Chemistry 30 Diploma key terms

The heat energy absorbed or released by a chemical reaction at constant pressure, usually in kilojoules per mole of a chosen reactant; a negative value means the system gave out heat.

A reaction that releases energy to its surroundings, mostly as heat, so the products store less chemical energy than the reactants and the temperature of the surroundings rises.

A reaction that takes in energy from its surroundings, so the products store more chemical energy than the reactants and the surroundings cool as heat is absorbed.

The enthalpy change when one mole of a compound is built from its elements, each in its standard state, under standard conditions; it is zero for an element in its standard state.

An experimental method for measuring the heat gained or lost during a change by tracking the temperature change of a known mass of material whose specific heat capacity is known.

The quantity of heat needed to raise the temperature of one gram of a substance by one degree Celsius; water has a high value, which makes it convenient in calorimetry experiments.

Because enthalpy is a state function, the overall enthalpy change of a reaction is the same whichever route it takes, so the steps of a multi-stage process can simply be added together.

The average energy needed to break one mole of a particular covalent bond in the gas phase; breaking bonds absorbs energy while forming new bonds releases it.

The minimum extra energy that colliding particles must possess before a reaction can take place; it controls how fast a reaction proceeds at a given temperature.

A graph plotting the stored energy of a reacting system against the progress of the reaction, showing reactant and product levels, the activation energy hump, and whether energy is released or absorbed overall.

A change in which a substance loses electrons, raises its oxidation number, or picks up oxygen; it never happens alone but always alongside reduction in the same reaction.

A change in which a substance gains electrons, lowers its oxidation number, or gives up oxygen; it is always matched by oxidation somewhere else in the same redox reaction.

The reactant that takes electrons from another substance and is therefore itself reduced; by accepting electrons it makes the oxidation of the other species possible.

One of the two parts of a redox change, written on its own to show either electron loss or electron gain, with electrons included as reactants or products so the charge balances.

A setup of two unlike electrodes dipped in electrolytes that turns the chemical energy of a spontaneous redox reaction into electrical energy, supplying a voltage until the reactants are consumed.

A device that uses an outside power supply to drive a redox reaction that would not happen on its own, for example splitting molten salt or depositing metal onto an object.

The voltage measured for a half-reaction when its electrode is compared with a standard hydrogen electrode under standard conditions; a more positive value means a stronger pull on electrons.

A tube of electrolyte connecting the two half-cells of a galvanic cell, allowing ions to flow and keep each solution electrically neutral so the electron flow does not stop.

The electrode at which oxidation takes place. It is the negative terminal in a galvanic cell but the positive terminal in an electrolytic cell.

The electrode at which reduction takes place. It is the positive terminal in a galvanic cell but the negative terminal in an electrolytic cell.

The voltage of an electrochemical cell, obtained by combining the cathode reduction potential with the anode half-reaction reversed; a positive result shows the reaction is spontaneous as written.

A particular cluster of atoms inside an organic molecule that governs its characteristic properties and reactions, such as the hydroxyl group found in alcohols.

A family of organic compounds sharing one functional group and one general formula, whose neighbouring members differ by a single carbon and two hydrogen atoms.

One of two or more compounds with the same molecular formula but a different arrangement of atoms and bonds, which gives them different physical and chemical behaviour.

An organic compound in which every carbon-carbon bond is single, so the molecule carries the greatest possible number of hydrogen atoms for its carbon skeleton.

An organic compound that contains at least one double or triple carbon-carbon bond, or an aromatic ring, and can therefore take on extra atoms in addition reactions.

A reaction in which two molecules combine into a single product, with the atoms of one molecule attaching to the two carbons that share a double or triple bond.

A reaction in which an atom or group attached to an organic molecule is swapped for a different atom or group, a pattern typical of alkanes and aromatic rings reacting with halogens.

A reaction that strips atoms from neighbouring carbons of an organic molecule, forming a double or triple bond and releasing a small molecule such as water or hydrogen chloride.

A condensation reaction between a carboxylic acid and an alcohol that yields an ester plus water, typically sped up by a strong acid catalyst and often recognised by a fruity odour.

A reaction in which many small monomer units link into very long chain molecules, either by addition across double bonds or by condensation that releases a small molecule each time.

The condition of a closed system in which forward and reverse reactions run at equal rates, so the amounts of all substances stay constant even though both reactions keep occurring.

The ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of its coefficient in the balanced equation; its size shows whether products or reactants dominate.

When a stress such as a change in concentration, pressure, or temperature is applied to a system at equilibrium, the system shifts in the direction that opposes the change and restores equilibrium.

A substance that gives a proton, meaning a hydrogen ion, to another substance during a reaction; once it has donated that proton it becomes its conjugate base.

Two species that differ by exactly one proton; the acid member hands that proton over, and the base member is what is left behind after the proton leaves.

Describes a substance that is able to donate a proton or accept a proton depending on what it is reacting with; water behaves this way in many acid-base reactions.

A number, usually between 0 and 14, that expresses how acidic or basic a solution is, calculated as the negative base-10 logarithm of the hydrogen ion concentration.

The equilibrium constant for the reaction in which a weak acid hands a proton to water; a larger number indicates a stronger acid that ionizes to a greater extent.

A solution holding a weak acid with its conjugate base, or a weak base with its conjugate acid, that resists large pH shifts when modest amounts of strong acid or strong base are added.

A procedure in which a solution of known concentration is delivered from a burette into a measured volume of unknown solution until the reaction is just complete, allowing the unknown concentration to be calculated.

The stage of a titration at which the titrant added has reacted exactly with the amount of substance in the sample, shown by an indicator colour change or a sharp jump on a pH curve.