A-level chemistry Ocr B, Oz

Dipole

A partial charge on an atom which is caused by the differing electronegativities of atoms in a covalent bond. It results in one end of the molecule being slightly positive and the other end being slightly negative.

Electronegativity

The ability of an atom to attract a bonding pair of electrons in a covalent bond.

Hydrogen Bonding

An interaction between a hydrogen atom and an electronegative atom, commonly nitrogen, fluorine or oxygen. The slightly positive hydrogen is attracted to the lone pair on the electronegative atom hydrogen bonds are stronger than van der Waals and dipole-dipole forces but weaker than ionic and covalent bonds.

Intermolecular Forces

Forces which act between molecules. These include permanent dipole-dipole forces and hydrogen bonding.

Polar Bond

A covalent bond that has a permanent dipole due to the different electronegativities of the atoms that make up the bond.

Van der Waals

Also known as induced dipole-dipole, van der Waals forces exist between all molecules. They arise due to fluctuations in electron density within a molecule. These fluctuations may temporarily result in an uneven distribution, producing an instantaneous dipole. This dipole can induce a dipole in another molecule, and so on.

Activation Enthalpy

The minimum enthalpy required for the reaction to occur between two colliding particles.

Amines

Compounds that contain the NR₃ functional group, (where R could be hydrogen atoms or alkyl chains). Amines can act as bases and nucleophiles.

Catalyst

A substance that increases the rate of reaction without being chemically changed or used up. They work by providing an alternate reaction pathway with a lower activation energy.

Collision Theory

The theory that particles must collide with sufficient energy and in the correct orientation to react.

Effect of concentration on reaction rate

As the concentration of reactants increases, the reacting particles get closer together meaning they will collide more often . As a result, there will be a higher rate of successful collisions and a faster rate of reaction.

Effect of pressure on rate of reaction

As the pressure of gaseous reactants increases, the reacting particles get closer together meaning they will collide more often. As a result, there will be a higher rate of successful collisions and a faster rate of reaction.

Effect of temperature on reaction rate

Increasing the temperature means the particles will have more kinetic energy and so will move faster. If the molecules are moving faster they will collide more often and, since they have more kinetic energy, a larger proportion of the particles will have energy equal to or greater than the activation energy. For both of these reasons the rate of reaction increases.

Enthalpy Profile

A diagram showing the energy change of a reaction as it moves from reactants to products.

Functional Group

An atom/group of atoms responsible for the characteristic reactions of a compound.

Halogenoalkanes

An organic compound containing a halogen atom (F/Cl/Br/I) bound to an alkyl chain. haloalkanes can be formed from alcohols via a substitution reaction with halide ions in the presence of acid.

Homogenous Catalyst

A catalyst which is in the same state as the reactants in the system.

Homologous Series

A series of organic compounds containing the same functional group with successive members differing by -CH₂.

Maxwell-Boltzmann Distribution

Shows the distribution of the molecular energies in a gas at a constant temperature. The area under the curve indicates the total number of particles present.

Nomenclature

The naming system for compounds

Rate of reaction

The measure of the amount of product formed or reactant used over time. The units of rate of reaction may be given as g/s, cm³/s or mol/s.

Boiling Point

The temperature at which a compound changes from a liquid state to a gaseous state.

Bond Enthalpy

The energy required to break one mole of gaseous bonds. Actual bond enthalpies may differ from the average as the average considers the bond across a range of molecules.

Curly Arrow

Shows the movement of a pair of electrons. Curly arrows must start from a bond, a lone pair of electrons or a negative charge.

Free Radical Substitution

A photochemical reaction between halogens and alkanes to form halogenoalkanes. The reaction requires UV light and involves three stages: initiation, propagation and termination. Initiation creates a radical species. Propagation involves a series of chain reactions where free radicals bond to molecules to form new free radicals. Termination involves the reaction of free radicals with other free radicals to form new molecules.

Free Radical

A species with an unpaired electron. These are represented in mechanisms by a single dot.

Heterolytic Fission

When a covalent bond breaks and one bonding atom receives both electrons from the bonded pair.

Homolytic Fission

When a covalent bond breaks each bonding atom receives one electron from the bonding pair, forming 2 radicals.

Initiation

The first step in a radical substitution mechanism, involving the formation of the radicals.

Nucleophile

An atom or molecule that donates an electron pair to form a covalent bond. Attracted to electron-deficient areas.

Nucleophilic Substitution

The reaction of an electron pair donor (the nucleophile) with an electron pair acceptor (the electrophile). Involves one species being replaced with another species.

Ozone

Chemical formed naturally in the upper atmosphere. It is beneficial because it absorbs ultraviolet radiation and prevents harmful radiation reaching earth. Ozone is formed in the reaction above.

Ozone Depletion

Chlorine atoms catalyse the decomposition of ozone and contribute to the destruction of the ozone layer.

Partial Charges

The slight charge on an atom due to the bonded atoms having different electronegativities

Photodissociation

The process where a molecule is broken down using photons. This process is significant in ozone depletion as UV light can cause ozone molecules to split, releasing oxygen and contributing to the reduction of the ozone layer.

Propagation

The intermediate steps in a radical substitution mechanism where a radical reacts with another species.

Substitution

A reaction where one atom/group of atoms is substituted for another atom/ group of atoms.

Termination

The final steps in a radical substitution mechanism in which 2 radicals react together to form a species that only contains paired electrons.

Ozone as sunscreen

Ozone block out the sun’s UV rays so acts as a natural sunscreen.

Pollution effect on Ozone

Man-made pollutants thin the ozone layer, meaning it doesn’t absorb as much UV radiation in those areas.

Electromagnetic Spectrum

A range of frequencies of electromagnetic radiation and the respective wavelengths.

Electron Promotion

When an electron absorbs a photon and moves from a low energy orbital to a vacant higher energy orbital.

Infrared

An area of the electromagnetic spectrum that has wavelengths between 780nm and 1mm.

Ultra-violet (UV)

An area of the electromagnetic spectrum that has wavelengths between 10nm and 400nm

Visible light

An area of the electromagnetic spectrum that has wavelengths between 380nm and 700nm.