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chemistry > R1.4 - entropy and spontaneity > Flashcards

R1.4 - entropy and spontaneity Flashcards

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1
Q

What is entropy?

A

Entropy (S), is the measure of the dispersal or distribution of matter and/or energy in a system; the degree of disorder in a system

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2
Q

What is the 2nd law of thermodynamics?

A

Spontaneous processes will always result in an increased entropy of the Universe

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3
Q

What are spontaneous reactions?

A

Reactions that occur without the need to do work or add energy into a system

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4
Q

How is heat transferred between hot and cold objects?

A
  • A hot object will lose heat to a cold object
  • A cold object will never heat up a hot object
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5
Q

What can increase entropy (S)?

A
  • The greater the disorder and the more ways in which energy can be dispersed or distributed, the higher the entropy
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6
Q

What are the entropies of different states?

A

Under the same conditions, the entropy of a gas is greater than a liquid which is greater than a solid.
gas < liquid < solid

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7
Q

What can cause entropy to increase?

A
  • Decrease in order
  • Increase in dispersal
  • Moving towards liquids or gases
  • ΔS is positive (+)
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8
Q

What can cause entropy to decrease?

A
  • Increase in order
  • Decrease in dispersal
  • Moving towards liquids or solids
  • ΔS is negative (-)
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9
Q

What is the entropy if you change from solid to liquid and why?

A

ΔS = +
- Decrease in order
- Increase in dispersal

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10
Q

What is the entropy if you change from solid to gas and why?

A

ΔS = +
- Decrease in order
- Increase in dispersal

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11
Q

What is the entropy if you change from liquid to gas and why?

A

ΔS = +
- Decrease in order
- Increase in dispersal

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12
Q

What is the entropy if you change from liquid to solid and why?

A

ΔS = -
- Increase in order
- Decrease in dispersal

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13
Q

What is the entropy if you change from gas to liquid and why?

A

ΔS = -
- Increase in order
- Decrease in dispersal

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14
Q

What is the entropy if you change from gas to solid and why?

A

ΔS = -
- Increase in order
- Decrease in dispersal

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15
Q

What 3 factors can affect entropy (S)?

A

1) Number of particles
2) Temperature
3) Pressure

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16
Q

What are the properties of entropy values?

A
  • Always positive
  • Absolute values
  • Measured in J K-1 mol-1
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17
Q

When is entropy 0?

A

A perfectly ordered solid at absolute temperature has an entropy of 0.

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18
Q

How do you calculate entropy change?

A

ΔS = (sum of ΔS in products ) - (sum of ΔS in reactants)

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19
Q

What is change in Gibbs energy?

A

ΔG relates to the energy that can be obtained from a chemical reaction to a change in enthalpy, ΔH, change in entropy, ΔS and absolute temperature

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20
Q

How do you calculate ΔS?

A

ΔS = -ΔH (system) / T

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21
Q

What is the relationship between ΔS and ΔH?

A

ΔS = -ΔH (system)
- Exothermic reactions (-ΔH) result in an increase in entropy of the surroundings (+ΔS)

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22
Q

Why are exothermic reactions more common?

A

Associated with increased entropy of the surroundings, not a decrease in energy of the system due to the 2nd law of thermodynamics

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23
Q

How will transferring heat to surroundings affect entropy?

A

Impact of addition of heat depends on the present state of dispersal in the system.

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24
Q

What equation links ΔS and absolute temperature?

A

ΔS (surroundings) = -1/T
- T is measured in K

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25
Q

What are the 2 equations for ΔS for a spontaneous change?

A

ΔS (total) = ΔS (system) + ΔS (surroundings) > 0
ΔS (total) = ΔS (system) - (ΔH (system) / T > 0

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26
Q

When do endothermic reactions occur?

A

Endothermic reactions occur if the change in entropy of the system can compensate for the negative entropy change of the surroundings produced as heat is transferred from the surroundings to the system

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27
Q

What is needed to predict feasibility of a chemical reaction?

A

ΔH and ΔS

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28
Q

How do you calculate Gibbs energy?

A

ΔG (system) = ΔH (system) - TΔS (system)
- must be negative for spontaneous reactions

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29
Q

What is the difference between ΔH and ΔG?

A

ΔH - measure of quantity of heat change during a chemical reaction
ΔG - measure of quality of energy available; a measure of energy which is free to do useful work rather than leave the system as heat

30
Q

Why is ΔG negative?

A

spontaneous reaction - ΔG is the useful work

31
Q

What is the equation for necessary energy transferred to surroundings?

A

Necessary energy transferred to surroundings = -ΔS (system) x T

32
Q

What is the equation for energy available to do work?

A

Energy available to do work = -ΔH (system) + TΔS (system)
- this is equal to -ΔG (system)

33
Q

How can ΔG be calculated from ΔGf?

A

ΔG (system) = ΔH (system) - TΔS (system)
- at 298K

34
Q

What is the Gibbs energy cycle?

A

ΔG = ΔG(products) - ΔG(reactants_

35
Q

How can ΔG be calculated from ΔS and ΔH?

A

ΔG = ΔH - TΔS
- at all temperatures

36
Q

When is a change spontaneous at constant pressure?

A

At constant pressure, a change is spontaneous if the change in Gibbs energy is negative.

37
Q

What is the equation for change if T is positive?

A

ΔH (system) < TΔS (system)

38
Q

When is this condition met at low temperatures when TΔS is 0?

A

For exothermic reactions as ΔH (system) is negative

39
Q

What is the significance of T in spontaneity of a reaction?

A

T adjusts the spontaneity for a reaction

40
Q

When are endothermic reactions spontaneous?

