thermodynamics

Question 1

electron affinity (2)

Answer 1

Enthalpy change/heat energy change when one mole of gaseous atoms

Form (one mole of) gaseous negative ions (with a single charge)

Question 2

why electronegativity of F greater than Cl (2)

Answer 2

Fluorine (atom) is smaller than chlorine/shielding is less/ outer electrons closer to
nucleus

(Bond pair of) electrons attracted more strongly to the NUCLEUS/PROTONS

Question 3

why hydration enthalpy of fluoride ion more neg than cl ion (2)

Answer 3

Fluoride (ions) smaller (than chloride) / have larger charge density
So (negative charge) attracts (δ+ hydrogen on) water more strongly

Question 4

how to calc ∆H(solution)

Answer 4

∆H(solution) = LE + Σ(hydration enthalpies)

Question 5

why … is always spontaneous process (2)

Answer 5

Entropy change is positive/entropy increases and enthalpy change negative/exothermic

So ∆G is (always) negative

Question 6

suggest condition under which element C (diamond) would have entropy value of 0

Answer 6

absolute 0/ 0K/-273C

REMEMEBR AT 0K entropy is 0

Question 7

enthalpy of atomisation (2)

Answer 7

Enthalpy change for the formation of 1 mol of gaseous atoms

From the element (in its standard state)

Question 8

lattice dissociation enthalpy (2)

Answer 8

Enthalpy change to separate 1 mol of an ionic lattice/solid/compound
Into (its component) gaseous ions

Question 9

perfect ionic model (2)

Answer 9

ions r perfect spheres
ONLY electrostatic attraction/ no covalent interaction

Question 10

enthalpy of lattice formation

Answer 10

enthalpy change when 1 mol of solid ionic compound/ ionic lattice is formed from its gaseous ions

Question 11

enthalpy of hydration

Answer 11

(enthalpy change when) 1 mol of gaseous (fluoride)
ions (is converted) into aqueous ions / an aqueous solution

Question 12

explain interactions bw h20 molecules + f- when f- become hydrated

Answer 12

water is polar / H on water is / is electron deficient / is unshielded

(F– ions) attract water / on H / hydrogen

Question 13

why … more spontaneous even tho change is endthermic

Answer 13

molecules become more disordered
so entropy change is pos
so TΔS>ΔH
ΔG<0

Question 14

why is it that if compound shows covalent character it has larger values than theoretical values

Answer 14

bc forces/bonds holding lattice tg are stronger

Question 15

Bond dissociation enthalpy (bond energy)

Answer 15

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)

Question 16

Enthalpy of sublimation

Answer 16

The enthalpy change for a solid metal turning to gaseous atoms can also be called the Enthalpy of sublimation and will numerically be the same as the enthalpy of atomisation

Question 17

for diatomic molecules what is enthalpy of diss

Answer 17

deltadissH of the molcule is the same as 2x deltaatH of the element

Question 18

Enthalpy of hydration hyd H
give formula example
is it exo or end why

Answer 18

Enthalpy change when one mole of gaseous ions become aqueous ions .
X+ (g) + aqX+ (aq) For Li+ hydH = -519 kJ mol-1
or
X- (g) + aq  X- (aq) For F- hydH= -506 kJ mol-1
This always gives out energy (exothermic, -ve) because bonds are made between the ions and the water molecules.

Question 19

Enthalpy of solution

Answer 19

The enthalpy of solution is the 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)

Question 20

why the second electron affinity for oxygen is endothermic

Answer 20

the second electron affinity for oxygen is endothermic because it take energy to overcome the repulsive force between the negative ion and the electron

Question 21

the strength of an enthalpy of lattice formation depends on the following 2 factors:

Answer 21

1.
2.
The sizes of the ions
The larger the ions, the less negative the enthalpies of lattice formation (i.e. a weaker lattice). As the ions are larger the charges become further apart and so have a weaker attractive force between them.
The charges on the ion
The bigger the charge of the ion, the greater the attraction between the ions so the stronger the lattice enthalpy (more negative values).

Question 22

Differences between theoretical and Born Haber (experimental) lattice enthalpies

Answer 22

The Born Haber lattice enthalpy is the real experimental value.
When a compound shows covalent character, the theoretical and the born Haber lattice enthalpies differ. The more the covalent character the bigger the difference between the values.

Question 23

which ion becomes polarised

Answer 23

When the negative ion becomes distorted and more covalent we say it becomes polarised. The metal cation is called polarising if it polarises the negative ion.

Question 24

There is a tendency towards covalent character in ionic substances when

Answer 24

*the positive ion is small
*the positive ion has multiple charges *the negative ion is large
*the negative ion has multiple negative charges.

Question 25

why the Born-Haber value would be larger than the theoretical value if covalent character

Answer 25

When a compound has some covalent character- it tends towards giant covalent so the lattice is stronger than if it was 100% ionic. Therefore the Born-Haber value would be larger than the theoretical value.

Question 26

which is most thermodynamically stable

Answer 26

The one with the most exothermic enthalpy of formation will be the one that forms as it will be the most thermodynamically stable

Question 27

Calculating ∆S ̊, unit

Answer 27

∆S ̊ = Σ S ̊products - ΣS ̊reactants J K-1 mol-1

Question 28

what has 0 entropy

Answer 28

Elements in their standard states do not have zero entropy. Only perfect crystals at absolute zero (T = 0 K) will have zero entropy.

Question 29

∆G eqn

Answer 29

∆G = ∆H - T∆S

Question 30

enthalpy of soln formula

Answer 30

Hsolution =  Hlatticedissociation + sum of hydH H solution = -  HL formation + sum of hydH

Question 31

why r hydration enthalpies exothermic

Answer 31

energy is given out when water molecules bond to the metal ions.

Question 32

what leads to greater hydration enthalpy

Answer 32

The higher the charge density the greater the hydration enthalpy (e.g. smaller ions or ions with larger charges) as the ions attract the water molecules more strongly. e.g. fluoride ions have more negative hydration enthalpies than chloride ions. Magnesium ions have a more negative hydration enthalpy than barium ions.

Question 33

What does ΔHSolution tell us?

Answer 33

Generally ΔH solution is not very exo or endothermic so the hydration enthalpy is about the same as lattice enthalpy. In general the substance is more likely to be soluble if the ΔH solution is exothermic. If a substance is insoluble it is often because the lattice enthalpy is much larger than the hydration enthalpy and it is not energetically favourable to break up the lattice, making ΔH solution endothermic.

Question 34

for salts where enthalpy of soln is exo will salt always dissolve

Answer 34

yes