13 - energetics 2

Question 1

what is lattice energy?

Answer 1

for ionic compounds
energy change when one mole of the ionic solid is formed from its gaseous ions
if under standard conditions then it is the standard lattice energy

Question 2

what factors affect the magnitude of lattice energy?

Answer 2

• charge - greater charge = greater le
• cation-to-anion interactions - more interactions = greater le
• distance between centre of cation and anion

covalent interactions between ions also affects lattice energy

Question 3

what is electron affinity?

Answer 3

the energy change when each atom in one mole of atoms in the gaseous state gain an electron to form a -1 ion

Question 4

what is experimental lattice energy?

Answer 4

value of the lattice energy calculated from the knowledge of other energy changes, all of which can be determined experimentally

Question 5

what assumptions are made for theoretical lattice energy?

Answer 5

• ions are in contact with one another
• ions are perfectly spherical
• charge on each ion is evenly distributed around the centre so that each ion can be considered as point charges

type of lattice and inter-ionic distance found through x-ray crystallography

Question 6

what two factors affect the extent at which anions are polarised?

Answer 6

• a high charge and small size of cation (high density)
• high charge and large size of anion

Question 7

if electrons from an anion are pulled towards the cation, the anion is said to be…

Answer 7

polarised

Question 8

what kind of cation has a high polarising power?

Answer 8

high charge and small radius

Question 9

which anions are easiest to polarise?

Answer 9

high charge and large radius

Question 10

when an ion is placed in water it immediately reacts. what kind of bond is formed?

Answer 10

water is polar
ion-dipole interaction
for transition metals - dative covalent bond

Question 11

what factors affect the magnitude of hydration enthalpy?

Answer 11

• charge - greater charge means electrostatic forces of attraction between ion and water molecules is stronger
• ionic radius - larger ions = lower electrostatic force of attraction between ion and water molecules

Question 12

what is a spontaneous process?

Answer 12

one that takes place without intervention (such as heating)
the reverse of a spontaneous reaction never happens on its own
e.g. two gases in a container will mix spontaneously however they will not separate on their own accord

total entropy increases in spontaneous process

Question 13

what is entropy?

Answer 13

property of matter (like density or energy)
it is a quantity associated with randomness or disorder
the greater the degree of disorder, the greater the entropy
e.g. entropy of a liquid is higher than that of a solid

Question 14

total entropy change ΔStotal =

Answer 14

ΔSsystem + ΔSsurroundings
for reaction to be spontaneous ΔStotal must be positive

Question 15

entropy change of system ΔSsystem =

Answer 15

ΣS(products) - ΣS(reactants)

Question 16

what units are entropy values in?

Answer 16

J K-1 mol-1
whereas enthalpy changes usually kJ mol-1

Question 17

entropy change of surroundings ΔSsurroundings =

Answer 17

• ΔH/T
  T in Kelvin
  ΔH = enthalpy change of reaction

Question 18

how does temperature affect entropy?

Answer 18

cold object with increased heat increases entropy
hot object with increased heat also increases entropy but not as much since the molecules were already vigorously moving around

Question 19

what happens to entropy if you increase the number of particles?

Answer 19

increased number of ways the particles can be arranged = increased entropy

Question 20

what changes take place when an ionic solid dissolves in water?

Answer 20

• lattice structure is broken down (endothermic)
• ions become hydrated (exothermic)

Question 21

what factors affect the solubility of a solid?

Answer 21

• entropy of the system
• enthalpy change of solution
• temperature

Question 22

ΔG =

Answer 22

ΔH - TΔSsystem
thermodynamically feasible if Gibbs is negative

Question 23

ΔG =
relationship between K and Gibbs

Answer 23

-RTlnK

K = e^(-ΔG/RT)
R is the gas constant

Question 24

what does is mean for K when ΔG is so large/so small?

Answer 24

ΔG is super big (positive) equilibrium lies to the reactant side
ΔG is so small (negative) equilibrium lies to the product side

+40kJ mol-1 and -40kJ mol-1 so large/small that either all reactants or all products

Question 25

what two factors need to be considered when applying Gibbs?

Answer 25

• kinetic stability - activation energy and how many collisions occur
• non standard conditions

Question 26

what does it mean if ΔsolG* is negative?

Answer 26

salt is soluble
if it is positive then salt is insoluble

Question 27

ΔsolG* =

Answer 27

ΔsolH* - TΔSsystem

Question 28

NOTE TO SELF

Answer 28

13.3 not complete as beyond scope - revisit textbook if not understanding solubility, strength of acids