Chemistry OCR B DF key definitions

Enthalpy Change (Delta H)

The amount of heat energy absorbed or released during a chemical reaction at constant pressure, typically measured in joules or kilojoules.

Ideal Gas

A model of gases that assumes that gas molecules are perfect spheres that move in straight lines at a constant speed. Ideal gases have no intermolecular forces and occupy no volume.

Displayed formula

A structural formula that shows the arrangement of atoms and the bonds between them in a molecule, typically representing all atoms explicitly.

Empirical formula

The simplest whole number ratio of atoms of each element present in a compound which does not indicate the actual number of atoms in the molecule.

Skeletal Formula

A simplified organic formula that shows the bonding between atoms in a molecule, omitting hydrogen atoms and representing carbon atoms as lines and the functional group.

Structural formula

A representation that shows the arrangement of atoms in a molecule.

Sigma bond

A type of covalent bond formed by the direct overlap of atomic orbitals, allowing for free rotation around the bond axis.

Pi bond

A type of covalent bond formed by the side-to-side overlap of p orbitals, which restricts rotation around the bond axis. As Pi bonds have low bond enthalpy, alkenes are more reactive then alkanes.

Average bond Enthalpy

The enthalpy change when one mole of a specified covalent bond is broken, averaged out across the range of compounds.

Endothermic

A reaction that takes in energy from the surroundings (Delta H is positive). The energy of the products is higher than that of the reactants.

Enthalpy (H)

A value that represents the heat content of a system.

Enthalpy change of neutralisation (Delta_NeutH)

The enthalpy change that occurs when one mole of water is formed from the reaction of an acid and a base (neutralisation Reaction).

Enthalpy change of reaction (Delta_rH)

The enthalpy change that is associated with a particular chemical reaction. It is defined as the difference in enthalpy between the products and reactants, indicating whether the reaction is exothermic or endothermic.

Exothermic

A reaction that releases energy into the surroundings (Delta H is negative). The energy of the reactants is higher than that of the products.

Hess’ law

The principle stating that the total enthalpy change during a chemical reaction is the same, regardless of the pathway taken, allowing for calculations of enthalpy changes using known values.

Standard conditions

Solution concentrations of 1.00 mol dm^-3, a pressure of 100 kPa, and a temperature of 298 K (25°C) under which standard enthalpy changes are measured.

Standard State

The physical state of a substance (s, l, g, aq) under standard conditions, typically at 1 atm pressure and a specified temperature, usually 298 K.

Standard enthalpy of combustion (Delta_cH^⦵)

The enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions.

Standard enthalpy of formation (Delta_fH^⦵)

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.

Activation energy

The minimum amount of energy required for particles to collide with for a successful reaction to occur.

Catalysis

The process of increasing the rate of a chemical reaction by adding a substance, called a catalyst, which is not consumed in the reaction.

Catalyst

A substance that increases the rate of a chemical reaction without being used up. A catalyst allows a reaction to proceed via an alternate pathway with a lower activation energy. Catalysts are important in terms of economics and sustainability as they enable the process to take place at lower temperatures meaning less energy is required (this decreases the combustion of fossil fuels so CO₂ emissions are reduced).

Catalyst poisoning

the decrease in activity of a heterogeneous catalyst due to another compound. As catalysts are regenerated in theory they can be used indefinitely, however in reality, the surface of a catalyst sometimes gets ‘poisoned’ by impurities and is, therefore, less exposed for reacting particles so the catalyst may need to be replaced or regenerated to restore its effectiveness.

Cracking

The process of breaking long chain alkanes into smaller, more useful hydrocarbons. Helps to convert low demand hydrocarbons into more highly demanded ones. This process can be achieved through thermal or catalytic methods.

Heterogeneous catalyst

A catalyst that exists in a different phase than the reactants, typically as a solid in a liquid or gas reaction. Heterogeneous catalysts facilitate reactions at their surface, allowing for easier separation and recycling after the reaction.

Hydrocarbon

A compound consisting entirely of hydrogen and carbon atoms, commonly found in fuels and organic materials.

Aliphatic compound

A type of organic compound that consists of straight or branched chains of carbon atoms, which can be saturated or unsaturated.

Alkane

A homologous series of saturated hydrocarbons with the general formula C_nH_2n+2.

Alkene

A homologous series of unsaturated hydrocarbons with at least one carbon-carbon double bond, represented by the general formula C_nH_2n.

