chemistry definitions

THE RELATIVE ATOMIC MASS

THE RELATIVE ATOMIC MASS OF A MOLECULE IS THE MASS OF THAT MLECULE COMPARED TO 1/12 THE RELATIVE ATOMIC MASS OF ONE ATOM F CARBON-12

IONISATION ENERGY- 1ST

THE ENERGY REQUIRED TO REMOVE 1 MOLE OF ELECTRONS FROM 1 MOLE OF AN ATOM IN GASESOUS STATE TO FORM 1 MOLE OF +1 IONS.

successive ionisation energy equations:

X(g) → X⁺(g) + e⁻

X⁺(g) → X²⁺(g) + e⁻

X²⁺(g) → X³⁺(g) + e⁻

Empirical formula

The simplest whole-number ratio of atoms of each element in a compound.

Molecular formula

The actual number of atoms of each element in a molecule.

Ionic bonding

Electrostatic attraction between oppositely charged ions in a lattice.

Covalent bond

A shared pair of electrons between atoms.

Metallic bonding

Attraction between delocalised electrons and positive ions arranged in a lattice.

what are the four crystal structures

Structure	Example
Ionic	NaCl
Metallic	Mg
Giant covalent	diamond, graphite
Molecular	iodine, ice

Mean bond enthalpy

The average enthalpy change required to break one mole of a particular type of bond in gaseous molecules.

Activation energy

The minimum energy required for a reaction to occur.

Rate of reaction

Change in concentration of a reactant or product per unit time.

Catalyst

A substance that increases the rate of a chemical reaction without being changed in chemical composition or amount.

Catalysts provide an alternative reaction route with a lower activation energy.

Maxwell–Boltzmann

Temperature ↑ → more particles have E ≥ Ea → more successful collisions → rate ↑

Catalyst → Ea ↓ → greater proportion have E ≥ Ea → rate ↑

Dative covalent bond

A covalent bond in which both electrons in the shared pair are donated by one atom.

lone pair–lone pair > lone pair–bond pair > bond pair–bond pair

because lone pairs occupy more space.

lone pair–lone pair > lone pair–bond pair > bond pair–bond pair

because lone pairs occupy more space.

Relative molecular mass

The weighted mean mass of a molecule compared with 1/12 of the mass of an atom of carbon-12.

For ionic compounds, AQA uses relative formula mass rather than relative molecular mass.

kc

Kc changes with temperature.

Kc does NOT change when concentration changes.

Kc does NOT change when a catalyst is added.

oxidation

loss of e

reduction

gain of e

oxidising agent

e acceptor

reducibg agent

e donor

across period 3,

atomic radius?

why?

Across Period 3:

Atomic radius

Generally decreases

Reason:

nuclear charge ↑ → stronger attraction for electrons → radius ↓

First ionisation energy

Generally increases

Exceptions:

Mg → Al

P → S

why are mg-al and p-s exeptions for the trend of decreasing first ionisation energy

Exception	Why?
Mg → Al	Al loses a 3p electron, which is in a higher energy level and easier to remove than Mg`s 3s electron Mg: 1s² 2s² 2p⁶ 3s² Al: 1s² 2s² 2p⁶ 3s² 3p¹
P → S	S has paired 3p electrons, causing extra electron-“electron repulsion, making an electron easier to remove P: 3s² 3p³ S: 3s² 3p⁴

trends in group 7

Electronegativity ↓

Boiling point ↑

Oxidising ability ↓

Reducing ability of halide ions ↑

ELECTRONEGATIVITY

THE ABILITY OF AN ATOM TO ATTARCT THE BONDING PAIR OF ELECTRONS IN A COVALENT BOND

DYNAMIC EQUILIBRIUM

THE FORWARD AND THE REVERSE REACTIONS HAPPEN AT EQAL RATES AND THEREFORE THE CONCENTRATION OF PROUDUCTS AND REACTANTS REMAIN CONSTANT.

HAS TO HAPPN IN A CLOSED SUYSTEM

LE CHATLIER`s principle

IF A REACTION AT EQUILIBRIUM IS SUBJECTED TO ANY CHANGE IN TEMP, CONC OR PRESSURE THE POSTION OF THE EQUILIBRUM WILL MOVE TO COUNTERACT/ OPPOSE THAT CHANGE

Must know

Catalyst → no change to equilibrium position.

Catalyst → no change to Kc.

It only makes equilibrium establish faster.

STANDARD ENTHAPLY OF FORMATION

THE ENTHALPY CHANGE WHEN 1 MOLE OF A SUSBTANCE IS FORMED FROM ITS CONSTITUENT ELEMENTS WHEN ALL THE REACTANTS AND THE PRODUCTS ARE IN THEIR STANDARD STATES AND CONDITIONS.

STANDARD ENTHALPY OF COMBUSTION

THE ENTHALPY CHANGE WHEN 1 MOLE OF A SUBSTANCE IS BURNT COMPLETELLY IN OXYGEN WHEN ALL THE REACTAS AND PRODUCTS ARE IN THEIR STANDARD STATES AND CONDITIONS.

HESS`S LAW

THE ENTHALPY CHNAGE FPR A CONDITION IS DEPENDENT ON THE ROUTE TAKEN.