As Chemistry Important Terms Definitions

AS Chemistry Important Terms & Definitions Acids are chemical species that donate protons/hydrogen ions. Strong acids completely dissociate into ions in aqueous solutions. Weak acids only dissociate partially in aqueous solutions. Activation energy is the minimum amount of energy needed before a reaction can occur. Addition reaction is the joining of two molecules to form a single product molecule. Addition Polymers are polymers formed by repeated addition reactions. Relative Atomic Mass is the weighted average of the masses of an element's isotopes relative to one twelth of the mass of a Carbon 12 atom. Atomic Number is the number of protons in the nucleus of each atom in an element. A Base is a chemical species which accept protons/hydrogen ions. Bond enthalpy is the amount of energy required to break one mole of a bond in a gaseous molecule Catalysts are substances that increase the rate of chemical reaction by providing an alternative pathway of lower activation energy, but which are left unchanged by the reaction. Chiral centres (also known as asymmetric carbon atoms) is an atom in a molecule which is bonded to four different chemical groups, giving rise to optical isomerism. Acid base Conjugate pairs are two molecules or ions that differ by one hydrogen ion Dative or co-ordinate covalent bonds is a covalent bond where both electrons come from the same atom. A Dynamic equilibrium occurs when, for a reversible reaction, the forward and reverse reactions occur at the same rate, so there is no change in net concentration. This must occur in a closed system Electronegativity is the ability of an atom to attract bonding electrons in a covalent bond. An electrophile is an atom or group of atoms which is attracted to an electron-rich centre or atom where it accepts a pair of electrons to form a covalent bond. Empirical formula shows the simplest whole number ratio of elements in a compound.

An enantiomer is a molecule with a chiral centre which can exist in two forms (two optical isomers). They can be distinguished by the direction in which they rotate a plane of polarised light or by certain enzyme reactions. Relative Formula Mass is the sum of the atomic masses in a compound, giving the mass of the compound relative to one twelth of the mass of a carbon-12 atom. A free radical is an atom or group of atoms with an unpaired electron. The Functional Group is the specific atom or group of atoms within a molecule which are responsible for the characteristic chemical reactions of that molecule other molecules in the same homologous series. General formula is the formula which may be written for members of the same homologous series. Ground state is the lowest allowed energy state of an atom, molecule or ion. Hess's Law states that the total energy change for a chemical reaction is independent of the route by which the energy change occurs. Heterolytic fission is when a compound breaks to form a positive ion and a negative ion. Homolytic Fission is when a compound breaks to form two free radicals. First ionisation energy is the energy required to remove one mole of electrons from each atom in one mole of gaseous atoms of an element Isomers are chemical species with the same number and types of atoms as another chemical species, but possessing different properties. Structural isomerism occurs when two species have the same molecular formula but a different structural formula. Chain isomers occur when the carbon skeletons of the molecules are different. Functional group isomers occur when the molecules belong to different homologous series. Positional isomers occur when the molecules have the same functional group but it occurs in a different place on the carbon skeleton Stereoisomers have the same structural formula but different spacial arrangements. Geometric cis-trans isomers occur when the carbon to carbon double bond cannot freely rotate, so some alkenes with additional functional groups either side of the double bond can exist in cis or trans form.

An isotope occurs when atoms of the same element with the same number of protons have different numbers of neutrons Molar mass is the mass of one mole of a substance A mole is a unit of amount for substances - 6.02 times 10 to the power of 23. Mole fraction is, for a mixture, the number of moles of a substance divided by the total number of moles of all components of the mixture. Relative molecular mass is the sum of the atomic masses in a molecule, giving the mass of the molecule relative to one twelth the mass of a carbon-12 atom. A nucleophile is a chemical with a lone pair of electrons which is attracted to a positively charged centre where it donates the electron pair forming a dative bond. Optical activity is the ability of a substance to rotate a plane of polarised light, a characteristic of a substance with a chiral centre. A racemic mixture is a fifty fifty mixture of positive and negative enantiomers which does not rotate a plane of polarised light. A saturated hydrocarbon only contains single carbon to carbon bonds Standard enthalpy change of atomisation is the energy change when one mole of gaseous atoms is formed from an element in standard conditions. Standard enthalpy change of combustion is the energy change when one mole of gaseous atoms reacts completely with oxygen under standard conditions Standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions with all elements in their standard states. Standard enthalpy change of reaction is the enthalpy change when the amounts of reactants as shown in the balanced equation react together under standard conditions to give products in their standard states. A substitution reaction occurs when an atom or group of atoms is replaced by a different atom or group of atoms. An unsaturated hydrocarbon is an organic compound containing at least one carbon to carbon double or triple bond.