Chapter 22- Enthalpy and Entropy

Question 1

Name two factors that impact lattice enthalpy.

Answer 1

Ionic radius and ionic charge.

Question 2

Describe how ionic charge impacts lattice enthalpy.

Answer 2

As the ionic radius increases the attraction between ions decreases and the lattice enthalpy becomes less negative/ exothermic. This means that the melting point decreases. e.g.
The melting point of Na^+ is much greater than that of RB^+.

Question 3

Describe how ionic charge can impact lattice enthalpy.

Answer 3

As ionic charge increases, the attraction between the ions increases making the lattice enthalpy more negative and causing the melting point to increase. e.g.A Na20 has a lower melting point than Ca0.

Question 4

Describe how charge and size of ion impact attraction across period three.

Answer 4

For the cations the charge increases as you move along the period creating greater attraction. The size of the ion also decreases as you move along also creating a greater attraction.
For the anions the charge increases the opposite way and as it increases more attraction is experienced. The increasing size however, provides less attraction.

Question 5

As ionic radius in cations increases hydration energy becomes…

Answer 5

… less exothermic because the attraction between the water molecules and ions decreases.

Question 6

As charge increases hydration energy becomes…

Answer 6

… more exothermic because the attraction with the water molecules increases.

Question 7

The greater the entropy the ______ the dispersal of energy and disorder.

Answer 7

greater

Question 8

What are the units for entropy?

Answer 8

JK^-1mol^-1

The greater the entropy the greater that energy is spread out per Kelvin per mole.

Question 9

In general what state has the greatest entropy value?

Answer 9

Gases generally have the greatest entropy value and solids the smallest.

Question 10

If a system changes to become more random, what is the entropy change?

Answer 10

It will be positive.

Question 11

If a system changes to become less random, what is the entropy change?

Answer 11

It will be negative.

Question 12

When a substance changes from a solid to a liquid what occurs?

Answer 12

The entropy value increases as…

• melting and boiling increase the randomness of particles.
• Energy is spread out more and entropy is positive.

Question 13

If there is a decrease in the number of gaseous molecules then what happens to the entropy value and why?

Answer 13

There is a decrease in the randomness of particles and the energy is spread out less making the entropy negative.

Question 14

How can entropy changes be calculated?

Answer 14

△S^⦵= ΣS^⦵(products)- ΣS^⦵(reactants)

Question 15

Define entropy.

Answer 15

The quantitative measure of dispersal of energy and disorder in chemical systems.

Question 16

How do you calculate free energy change?

Answer 16

△G = △H - T△S
△G - free energy change
△H - enthalpy change - KJmol^-1
T - temperature - K
△S - entropy change- remember to divide by 1000