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A2 OCR Chemistry F325/EEE Glossary

EEE Glossary (extended definitions list)
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Acid dissociation constant (Ka)
The equilibrium constant for the dissociation of a weak acid.
Adsorption
Process whereby weak bonds are formed between the molecules of a gas, liquid or aqueous substance and the surface of a solid.
Anode
The negative terminal of a fuel cell, where the oxidation happens.
Bidentate Ligand
Ligand that uses 2 lone pairs of electrons to form 2 dative covalent bonds to a single metal ion.
Born-Haber Cycle
An application of Hess's Law which enables us to calculate lattice enthalpies, which can't be measured directly. It identifies all the enthalpy changes which contribute to form a solid ionic compound from its gaseous ions and how they fit together to obtain lattice enthalpy.
Brønsted-Lowry Acid
A proton donor.
Brønsted-Lowry Base
A proton acceptor.
Buffer Solution
A solution which minimises the pH change when a small amount of a acid or base is added.
Cathode
The positive terminal of a fuel cell, where the reduction happens.
Cell Potential
It is the difference (as a positive number) between the electrode potential values of the two half cells that are coupled to build an electrochemical cell.
Cis-platin
Anti-cancer drug consisting of a square-planar complex ion between a platinum 2+ ion an 2 chloride and 2 ammonia ligands. The position of the identical ligands is cis with respect to each other, hence the name. It binds to DNA, distorting its shape and thus preventing its replication.
Complex Ion
Metal ion bonded to a number of ligands by dative covalent bonds (also known as coordinate bonds).
Conjugate Acid Base-pairs
Two species that transform into each other by loss or gain of a proton.
Conjugate Acid
A species formed when a proton is added to a base.
Conjugate Base
A species formed when a proton is removed from an acid.
Coordination Number
Number of lone pairs of electrons accepted by a metal ion from its ligands in a complex ion.
Disproportionation
The oxidation and reduction of the same species in a redox reaction.
Electrochemical Cell
Redox reaction set up as 2 half-cells in 2 separate containers connected by a wire and a salt-bridge. The energy released can produce an electric current between the 2 containers running through the wire.
End Point
The point in a titration at which there are equal concentrations of the weak acid and conjugate base forms of the indicator. At this point the colour change happens. Also see Indicator.
Energetically Feasible
The reaction is able to happen spontaneously (but it does not mean it will necessarily happen). Any reaction can only be feasible if the free energy change (∆G) is negative.
Enthalpy Change of Atomisation
Enthalpy change when 1 mole of gaseous atoms are formed from the element in its standard state.
Enthalpy Change of Formation
Enthalpy change when 1 mole of a compound is formed from its elements under standard conditions.
Enthalpy Change of Hydration
Enthalpy change when 1 mole of gaseous ions form 1 mole of hydrated ions.
Enthalpy Change of Solution
Enthalpy change when 1 mole of a compound dissolves in water. (Standard enthalpy change would be under standard conditions, 25°C, 1 atm of pressure).
Enthalpy of Neutralisation
The energy change when one mole of water is produced by the neutralisation of an acid with an alkali under standard conditions (25°C, 1 atm pressure).
Entropy (S)
Measure of the disorder of a system.
Entropy Change of Reaction
The entropy change that accompanies a reaction (it is given per mole of one of the reagents involved). It is normally calculated adding all the entropies of the products and subtracting the entropies of all the reactants.
Equilibrium Constant(Kc)
For a reversible reaction in equilibrium, nA+ mB ↔ pC + qD, Kc = ([C]p x [D]q) ÷ ([A]n x [B]m).
Equivalence Point
The point at which the two solutions used in a titration are present at the mole ratio at which they react with each other.
First Electron Affinity
Enthalpy change when 1 electron is added to each atom in 1 mole of gaseous atoms of an element.
First Ionisation Energy
Energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms of an element.
Free Energy Change (∆G)
∆H - T∆S. It reflects the balance between the enthalpy and entropy changes. It allows us to predict whether a reaction is feasible.
Fuel Cell
An electrochemical cell that uses external fuel and an oxidant to provide electrical energy. It needs both in a constant supply to continue working. At the same time, the waste products are removed.
Fuel Cell Vehicles
Vehicle powered by fuel cells, e.g. hydrogen fuel cells.
Half Cell
Half an electrochemical cell. One half-cell provides electrons, the other one receives them.
Half Life
The time it takes for the concentration of a reactant to decrease by half.
Hexaaqua Ion
A complex ion in which a central ion forms six dative covalent bonds with six water molecules.
Hydrogen Economy
An economy in which society's energy needs are met to a substantial extent by hydrogen in place of fossil fuels.
Hydrogen Fuel Cell
Fuel cell in which the fuel is hydrogen.
Hydrogen Rich Fuel
A substance with many hydrogen atoms and from which hydrogen gas can be formed in a chemical reaction. It is a possible solution to the problem of how to carry hydrogen on a hydrogen fuel cell vehicle in which hydrogen rich fuel is carried instead of hydrogen.
