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Glossary

constant, Ka the equilibrium constant for ATP (adenosine triphosphate) a 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 ( ) donor. provide energy for enzyme-catalysed reactions, for muscle acid– indicator a substance which changes colour over contraction or to drive 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 is slightly different in different catalytic reaction. compounds. active transport the movement of a substance against a Avogadro constant the number of (or , concentration gradient. molecules or ) in a mole of atoms (or ions, 23 acyl a reactive organic compound related to molecules or electrons): its numerical value is 6.02 × 10 . a , 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

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 () 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. the temperature at which the vapour pressure a base which is soluble in . 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 saturated with the general temperature.

formula CnH2n+2. bond energy/bond enthalpy the energy needed to break 1 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 . element. Graphite and diamond are allotropes of carbon. Brønsted–Lowry theory of 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 and its Aluminium is amphoteric. . anion a negatively charged ion. carbocation an group carrying a single positive charge + anode the positive . 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 © Cambridge University Press Glossary 1 cell membrane a membrane surrounding each cell which 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 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 to give an . 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 of a denaturation the process by which the three-dimensional dissolved 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 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 . at during , for example, during the condensation reaction a reaction in which two organic electrolysis of concentrated 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. to show their origin. endothermic term used to describe a reaction where energy double covalent bond two shared pairs of electrons is absorbed from the surroundings: the enthalpy change bonding two atoms together. is positive. dynamic (equilibrium) in an equilibrium mixture, energy levels (of electrons) the regions at various distances molecules of reactants are being converted to products from the nucleus where electrons have a particular amount at the same rate as products are being converted to of energy. Electrons further from the nucleus have more reactants. energy. See principal quantum shells. electrochemical cell two half-cells in separate enhanced global warming the increase in average compartments joined by a salt bridge. When the poles of temperatures around the world as a consequence of

the half-cells are joined by a wire, electrons travel in the the huge increase in the amounts of CO2 and other external circuit from the half-cell with the more negative greenhouse gases produced by human activity.

E value to the half-cell with the more positive E value. enthalpy change of atomisation ∆H at; the enthalpy change electrode potential the voltage measured for a half-cell when 1 mole of gaseous atoms is formed from its element compared with another half-cell. under standard conditions. electrode a rod of metal or carbon (graphite) which enthalpy change of hydration ∆H hyd; the enthalpy change conducts electricity to or from an . when 1 mole of a specified gaseous ion dissolves in electrolysis the decomposition of a compound into its sufficient water to form a very dilute solution.

elements by an electric current. enthalpy change of solution ∆H sol; the energy absorbed electrolyte a molten ionic compound or an aqueous or released when 1 mole of an ionic solid dissolves in solution of ions that is decomposed during electrolysis. sufficient water to form a very dilute solution. affinity (first electron affinity) ∆H ea1; the enthalpy change the energy transferred in a chemical enthalpy change when 1 mole of electrons is added to 1 reaction (symbol ∆H). mole of gaseous atoms to form 1 mole of gaseous 1– ions enthalpy cycle a diagram showing alternative routes under standard conditions. between reactants and products that allows the electron affinity (second electron affinity) ∆H ea2; the determination of one enthalpy change from other known enthalpy change when 1 mole of electrons is added to 1 enthalpy changes by using Hess’s law. mole of gaseous 1– ions to form 1 mole of gaseous 2– ions enthalpy profile diagram a diagram showing the enthalpy under standard conditions. change from reactants to products along the reaction electron tiny subatomic particles found in orbitals around pathway. the nucleus. They have a negative charge but have enzyme activity a measure of the rate at which substrate is negligible mass. converted to product in an enzyme-catalysed reaction. the ability of an atom to attract the enzyme a protein molecule that is a biological catalyst. Most bonding electrons in a covalent bond. act on a specific substrate. electronic configuration a way of representing the equilibrium constant a constant calculated from the arrangement of the electrons in atoms showing the equilibrium expression for a reaction. principal quantum shells, the sub-shells and the number equilibrium expression a simple relationship that links 2 2 3 of electrons present, e.g. 1s 2s 2p . The electrons may also Kc to the equilibrium concentrations of reactants and be shown in boxes. products and the stoichiometric equation. electropherogram the physical results of electrophoresis. equilibrium reaction a reaction that does not go to electrophile a species which can act as an acceptor of a pair completion and in which reactants and products are of electrons in an organic mechanism. present in fixed concentration ratios.

