Entropy

Question 1

What is the second law of thermodynamics?

Answer 1

Spontaneous changes result in an increase in disorder or entropy

Question 2

What does the second law of thermodynamics determine?

Answer 2

• Whether a physical or chemical change is likely to happen at a particular temperature
• Whether redox reactions will take place
• The position of equilibrum

Question 3

What is standard molar entropy?

Answer 3

The entropy per mole of substance under standard conditions. Usually given the units J mol^-1 K^-1, and usually given per mole of atoms, not molecules

Question 4

Diamond has a lower value for standard molar entropy than lead, why is this?

Answer 4

The carbon atoms in diamond are held in place by strong, highly directional covalent bonds, but the metallic bonding in lead is not directional so atoms can vibrate more freely and share out their energy in more ways than carbon atoms in diamond

Question 5

Lead has a higher standard molar entropy than iron, why is this?

Answer 5

In hard solids, the atoms have rigid structures with stiff bonds and the thermal motion of the atoms is very restricted so entropy of a hard solid like iron is lower than lead which is soft. In soft solids, there is more thermal disorder and entropy is larger

Question 6

Why is the standard molar entropy of liquids generally higher than solids?

Answer 6

Atoms or molecules are freer to move and there are more ways of distributing the particles and energy so there is more disorder.

Question 7

Why is the standard molar entropy of gases generally higher than liquids?

Answer 7

The atoms or molecules are free to move but are also very widely spaced, so there are even more ways of distributing the particles and energy so the disorder is even greater

Question 8

Why are more complicated molecules higher in entropy than simpler molecules?

Answer 8

More energy levels are available in a complicated molecule, so they can vibrate, rotate and arrange themselves in more ways

Question 9

What is the equation for ΔSsystem which concerns standard molar entropies of reactants and products?

Answer 9

ΔSsys = ΣSproducts - ΣSreactants

Question 10

What is the equation for ΔStotal which concerns ΔSsys and ΔSsurr?

Answer 10

ΔStotal = ΔSsys + ΔSsurr

Question 11

ΔSsurr is positive for which type of reaction?

Answer 11

Exothermic reactions

Question 12

ΔSsurr is negative for which type of reaction?

Answer 12

Endothermic reactions

Question 13

What effect does the temperature of the surroundings have on the value for increase in entropy?

Answer 13

Entropy increase is small because the molecules have high entropy and the area is already in chaotic motion.
The opposite is true if the surroundings are cold.
The entropy change in the surroundings caused by transfer of heat depends on the value of the heat change and is also inversely proportional to the temperature of the surroundings

Question 14

What is the equation which concerns ΔSsurr, ΔH and temperature?

Answer 14

ΔSsurr = -ΔH / T

Temperature measured in K, enthalpy has to be in Jmol^-1

Question 15

What is the equation which concerns ΔStotal, ΔSsys, ΔH and T?

Answer 15

ΔStotal = ΔSsys - (ΔH/T)

Question 16

For a spontaneous reaction, is ΔStotal positive or negative?

Answer 16

Positive

Question 17

What happens if ΔStotal is zero?

Answer 17

It is in equilibrium/balanced - becomes very sensitive to temperature and could go either way

Question 18

What is the definition of a spontaneous reaction?

Answer 18

A spontaneous reaction is one for which the total entropy change is positive

Question 19

Mathematically, why do most exothermic reactions tend to go at about room temperature?

Answer 19

-ΔH/T is much larger and more positive than ΔSsys, so ΔStotal is positive

Question 20

Under what conditions (to do with the equation for entropy) can an endothermic reaction be feasible?

Answer 20

If the increase in the entropy of the system is greater than the decrease of the entropy of the surroundings

Question 21

Mathematically, why might a reaction that doesn’t go at room temperature become feasible as temperature rises?

Answer 21

ΔSsurr decreases in magnitude as T increases

Question 22

What is the minimum value for ΔStotal for which reactions tend to go to completion?

Answer 22

200 J mol^-1 K^-1

Question 23

What is the equation for ΔG that concerns ΔH, T and ΔSsys?

Answer 23

ΔG = ΔH - TΔSsys

Question 24

Does a positive or negative value for ΔG indicate a feasible reaction?

Answer 24

Negative

Question 25

What happens if ΔG = 0?

Answer 25

The reaction is at equilibrium

Question 26

What is the equation for ΔG that concerns T and K (equilibrium constant)?

Answer 26

ΔG = -RTlnK

Question 27

When ΔG becomes more negative than a certain value (approx. -40kJ mol^-1), what happens to K and what does that indicate?

Answer 27

K becomes so large that the reaction has effectively gone to completion

Question 28

When ΔG becomes more positive than a certain value (approx. 40 kJ mol^-1), what happens to K and what does that indicate?

Answer 28

K becomes so small that equilibrium lies almost entirely to the reactants’ side, so hardly any of the reactants wil actually react

Question 29

What are the two main reasons that a reaction can be thermodynamically feasible but not take place?

Answer 29

• Kinetic stability - if the activation energy is too high/if reactants are kinetically stable
• Non-standard conditions - ΔG is calculated using values calculated under standard conditions