Chemistry definitions and some knowledge

Relative Atomic Masses A_r

weighted average of the masses of the isotopes in a naturally occurring sample of the element relative to 1/12 of the mass of a carbon 12 atom 

First ionisation energy

The energy required to remove one mole of electrons from one mole of isolated gaseous atoms

Relative isotopic mass Ir

The mass of an isotope relative to 1/12th of the mass of an atom of carbon-12

Relative molecular mass M_r

 the mass of that molecule compared to 1/12 the relative atomic mass of an atom of carbon-12

Avogadro’s constant

the number of atoms in 12g of carbon-12

Mole

The amount of substance that contains as many particles as there are in 12g of carbon-12

Empirical formula

The simplest whole number ratio of the atoms of each element present

Molecular formula

The actual number of atoms of each element in one molecule of the compound 

Ideal gas

A gas composed of identical particles in continuous, random motion. The particles are considered point like. Collisions are elastic and do not result in reaction.

Boyle’s Law

At constant temperature T, the volume V, of a fixed gas is inversely proportional to the pressure, p, applied to it

Ideal gas law

Equal volumes of all gases at the same pressure and temperature contain the same number of particles

VESPR theory

Charge clouds in valence shell of an atom repel each other and this forces the pairs of electrons to move themselves as far as possible to minimise repulsion.

Electronegativity

Electronegativity is the power of an atom (relative ability) to attract the bonding pair of electrons in a covalent bond to itself.

Hydrogen bonds

The hydrogen bond is the force of attraction between the lone pair of electrons on the neighbouring electronegative atom and the δ+ charge on the hydrogen atoms.

Temperature

A measure of the average kinetic energy of particles

Heat

Heat is the form of energy that flows from something at a higher temperature to something at a lower temperature

Enthalpy change

heat change of a reaction measured under conditions of constant pressure.

Standard enthalpy of combustion ΔH_c^Ꝋ

The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions, all reactants and products being in their standard states.

Standard enthalpy of formation ΔH_f^Ꝋ

The enthalpy change when 1 mole of a substance is formed from its constituent elements under standard conditions, all reactants and products being in their standard states.

Standard enthalpy change of reaction ΔH_r^Ꝋ

The enthalpy change when molar amounts of reactants, as shown in the stoichiometric equation, react together under standard conditions.

Enthalpy change of neutralisation ΔH_neut^Ꝋ

The enthalpy change when one mole of water molecules is formed when and acid (H+) reacts with an alkali (OH-) under standard conditions

Enthalpy change of solution

The enthalpy change when one mole of solute is dissolved in excess solvent to form a solution of ‘infinite dilution’ under standard conditions

Specific heat capacity

The energy required to raise the temperatures of 1g of substances by 1K, measures inJg-1K-1

First law of thermodynamics

The enthalpy change accompanying a chemical reaction is independent of the pathway between the initial and final states

Mean bond enthalpy

The average amount of energy required to break one mole of a particular type of covalent bond in a gaseous molecule averaged over a range of similar compounds

Structural isomers

Molecules with the same molecular formula but a different structural formula

Homologous series

A homologous series is a group of compounds of the same functional group with the same general formula

Functional group

An atom or group of atoms that is typical fro a particular organic family and gives rise to its chemical properties

General formula

The general formula is an algebraic formula which can describe members of a homologous series

Nucleophile

An electron pair donor

Elimination

Removal of an atom/group of atoms from a parent molecule

Geometric isomers

All atoms are connected to each other in the same way and have the same molecular formula ,however the atoms are arranged differently in space

Homolytic fission

When a covalent bond is broken equally such that each atom in the bond accepts one electron

Heterolytic fission

When a covalent bond is broken unequally so that an atom in the bond receives both electrons to become an anion, the other a cation

Monomer

A small reactive molecule that contains the smallest group of atoms repeated throughout the chain (basic unit of a polymer)

Oxidising agent

A substance which oxidises another atom / molecule by gaining electrons (being reduced) causing other substances to lose electrons and be oxidised

Reducing agent

A substance which reduces another atom / molecule by losing electrons (being reduced) causing other substances to gain electrons and be reduced 

Structural isomer

A structural isomer is a molecule that has the same molecular formula but different structural formulas

Position isomer

Position isomers are structural isomers which have different structural formula due to the functional group being in a different place along the chain

Functional group isomer

Functional group isomers have the same molecular formula but a different structural formula due to having different functional groups

Electron-releasing groups

(CH₃, OCH₃, NH₂) Releases electrons onto the benzene ring and hence increases the electron density on the ring, hence the electrophile is more strongly attracted and so the reaction is much faster (direct further substitution to the 2,4 and 6 positions.