AS and A Level Chemistry © Cambridge University Press Glossary 3 esterification the reaction between an alcohol and a a number for n in the general formula you get the carboxylic acid (or acyl chloride) to produce an ester molecular formula of a particular compound in that and water. homologous series. eutrophication an environmental problem caused by general gas equation an equation relating the volume of a fertilisers leached from fields into rivers and lakes. The gas to the temperature, pressure and number of moles of fertiliser then promotes the growth of algae on the gas. Also called the ideal gas equation. surface of water. When the algae die, bacteria thrive pV = nRT and use up the dissolved oxygen in the water, killing genetic code a code made up of sets of three consecutive aquatic life. nitrogenous bases that provides the information to make exothermic the term used to describe a reaction where specific proteins. energy is released to the surroundings: the enthalpy genetic engineering the deliberate alteration of one or change is negative. more bases in the DNA of an organism, leading to an Faraday constant the charge carried by 1 mole of electrons altered protein with improved properties. Scientists hope (or 1 mole of singly charged ions). It has a value of 96 500 to be able to use genetic engineering to eliminate genetic coulombs per mol (C mol−1). diseases which are caused by mutations in DNA. Faraday’s laws first law: the mass of a substance produced genetic fingerprinting a technique based on matching the at an electrode during electrolysis is proportional to the minisatellite regions of a person’s DNA to a database of quantity of electricity passed in coulombs. Second law: the reference samples. number of Faradays needed to discharge 1 mole of an ion giant molecular structure/giant covalent at an electrode equals the number of charges on the ion. structure structures having a three-dimensional feasibility (of reaction) the likelihood or not of a reaction network of covalent bonds throughout the whole occurring when reactants are mixed. We can use E values structure. to assess the feasibility of a reaction. GLC gas–liquid chromatography. Fehling’s solution an alkaline solution containing GLC/MS a technique in which a mass spectrometer is copper(II) ions used to distinguish between connected directly to a gas–liquid chromatograph to and ketones. A positive test is one in which the clear blue identify the components in a mixture. solution gives a red/orange precipitate when warmed with haemoglobin the iron-containing protein found in red aldehydes, but no change is observed with ketones. blood cells which transports oxygen around the body. fragmentation the breaking up of a molecule into smaller half-cell half of an electrochemical cell. The half-cell parts by the breaking of covalent bonds in a mass with the more negative E value supplies electrons. spectrometer. The half-cell with the more positiveE value receives free radical very reactive atom or molecule which has a electrons. single unpaired electron. half-equation in a reaction, an equation showing free-radical substitution the reaction in which either an oxidation or a reduction. atoms substitute for hydrogen atoms in alkanes. The half-life the time taken for the amount (or concentration) mechanism involves steps in which reactive free-radicals of the limiting reactant in a reaction to decrease to half are produced (initiation), regenerated (propagation) and its value. consumed (termination). Group VII elements. a source of electrical energy which comes directly Hess’s law the total enthalpy change for a chemical reaction from the energy stored in the chemicals in the cell, one of is independent of the route by which the reaction which is oxygen (which may come from the air). takes place. an atom or group of atoms in an organic heterogeneous catalysis the type of catalysis in which the molecule that determine the characteristic reactions of a catalyst is in a different phase from the reactants. For homologous series. example, iron in the Haber process. gene a length of DNA that carries a code for making a homogeneous catalysis the type of catalysis in which the particular protein. catalyst and reactants are in the same phase. For example, general formula a formula that represents a homologous catalysing the formation of an ester from an series of compounds using letters and numbers; e.g. the alcohol and carboxylic acid.

general formula for the alkanes is CnH2n+2. By substituting HPLC high-performance liquid chromatography.

