Energetics II

Question 1

Define Lattice Energy

Answer 1

This is the energy change when one mole of an ionic solid is formed from its gaseous ions

Question 2

The more negative the lattice energy the ……….. the bonding

Answer 2

The more negative the lattice energy the ..Stronger.. the bonding

Question 3

What is enthalpy change of formation

Answer 3

When 1 mole of the compound is formed from its elements under standard conditions (298K and
100kpa) with all reactants and products being in their standard states

Question 4

Define Enthalpy of atomization

Answer 4

When 1 mole of gaseous atoms is formed from the element in its standard state

Na (s) —–> Na(g) [Delta at H = +148 kJ mol^-1]

Question 5

What is first ionisation enthalpy

Answer 5

This is the 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 under standard conditions

Mg (g) —–> Mg^+ (g) + e

Question 6

What is Lattice Enthalpy ?

Answer 6

It is the sum of the energy needed to break the bonds within the crystal lattice and the energy released when the ions come together to form the lattice.

Question 7

Enthalpy of hydration

Answer 7

when 1 mole of GASEOUS ions becomes hydrated or dissolves in water

X^+(G) + aq —-> X^+ (aq)

Question 8

Enthalpy of lattice formation

Answer 8

is the standard enthalpy change when
1 mole of an ionic crystal lattice is formed
from its constituent ions in gaseous form.

Question 9

enthalpy change of solution

Answer 9

This is the enthalpy change when 1 mole of SOLUTE dissolves in water.

Question 10

What is first electron Affinity

Answer 10

This is the 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

eg . O(g) + e^- —-> O^-(g)

Question 11

What is Second Electron Affinity

Answer 11

This is the enthalpy change that occurs when 1 mole of gaseous ions with -1 charge forms a gaseous ion with a -2 charge

Question 12

Why is first electron affinity exothermic (negative delta H ) but second and third electron affinity is endothermic?

Answer 12

This is because the nucleus initially attracts electrons hence energy is released when an electron is added to it, however after then it requires energy (endothermic) to overcome repulsive force between the negative ion formed and the electron

Question 13

What is the difference between enthalpy change of hydration and solution

Answer 13

Hydration is when 1 mole of GASEOUS ions dissolve in water while
Solution is when 1 mole of SOLUTE dissolves in water

Question 14

Why is hydration exothermic ?

Answer 14

Because energy is being given out as water molecules form bonds with the ions that is the delta negative oxygen with positive ions and delta positive hydrogen with negative ions

Question 15

Formula for enthalpy change of solution

Answer 15

-lattice energy + hydration enthalpy

Question 16

Why is the theoretical value of the perfect ionic model less that the actual calculated value

Answer 16

Because it assumes that that the compound is made of 100% ionic bonds therefore there no covalent bonding letting it have a smaller value and weaker bond but because most ionic compounds have a covalent property the bonds are stronger and hence larger

Question 17

The longer the atomic radii

Answer 17

the lower the melting point

Question 18

Look at a born haber cycle diagram and try to fill in the blanks

Question 19

TREND IN LATTICE ENERGY BASED ON SIZE OF IONS

As you go down the group of a group 2 halide what do we notice in terms of covalency (they are ionic compounds)

Answer 19

As we go down the group it would become more covalent because :
- It is more highly distorted
- Electrons are further away from the nucleus hence there’s more shielding hence there’s more covalency in MgI2 compared to MgF2 which is more ionic

Question 20

Trend in Lattice enthalpy based on ionic radius

Answer 20

• The smaller the ions
• the more negative the enthalpies of the lattice is (stronger lattice enthalpy)
• The smaller the distance between oppositely charged ions
• This increases electrostatic attraction and hence
• Increases Lattice energy

Question 21

Trend in Lattice Energy is based on the charges on ions

Answer 21

• The smaller the charge of an ion
• The smaller the attraction between the ions
• The weaker the lattice enthalpy (less negative values)

Question 22

What is entropy

Answer 22

This is the measure of the disorder of a system

Question 23

The more disordered something is the

Answer 23

The higher the entropy

Question 24

What is the formula of entropy of total

Answer 24

Stotal = Ssystem + Ssurroundings

Question 25

What is the formula for delta s System

Answer 25

Sum of products - Sum of reactants

Question 26

What is the formula of delta s Surroundings

Answer 26

-(enthalpy of reaction or delta h)/ Temperature in kelvin

Question 27

What is the unit for deltaS surroundings

Answer 27

Jmol^-1k^-1

Question 28

How do you convert from Celsius to kelvin ?

Answer 28

Celsius + 273 = Kelvin

Question 29

Temperature in Kelvin can never be

Answer 29

Negative

Question 30

How does size affect entropy

Answer 30

The bigger a molecule
The more disordered it is
This is because it bends more

Question 31

How does state affect entropy

Answer 31

A molecule becomes more disorderly the more gaseous is becomes
Gas - The most disorderly
Liquid in the middle
Solid - The most orderly

Question 32

Which attribute takes priority in entropy

Answer 32

State take priority over size and other attributes

Question 33

What three things cause changes in entropy

Answer 33

• Change of state from solid to liquid to gas
• Dissolving a solid ionic lattice this converts it from solid to liquid or (aqueous)
• Change in number of moles from reactants to products (If the number of moles increases from reactants to products the entropy increases)

Question 34

Entropy values are always…

Answer 34

positive

Question 35

What is the formula for delta G

Answer 35

∆H - Temperature x ∆S (system)

Question 36

How do you calculate the temperature in which a reaction is feasible

Answer 36

We rearrange the formula for delta G when it is 0 as that’s the value it’s just feasible and calculate with this formula

Temperature = delta H / delta S system

Question 37

When lattice enthalpy is greater than hydration enthalpy then the reaction is

Answer 37

exothermic

Question 38

When hydration enthalpy is greater than lattice enthalpy the rection is

Answer 38

endothermic

Question 39

What is enthalpy change ?

Answer 39

Enthalpy change, ∆H (delta H), is the heat energy change in a reaction at constant pressure.

Question 40

State Hess’ Law

Answer 40

The total enthalpy change of a reaction is always the same, no matter which route is taken.

Question 41

An increase in entropy (positive entropy value eg + 40 J K^-1mol^-1) means the

Answer 41

Less ordered the system is

Question 42

A decrease in entropy value means (negative entropy )

Answer 42

An increase in orderliness

Question 43

A reaction is only feasible when delta G is

Answer 43

Less than or equal to 0

Question 44

How do you calculate delta G (Gibbs Free energy) using gas constant and equilibrium constant

Answer 44

R = gas constant, 8.31 J K–1 mol–1
T = temperature (in K)
ln K = the natural log of the equilibrium constant

∆G = −RT ln K

Question 45

What is equilibrium constant

Answer 45

the ratio of the concentration of products to reactants at equilibria for a reversible reaction at a specific temperature

Question 46

Feasible reactions with negative ∆G have a ……. as their equilibrium constant value

Answer 46

large value greater than 1

Question 47

Not Feasible reactions with positive ∆G have a ……. as their equilibrium constant value

Answer 47

smaller value less than 1

Question 48

if the ∆G (free energy) of a reaction is less than the activation energy for a reaction then it is kinetically stable even though it is not feasible based on free energy hence the reaction will
a. Occur
b. Not Occur

Answer 48

occur