Glossary acid dissociation constant, Ka the equilibrium constant for ATP (adenosine triphosphate) a molecule involved []HA+ []‒ in energy transfers in cells. The hydrolysis of ATP a weak acid: Ka = [HA] releases energy which can be used to do useful work, e.g. acid a proton (hydrogen ion) donor. provide energy for enzyme-catalysed reactions, for muscle acid–base indicator a substance which changes colour over contraction or to drive molecules through cell membranes a narrow range of pH values. against a concentration gradient. activation energy the minimum energy that colliding average bond energy a general bond energy value used particles must possess for a successful collision that results for a particular bond, e.g. a C H, when the exact in a reaction to take place. bond energy is not required. Average bond energies are active site (of an enzyme) the ‘pocket’ on an enzyme often used because the strength of a bond between two surface where the substrate binds and undergoes particular types of atom is slightly different in different catalytic reaction. compounds. active transport the movement of a substance against a Avogadro constant the number of atoms (or ions, concentration gradient. molecules or electrons) in a mole of atoms (or ions, 23 acyl chloride a reactive organic compound related to molecules or electrons): its numerical value is 6.02 × 10 . a carboxylic acid, with the OH group in the acid azo dyes coloured compounds formed on the addition replaced by a Cl atom, for example ethanoyl chloride, of phenol (or another aryl compound) to a solution CH3COCl. containing a diazonium ion. They contain the addition reaction an organic reaction in which two reactant N N group. molecules combine to give a single product molecule. base a proton (hydrogen ion) acceptor. addition polymerisation the reaction in which monomers bidentate ligands which can form two co-ordinate bonds containing carbon-to-carbon double bonds react together from each ion or molecule to the central transition to form long-chain molecules called polymers. metal ion. adsorption (in catalysis) the first stage in heterogeneous biofuels renewable fuels, sourced from plant or catalysis – molecules of reactants (usually gases) form animal materials. bonds with atoms on the surface of the catalyst. boiling point the temperature at which the vapour pressure alkali a base which is soluble in water. is equal to the atmospheric pressure. alkaline earth metals the elements in Group II of the Boltzmann distribution a graph showing the distribution Periodic Table. of energies of the particles in a sample at a given alkanes saturated hydrocarbons with the general temperature. formula CnH2n+2. bond energy/bond enthalpy the energy needed to break 1 alkenes unsaturated hydrocarbons with a carbon–carbon mole of a particular bond in 1 mole of gaseous molecules. double bond. Their general formula is nC H2n. Born–Haber cycle a type of enthalpy cycle used to calculate allotrope different crystalline or molecular forms of the same lattice energy. element. Graphite and diamond are allotropes of carbon. Brønsted–Lowry theory of acids acids are proton donors alloy a mixture of two or more metals or a metal with and bases are proton acceptors. a non-metal. buffer solution a solution that minimises changes in amino acid residue an amino acid unit within a pH when moderate amounts of acid or base are added. polypeptide chain. Common forms of buffer consist of either a weak amphoteric able to behave as both an acid and a base. acid and its conjugate base or a weak base and its Aluminium oxide is amphoteric. conjugate acid. anion a negatively charged ion. carbocation an alkyl group carrying a single positive charge + anode the positive electrode. on one of its carbon atoms, e.g. CH2CH3 arenes hydrocarbons containing one or more benzene rings. catalyst a substance that increases the rate of a reaction atomic orbitals regions of space outside the nucleus but remains chemically unchanged itself at the end of which can be occupied by one or, at most, two electrons. the reaction. Orbitals are named s, p, d and f. They have different cathode the negative electrode. shapes. cation a positively charged ion. AS and A Level Chemistry © Cambridge University Press Glossary 1 cell membrane a membrane surrounding each cell which covalent bond a bond formed by the sharing of pairs of controls the entry and exit of particular biological electrons between two atoms. molecules and ions. cracking the process in which large, less useful hydrocarbon ceramic an inorganic non-metallic solid which is prepared molecules are broken down into smaller, more useful by heating a substance or mixture of substances to a high molecules. temperature. dative covalent bond another name for a co-ordinate bond. chiral centre a carbon atom with four different groups degenerate orbitals atomic orbitals at the same