Aromatic compound/Arene

A compound containing at least one benzene ring

Functional group

A specific group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule.

General formula

A simplified expression that shows the ratio of atoms of each element in a compound, typically represented as C_nH_n or similar formats.

Global warming

The long-term rise in Earth's average surface temperature due to human activities, primarily fossil fuel burning, which increases heat-trapping greenhouse gas levels in the atmosphere.

Homologous series

A series of compounds with the same functional group similar chemical properties. For example, all alkenes belong to the same homologous series. They all share the same general formula.

Nomenclature

The naming of a molecule/compound in chemistry.

Organic compound

A carbon-containing compound

Pollutant

A substance that has undesired effects on the air, water or environment.

Saturated

An organic compound that only contains C-C single bonds

Structural isomers

Compounds with the same molecular formula but different structural formulae.

Unsaturated

An organic compound that contains at least one C=C double bond, a C≡C triple bond or an aromatic ring.

Addition polymer

A polymer formed by addition polymerisation. Formed from monomers with CC double bonds.

Addition polymerisation

The process by which addition polymers are formed. Lots of short chain monomers (alkenes) join together to form a long chain polymer by the ‘opening up’ of double carbon bonds.

Monomer

A small molecule that is used to form polymers.

Polymer

A large molecule made from many small units (monomers) that have been bonded together.

Polymerisation

The process of making a polymer from its monomers. There are two types: addition polymerisation and condensation polymerisation.

Repeat unit

A structure within a polymer that appears over and over again. Joining many repeat units together would form the polymer.

Addition reaction

A chemical reaction where two or more substances combine to form a single product, often involving the addition of atoms or groups to a molecule without the loss of any atoms.

Carbocation

A positively charged ion that has a carbon atom with three bonds and a vacant p-orbital, making it highly reactive.

Curly arrow

A notation used in chemical reaction mechanisms to represent the movement of electrons from one atom or bond to another, indicating the direction of electron flow.

Electrophile

A species that accepts an electron pair from a nucleophile to form a chemical bond, typically possessing a positive charge or a partial positive charge.

Electrophilic addition

A reaction in which a Pi bond is broken and two new Sigma bonds form as a result of the addition of an electron pair acceptor.

Major/Minor products

Formed in electrophilic addition reactions due to the differing relative stabilities of the primary/secondary/tertiary carbocation intermediates. The major product is formed from the least stable intermediate.

Primary carbocation

A molecule in which the carbon with the positive charge is only attached to one alkyl group. this is the least stable carbocation.

Secondary carbocation

A molecule in which the carbon with the positive charge is attached to two alkyl groups, making it more stable than a primary carbocation but less stable than a tertiary carbocation.

Tertiary carbocation

A molecule in which the carbon with the positive charge is attached to three alkyl groups, making it the most stable type of carbocation.

Cahn-Ingold-Prelog (CIP) priority rules

A set of rules used to identify whether a stereoisomer is E or Z. Atoms with a higher atomic number have a higher atomic number have a higher priority. If the highest priority substituents on the same side of the double bond (both above or below), it’s the Z isomer whereas if the highest priority groups are on the opposite sides ( one above and one below) it’s the E isomer.

Cis-trans isomerism

A type of stereoisomerism in which the two substituent groups attached to the carbon atoms are the same. According to the CIP priority rules, cis isomers have the highest priority substituents on the same side of the double bond (both above or below) while trans isomers have them on the opposite sides (one above and one below).

E/Z isomerism

A type of stereoisomerism caused by the restricted rotation of Pi bonds. Two different groups must be attaches to each carbon atom of the CC double bond group. According to the CIP priority rules, Z isomers have the highest priority substituents on the same side of the double bond while E isomers have them on the opposite sides.

Stereoisomers

Compounds with the same structural formula but a different arrangement of atoms in space.

Biofuels

Fuels made from once-living organic matter. These fuels are renewable and sustainable and are an alternative to fossil fuels.

Complete combustion

When a compound is burnt in an excess of oxygen. When alkanes are completely combusted, the only products are water and carbon dioxide.

Hydrogen fuel cell

An electrochemical cell that converts the chemical energy of hydrogen and oxygen, which is an oxidising agent, into electricity through redox reactions. they do not need to be recharged but require a constant supply of fuel to maintain the potential difference.

Incomplete combustion

When a compound is burnt in a limited supply of oxygen. When alkanes are incompletely combusted water, carbon monoxide, carbon particulates and some carbon dioxide may be produced.