Indicator
A weak acid or base in which the conjugate base or acid has a different colour. Different indicators have different ranges of pH for their colour change. The right indicator for a titration is one that changes colour within the pH range in the vertical part of the titration curve.
Ion-dipole Interactions
The type of attraction between aqueous ions and water molecules. Positive ions are attracted to the negative pole of water and vice versa.
Ionic Product of Water (kw)
It is the concentration of H+ multiplied by the concentration of OH- and at 25°C it equals 1 x 10 to the power of -14. It means that it is always possible to know [H+] when [OH-] is known and vice versa.
Initial Rate of Reaction
The rate of a reaction (change in concentration of a reactant, or product per unit time) at the start of the reaction when t=0.
Intermediate
A species formed in one step of a multi-step reaction that is used up in the subsequent step, and is neither a reactant nor a product of the overall equation.
Lattice Enthalpy
Enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions.
Le Chatelier's Principle
When a condition affecting a dynamic equilibrium is changed the equilibrium shifts to oppose the change.
Ligand Substitution
A reaction where one ligand is replaced by another. It is normally a reversible reaction.
Ligand
Ion or neutral molecule with a lone pair of electrons which it donates to a central metal ion forming a dative covalent bond (coordinate bond) with it. The metal ion accepts the electron pair.
Octahedral
The shape formed by a complex ion when there are 6 ligands bonded to the central ion. The angles between the ligands are 90 and 180°.
Optical Isomers
Non-superimposable mirror images.
Overall Order of Reaction
The sum of the individual orders for all the reactants whose concentrations affect the rate of the reaction.
Oxidant
The oxidising agent in a fuel cell, normally oxygen from the air. As an oxidising agent, it gets reduced and so it is placed at the positive electrode (the cathode).
Oxidising Agent
A substance that gets reduced in a redox reaction. Example: MnO4-, in which the oxidation number of manganese is +7, is reduced to Mn2+, in which the oxidation number of manganese is +2. Note that it is MnO4- and not manganese that is the oxidising agent (an agent is a substance, it might be an ion, a compound, etc, not just the element).
Proton Exchange Membrane (PEM)
A semi permeable membrane that separates the cathode from the anode in a hydrogen fuel cell. Only protons and not electrons can pass through it.
pH
pH = -log[H+]
pKa
pKa = -logKa
Rate Equation
Equation that relates the rate of a reaction with the concentration of the reactants that affect it. For a reaction A+B → C; the rate = k[A] to the power of m x[B] to the power of n, where m is the order of reaction with respect to A and n is the order of reaction with respect to B.
Rate Determining Step
The slowest step in a multi-step reaction.
Rate of Reaction
The rate of change of concentration of a reactant or product over time.
Reaction Mechanism
A series of steps that, added together, make up the overall reaction. It reflects the Rate equation of the reaction.
Reducing Agent
A substance that gets oxidised in a redox reaction. Note that it is the substance containing the element that gets oxidised (it could be an ion, a compound, etc.) and not just the element.
Salt Bridge
It allows current to flow by means of controlled movement of ions between half cells.
Square Planar
Shape formed by a complex ion with 4 ligands in which the ligands form 90 and 180° angles between them (they are placed in the corners of a square).
Standard Electrode Potential (of a half cell)
Voltage measured under standard conditions when the half cell is coupled in an electrochemical cell with a standard hydrogen electrode.
Standard Hydrogen Electrode
A half cell consisting of a platinum electrode submerged in a 1 moldm-3 H+(aq) solution and on which H2 gas is bubbled through under standard conditions (25°C and 1 atm of pressure).
Stability Constant (Kstab)
The equilibrium constant for an equilibrium between a transition metal ion forming a complex ion and the complex formed when the ion has undergone a ligand substitution reaction and is now forming a different complex (the concentration of water is not part of the Kstab calculation because its concentration is considered to be constant).
Stereoisomers
Compounds that have structures with the same sequence of bonds but a different three dimensional arrangement (this includes both cis/trans and optical isomerism).
Storage Cell
Sealed system containing all the chemical components necessary for a redox reaction but with physical separation which means that the reaction only proceeds when electrons can travel through an external wire. The free energy released when the cell reaction takes place is used to power an external device.
Strong Acid
Acid that is completely ionised in aqueous solution. Examples are hydrochloric acid and nitric acid.
Titration Curve
A curve on a pH v/s added acid or added alkali graph. It shows how the pH changes during a titration. It might vary according to whether an alkali or base is being titrated or whether strong or weak acids or alkalis are being used, but they all have a vertical area wich indicates rapid change in pH when the equivalence point is reached.
Transition Element
Element that forms one or more stable ions with a partially filled d sub-shell.
Weak Acid
Acid that is not completely ionised in aqueous solution. Most organic acids are weak (ethanoic acid, methanoic acid, phenol, etc.)
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