attached, creating the possibility of optical isomers. energy level. closed system a system in which matter or energy is not lost dehydration a reaction in which a water molecule is or gained, e.g. gases in a closed vessel. removed from a molecule, e.g. in the dehydration of an codon a set of three successive bases in mRNA which codes alcohol to give an alkene. for a specific amino acid in protein synthesis. delocalised electrons electrons which are not associated cofactor a small molecule which is not a substrate but which with a particular atom – they can move between three or is essential for an enzyme-catalysed reaction. more adjacent atoms. common ion effect the reduction in the solubility of a denaturation the process by which the three-dimensional dissolved salt by adding a compound which has an ion structure of a protein or other biological macromolecule is in common with the dissolved salt. This often results in changed, often irreversibly. Relatively high temperatures, precipitation of the salt. extremes of pH and organic solvents often cause competitive inhibition enzyme inhibition by molecules denaturation. that bind to the active site, preventing the normal desorption the last stage in heterogeneous catalysis. The substrate from reacting. They have a structure similar to bonds holding the molecule(s) of product(s) to the the substrate molecule. The inhibition is reversible. surface of the catalyst are broken and the product complementary base pairing In nucleic acids, bases are molecules diffuse away from the surface of the catalyst. said to be complementary to each other if they form diazotisation the reaction between phenylamine and specific hydrogen-bonded pairs. In DNA adenine (A) nitrous acid (nitric(III) acid), HNO2, to give a always pairs with thymine (T) and cytosine (C) always diazonium salt in the first step in making an azo dye. pairs with guanine (G). dipeptide the product formed when two amino acids complex a central transition metal ion surrounded by react together. ligands. dipole a separation of charge in a molecule. One end of the compound a substance made up of two or more elements molecule is permanently positively charged and the other bonded (chemically joined) together. is negatively charged. condensation the change in state when a vapour changes to discharge(d) the conversion of ions to atoms or molecules a liquid. at electrodes during electrolysis, for example, during the condensation reaction a reaction in which two organic electrolysis of concentrated sodium chloride solution, molecules join together and in the process eliminate a chlorine is discharged at the anode by the conversion of − small molecule, such as water or hydrogen chloride. Cl ions to Cl atoms which then combine to form Cl2 conjugate pair (acid/base) an acid and base on each side molecules. of an equilibrium equation that are related to each other displayed formula a drawing of a molecule that shows all by the difference of a proton; e.g. the acid in the forward the atoms and bonds within the molecule. reaction and the base in the reverse reaction or the base in disproportionation the simultaneous reduction and the forward reaction and the acid in the reverse reaction. oxidation of the same species in a chemical reaction. co-ordinate bond a covalent bond where both electrons in dissociation the break-up of a molecule into ions, for the bond come from the same atom. example, when HCl molecules dissolve in aqueous co-ordination number the number of co-ordinate (dative) solution, they dissociate completely into H+ and bonds formed by ligands to the central transition metal Cl− ions. ion in a complex. disulfide bridge an S S bond formed when the SH coupling reaction when a diazonium ion reacts with an groups on the side-chain of two cysteine residues in a alkaline solution of phenol (or similar compound) to protein combine. Disulfide bridges help maintain the make an azo-dye. tertiary structure of some proteins. 2 Glossary AS and A Level Chemistry © Cambridge University Press DNA (deoxyribonucleic acid) a polymer with a double electrophoresis the separation of charged particles by their helical structure containing two sugar–phosphate chains different rates of movement in an electric field. with nitrogenous bases attached to them. The sequence electrovalent bond another name for an ionic bond. of bases forms a code which is used to form more DNA element a substance made of only one type of atom. by replication or to encode mRNA (transcription). elimination a reaction in which a small molecule, such as dot-and-cross diagram a diagram showing the arrangement H2O or HCl, is removed from an organic molecule. of the outer-shell electrons in an ionic or covalent element empirical formula the formula that tells us the simplest or compound. The electrons are shown as dots or crosses ratio of the different atoms present in a molecule.