Electron withdrawing groups

(NO₂, COCl) These withdraw electrons from the ring making it less susceptible to further electrophilic substitutions (encourage substitutions on 3 and  5 positions) 

 

Catalysts

A catalyst speeds up the rate of reaction without getting used u in the process. It does this by providing an alternate reaction pathway with a lower activation energy

Heterogenous catalyst

A heterogeneous catalyst is in a different phase from the reactants

Homogenous catalyst

A homogeneous catalyst is in the same phase as the reactant

Activation energy

The minimum energy needed for a successful collision

Rate of reaction

Measured change in concentration of reactant or product in time

Rate determining step

The rate of the slowest step which will govern the rate of the reaction

Equivalence point

The volume at which exactly the same number of moles of hydrogen ions and hydroxide ions have been added

End -point

The volume of acid or alkali added so that the indicator just changes colour

Buffers

Buffers are solutions that resist changes in pH when small amounts of acid or alkali are added to them (maintains a constant pH, despite dilution or addition of small amounts of acid or base

Relative mass of an electron

1/1840

Principle quantum number

The number of energy levels/shells

How many electrons can fit in each shell in ascending order

2, 8, 18, 32

What are sub shells?

principal quantum shells are split into subshells, s, p, d and f they contain multiple orbitals, which exist at specific energy levels and can contain up to 2 electrons. The s subshells has 1 orbital, p has 3, d has 5 and f has 7.

What are the exceptions in ionisation energy in?

Between Beryllium and Boron (and Mg and Al) there is decrease in ionisation energy. And between Nitrogen and Oxygen (and P and S) there is a decrease in ionisation energy.

What is the reason for the exceptions in ionisation energy between Be/B and Mg/Al?

5th electron in B/13th electron in Al are in the 2p/3p subshells which is further from the nucleus than the 2s/3s subshells. Hence lower effective nuclear charge and it takes less energy to remove the electron.

What is the reason for the exceptions in ionisation energy between N/O and P/S?

In N/P all the electrons in the 2p/3p subshells are unpaired while in O/S there is on orbital with paired electrons. SInce there electrons are both negative they will repel each other which will make it easier for one of these electrons to be removed.

2 bonding pairs and 0 lone pairs

Linear 180°

2 bonding pairs 2 lone pairs

V- shaped 104.5° (water)

3 bonding pairs 0 lone pairs

Trigonal planar 120°

3 bonding pairs 1 lone pair

Trigonal pyramidal 107°

4 bonding pairs 0 lone pairs

Tetrahedral 109.5°

5 bonding pairs 0 lone pairs

Trigonal bipyramidal 90° and 120°

6 bonding pairs 0 lone pairs

90°

What is a permanent dipole?

Two atoms that are bonded have different electronegativities the more elctronegative atom draws more of the negative charge towards itself producing a δ+ and δ- atom.

What is an induced dipole?

A δ+ /δ- is created due to proximity with a different charged particle.

Bond breaking is…

endothermic

Bond making is …

exothermic

What is Hess’s law?

the energy change that occurs when a reaction takes place is the same, regardless of the route taken

What are the properties of chemical equilibria?

• rate of the forwards and backwards reaction are the same

• the concentrations of reactants and products are constant

What is a closed system?

When none of the reactants or products can escape the reaction mixture. (however energy can)

What is an open system?

Matter and energy can be lost to the surroundings

What is an isolated system?

No matter or energy can escape to the surroundings

Lewis acid

electron pair acceptor

Lewis base

electron pair donor

Bronsted-Lowry acid

proton donor

Bronsted Lowry base

proton acceptor

sodium potassium and ammonium compounds

soluble

nitrates

soluble

chlorides except silver and lead(II)

soluble

silver and lead(II) chlorides

insoluble

sulfates except barium calcium and lead(II)

soluble

barium calcium and lead(II) sulfates

insoluble

Sodium potassium and ammonium carbonates

soluble

carbonates except sodium potassium and ammonium

insoluble

sodium potassium and calcium hydroxides

soluble

hydroxides except for sodium potassium and calcium

insoluble

ionisation enthalpy

the amount of energy required to remove an electron from a gaseous atom of an element to form a gaseous ion under standard conditions

electron affinity

the energy change when one mole of electrons is gained by one mole of gaseous atoms of an element to form one mole of gaseous ions under standard conditions

Enthalpy change of atomisation

the enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions

Entropy (S)

a measure of how disordered or chaotic a system is

What causes an increase in entropy?

• more gaseous products

• more aqeous/liquid products

• a greater number of moles of products

Spontanous

a reaction that will take place of its own accord / is energetically favourable under standard conditions.

Feasible

a reaction that will of its own accord / is energetically favourable however may not necessarily take place at a high enough rate under standard conditions.

ΔH_r^ꝋ<0 and ΔS^ꝋ>0

ΔG^ꝋ<0 and spontaneous (forward reaction spontaneous at any T)

ΔH_r^ꝋ>0 and ΔS^ꝋ<0

ΔG^ꝋ>0 never takes place (backwards reaction spontaneous at and T)

ΔH_r^ꝋ<0 and ΔS^ꝋ<0

ΔG^ꝋ<0 at low T >0 at high T (only spontaneous at low T)

ΔH_r^ꝋ>0 and ΔS^ꝋ>0

ΔG^ꝋ<0 at high T >0 at low T (only spontaneous at high T)