4 Glossary AS and A Level Chemistry © Cambridge University Press hydrocarbon a compound made up of carbon and lock-and-key mechanism a model used to explain why hydrogen only. enzymes are so specific in their activity. It is suggested that hydrogen bond the strongest type of intermolecular the active site of the enzyme has a shape into which the force – it is formed between molecules having a hydrogen substrate fits exactly – rather like a particular key fits a atom bonded to one of the most electronegative elements particular lock. (F, O or N). lone pairs (of electrons) pairs of electrons in the outer shell hydrolysis the breakdown of a compound by water, which of an atom that are not bonded. is often speeded up by reacting with acid or alkali. mass number see nucleon number. hydrophobic the non-polar part of a molecule that has no mass spectrometer an instrument for finding the relevant attraction for water molecules (‘water hating’). isotopic abundance of elements and to help identify hydroxynitrile an organic compound containing both an unknown organic compounds. OH and a CN group, e.g. 2-hydroxypropanenitrile, messenger RNA a type of RNA which is synthesised using

CH3CH(OH)CN. part of the DNA strand as a template. The sequence of ideal gas a gas whose volume varies in proportion to the triplet bases along the mRNA codes for the sequence of temperature and in inverse proportion to the pressure. amino acids in a protein. Noble gases such as helium and neon approach ideal metabolism the series of linked chemical reactions taking behaviour because of their low intermolecular forces. place in living organisms. initiation step the first step in the mechanism of free- metalloid elements which have a low electrical radical substitution of alkanes by halogens. It involves the conductivity at room temperature but whose breaking of the halogen–halogen bond by UV light from conductivity increases with increasing temperature. the Sun. Metalloids are found in a diagonal band running from intermolecular forces the weak forces between molecules. the top left to nearly the bottom right of the p-block in ion polarisation the distortion of the electron cloud on an the Periodic Table. anion by a neighbouring cation. The distortion is greatest mobile phase the in the chromatography process, when the cation is small and highly charged. which moves through the column or over the paper or ionic bond the electrostatic attraction between oppositely thin layer. charged ions. molar mass the mass of a mole of substance in grams. ionic product of water, Kw The equilibrium constant for mole the unit of amount of substance. It is the amount of the ionisation of water. substance that has the same number of particles (atoms, + − Kw = [H ][OH ] ions, molecules or electrons) as there are atoms in exactly ionisation energy, ∆Hi the energy needed to remove 1 mole 12 g of the carbon-12 isotope. of electrons from 1 mole of atoms of an element in the molecular formula the formula that tells us the actual gaseous state to form 1 mole of gaseous ions. numbers of each type of atom in a molecule. isotopes atoms of an element with the same number of molecular ion the ion formed by the loss of an electron protons but different numbers of neutrons. from the original complete molecule during mass kinetic theory the theory that particles in gases and spectrometry and which gives us the relative molecular are in constant movement. The kinetic theory can be used mass of an unknown compound. to explain the effect of temperature and pressure on the monodendate ligands, such as water and ammonia, which volume of a gas as well as rates of chemical reactions. can form only one co-ordinate bond from each ion or lattice a regularly repeating arrangement of ions, atoms or molecule to the central transition metal ion. molecules in three dimensions. monomer a small, reactive molecule that reacts to make lattice energy the enthalpy change when 1 mole of an ionic long-chain molecules called polymers. compound is formed from its gaseous ions under standard mutation a change in the structure of DNA that results in conditions. an alteration of the genetic code. le Chatelier’s principle when any of the conditions affecting nanotechnology the design and production of machines the position of equilibrium are changed, the position of that are so small we measure them in nanometres (nm), that equilibrium shifts to minimise the change. where 1 nm = 1 × 10−9 m. ligand a molecule or ion with one or more lone pairs of neutron a subatomic particle found in the nucleus of an electrons available to donate to a transition metal ion. atom. It has no charge and has the same mass as a proton.

