4.4 How Far? - Entropy

Question 1

What is entropy?

Answer 1

Entropy is a measure of the number of way that particles can be arranged and the number of ways that the energy can be shared out between particles.

Question 2

Why can some endothermic reactions occur spontaneously at room temperate?

Answer 2

Enthalpy changes alone do not control whether a reaction can occur. For these reactions the total change in entropy is positive.

Question 3

Order the physical states in terms of their entropy.

Answer 3

Solids have the lowest entropy, gasses have the highest entropy.

Question 4

Why do solid particles have the lowest entropy?

Answer 4

They have little randomness to their arrangement and only vibrate about a fixed point.

Question 5

Why do gasses have the highest entropy?

Answer 5

Gas particles move at high speeds in random directions. They have the most random arrangements of particles.

Question 6

What effect, if any, does dissolving a solid have on its entropy and why?

Answer 6

It increases its entropy because dissolved particles can now move freely since they are no longer held in place.

Question 7

What are energy quanta?

Answer 7

Small packets of energy with fixed amounts.

Question 8

How does the number or particles of a substance affect its entropy?

Answer 8

More particles results in more entropy since there are more ways that these particles can be arranged.

Question 9

Describe a substance that would have zero entropy.

Answer 9

A perfect crystal at zero kelvin.

Question 10

What effect does temperature have on entropy?

Answer 10

Increasing the temperature of a substance increases its entropy because the substance has more energy quanta.

Question 11

What is the standard entropy of a substance?

Answer 11

The entropy of 1 mole of that substance at 100kPa and 298K.

Question 12

What are the units of standard entropy?

Answer 12

JK^-1mol^-1

Question 13

Why does carbon dioxide gas have a higher standard entropy than carbon monoxide gas?

Answer 13

Because a carbon dioxide molecule is a more complicated molecule since it contains more atoms.

Question 14

What are the units of entropy?

Answer 14

JK^-1

Question 15

How do you calculate the total entropy of a system, ∆S total?

Answer 15

∆S total = ∆S system + ∆S surroundings

Question 16

How do you calculate ∆S system?

Answer 16

∆S system = S products - S reactants

Question 17

How do you calculate ∆S surroundings?

Answer 17

∆S surroundings = -∆H/T

Question 18

What does it mean if reactants are said to be kinetically stable?

Answer 18

∆S total is negative and therefore the reactants will not react on their own.

Question 19

The conversion of diamond to graphite has a total entropy of +9.7 JK^-1. Why is it kinetically inert?

Answer 19

Under standard conditions the rate of reaction is extremely slow. It takes millions of years to react because the activation energy needed to start the reaction is so high.

Question 20

What is the enthalpy change of solution, ∆H solution?

Answer 20

The enthalpy change when 1 mole of solute is dissolved in sufficient solvent that no further enthalpy change occurs on further dilution.

Question 21

What is the standard lattice enthalpy, ∆Hº latt?

Answer 21

The enthalpy change when 1 mole of a solid ionic compound is formed from gaseous ions under standard conditions.

Question 22

What is the enthalpy change of hydration, ∆H hyd?

Answer 22

The enthalpy change when 1 mole of gaseous ions dissolve in sufficient water to give an infinitely dilute solution. Hydration enthalpies are always negative.

Question 23

How do you calculate ∆H solution?

Answer 23

∆H solution = ∆H hyd - ∆H latt

Question 24

How does ionic charge affect lattice enthalpy and why?

Answer 24

Higher charge means more negative lattice enthalpy because more energy is related when the lattice is formed.

Question 25

How does ionic radii affect lattice enthalpy and why?

Answer 25

The smaller the ionic radii of the ions involved, the more exothermic the lattice enthalpy because smaller ions attract more strongly because their charge density is higher.

Question 26

The dissolution of silver bromide has a ∆S total of -235.5 JK^-1mol^-1. What can you infer form this?

Answer 26

Silver bromide will not dissolve in water at 298 K.