Chemistry Definitions - AS & A Level Cambridge

Essential definitions found in examination papers

Mole

1 mole of any substance has the same number of particles as 12g of carbon-12.

Atomic radius

The distance between the nucleus and the outer most electron of an atom

Ionic radius

The distance between the nucleus and the outer most electron of an ion

Periodicity

The repeating patterns of chemical and physical properties shown by elements across the periods

Hydrophobic

A substance that “likes” water; is able to be dissolved in water.

Hydrophilic

A substance that “dislikes” water; is unable to be dissolved in water.

The law of conservation of energy

Energy cannot be created or destroyed. It can only be transformed from one form of energy to another.

Hess’s Law

The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same.

Enthalpy Change of Reaction

The enthalpy change when the reactants in the stoichiometric equation react to give the products (under standard conditions)

Enthalpy Change of Formation

The enthalpy change when 1 mole of a compound is formed from its elements (under standard conditions)

Enthalpy Change of Combustion

The enthalpy change when 1 mole of a substance is reacted/burnt in excess oxygen (under standard conditions)

Enthalpy Change of Neutralisation

The enthalpy change when 1 mole of water is formed by reacting and acid with an alkali (under standard conditions)

Enthalpy Change of Hydration

The enthalpy change when 1 mole of a specified gaseous ion dissolves in sufficient water to form a very dilute solution (under standard conditions)

Enthalpy Change of Solution

The enthalpy change when 1 mole of an ionic solid compound fully dissolves in sufficient water to form a very dilute solution (under standard conditions)

Enthalpy Change of Atomisation

The enthalpy change when one mole of gaseous atoms is formed from its element (under standard conditions)

Bond Energy

The energy required to break one mole of a particular covalent bond in the gaseous states (under standard conditions)

Bond Length

the internuclear distance of two covalently bonded atoms (the distance from the nucleus of one atom to another atom that forms the covalent bond)

(under standard conditions)

Standard Conditions

Pressure of 101kPa and a temperature of 298 K (25 ℃)

Exothermic Reactions

A reaction is exothermic when the products have less energy than the reactants.

Endothermic Reactions

A reaction is endothermic when the products have more energy than the reactants.

The Pauli Exclusion Principle

No two electrons can be identified by the same set of quantum numbers. In other words, two electrons in an orbital must have opposite spins.

Ground state

The most stable electronic configuration of an atom which has the lowest amount of energy.

Nucleon Number

The number of protons and neutrons (excludes number of electrons)

First ionisation energy

The energy required to remove 1 mole of electrons from 1 mole of gaseous atoms of a specific element (under standard conditions)

Second ionisation energy

The energy required to remove 1 mole of electrons from 1 mole of gaseous ions of a specific element (under standard conditions)

Dynamic equilibrium

The molecules or ions of reactants and products are continuously reacting.

Equilibrium position

The balance point where the amount of reactants and products formed remains constant.

Le Chatelier’s Principle

When a chemical system in dynamic equilibrium is disturbed (conditions change), the system tends to shift the equilibrium to oppose the change and a new equilibrium is set up.

Collision theory

For a chemical reaction to occur, particles of the reactants must collide with sufficient energy and at the correct orientation.

Mole fraction

The mole fraction for a specific gas in chemical reaction is the amount of moles of that gas divided by total number of moles of gases

Partial Pressure

Partial Pressure of a specific gas in a mixture is the pressure which it would exert if it occupied the whole container alone.

The Contact Process

The Contact process makes sulphur dioxide – which converts sulphur dioxide into sulphur trioxide (reversible reaction) – which then converts sulphur trioxide into concentrated sulfuric acid.

The Haber Process

The Haber process creates ammonia by combining nitrogen from the air with hydrogen derived from methane gas.

Catalyst

A substance used to reduce/lower the activation energy of a chemical reaction

Oxidation

Oxidation is a loss of electrons. Oxidation is an increase in oxidation number.

Reduction

Reduction is a gain in electrons. Reduction is a decrease in oxidation number.

Oxidising agent

A substance that oxidises another atom or ion by causing it to lose electrons.

