Thermodynamics

Question 1

What is the formula for delta H?

Answer 1

Sum of bonds broken - sum of bonds formed

Question 2

What are the 3 stages involved in the formation of ionic compounds from their elements?

Answer 2

1) Formation of free gaseous atoms from the elements
2) Addition or removal of electrons to form an ion
3) Attraction of ions to form ionic compounds

Question 3

What are the 2 ways of obtaining enthalpy changes to form free gaseous atoms?

Answer 3

1) Bond dissociation enthalpy

2) Atomisation enthalpy

Question 4

What is the definition of Bond dissociation enthalpy?

Answer 4

The enthalpy change when 1 mole of covalent bond is broken homolytically in the gaseous state

Question 5

What is the definition of Atomisation enthalpy?

Answer 5

The energy required to produce 1 mole of free gaseous atoms of the same atom

Question 6

What is the 1st ionisation energy?

Answer 6

The energy required to remove 1 electron from each mole of free gaseous atoms of that element

Question 7

What is the 1st electron affinity?

Answer 7

The energy change when 1 electron is added to each mole of free gaseous atoms of that element

Question 8

What changes in the definition as the ionisation energy/electron affinity increases?

Answer 8

The ions become unipositive/negative

Question 9

What is the definition of enthalpy of lattice formation?

Answer 9

The energy released when 1 mole of the ionic compound is formed from its free gaseous atoms under standard conditions

Question 10

What is the definition of enthalpy of lattice dissociation?

Answer 10

The energy required to convert 1 mole of the ionic compound to form its free gaseous atoms

Question 11

Which of the following are endothermic or exothermic:

1) Atomisation enthalpy
2) 1st ionisation energy
3) 2nd ionisation energy
4) 1st electron affinity
5) 2nd electron affinity
6) Enthalpy of lattice formation
7) Enthalpy of lattice dissociation

Answer 11

1) Endothermic as bonds need to be broken
2) Endothermic
3) More endothermic that 1st ionisation energy as their is less repulsion on the remaining electron
4) Exothermic because the incoming electron is attracted to the nucleus
5) Endothermic as their is a negative electron approaching a negative ion thus more repulsion
6) Exothermic as bonds are being formed
7) Endothermic as bonds are being broken

Question 12

What is the hydration enthalpy?

Answer 12

The energy released when a gaseous ion is dissolved in water.

Question 13

What is the enthalpy of solution?

Answer 13

The enthalpy change when 1 mole of an ionic compound dissolves in excess water.

Question 14

What does the enthalpy of solution value indicate?

Answer 14

The more exothermic the enthalpy of solution the more likely the compound is to dissolve.

Question 15

What does the hydration energy depend on?

Answer 15

1) Charge and size of ion

The larger the charge and smaller the size = larger hydration energy

Question 16

What is the equation for enthalpy of solution?

Answer 16

The sum of hydration energies - the sum of the lattice formation energies

Question 17

What is entropy?

Answer 17

A measure of the degree of disorder of a system.

Question 18

How do you calculate entropy change?

Answer 18

Entropy change= the sum of the products - the sum of the reactants

Question 19

What is the interpretation of entropy change values?

Answer 19

Positive results = likely

Negative results = unlikely

Question 20

What is the formula for free energy?

Answer 20

The change in free energy = change in enthalpy - (T x change in energy) ^G= ^H-T^S

Question 21

What is the interpretation of free energy value?

Answer 21

< 0 = Likely

> 0 = Unlikely

Question 22

How do you calculate critical temperature?

Answer 22

T= ^H/^S

Question 23

For a chemical reaction to be feasible what has to happen to the amount of disorder?

Answer 23

The amount of disorder must not decrease

Question 24

When is a reaction feasible at all temperatures?

Answer 24

When Delta S is positive and the reaction is exothermic

Question 25

When is a reaction infeasible at all temperatures?

Answer 25

When Delta S is negative and the reaction is endothermic

Question 26

What happens when Delta S is negative and the reaction is exothermic?

Answer 26

The reaction will become feasible at low temperatures i.e. when Delta H outweighs T Delta S

Question 27

What happens when Delta S is positive and the reaction is endothermic?

Answer 27

The reaction will become feasible at high temperatures i.e. when T Delta S outweighs Delta H