AS and A Level Chemistry © Cambridge University Press Glossary 5 nitrogenous bases nitrogen-containing bases found in oxidation number () a number given to an DNA and RNA. In DNA they are adenine (A), guanine atom in a compound which describes how oxidised or (G), thymine (T) and cytosine (C). In RNA uracil (U) reduced it is. replaces thymine. oxidising agent a reactant that increases the oxidation NMR nuclear magnetic resonance spectroscopy. number of (or removes electrons from) another non-competitive inhibition a type of enzyme inhibition reactant. in which the inhibitor molecule binds to a region of the partial pressure the pressure that an individual gas enzyme surface, often at a region other than the active contributes to the overall pressure in a mixture of gases. site. It distorts the shape of the active site or blocks the partition coefficient the ratio of the concentrations of active site permanently so that the active site no longer a solute in two different immiscible solvents when an functions. equilibrium has been established. non-degenerate orbitals atomic orbitals that have been peptide bond the link between the amino acid residues split to occupy slightly different energy levels. in a polypeptide or protein chain. The link is formed non-polar (molecule) a molecule with no separation of by a condensation reaction between the NH2 group charge; it will not be attracted to a positive or negative of one amino acid and the COOH group of another charge. amino acid. nucleon number the total number of protons and periodicity the repeating patterns in the physical and neutrons in the nucleus of an atom. chemical properties of the elements across the periods of nucleophile species that can act as a donor of a pair of the Periodic Table. electrons. permanent dipole–dipole forces a type of intermolecular nucleophilic addition the mechanism of the reaction in force between molecules which have permanent dipoles. which a nucleophile attacks the carbon atom in a pH the hydrogen ion concentration expressed as a logarithm carbonyl group and adds across the C O bond, to base 10. + e.g. aldehydes or ketones reacting with hydrogen pH = −log10[H ] cyanide. pi (π) bonds multiple covalent bonds involving the sideways nucleotide a compound consisting of a nitrogenous base, overlap of p atomic orbitals. a sugar (ribose or deoxyribose) and a phosphate group. pKa values of Ka expressed as a logarithm to base 10. + Nucleotides form the basic structural units of DNA pKa = −log10[H ] and RNA. polar (covalent bond) a covalent bond in which the two nucleus the small dense core at the centre of every atom, bonding electrons are not shared equally by the atoms in containing protons (positively charged) and neutrons the bond. The atom with the greater share of the electrons (no charge). Nuclei are therefore always positively has a partial negative charge, δ− and the other has a partial charged. positive charge, δ+. open system a system in which matter is lost or gained, polarising power (of a cation) the ability of a cation to e.g. a mixture of solids and gases in an open beaker. attract electrons and distort an anion. optical resolution the separation of optically active isomers polyamides polymers whose monomers are bonded to each (enantiomers) from a mixture. other via the amide link, CONH . order of reaction the power to which the concentration of a polyesters polymers whose monomers are bonded to each reactant is raised in the rate equation. If the concentration other via the ester link, COO . does not affect the rate, the reaction is zero order. If the polymer a long-chain molecule made up of many rate is directly proportional to the reactant concentration, repeating units. the reaction is first order. If the rate is directly proportional polypeptides natural polymers whose monomers are to the square of the reactant concentration, the reaction is bonded to each other via the amide link, CONH , and second order. whose monomers are amino acids. oxidation the addition of oxygen, removal of electrons or primary alcohol an alcohol in which the carbon atom increase in oxidation number of a substance; in organic bonded to the OH group is attached to one other chemistry refers to a reaction in which oxygen atoms are carbon atom (or alkyl group). added to a molecule and/or hydrogen atoms are removed primary structure (of proteins) the sequence of amino acids from a molecule. in a polypeptide chain.

6 Glossary AS and A Level Chemistry © Cambridge University Press principal quantum shells, n regions at various distances relative formula mass the mass of one from the nucleus which may contain up to a certain of a compound measured on a scale on which an number of electrons. The first quantum shell contains atom of the carbon-12 isotope has a mass of exactly up to 2 electrons, the second up to 8 and the third 12 units. up to 18. relative isotopic mass the mass of a particular isotope of propagation step a step in a free-radical mechanism in an element on a scale where an atom of the carbon-12 which the radicals formed can then attack reactant isotope has a mass of exactly 12 units. molecules generating more free-radicals, and so on. relative molecular mass the mass of a molecule measured on prosthetic group an ion or molecule that is permanently a scale on which an atom of the carbon-12 isotope has a bound to part of an enzyme, for example, a Zn2+ ion mass of exactly 12 units. is the prosthetic group in the enzyme carbonic anhydrase. replication the process of copying DNA during cell protein condensation polymer formed from amino acids division; it is a complex process requiring a number of and joined together by peptide bonds. Proteins can be different enzymes and other compounds. structural (e.g. cartilage), catalysts (enzymes), hormones residue see amino acid residue. (e.g. insulin) or antibodies. retention time the time taken for a component in a mixture proton a positively charged subatomic particle in the to travel through the column in GLC or HPLC. nucleus. reversible reaction a reaction in which products can be rate constant the proportionality constant in the rate changed back to reactants by reversing the conditions.

