Topic 13.2 - Energetics II: Entropy Flashcards

1
Q

What is the equation for the energy change (J) of a reaction?

A

Q = mc(delta)T

Q - energy in Joules, J
m - mass of solution, g (equivalent to the volume of solution as we assume density = 1)
(delta)T - change in temperature, K

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2
Q

What is the equation for the enthalpy change of reaction when you know the energy change of the reaction?

A

(delta) H = Q/n

(delta)H - Enthalpy change of reaction, kJmol(-1)
Q - energy in Kilojoules, kJ
n - Number of moles of limiting reagant

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3
Q

What is the equation for the enthalpy change of reaction when you know the enthalpy change of formation for your products and reactants?

A

(delta) H reaction = (delta) H products - (delta) H reactants

All units are kJmol(-1)

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4
Q

What is the equation for the standard total entropy change of a reaction?

A

(delta) S total = (delta) S system + (delta) S surroundings

All units JK(-1)mol(-1)
Under standard conditions

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5
Q

What is the equation for the standard entropy of the system?

A

(delta) S system = (delta) S products - (delta) S reactants

All units JK(-1)mol(-1)
Under standard conditions

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6
Q

What is the equation for the standard entropy of the surroundings?

A

(delta) S surroundings = - (delta)H reaction/ T

(delta) S - Entropy change of surroundings, JK(-1)mol(-1)
(delta) H - Enthalpy change of reaction, convert to Jmol(-1)
T - Temperature, K

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7
Q

What is the standard equation for Gibbs Free energy?

A

(delta) G = (delta) H reaction - T(delta) S system

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8
Q

What is the equation for Gibbs Free energy that includes the equilibrium constant of a reaction?

A

(delta) G = -RTlnKc

(delta) G - Gibbs free energy of the reaction, KJmol(-1)
R - Gas constant, 8.31 Jmol(-1)K(-1)
T - Absolute temperature, K
Kc - Equilibrium constant, from this equation you cant know the units of Kc

Under standard conditions

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9
Q

What is the equation for Gibbs Free energy that includes the Ecell value of a reaction?

A

(delta) G = -nFEcell

(delta) G - Gibbs Free energy change, KJmol(-1)
n - the number of electrons involved in the redox reaction
F - Faraday constant, 9.65 x10^4 Cmol(-1)
Ecell - Standard cell potential, V

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10
Q

What is the equation for the standard total change in entropy that includes the Ecell value of the reaction?

A

(delta) S total =(nFEcell)/T

(delta) S total - Total entropy change of the reaction, JK(-1)mol(-1)
n - the number of electrons involved in the redox reaction
F - Faraday constant, 9.65 x10^4 Cmol(-1)
Ecell - Standard cell potential, V

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11
Q

What is the definition of bond enthalpy?

A

The energy required to break 1 mole of that bond

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12
Q

What is the equation for the standard total enthalpy change of a reaction when you are given the bond energies?

A

(delta) H total = (sum of) Bonds broken - (sum of ) Bonds made

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13
Q

What sign does entropy always have? Why?

A

+ve, as particles are always in constant motion so there is always a degree of disorder

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14
Q

What is the definition of Entropy?

A

Entropy, S, is a quantitative measure of the degree of disorder of a system

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15
Q

When is the only time that entropy of a system = 0?

A

At Absolute Zero

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16
Q

What is the definition of a Spontaneous reaction?

A

A reaction that does not require any external input of energy in order to occur

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17
Q

What is the definition of a Feasible reaction?

A

A reaction that naturally tends to happen, even if it is very slow as the activation is high

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18
Q

What is required, in terms of entropy, for a reaction to be feasible?

A
  • For (delta) S total > 0
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19
Q

Are spontaneous reactions always feasible?

A

Yes

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20
Q

Are feasible reactions always spontaneous?

A

No - A reaction can be thermodynamically feasible but not occur due to a very high activation energy (kinetics)

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21
Q

As entropy increases…

A
  • Disorder tends to increase (number of moles of substances)
  • Energy tends to become dispersed
  • A reaction is more likely to happen
22
Q

What is the universe made up of?

