1.8 Thermodynamics Flashcards
(21 cards)
Define enthalpy of atomisation
The enthalpy of atomisation of an element is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state
examples :
Na (s) -> Na(g)
½ O2(g) -> O (g)
Define Hess’ law
The enthalpy change for a chemical reaction is the same, regardless of the route taken from reactants to products.
Define mean bond enthalpy / bond dissociation enthalpy
The bond dissociation enthalpy is the standard molar enthalpy change
when one mole of a covalent bond is broken into two gaseous
atoms (or free radicals), averaged over a range of compounds
Examples :
Cl2 (g) -> 2Cl (g)
CH4 (g) -> CH3 (g) + H(g
Define second ionisation energy
The second ionisation enthalpy is the enthalpy change to remove 1 mole of electrons from one mole of gaseous 1+ ions
to produces one mole of gaseous 2+ ions.
Define first electron affinity
The first electron affinity is the enthalpy change that occurs when 1
mole of gaseous atoms gain 1 mole of electrons to form 1 mole of
gaseous ions with a –1 charge, under standard conditions
example :
O (g) + e- -> O- (g)
Define second electron affinity
The second electron affinity is the enthalpy change when one
mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions, under standard conditions
example
O - (g) + e- -> O2- (g
Define lattice formation enthalpy
The standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions.
Na+(g) + Cl- (g) -> NaCl (s)
Why is first electron affinity exothermic
This is because the ion is more stable than the
atom, and there is an attraction between the nucleus and the
electron.
Why is second electron affinity endothermic
because it take energy to overcome the repulsive force between the negative ion and the electron.
Enthalpy of lattice dissociation
The standard enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions.
NaCl (s) -> Na+(g) + Cl- (g
Define enthalpy of hydration
Enthalpy change when one mole of gaseous ions become
aqueous ions .
Define enthalpy of solution
standard enthalpy change
when one mole of an ionic solid dissolves in a large enough amount of water to ensure that the dissolved ions are well
separated and do not interact with one another.
NaCl (s) + aq ->
Na+(aq) + Cl-(aq)
What is a perfectly ionic model
- ions act as point charges
- ions are perfect spheres
- ions show purely ionic bonding with no covalent character
Difference between theoretical and Born Haber lattice enthalpies
The Born Haber lattice enthalpy is the real experimental value.
Born haber takes covalent character into account
How to calculate entropy
Entropy of products - entropy of reactants
entropy = delta s
What happens to entropy if a system is more random, and what happens if its more ordered
more random = positive
more ordered = delta s is negative
For entropy and gibbs, how does a reaction become feasible
Delta G must be equal to or greater than zero
Why is Lattice dissociation enthalpy of magnesium chloride higher calcium chloride
Same charge
Mg^2+ has a smaller ionic radius
Therefore more strongly attracts Cl^- ions