equation (see rate equation). Rf value the ratio of the distance a component has travelled rate equation an equation showing the relationship between compared to the distance travelled by the solvent front the rate constant and the concentrations of those reactants during paper chromatography or TLC. which affect the rate of reaction. The general form of the ribosomes small units about 20 nm in diameter present in rate equation is: most cells, on which protein synthesis takes place. They rate = k[A]m[B]n contain rRNA and protein. where k is the rate constant, [A] and [B] are the RNA (ribonucleic acid) a polynucleotide molecule which, concentrations of those reactants which affect the rate in most organisms, is single stranded. There are several of reaction, m is the order of the reaction with respect types of RNA including messenger RNA (mRNA), to A and n is the order of reaction with respect to B. ribosomal RNA (rRNA) and transfer RNA (tRNA). rate of reaction a measure of the rate at which reactants are salt bridge a piece of filter paper soaked in nitrate used up or the rate at which products are formed. The solution used to make electrical contact between the half- units of rate are mol dm−3 s −1. cells in an electrochemical cell. rate-determining step the slowest step in a reaction saturated hydrocarbons compounds of hydrogen and mechanism. carbon only in which the carbon–carbon bonds are all real gases gases which do not obey the ideal gas law single covalent bonds, resulting in the maximum number especially at low temperatures and high pressures. of hydrogen atoms in their molecules. redox reaction a reaction where oxidation and reduction secondary alcohol an alcohol in which the carbon atom take place at the same time. bonded to the O H group is attached to two other reducing agent a reactant that decreases the oxidation carbon atoms (or alkyl groups). number of (or adds electrons to) another reactant. secondary structure (of proteins) the second level of protein reduction the removal of oxygen, addition of electrons or structure. The folding of a polypeptide chain into specific decrease in oxidation number of a substance; in organic structures (e.g. α-helix and β-pleated sheet) which are chemistry it is the removal of oxygen atoms from a stabilised by hydrogen bonds formed between CO and molecule and/or the addition of hydrogen atoms to NH groups in peptide bonds. a molecule. shielding the ability of inner shells of electrons to reduce the relative atomic mass the weighted average mass of the effective nuclear charge on electrons in the outer shell. atoms of an element, taking into account the proportions short tandem repeat (STR) analysis a DNA fingerprinting of naturally occurring isotopes, measured on a scale on technique in which the enzyme DNA polymerase is used which an atom of the carbon-12 isotope has a mass of to copy selected short sequences of bases over a million exactly 12 units. times. This gives enough sample material to separate and

AS and A Level Chemistry © Cambridge University Press Glossary 7 detect using electrophoresis, even if the original sample standard electrode potential the electrode potential of was very small. a half-cell when measured with a standard hydrogen sigma (σ) bonds single covalent bonds, formed by the ‘end- electrode as the other half-cell. on’ overlap of atomic orbitals. standard enthalpy change an enthalpy change which takes single covalent bond a shared pair of electrons bonding two place under the standard conditions of pressure (105 Pa) atoms together. and temperature (298 k). skeletal formula a simplified version of the displayed standard hydrogen electrode a half-cell in which hydrogen formula which has all the symbols for carbon and gas at a pressure of 1 atmosphere (101 kPa) bubbles into a hydrogen atoms removed, as well as the carbon to solution of 1.00 mol dm−3 H+ ions. This electrode is given hydrogen bonds. The carbon to carbon bonds are left in a standard electrode potential of 0.00 V. All other standard place as are the bonds to other atoms. electrode potentials are measured relative to this value.

SN1 mechanism the steps in a nucleophilic substitution state symbol a symbol used in a chemical equation which reaction in which the rate of the reaction (which is describes the state of each reactant and product: (s) for determined by the slow step in the mechanism) involves solid, (l) for liquid, (g) for gas and (aq) for substances in only the organic reactant, e.g. in the hydrolysis of a aqueous solution. tertiary halogenoalkane. stationary phase the immobile phase in chromatography

