THERMODYNAMICS Flashcards
^H f
The enthalpy change when one mole of a compound is formed from its elements under standard conditions, all reactants and products in their standard states.
^H c
The enthalpy change when one mole of a compound is completely burned in oxygen under standard conditions, all reactants and products in their standard states.
^H at
The enthalpy change when one of gaseous atoms is formed from an element in its standard state.
^H BE
The enthalpy change when one mole of gaseous molecules each break a covalent bond to form two free radicals, averaged over a range of compounds.
1st ^H i
The standard enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to give one mole of gaseous ions each with a single positive charge.
2nd ^H i
The standard enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to give one mole of gaseous ions each with a 2+ charge.
1st ^H aff
The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions, each with a single negative charge under standard conditions.
2nd ^H aff
The standard enthalpy change when one mole of electrons is added to a mole of gaseous ions each with a single negative charge, to form a mole of ions each with a two negative charge.
^H LF
The standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions.
^H Hyd
The standard enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions.
^H Sol
The standard enthalpy change when one mole of solute dissolves in enough solvent to form a solution in which the ions are far enough apart not to interact with each other.
^H LD
The standard enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions.
C(s) + O2(g) à CO2(g) is an example of:
enthalpy change of formation
C2H5OH(l) + 3O2(g) à 2CO2(g) + 3H2O(l) is an example of
^H combustion
½ Br2(l) à Br(g)
atomisation
Mg(s) à Mg(g)
atomisation
CH4(g) à H(g) + CH3(g)
bond enthalpy
how are bond enthalpy and atomisation linked
AT x2 = BE
which two enthalpies are linked
bond enthalpy and atomisation
Mg2+(g) + 2Br-(g) à MgBr2(s)
lattice formation
MgBr2(s) à Mg2+(g) + 2Br-(g)
lattice dissociation
NaCl(s) à Na+(g) + Cl-(g)
lattice dissociation
Cl-(g) + aq à Cl-(aq)
hydrolysis
NaCl(s) + aq à Na+(aq) + Cl-(aq)
solution
equation for ^H
∆H = ∑Bonds Broken – ∑Bonds Formed
factors of lattice enthalpies
- Charge on the ions
- Size of the ions (ionic radius)
why does charge and radius affect lattice enthalpy
As the ionic radius increases:
- The attraction between the ions decreases
- The lattice enthalpy becomes less negative (less exothermic)
NaCl has a theoretical value of ∆HLF = -777 kJmol-1. Experimentally this value has been determined to be -780 kJmol-1
what does this mean?
the lattice is almost purely ionic bonding
why can the theoretical ^H differ from the actual ^H
actual has covalent character
comparing covalent character
The Zn2+ ion is quite small with quite a strong +ve charge.
The Se2- ion is large with a diffuse electron cloud.
The two will ionically bond because of the electrostatic attraction.
The Zn2+ ion is strongly polarising
The electron cloud about the Se2- ion becomes distorted
Some of the electron density is shared.
This is covalent character
what is covalent character
an atoms ability to attract a pair of electrons in a covalent bond
what is the perfect ionic model
The term ‘Perfect Ionic Model’ refers to compounds which display purely ionic bonding with no covalent character.
what can be calculated with the perfect ionic model
theoretical lattice enthalpies
features of the perfect ionic model include
- Ions act as point charges
- Ions are perfect spheres which cannot be distorted
- The ions show purely ionic bonding with no covalent character.
why affects a reaction occuring
Temperature
- Enthalpy – most reactions which are feasible are exothermic because the reactants want to release energy to the environment. This means the products will be lower in energy and more stable.
- The third factor which determines whether a reaction is feasible or not is called ENTROPY.
what is entropy
disorder
if entropy decreases
less disorder
if entropy increases
more disorder
what has the highest entropies
gases
does entropy increase or decrease when gas is produced
increases- more disorder
does entropy increase or decrease when fewer moles are produced
decreases- less disorder
formula for entropy change
∆Sᶿ = ∑Sᶿ (products) - ∑Sᶿ (reactants)
formula for Gibbs
∆G = ∆H - T∆S
what do you need to do to ^s when calculating Gibbs
x 1x10-3
when is a reaction feasible
^G< 0
