22. Enthalpy and Entropy Flashcards

1
Q

What does entropy (S) represent?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What does a higher amount of disorder mean for enthalpy

A

higher ( more +ve) enthalpy

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

When is there no entropy?

A

0K / -273C

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

How do you calculate delta S?

A

S (products) - S (reactants)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

When does entropy increase?

A
when: 
solids melt
liquids boil
ionic solids dissolve in water
the number of gaseous molecules increases
temperature increases
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is standard entropy?

A

the entropy of one mole of a substance at standard conditions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What are the units for standard entropy?

A

J K-1 mol-1

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Why are standard entropy values always +ve

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is the Gibbs Free Energy equation?

A

delta G = delta H - Tdelta S

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

At what point does a reaction become feasible?

A

when delta G < 0

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

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

A

activation energy is too high, catalyst needed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

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

A

it is at equilibrium and the reaction is just becoming feasible

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

When will a reaction never be feasible?

A

when delta H is positive and delta S is negative

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

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?

A

-ve

always feasible

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

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?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

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?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

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?

A

+ve

never feasible

18
Q

Rearrange Gibbs Free Energy equation for delta S

A

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

19
Q

When does delta G = 0

A

at changes of state

20
Q

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

A

delta S = (delta H)/T

21
Q

What is the limitation of delta G?

A

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)

22
Q

Define lattice formation enthalpy

A

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

23
Q

Write an equation for the lattice formation enthalpy of NaCl

A

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

24
Q

Define enthalpy of sublimation

A

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

25
Define enthalpy of atomisation
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
26
What is the sign for enthalpy of atomisation?
+ve (endothermic)
27
What enthalpy change does Cl2(g) => 2Cl(g) represent?
2x the enthalpy of atomisation // bond enthalpy
28
Define 1st electron affinity
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
29
What is the sign of 1st electron affinity?
-ve (exothermic)
30
Define hydration enthalpy
the enthalpy change when 1 mole of gaseous ions are dissolved in water to form 1 mole of aqueous ions
31
Define enthalpy of solution
the enthalpy change when 1 mole of a solute is completely dissolved in water (for ionic solids)
32
What is the difference between enthalpy of solution and hydration enthalpy?
hydration enthalpy: gaseous ions dissolving | enthalpy of solution: ionic solid dissolving
33
Define lattice dissociation enthalpy
the enthalpy change when 1 mole of an ionic lattice dissociates into gaseous ions
34
What is the sign of lattice dissociation enthalpy?
+ve (endothermic)
35
What is the sign of lattice formation enthalpy?
-ve (exothermic)
36
What happens to lattice enthalpy values as cation size increases?
become less negative
37
Why do lattice enthalpy values become less negative as cation size increases?
``` 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) ```
38
What happens to lattice enthalpies (and hydration enthalpies) and melting points across the a period? and why?
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
39
What are the exothermic and endothermic components of enthalpy of solution?
endothermic: ionic lattice must break exothermic: water molecules attracted to the ions and surround them
40
How can enthalpy of solution be +ve or -ve?
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)
41
How does hydration enthalpy change as cation size increases? and why?
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
42
What is the formula for enthalpy of hydration?
delta hyd (cation) + delta hyd (anion) = delta LE + delta SOL