Lattice Enthalpy, Entropy and Free Energy

Question 1

what is meant by lattice enthalpy?

Answer 1

enthalpy change that accompanies the formation of one mole of a solid ionic lattice from its gaseous ions under standard conditions

Question 2

what is meant by enthalpy change of atomisation?

Answer 2

enthalpy change that acoompanies the formation of one mole of gaseous atoms from the element in its standard state under standard conditions

Question 3

what is first electron affinity?

Answer 3

enthalpy change that takes place when one mole of electrons is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions

Question 4

is the first electron affinity endothermic or exothermic? why?

Answer 4

exothermic because an electron is being attracted towards the nucleus, so adds easier

Question 5

is the second electron affinity endothermic or exothermic? why?

Answer 5

endothermic because the second electron is being gained by a negative ion, so repels the electron away
- so more energy is put in to force negatively charged electron onto a negative ion

Question 6

what is the lattice enthalpy for KCl and MgCl2?

Answer 6

KCl: K+(g) + Cl-(g) –> KCl(s)
MgCl2: Mg2+(g) + 2Cl-(g) –> MgCl2(s)

Question 7

what is the enthalpy change of atomisation of sodium and chlorine?

Answer 7

Na(s) –> Na(g)
1/2Cl2(g) –> Cl(g)

Question 8

is enthalpy change of atomisation exothermic or endothermic?

Answer 8

always endothermic
- bonds are broken to form gaseous atoms

Question 9

what is the electron affinity for chlorine?

Answer 9

Cl(g) + e- –> Cl-(g)

Question 10

what is the ionisation energy for sodium?

Answer 10

Na(g) –> Na+(g) + e-

Question 11

what is the order for the born haber cycle?

Answer 11

formation, atomisation. atomisation. ionisation energy, electron affinity, lattice enthalpy

Question 12

what is enthalpy change of solution?

Answer 12

enthalpy change that takes place when one mole of a solute dissolves in a solvent

Question 13

what is enthalpy change of hydration?

Answer 13

enthalpy change that accompanies the dissolving of 1 mole of gaseous ions in water to form 1 mole of aqueous ions

Question 14

what is the dissolving process?

Answer 14

• the solid ionic lattice breaks up
• water molecules are attracted to and surround the ions

Question 15

what is the enthalpy change of sodium chloride in water?

Answer 15

NaCl(s) + aq –> Na+(aq) + Cl-(aq)

Question 16

what is the enthalpy changes of hydration during the dissolving of NaCl?

Answer 16

Na+(g) + aq –> Na+(aq)
Cl-(g) + aq –> Cl-(aq)

Question 17

what are the factors affecting lattice enthalpy?

Answer 17

• ionic charge
• ionic radius

Question 18

why does ionic charge affect lattice enthalpy?

Answer 18

• higher the charges on the ions = stronger the electrostatic attractions between the ions so the more energy is released when the ionic lattice forms
• more energy is released means the lattice enthalpy is more negative

Question 19

why does ionic radius affect lattice enthalpy?

Answer 19

• the smaller the ionic radii of the ions = higher the charge density of the ion
• this means that the electrostatic attraction between the ions is greater so the lattice enthalpy is more exothermic

Question 20

how can we predict that a compound should dissolve?

Answer 20

• sum of the hydration enthalpies is larger than the magnitude of the lattice enthalpy, the overall enthalpy change will be exothermic
• compound should dissolve

Question 21

what is entropy?

Answer 21

measure of the dipersal of energy in a system
- the entropy is greater the more disordered a system is

Question 22

order the states from the smallest entropy to the greatest entropy?

Answer 22

• solids
• liquids
• gases

Question 23

how do we predict entropy changes?

Answer 23

• if a system changes to be more random, energy is more spread out and entropy is positive
• if a system changes to be less random, energy is more concentrated and entropy is negative

Question 24

what ways can we see that there will be a change in entropy?

Answer 24

• changes in state
• change in the number of gaseous molecules

Question 25

how can we see a change in entropy in the number of gaseous molecules?

Answer 25

• more moles of gas = positive value of entropy
• same mole of gas = entropy value is close to 0

Question 26

what is the feasibility of a process?

Answer 26

the feasibility of a process depends on the entropy change and temperature in the system and the enthalpy change of the system

Question 27

what is feasibility used to describe?

Answer 27

whether a reaction is able to happen

Question 28

what is the Gibbs equation?

Answer 28

^G = ^H - T^S

Question 29

for a reaction to be feasible what must happen?

Answer 29

• decrease in free energy
• negative ^G
• if ^G is zero the system is at equilibrium

Question 30

what is the equation to find the temperature that the reaction is feasible at?

Answer 30

T = ^H divided by ^S

Question 31

what are the units for ^G in Gibbs equation?

Answer 31

kJmol-1

Question 32

what are the units for ^H in Gibbs equation?

Answer 32

kJmol-1

Question 33

what are the units for temperature in Gibbs equation?

Answer 33

kelvin

Question 34

what do we have to do after we find the value of ^S?

Answer 34

divide by 1000

Question 35

what is the equation for ^S?

Answer 35

products- reactants

Question 36

how can we link the Gibbs equation to the equation of a straight line?

Answer 36

y = mx + c
^G = -^ST + ^H

Question 37

what does each part of the equation of a straight line show?

Answer 37

y = ^G
m = gradient = -^S
x = T
c = y intercept, ^H

Question 38

why may a negative value of the Gibbs equation may not truly be feasible?

Answer 38

• slow rate of reaction
• high activation energy

Question 39

when asked to give a range of the temperature for which a reaction is feasible what would you write if the reaction was exo/endothermic?

Answer 39

endothermic = temp and above
exothermic = temp and below