A

The system and the surroundings

23
Q

Describe the molar entropy in solids

A
  • Relatively low as atoms are held in fixed positions and only vibrate slightly
24
Q

Describe the molar entropy in liquids

A
  • Higher entropy than solids as the atoms/particles are free to move
  • There are many more ways of distributing the particles and energy (the complexity)
  • Molecules with more atoms have higher molar entropies as they can vibrate, rotate and arrange themselves in more ways
25
Q

Describe the molar entropy in gases

A
  • Gases have the highest molar entropy as they are both free to move and are widely spaced (so the energy is dispersed)
  • There are the most ways of distributing the particles and energy
  • Molecules with more atoms have higher molar entropies as they can vibrate, rotate and arrange themselves in more ways
26
Q

What is a fundamental principal of Entropy?

A

Random changes always tend to go in the way that increases the number of ways of distributing the molecules, energy and quanta (entropy)

27
Q

If (delta) S system is > 0 then…

A

there has been an increase in disorder (entropy)

28
Q

If (delta) S system is < 0 then…

A

there has been a decrease in disorder (entropy)

29
Q

If a reaction is exothermic then the (delta) S of the surroundings will be…?

A

+ve

30
Q

If a reaction is endothermic then the (delta) S of the surroundings will be…?

A

-ve

31
Q

If you lower the temperature of the surroundings how will that affect the value of (delta) S of surroundings? Why?

A

It will increase the value of the change in entropy of the surroundings.

  • Using the equation -(delta)H/T a decrease in temperature will cause and increase in (delta) S
  • For a given quantity of energy, the increase in entropy is greater when the surroundings are cool than when they are hot
32
Q

What is the relationship between (delta) S surroundings and T, temperature?

A

(delta) S is inversely proportional to T

33
Q

In what situation can an endothermic reaction be feasible?

A

If the (delta) S of the system is more +ve than the -ve entropy of the surroundings

34
Q

In what situation can a non-feasible reaction at room temperature become feasible?

A

An increase in temperature

35
Q

For all spontaneous processes (delta) G must be…?

A

-ve

36
Q

If (delta) H is -ve and (delta) S is +ve then (delta) G will be…?

A

-ve - ALWAYS FEASIBLE

37
Q

If (delta) H is -ve and (delta) S is -ve then (delta) G will be…?

A

-ve - feasible at low temperatures for (delta) H > T(delta) S system

38
Q

If (delta) H is +ve and (delta) S is +ve then (delta) G will be…?

A

-ve - feasible at high temperatures for T(delta) S system > (delta) H

39
Q

If (delta) H is +ve and (delta) S is -ve then (delta) G will be…?

A

+ve - NEVER FEASIBLE

40
Q

At the point that a reaction becomes feasible…

A

(delta) G/ (delta) S = 0

This means you can determine at what temperature a reaction becomes feasible

41
Q

What are the units of (delta) G?

A

Kjmol(-1)

42
Q

Can a feasible reaction still be called feasible even if it is slow?

A

Yes

43
Q

In equilibria are both the forwards and backwards happening? What does this suggest about the feasibility of those reactions?

A

Yes, both are happening. This suggests that both the forward and back reactions are feasible (even though one will give a +ve (delta) G and the other -ve (delta) G)

44
Q

Describe what a graph of total entropy change against extent of reaction would look like.

A

Endothermic: For an endothermic reaction the line would increase and reach a maximum quickly at a +ve entropy and then decrease to a -ve value

Exothermic: For an exothermic reaction the line would increase a lot first and meet a peak close to the end before decreasing slightly to a final +ve entropy value

45
Q

Why are both the forwards and backwards reaction feasible in equilibria?

A

As both the forwards and backwards reaction go via an increase in entropy

46
Q

Why is there a peak of total change in entropy during an equilibria reaction?

A

As a mixture of reactants and products gives a higher entropy value than one on their own

47
Q

When does the maximum total entropy value occur?

A

At equilibrium - when (delta) S forwards = (delta) S backwards

48
Q

At Kc = 1, (delta) G of the forwards reaction is…?

A

0 - The position of equilibrium is in the middle and neither the reactants nor products are favoured

49
Q

At Kc > 1, (delta) G of the forwards reaction is…?

A

-ve - The position of equilibrium lies to the right, more products than reactants are being made

50
Q

At Kc < 1 , (delta) G of the forwards reaction is…?

A

+ve - The position of equilibrium lies to the left, more reactants than products are in the equilibrium mixture