal ion or atom to form one coordinate bond Bidentate A ligand which donates two lone pair of electrons to the central metal ion or atom to form two coordinate bond Multidentate A ligand which forms one or more coordinate bond with the central metal atom or ion Optical Isomers/Enantiomers Stereoisomers that are non-superimposable morrow images of each other caused by the presence of a chiral centre in their structure Chiral Carbon Atom A carbon atom attached to 4 different atoms or groups of atoms Optically active Optical isomers rotate the plane of polarisation of plane-polarised monochromatic light