A

ΔS (system) = +
- spontaneous at high temperatures when TΔS (system) > ΔH (system)

41
Q

How can the temperature of an endothermic process be determined?

A

Tspontaneous TΔS (system) = ΔH (system)

42
Q

What can values of ΔG give information about?

A

Values of ΔG give information about the feasibility of a reaction, but don’t give information on the rate of the reaction or activation energy.

43
Q

What can cell potentials be used for?

A

Predicting whether a reaction is spontaneous or not as it measured useful energy that can be obtained from a reaction

44
Q

What happens to ΔG as a reaction approaches equilibrium?

A

ΔG becomes less negative and reaches zero.

45
Q

What happens when ΔG is zero?

A

One mole of reactants react to form one mole of a product without breaking or making any bonds.
As ΔH=0, and ΔS is 0, this means that ΔG is zero.
ΔG = ΔH - TΔS
Q = 1

46
Q

What does the position of equilibrium correspond to in terms of entropy and gibbs energy?

A
  • Maximum entropy
  • Minimum Gibbs energy
47
Q

When is there a reversible reaction?

A

There is a change or a curve in entropy or gibbs free energy.

48
Q

What does a positive and negative standard ΔG mean?

A

+ = non-spontaneous reaction
- = spontaneous reaction - wants to be at equilibrium

49
Q

What happens to the equilibrium mixture when ΔG is negative?

A

1) At the start of the reaction, total ΔG is greater than that of products, so the reaction proceeds for the forward reaction and Q increases to make more products.
2) As the reaction continues to convert reactants to products, ΔG decreases until it reaches equilibrium , where it is equal to K.
3) Once equilibrium is reached, all possible changes lead to an increase in ΔG. This means that the total entropy of the universe would decreases. Therefore, this does not happen.
4) The position of equilibrium corresponds to a mixture with more products than reactants.

50
Q

What happens to the equilibrium mixture when ΔG is positive?

A

1) At the start of the reaction, total ΔG is smaller than that of products, so the reaction proceeds for the backward reaction and Q decreases to make more reactants.
2) As the reaction continues to convert products to reactants , ΔG increases until it reaches equilibrium , where it is equal to K.
3) Once equilibrium is reached, all possible changes lead to an decrease in ΔG. This means that the total entropy of the universe would increases. Therefore, this does not happen.
4) The position of equilibrium corresponds to a mixture with more reactants than products.

51
Q

How can the graph when ΔG is negative be explained?

A

1) ΔG is high for reactants.
2) ΔG is lower for products.
3) ΔG is negative, so is a spontaneous reaction.
4) System of maximum entropy (at equilibrium) consists of mainly products, so has a lowest ΔG value.
5) K>1 as there are more products.

52
Q

How can the graph when ΔG is positive be explained?

A

1) ΔG is low for reactants.
2) ΔG is high for products.
3) ΔG is positive so is a non-spontaneous reaction.
4)System of maximum entropy (at equilibrium) consists of mainly reactants, so has a highest ΔG value.
- a large amount of energy is formed.
5) K<1 as there are more reactants.

53
Q

What happens to K as ΔG is negative?

A

-ΔG - mainly products at equilibrium -K>1
- spontaneous reaction

54
Q

What happens to K as ΔG is positive?

A

+ΔG - mainly reactants at equilibrium - K<1
- non-spontaneous reaction

55
Q

What happens to K as ΔG is o?

A

ΔG = 0 - reactants and products are at equilibrium - K=1

56
Q

What is the equation that links K and ΔG?

A

ΔG = -RT ln K

57
Q

What does the magnitude of ΔG tell us?

A

The magnitude tells us the composition of reactants and products in a reversible reaction.

58
Q

What occurs when ΔG is highly positive?

A

No reaction - non-spontaneous
>30

59
Q

What occurs when ΔG is slightly positive?

A

Partial reaction - mainly reactants
0 < ΔG < 30

60
Q

What happens when ΔG is 0?

A

Reaction is at equilibrium

61
Q

What happens when ΔG is slightly negative?

A

Partial reaction - mainly products

62
Q

What happens when ΔG is highly negative?

A

Complete reaction - spontaneous

63
Q

When does ΔG change with temperature in relation to ΔH and ΔS?

A

When both ΔH and ΔS are the same sign; they are both positive or both negative?

64
Q

When will ΔG be negative, in relation to ΔH and ΔS?

A
  • ΔH
    + ΔS
  • spontaneous reaction
  • exothermic
65
Q

When will ΔG be positive, in relation to ΔH and ΔS?

A

+ ΔH
- ΔS
- non-spontaneous reaction
- endothermic

66
Q

Where does equilibrium lie if standard ΔG is positive?

A

Equilibrium lies on the left.

67
Q

Where does equilibrium lie if standard ΔG is negative?

A

Equilibrium lies on the right.

68
Q

What is the equation that links ΔG and standard ΔG?

A

ΔG = ΔG (standard) + RT ln Q

69
Q

What are the values of K and Q when ΔG = 0?

A

K=Q as energy changes in both directions cancel out.

70
Q

What happens to Q as it increases after K=0, where it is at equilibrium?

A

Q continues to increase.

71
Q

What is the difference between ΔG and standard ΔG?

A

ΔG - difference between the Gibbs energy possessed by the products and reactants in a particular ratio.
- It varies as the ratio of product to reactant changes, but has the value of 0 at equilibrium.
standard ΔG - the change in Gibbs energy when the reactants specified in a reaction equation are completely transformed into products under standard conditions.
- It is constant at a constant temperature.

72
Q
A

chemistry (22 decks)

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