22. Enthalpy and Entropy

Question 1

What does entropy (S) represent?

Answer 1

the dispersal of energy and disorder within a chemical making up a chemical system

Question 2

What does a higher amount of disorder mean for enthalpy

Answer 2

higher ( more +ve) enthalpy

Question 3

When is there no entropy?

Answer 3

0K / -273C

Question 4

How do you calculate delta S?

Answer 4

S (products) - S (reactants)

Question 5

When does entropy increase?

Answer 5

when: 
solids melt
liquids boil
ionic solids dissolve in water
the number of gaseous molecules increases
temperature increases

Question 6

What is standard entropy?

Answer 6

the entropy of one mole of a substance at standard conditions

Question 7

What are the units for standard entropy?

Answer 7

J K-1 mol-1

Question 8

Why are standard entropy values always +ve

Answer 8

because absolute zero is the most ordered possible state, so at any temperature greater than that, the entropy will be greater, therefore more +ve

Question 9

What is the Gibbs Free Energy equation?

Answer 9

delta G = delta H - Tdelta S

Question 10

At what point does a reaction become feasible?

Answer 10

when delta G < 0

Question 11

Why may some reactions, although theoretically feasible, never take place?

Answer 11

activation energy is too high, catalyst needed

Question 12

What does delta G being equal to zero mean for the reaction?

Answer 12

it is at equilibrium and the reaction is just becoming feasible

Question 13

When will a reaction never be feasible?

Answer 13

when delta H is positive and delta S is negative

Question 14

If delta H is -ve and delta S is +ve what will the sign of delta G be, and what effect will temperature have on feasibility?

Answer 14

-ve

always feasible

Question 15

If delta H is -ve and delta S is -ve what will the sign of delta G be, and what effect will temperature have on feasibility?

Answer 15

probably -ve
at low temperatures the reaction will be feasible
as temperature increases, feasibility decreases

Question 16

If delta H is +ve and delta S is +ve what will the sign of delta G be, and what effect will temperature have on feasibility?

Answer 16

probably +ve
will not be feasible at low temperatures
as temperature increases, feasibility increases

Question 17

If delta H is +ve and delta S is -ve what will the sign of delta G be, and what effect will temperature have on feasibility?

Answer 17

+ve

never feasible

Question 18

Rearrange Gibbs Free Energy equation for delta S

Answer 18

delta S = (delta H - delta G)/T

Question 19

When does delta G = 0

Answer 19

at changes of state

Question 20

What is the equation for delta S at changes of state?

Answer 20

delta S = (delta H)/T

Question 21

What is the limitation of delta G?

Answer 21

indicates thermodynamic feasibility but does not take into account kinetics or the rate of reaction (i.e. whether the activation energy is too high, or whether a catalyst is needed)

Question 22

Define lattice formation enthalpy

Answer 22

the enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions

Question 23

Write an equation for the lattice formation enthalpy of NaCl

Answer 23

Na+(g) + Cl- (g) => NaCl (s)

Question 24

Define enthalpy of sublimation

Answer 24

the standard enthalpy change when 1 mole of a solid is changed into gaseous atoms under standard conditions

Question 25

Define enthalpy of atomisation

Answer 25

the enthalpy change when one mole of gaseous atoms of a substance are formed from its element(s) in their standard states under standard conditions

Question 26

What is the sign for enthalpy of atomisation?

Answer 26

+ve (endothermic)

Question 27

What enthalpy change does
Cl2(g) => 2Cl(g)
represent?

Answer 27

2x the enthalpy of atomisation // bond enthalpy

Question 28

Define 1st electron affinity

Answer 28

the enthalpy change when 1 e- is added to each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1- ions

Question 29

What is the sign of 1st electron affinity?

Answer 29

-ve (exothermic)

Question 30

Define hydration enthalpy

Answer 30

the enthalpy change when 1 mole of gaseous ions are dissolved in water to form 1 mole of aqueous ions

Question 31

Define enthalpy of solution

Answer 31

the enthalpy change when 1 mole of a solute is completely dissolved in water (for ionic solids)

Question 32

What is the difference between enthalpy of solution and hydration enthalpy?

Answer 32

hydration enthalpy: gaseous ions dissolving

enthalpy of solution: ionic solid dissolving

Question 33

Define lattice dissociation enthalpy

Answer 33

the enthalpy change when 1 mole of an ionic lattice dissociates into gaseous ions

Question 34

What is the sign of lattice dissociation enthalpy?

Answer 34

+ve (endothermic)

Question 35

What is the sign of lattice formation enthalpy?

Answer 35

-ve (exothermic)

Question 36

What happens to lattice enthalpy values as cation size increases?

Answer 36

become less negative

Question 37

Why do lattice enthalpy values become less negative as cation size increases?

Answer 37

ionic radius increases
lower charge density: same charge spread across a larger surface area
attraction between ions decreases
lattice enthalpy becomes less negative
(melting point decreases)

Question 38

What happens to lattice enthalpies (and hydration enthalpies) and melting points across the a period? and why?

Answer 38

lattice enthalpy becomes more negative, melting point increases
ionic charge increases (Na+ -> Ca2+)
charge density increases
attraction between opposite ions increases
lattice enthalpy = more -ve

Question 39

What are the exothermic and endothermic components of enthalpy of solution?

Answer 39

endothermic: ionic lattice must break
exothermic: water molecules attracted to the ions and surround them

Question 40

How can enthalpy of solution be +ve or -ve?

Answer 40

if exothermic component of water molecules surrounding ions is greater than lattice enthalpy of dissociation, delta SOL = -ve
ionic compound should dissolve
(but enthalpy is not the only factor to consider)

Question 41

How does hydration enthalpy change as cation size increases? and why?

Answer 41

becomes less negative
ionic radius increases
charge density decreases (charge stays the same but surface area of ion increases)
attraction between ions and water molecules decreases
hydration enthalpy = less -ve

Question 42

What is the formula for enthalpy of hydration?

Answer 42

delta hyd (cation) + delta hyd (anion) = delta LE + delta SOL