Reducing agent

A substance that reduces another atom or ion by causing it to gain electrons

Disproportionation reaction

When a substance undergoes oxidation and reduction in the same reaction

Bronsted-Lowry theory

Acids can be defined as proton donors. A base can be defined as a proton acceptor.

Ionisation of a compound

The process in which a neutral molecule splits/dissociates into charged ions when exposed in a solution.

Amphoteric

A substance that can act as both a base and an acid

Volatility

How easily a substance can evaporate

Functional Groups

The atom, group of atoms, or structural feature in a molecule that is the site of reactivity

Aliphatic Compounds

Compounds which do not have aromatic rings (benzene).

Homologous series

A family of organic compounds that have similar features and chemical properties due to having the same functional groups.

Chiral Centre

The asymetric central carbon atom is bonded to 4 different functional groups

Enantiomer

Two optical isomers

Racemic mixture

A mixture of enantiomers

Saturated Hydrocarbons

Hydrocarbons that only contain (C-C) carbon single bonds

Unsaturated Hydrocarbons

Hydrocarbons that contain one or more (C=C) carbon double bonds or carbon triple bonds

Homolytic Fission

Breaking of a covalent bond in such a way that each atom takes half the number of electrons from the bond to form 2 radicals

Heterolytic Fission

Breaking of a covalent bond in such a way that the more electronegative atom takes the electrons from the bond to form a negative ion and leaving behind a positive ion

Nucleophile

An electron-rich species that can donate a pair of electrons

Electrophile

An electron-deficient species that can accept a pair of electrons

Monomer

A molecule, generally organic, that can react with other molecules to form very large molecules called polymers

Polymer

A large molecule made up of a very large number of repeating units called monomers

Markovnikov’s Rule

When a hydrogen halide compound (H-X) is added to an unsymmetrical alkene, the hydrogen becomes attached to the carbon with the most hydrogen attached to it already

Zaitsev’s rule

Elimination will occur such that the hydrogen is removed from the carbon with the least hydrogens

Lattice Energy

The enthalpy change when one more of an ionic compound is formed from its gaseous ions (under standard conditions)

The first Electron Affinity

The enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms, to form 1 mole of gaseous ions each with a single negative charge (under standard conditions)

Entropy of a given system

The number of possible arrangements of the particles and their energy in a given system

Electrolysis

The process by which ionic compounds are decomposed/broken down into simpler substances when an electric current passes through them

Electrode Potential

The electrode (reduction) potential E is a value which shows how easily a substance is reduced.

The standard electrode potential (Eꝋ)

The voltage produced when a standard half-cell is connected to a standard hydrogen cell under standard conditions

The standard cell potential (Ecellꝋ)

The voltage difference between two standard half-cells

The acidic dissociation constant

The equilibrium constant for the dissociation of a weak acid at 298K.

The ionic product of water

The equilibrium constant for the dissociation of water at 298K which has a value of 1.00x10-14 mol2dm-6.

Buffer solution

A solution in which the pH does not change a lot when small amounts of acids or alkalis are added.

Solubility

The maximum amount of solute that can be dissolved into a known amount of solvent at a given temperature, producing a saturated solution

Solubility product

The product of the concentrations of each ion in a saturate solution of a relatively soluble salt, at 298K, raised to the power of their relative concentrations

Saturated Solution

A solution that contains the maximum amount of dissolved salt

Partition coefficient (Kpc)

The ratio of the concentrations of a solute in two different immiscible solvents in contact with each other when equilibrium has been established

Half-life

The time taken for the concentration of a limiting reactant to become half of its initial value

A complex molecule/ion

A molecule/ion formed by a central metal atom/ion surrounded by one or more ligands

Ligand

A molecule/ion that has one or more lone pairs of electrons

Monodentate ligand

Form only one dative bond to the central metal ion

Bidentate ligand

Form 2 dative bonds to the central metal ion

Polydentate ligand

Form more than 2 dative bonds to the central metal ion

Ligand exchange/substitution

When one ligand complex is replaced by another

The stability constant

The equilibrium constant for the formation of the complex ion in a solvent from its constituent ions or molecules