17. Thermodynamics Flashcards
Define the standard molar enthalpy of formation
- the enthalpy change
- when one mole of a compound is formed from its constituent elements under standard conditions
- all reactants and products in their standard states
Define the standard molar enthalpy of combustion
- enthalpy change
- when one mole of substance is completely burnt in oxygen
- all reactants and products in their standard states
Define the standard enthalpy of atomisation
- the enthalpy change
- which accompanies the formation of one mole of gaseous atoms
- from the element in its standard state
- under standard conditions
Write an equation for the formation of water
H2 (g) + 1/2 O2 (g) —> H2O (l)
Write an equation for the combustion of methane
CH4 (g) +2O2 (g) —-> CO2 (g) + H2O (l)
Write an equation for the atomisation of magnesium and then chlorine
Mg (s) —-> Mg (g)
1/2Cl2 (g) —-> Cl (g)
Define the first ionisation energy
- standard enthalpy change
- when one mole of electrons
- is removed from one mole of gaseous atoms
- leaving one mole of +1 ions
Write an equation for the first ionisation of Mg
Mg (g) —> Mg+ + e-
Define the second ionisation energy
- standard enthalpy change
- when one mole of electrons
- is removed from one mole of gaseous +1 ions
- forming one mole of +2 ions
Write an equation for the second ionisation energy of Mg
Mg+ (g) —-> Mg2+ (g) + e-
Define first electron affinity
- standard enthalpy change
- when a mole of electrons
- is added to a mole of gaseous atoms
- forming one mole of -1 ions
Define second electron affinity
- standard enthalpy change
- when a mole of electrons
- is added to a mole of gaseous -1 ions
- forming one mole of -2 ions
Write an equation for the first electron affinity of oxygen
O (g) + e- —-> O- (g)
Write an equation for the second electron affinity of oxygen
O- (g) +e- —–> O2- (g)
Define lattice enthalpy of formation
- the standard enthalpy change
- when one mole of solid ionic compound
- is formed from its gaseous ions
Write an equation for the lattice enthalpy of formation of NaCl
Na+ (g) + Cl- (g) —->NaCl (s)
Define enthalpy of lattice dissociation
- standard enthalpy change
- when one mole of solid ionic compound
- dissociated into its gaseous ions
Write an equation for the lattice dissociation of NaCl
NaCl (s) —> Na+ (g) + Cl- (g)
Define enthalpy of hydration
- standard enthalpy change
- when water molecules surround
- one mole of gaseous ions
Write an equation for the hydration of Na+
Na + (g) +aq —> Na+ (aq)
Define enthalpy of solution
- standard enthalpy change
- when one mole of solute dissolves completely in sufficient solvent
- to form a solution in which the molecules or ions are far enough apart
- to not interact with each other
Write an equation for the enthalpy of solution of NaCl
NaCl (s) + aq —> Na+ (aq) + Cl- (aq)
Define mean bond enthalpy
- standard enthalpy change
- when one mole of gaseous molecules
- each break a covalent bond to form
- two free radicals
- averaged over a range of compound
What is a Born-Haber cycle
A thermochemical cycle that includes all the enthalpy changes involved in the formation of an ionic compound
Write all the equation that has the equivalent enthalpy to the formation of NaCl (Na (s) + 1/2Cl2 (g) —->NaCl (s)
Atomisation of Na:
Na (s) + 1/2Cl2 (g) —> Na (g) + 1/2Cl2 (g)
Atomisation of Cl:
Na (g) + 1/2Cl2 (g) —–> Na(g) + Cl (g)
First ionisation energy of Na
Na(g) + Cl (g) —-> Na + (g) + e- + Cl (g)
First electron affinity of Cl:
Na + (g) + e- + Cl (g) —-> Na+ (g) + Cl- (g)
What is the trend in lattice enthalpies (ion size)
Larger ions lead to smaller lattice enthalpies
Why does larger ions lead to smaller lattice enthalpies
The oppositely charges do not approach each other as closely when the ions are larger so less attraction
What is the trend in lattice enthalpies (charge size)
Lattice enthalpy increases with the size of the charge
Why does lattice enthalpy increase with the size of the charge
Ions with higher charge give out more energy when they come together
Enthalpy of hydration has the same enthalpy trend as what
Lattice enthalpy
The dissolving of ionic compounds in water is the sum of what three processes
- breaking the ionic lattice (to give separate ions- the lattice dissociation enthalpy has to be put in)
- hydrating the positive ions (enthalpy of hydration is given out)
- hydrating the negative ions (enthalpy of hydration is given out)
Write the all the equations equivalent enthalpy change for the solution of NaCl (NaCl (s) + aq —> Na+ (aq) + Cl- (aq) )
NaCl (s) —> Na + (g) + Cl- (g)
Na+ (g) + aq + Cl- (g) —> Na+ (aq) + Cl- (g)
Na + (aq) + Cl- (g) + aq —> Na+ (aq) + Cl- (aq)
Why in some compounds are there large discrepancies between the experimental and theoretical values for lattice formation enthalpy
Covalent character
Describe how covalent character occurs
- when the positive ion is relatively small and high charge
- and the negative ion is relatively large and has a high negative charge
- the positive ion can approach more closely to the negative ions electron cloud
- and distort them towards the positive ion
- the positive ion can easily distort the negative as the negative has a large size so electrons far from nucleus -and large negative charge means lots of negative charge to distort
- this means more electrons than expected are concentrated between the two ions
- the negative ion is said to be polarised
What factors increase polarisation and therefore covalent character
positive ion: small size, high charge
negative ion: large size, high charge
What do the words feasible or spontaneous describe
Describes reactions which could take place of their own accord
Reactions often occur spontaneously when the enthalpy change is
Negative
What is the order of randomness of the states
Solid
What is entropy
A numerical measure of disorder in a chemical system
Apart from negative enthalpy change what else makes a reaction feasible
An increase (positive) entropy
How do you calculate entropy change
Entropy of the products - entropy of the reactants
What two factors govern the feasibility of a reaction
- enthalpy change
- entropy change
What is Gibbs free energy
A quantity that combines enthalpy and entropy change
If the change in G is negative…
then the reaction is feasible
If the change in G is positive
then the reaction is not feasible
What is the equation for ΔG
ΔG= ΔH -TΔS
What happens when ΔG is 0
This is the temperature which the reaction is just feasible
Why might spontaneous reactions not occur (such as the oxidation of graphite)
- the reaction takes place so slowly that for practical purposes it does not occur at all
- when this happens we say graphite is thermodynamically unstable but kinetically stable
