Entropy

Question 1

SPONTANEOUS Reactions

Answer 1

• tend to ‘go’ without external influence
• proceed in ONE DIRECTION & require an input of energy to reverse them
• take place QUICKLY upon mixing reactants at room temperature
• the word spontaneous can be ambiguous so FEASIBLE is used instead

Question 2

Can SLOW reactions be spontaneous?

Answer 2

YES, despite their high activation energies.

e.g diamond converting to graphite

• reaction is extremely slow
• graphite is more stable than diamond

Question 3

Entropy

Answer 3

• a measure of the DISORDER or RANDOMNESS of a chemical system
• number of possible ARRANGEMENTS of substances in space
• how the ENERGY of a system is DISTRIBUTED across the energy levels

Question 4

Standard Molar entropy, S

Answer 4

The entropy per mole of a substance under standard conditions.

Units ~ JK^-1mol^-1

Question 5

Which has the highest entropy and why?

C(s) & Hg(l)

Answer 5

C(s):

• atoms held firmly in fixed lattice positions by strong covalent bonds
• atoms can only vibrate

Hg(l):

• atoms are free to move
• many more ways to distribute the atoms & hence the energy

The disorder (entropy) INCREASES going from a solid to a liquid

Question 6

Which has the highest entropy and why?

Br2(l) or Br2(g)

Answer 6

Br2(g):

• gaseous bromine is free to move
• widely spaced
• higher entropy than liquid bromine

Diatomic molecule ~ bonds can rotate & vibrate

Question 7

Why do DIATOMIC MOLECULES have a higher entropy than atoms?

Answer 7

The bonds can ROTATE & VIBRATE

Question 8

Which has the highest entropy and why?

HF(g) or CH4(g)

Answer 8

CH4(g):

• vibrate
• rotate
• stretch
• more single bonds which can rotate & they can arrange themselves in more ways

Question 9

Explain why the ENTROPY increases:

1) MnSO4.4H2O(s) + aq — Mn2+(aq) + SO42-(aq) + 4H2O(l)

2) H2O(s) ————— H2O(g)

3) CaCO3(s) ——— CaO(s) + CO2(g)

4) CaCO3(s) + 2HCl(aq) —– CaCl2(aq) + CO2(g) + H2O(l)

Answer 9

1) ionic lattice dissolves

2) state changes to one which has more randomness

3)a gas, which has a much higher entropy than a solid, is produced

4) a gas is produced

Question 10

The standard entropy change for a reaction

• definition
• Formula
• Units

Answer 10

• The change in ENTROPY that occurs when a reaction takes place
• under standard conditions
• with all reactants & products in their standard states

sum of PRODUCTS - sum of REACTANTS

J K ^-1mol^-1

Question 11

The meaning of the SIGN of the ENTROPY CHANGE

Answer 11

POSITIVE ~ system becomes more random (e.g solid to gas)

NEGATIVE ~ system becomes more ordered ( e.g gas to solid)

Question 12

Feasible reactions

Answer 12

• both EXOTHERMIC & ENDOTHERMIC reactions can be feasible at room temperature
• the chemical system must become MORE STABLE & LOWER in energy if the reaction is to be feasible

The overall energy change comes from the change in ENTHALPY & ENTROPY.

Question 13

The feasibility of a reaction depends up on the system’s …

These combine to give the FREE ENERGY CHANGE, delta G …

Answer 13

• Temperature, T, in Kelvin (K)
• Change in entropy, delta S
• Change in enthalpy, delta H

delta G = delta H - TdeltaS

• divide value of delta S by 1000 to get in KJ

Question 14

For a reaction to be feasible what value must delta G be?

Answer 14

delta G>0

Value of G BELOW 0

The overall decrease in energy comes from both the enthalpy & entropy changes of the systems

Question 15

delta H = -ve
delta S = +ve

What sign does delta G have, and therefore is it feasible?

Answer 15

ALWAYS NEGATIVE

FEASIBLE

Question 16

delta H = +ve
delta S = -ve

What sign does delta G have, and therefore is it feasible?

Answer 16

ALWAYS POSITIVE

NEVER FEASIBLE

Question 17

delta H = -ve
delta S = -ve

What sign does delta G have, and therefore is it feasible?

Answer 17

• NEGATIVE at LOW temperatures
• the magnitude of TdeltaS is LESS than deltaH

FEASIBLE AT LOW TEMPERATURES

Question 18

delta H = +ve
delta S = +ve

What sign does delta G have, and therefore is it feasible?

Answer 18

• NEGATIVE at VERY HIGH temperatures
• the magnitude of TdeltaS is GREATER than deltaH

FEASIBLE AT HIGH TEMPERATURES

Question 19

LIMITATIONS of predictions made for feasibility

Answer 19

The sign of deltaG indicates the:

• THERMODYNAMIC FEASIBILITY of a reaction
• NOT its RATE

Example:

• decomposition of H2O2 to form O2 & H2O at room temperature
• reaction does not take place as the Ea is too large
• would take place at a very SLOW RATE
• a CATALYST, e.g MnO2 would increase ROR

Question 20

Calculate the temperature at which the reaction becomes feasible

Answer 20

deltaG = deltaH - TdletaS = 0
(for a reaction to be feasible)

T = deltaH / (deltS / 1000)

Question 21

Unit conversions in the Gibbs equation

Answer 21

T(K) = T(degrees Celsius) + 273

deltaS ( J^-1K^-1mol^-1) = divide by 1000 to get KJ