Thermodynamics

Question 1

ΔfH definition

Answer 1

Energy transferred when 1 mol of a compound is formed from its elements under standard conditions and all reactants and products being in their standard states

Question 2

ΔatH definition

Answer 2

Enthalpy change when 1 mol of gaseous atoms is formed from its elements in their standard states

Question 3

ΔsublimationH definition

Answer 3

enthalpy change for a solid metal turning to gaseous atoms and equals ΔatH

Question 4

ΔdissH (bond dissociation) definition

Answer 4

standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms

Question 5

First IE definition

Answer 5

ΔH required to remove 1 mol of e- from 1 mol gaseous atoms to form 1 mol gaseous ions with a 1+ charge

Question 6

First electron affinity definition

Answer 6

ΔH when 1 mol of gaseous atoms gain 1 mol of electrons to form 1 mol of gaseous ions with a 1- charge

Question 7

Why is first electron affinity exothermic for atoms that normally form 1- ions (group 7 for instance)

Answer 7

The ion is more stable than the atom
There is an attraction between the nucleus and the electron

Question 8

Why is the second electron affinity for oxygen endothermic?

Answer 8

Energy is required to overcome the repulsive force between the negative ion and the electron

Question 9

ΔlatformH definition

Answer 9

standard ΔH when 1 mol of a solid ionic crystal lattice is fromed from its consituent ions in gaseous form

Question 10

ΔlatdissH definition

Answer 10

standard ΔH when 1 mole of an ionic crystal lattice forms its consituent ions in gaseous form

Question 11

ΔhydH definition
(exo or endothermic?)

Answer 11

enthalpy change when 1 mol of gaseous ions become aqueous ions

always exothermic as bonds are formed between ions and water

Question 12

ΔsolH definition

Answer 12

standard enthalpy change when 1 mol of an ionic solid dissolves in a large enough amount of water to ensure that the ions are well separated and do not interact with each other

Question 13

Opposite of enthalpy of lattice formation?

Answer 13

enthalpy of lattice dissociation

Question 14

Why use born haber cycles to calculate lattice enthalpies?

Answer 14

It cannot be determined directly

Question 15

Which lattice enthalpy would point down in a born haber cycle?

Answer 15

formation

Question 16

What’s always at the bottom row of a born haber cycle?

Answer 16

The solid compound

Question 17

What’s always at the top row of a Born Haber cycle?

Answer 17

positive ions, electrons and gaseous atoms (which turn into negative ions next)

Question 18

ΔatH(Cl2) is the same thing as…

Answer 18

the mean bond energy for Cl2

Question 19

Explain 2 trends in lattice enthalpies

Answer 19

ion size: the larger the ions, the less -ve the ΔlatformH will be, the weaker the lattice will be

ion charge: the bigger the charge, the greater the attraction so the stronger the lattice enthalpy (more -ve values)

Question 20

Perfect ionic model

Answer 20

assumed by theoretical lattice enthalpies
ions are 100% spherical and ionic
attractions are purely electrostatic

Question 21

What 4 things cause covalent character?

Answer 21

Small +ve ion
multiple +ve charges on +ve ion
large -ve ion
multiple -ve charges on -ve ion

Question 22

The more ________ the bigger the difference between the theoretical and born haber lattice enthalpies

Answer 22

covalent character

Question 23

Why would the born haber value be larger than the theoretical value for a compound?

Answer 23

if it has some covalent character
it tends towards giant covalent structure
lattice + is stronger than if it was 100% ionic

Question 24

What is different in the shapes of purely ionic ions and ions with covalent character?

Answer 24

purely ionic- 100% spherical
covalent character- electron charge cloud is distorted

Question 25

Substances with more ways of arranging their atoms and energy (more disordered) have a higher/lower entropy?

Answer 25

higher

Question 26

Why do solids have lower entropies than gases?

Answer 26

as the solid increases in temperature its entropy increases as particles vibrate more
gases move around even more–> more disordered

Question 27

At 0K substances have ______ entropy and the reason why

Answer 27

0

no disorder
particles are stationary

Question 28

When do significant increases in entropy occur?

Answer 28

change of state from s to l to g
increase in no of molecules from r to p

Question 29

entropy unit

Answer 29

JK−1mol−1

Question 30

ΔS =

Answer 30

Sprod - Sreact

Question 31

Gibbs free energy purpose

Answer 31

combines effect of entropy and enthalpy into 1 number

Question 32

ΔG=

Answer 32

ΔH-TΔS

ALWAYS USE S/1000

Question 33

ΔG units

Answer 33

KJmol-1

Question 34

ΔG has to be ____ve for a reaction to occur

Answer 34

-

Question 35

What is ΔG for melting/boiling? Why?

Answer 35

0

physical phase changes are equillibria

Question 36

If the reaction involves an increase in entropy what does increasing temperature do to ΔG?

Answer 36

makes it more likely that ΔG is negative and the reaction occurs

Question 37

If the reaction involves a decrease in entropy what would an increase in temperature do to ΔG

Answer 37

Less likely that ΔG is negative and the reaction is less likely to occur

Question 38

ΔG = ΔH - TΔS as a graph’s y, m, x and c values

Answer 38

ΔG as y
T as x
ΔH as c
-ΔS as m

Question 39

ΔG = ΔH - TΔS positive gradient means

Answer 39

ΔS is negative