SN2 mechanism the steps in a nucleophilic substitution that the mobile phase passes over or through. Examples reaction in which the rate of the reaction (which is are the surface of the thin-layer particles in TLC or the determined by the slow step in the mechanism) involves involatile liquid adsorbed onto the column in GLC or two reacting species, e.g. in the hydrolysis of a primary HPLC. halogenoalkane. stereoisomers compounds whose molecules have the solubility product, Ksp the equilibrium expression showing same atoms bonded to each other but with different the product of the concentrations of each ion in a arrangements of the atoms in space. saturated solution of a sparingly soluble salt at 298 K, the mole ratio of the reactants and products raised to the power of the relative concentrations: in the balanced equation for a reaction. y+ a x− b Ksp = [C (aq)] [A (aq)] strong acid/base an acid or base which is (almost) where a is the number of Cy+ ions in one formula unit completely ionised in water. of the compound and b is the number of Ax− ions in one structural formula the formula that tells us about the atoms formula unit of the compound. bonded to each carbon atom in an organic molecule, e.g. solute a substance that is dissolved in a solution. CH3CH CH2 specific most enzymes are described as specific because they structural isomers compounds with the same molecular will only catalyse one reaction involving one particular formula but different structural formulae. molecule or pair of molecules. sub-shells regions within the principal quantum shells spectator ions ions present in a reaction mixture which do where electrons have more or less energy depending on not take part in the reaction. their distance from the nucleus. Sub-shells are given the spin pair repulsion electrons repel each other since they letters s, p, d and f. have the same charge. Electrons arrange themselves so substitution a reaction which involves the replacement of that they first singly occupy different orbitals in the same one atom, or group of atoms, by another. sub-level. After that they pair up with their spins opposed substrate a molecule that fits into the active site of an to each other. enzyme and reacts. splitting pattern the pattern of peaks that main signals are successive ionisation energy ∆Hi1, ∆Hi2, etc: the energy divided into in high-resolution NMR. required to remove the first, then the second, then the standard cell potential the difference in standard electrode third electrons and so on from a gaseous atom or ion, potential between two half-cells. producing an ion with one more positive charge each standard conditions conditions of temperature and time. Measured in kJ per mole of ions produced. pressure which must be the same in order to compare surroundings in enthalpy changes, anything other than the moles of gases or enthalpy changes accurately. Standard chemical reactants and products, for example the solvent, conditions are a pressure of 105 pascals (100 kPa) and a the test tube in which the reaction takes place, the air temperature of 298 K (25 °C). around the test tube.

8 Glossary AS and A Level Chemistry © Cambridge University Press termination step the final step in a free-radical mechanism translation (in protein synthesis) the process by which in which two free radicals react together to form a the genetic message encoded by mRNA is translated molecule. at the ribosomes into a sequence of amino acids in the tertiary alcohol an alcohol in which the carbon atom polypeptide chain. mRNA, tRNA and ribosomes are all bonded to the O H group is attached to three other involved in the process. carbon atoms (or alkyl groups). triple covalent bond three shared pairs of electrons bonding tertiary structure (of proteins) the third level of protein two atoms together. structure. It involves further folding of the polypeptide turnover number the number of substrate molecules chain which is stabilised by interactions between the converted to product per minute per enzyme molecule. amino acid side-chains (ionic interactions, hydrogen two-way chromatography a technique used in paper or bonding, van der Waals’ forces and disulfide bonds). thin-layer chromatography in which one spot of a mixture titre in a titration, the final burette reading minus the is placed at the corner of a square sheet and is developed initial burette reading. in the first solvent as usual. The sheet is then turned TLC thin-layer chromatography. through 90° and developed in the second solvent, giving a

TMS tetramethylsilane. An inert, volatile liquid used as a better separation of components having similar Rf values. reference in NMR, given a chemical shift of zero. unsaturated hydrocarbons compounds of hydrogen and Tollens’ reagent an aqueous solution of silver nitrate in carbon only whose molecules contain carbon-to-carbon excess ammonia solution, sometimes called ammoniacal double bonds (or triple bonds). silver nitrate solution. It is used to distinguish between van der Waals’ forces the weak forces of attraction between aldehydes and ketones. It gives a positive ‘silver mirror’ molecules caused by the formation of temporary dipoles. test when warmed with aldehydes but no change is vaporisation the change in state when a liquid changes observed with ketones. to vapour. transcription the process by which the genetic message vapour pressure The pressure exerted by a vapour in encoded on the template strand of DNA is ‘copied’ to equilibrium with a liquid. make a strand of messenger RNA. weak acid/base an acid or base which is only slightly ionised transfer RNA a type of RNA which transfers amino in water. acids to a lengthening peptide chain during protein X-ray crystallography an analytical technique which uses synthesis by interaction with mRNA and the the diffraction pattern of X-rays passed through a solid ribosomes. sample to elucidate its structure.

AS and A Level Chemistry © Cambridge University Press Glossary 9