Slides in this set
Enthalpy Change (H)
· Enthalpy of formation
is the enthalpy change that occurs when one mole of compound in its standard
state if formed from its element in their standard states under standard
· Ionisation enthalpy
of an element is the enthalpy change that accompanies the removal of 1 electron
in each atom of one mole of gaseous atoms
· Enthalpy of atomisation
of an element is the enthalpy change that occurs when 1 mole of gaseous atoms
are formed from the element in its standard state.
· Bond dissociation enthalpy
the enthalpy required to break and separate one mole of bond so that the
gaseous atoms exert no force on each other.
· Electron affinity
of an element is the enthalpy change that accompanies the addition of one
electron to each atom in one mole of gaseous atoms.
· Lattice enthalpy
of an ionic compound is the enthalpy change which accompanies the formation
of one mole of ionic compound from its constituent gaseous ions.
· You will need to be able to use the quantities in energy cycle calculations,
by applying Hess's Law. Remember it doesn't matter which route a
chemical change takes the net energy change stays the same.…read more
Free-Energy Change (G) and
Entropy Change (S)
· H, whilst important, is not sufficient to explain spontaneous
· The concept of increasing disorder (entropy change, S) accounts
for this phenomenon.
· E.g. the production of CO2 from hydrogencarbonates with acid is
endothermic but occur without heating, because the products are
significantly more disordered than the products.
· The balance between entropy and enthalpy determines the
feasibility of a reaction and is called the change in free energy.
· G = H - TS (derivation not required).
· G must be negative for a reaction to proceed spontaneously…read more
· All trends listed here are for progression from left to right
of the periodic table.
· Reactions with water Na and Mg
· The elements progress from metals to non-metals.
· Period 3 oxides
The amount of oxygen per mole of the element increases.
The oxides go from ionic to covalent in nature
The oxides go from basic to acidic
Boiling points start high because of the strong ionic forces but
become higher in the middle oxides have mixed ionic covalent
nature but still form giant lattices so have high boiling points. The
latter few elements form simple molecules and so have relatively
low boiling points.
With water the metal oxides form alkalis, aluminium and silicon
oxide are insoluble, the non metal oxides form acidic solutions…read more