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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)

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

20
Q

How do you calculate ΔS?

A

ΔS = -ΔH (system) / T

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)

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

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.

24
Q

What equation links ΔS and absolute temperature?

A

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

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

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

27
Q

What is needed to predict feasibility of a chemical reaction?

A

ΔH and ΔS

28
Q

How do you calculate Gibbs energy?

A

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

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
A

chemistry (22 decks)

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