Thermodynamics

Question 1

Define

Enthalpy of formation

Answer 1

The standard enthalpy change is when 1 mole of

the compound is formed from its elements under standard conditions under standard states

Question 2

Define enthalpy of atomisation

Answer 2

Enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard states

Na (s) -> Na (g)

Question 3

Define

Enthalpy of bond dissociation

Answer 3

Enthalpy change when one mole of a covalent bond is broken into two gaseous atoms (or free radicals

Question 4

Define

First ionisation energy

Answer 4

enthalpy change required to remove 1 mole of electrons from 1 mole of gaseous atoms to form
1 mole of gaseous ions with a +1 charge

Question 5

Define

First ionisation energy

Answer 5

enthalpy change required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a 1+ charge

Question 6

Define

Second ionisation energy

Answer 6

enthalpy change required to remove 1 mole of electrons from 1 mole of 1+ gaseous ions to form 2 mole of gaseous ions with a 2+ charge

Question 7

Define

first electron affinity

Answer 7

enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous ions with a 1- charge

Question 8

Define

second electron affinity

Answer 8

enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions.

Question 9

Define

enthalpy of the lattice of formation

Answer 9

Enthalpy change when one mole of a solid ionic compound is formed from its constituent ions in the gas phase

Question 10

Define

enthalpy of lattice dissociation

Answer 10

Enthalpy change when one mole of a solid ionic compound is broken up into its constituent ions in the gas phase

Question 11

Define

enthalpy of hydration

Answer 11

Enthalpy change when one mole of gaseous ions become aqueous ions.

Question 12

Define

enthalpy of solution

Answer 12

Enthalpy change when one mole of an ionic solid dissolve in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other.

Question 13

Why is the first electron affinity exothermic

Answer 13

The first electron affinity is exothermic for atoms that normally form negative ions. This is because the ion is more stable than the atom, and there is an attraction between the nucleus and the electron.

Question 14

Why is the second electron affinity endothermic

Answer 14

The second electron affinity is endothermic

because it takes a lot of energy to overcome the repulsive force between the negative ion and the electron.

Question 15

What does the perfect ionic model assume

Answer 15

• ions are 100% ionic
• are spherical
• the attractions are purely electrostatic.

Question 16

What does the strength of lattice of formation energy depend on

Answer 16

1. The sizes of the ions
   The larger the ions, the less negative the enthalpies of lattice formation. As the ions are larger the charges become further apart and so have a weaker attractive force between them.
2. The charges on the ion
   The bigger the charge of the ion, the greater the attraction between the ions so the stronger the lattice enthalpy.

Question 17

When may there be tendencies of covalent character in an ionic lattice

Answer 17

•the positive ion is small 
•the positive ion has multiple charges
 •the negative ion is large 
•the negative ion has multiple negative
charges.

Question 18

What does it mean when there is a difference between the experimental value (born Haber cycle value) and the theoretical value

Answer 18

The more the covalent character the bigger

the difference between the values.

Question 19

What happens if a molecule has more covalent character

Answer 19

it means that either the negative ion are large which means it was easier to distort as it is more attracted to the positive nucleus

or

positive ion are small and are good at polarizing negative larger ions

Question 20

State the meaning of entropy

Answer 20

a measure of how disordered or random a system is

Question 21

What has a higher entropy a solid, liquid or gas and why

Answer 21

Gas

partciles move rapidly and randomally compared to a solid where they are in a fized position

Question 22

What is the equation for entropy and what are the units

Answer 22

ΔSsystem = ΣΔSproducts - ΣΔSreactants

J k-1 mol-1

Question 23

What is the equation for the formation

Answer 23

products- reactants

Question 24

What is the gibbs free energy equation

Answer 24

∆G = ∆H – T∆S

∆G= Gibbs free energy
∆H = enthalpy of formation 
T= temperature in kelvins
∆S = entropy (you divide this by 1000 so its in kj)

Question 25

What number does the Gibbs free energy have to be under for it to be feasible

Answer 25

<0

less than 0 kelvins so in the minuses

0 kelvins is the perfect ionic solid, which means the particles are in perfect order

Question 26

is the reaction feasible

If there is a positive ΔH and a positive ΔS

Answer 26

Yes if it is above a certain temperature

Question 27

is the reaction feasible

If there is a negative ΔH and a positive ΔS

Answer 27

always feasible

because as you increase temp gibbs free energy will become more negative

Question 28

is the reaction feasible

If there is a positive ΔH and a negative ΔS

Answer 28

Never feasible

why?- because as we increase temp gibbs free energy will always be positive

Question 29

is the reaction feasible

If there is a negative ΔH and a negative ΔS

Answer 29

Yes below a certain temp

Why?- gibbs free energy will become more poistive as you increase temp

Question 30

What is the equation to work out which temperature a reaction will be feasible at

Answer 30

ΔH /ΔS