Definitions - All Topics
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Relative Atomic Mass (AR).
The average mass of an atom of an element compared to 1/12th of the mass of a 12C atom
Relative Molecular Mass (MR)
The average mass of a molecule compared to 1/12th of the mass of a 12C atom
Empirical Formula
The simplest ration of atoms of each element in a compound
Atom economy
Measure of what proportion of the product of a reaction is desired, and how much is waste
Homologous series
A group of chemicals which have similar chemical properties, the same functional group and can be represented by a general formula
Isomers
Chemicals with the same molecular formula, but a different structural formula
Structural isomerism
Compounds with the same molecular formula, but different structural/skeletal/displayed formula. Eg of structural isomerism = positional isomerism
Chain isomerism
Compounds with the same molecular formula but a different carbon skeleton
Position isomerism
Compounds with the same molecular formula and carbon skeleton, but a different position of the same type of functional group
Functional group isomerism
Same molecular formula, different functional group
Stereo isomerism
When molecules have the same structural formula but a different spatial arrangement of atoms
Enthalpy change of formation
The enthalpy change when one mole of a substance is formed from its elements with all products and reactants in their standard states under standard conditions
Enthalpy change of combustion
The enthalpy change when one mole of a substance is burned completely in oxygen with all reactants and products in their standard states under standard conditions
Standard conditions
298K, 100kPa, 1 moldm^-3 if any solutions involved
Mean bond enthalpy
The average heat needed to break one mole of covalent bonds of a molecule in a gaseous state under standard conditions, averaged over a range of compounds
Define “dynamic” (in terms of equilibria)
Forward and reverse reactions both take place
Equilibrium
Both forward and reverse reactions take place at equal rates. The concentration of all reactants remain constant
Catalyst
A substance that increases the rate or reaction without being changed in chemical composition or amount. They work by providing an alternative pathway/reaction route of lower activation energy
Activation energy
The minimum amount of energy needed for a reaction to occur/for a successful collision
Equilibrium constant (Kc)
A value calculated from the equilibrium concentrations for a system at constant temperature Kc = concentration of products/concentration of reactants
Oxidation
Addition of oxygen or loss of electrons to a substance
Reduction
Removal of oxygen or addition of electrons to a substance
Disproportionation
When atoms of the same element are simultaneously oxidised and reduced
Reducing agent
A species which aids the reduction of another species by donating the electrons which the other species gains. The reducing agent’s oxidation state will become more positive (more oxidised)