15.1 Energy cycles (HL)

Question 1

Lattice enthalpy

Answer 1

Enthalpy change when one mole of an ionic compound is broken apart into its constituent gaseous ions under standard conditions

Question 2

Is lattice enthalpy positive or negative?

Answer 2

Always positive as energy is absorbed to break the bonds

Question 3

Opposite of lattice enthalpy

Answer 3

Lattice formation enthalpy

Question 4

What does an increase in lattice enthalpy mean?

Answer 4

The higher it is, the stronger the electrostatic bonds

Question 5

What does lattice enthalpy depend on?

Answer 5

Ionic radii and ionic charge

Question 6

What values do you need to calculate lattice enthalpy?

Answer 6

• Enthalpy of formation
• Enthalpy of atomization
• Bond dissociation energy
• First ionization energy
• First electron affinity

Question 7

What is enthalpy of atomization?

Answer 7

When one mole of gaseous atoms is formed from an element

Question 8

If bond dissociation energy of O2 is +498kJ/mol, what is enthalpy of atomization?

Answer 8

It will be half as only 1 oxygen is made i.e. 249kJ/mol

Question 9

What is the general formula for Born-Haber cycle? eg. NaCl

Answer 9

Lattice enthalpy= Enthalpy of atomization(Na) + Enthalpy of atomization(Cl) + First ionization energy (Na) + First electron affinity (Cl) - Enthalpy of formation (NaCl)

Question 10

What needs to be taken into account for Born-Haber cycle of MgF2?

Answer 10

Second ionization of energy of Mg is required and first electron affinity needs to be doubled for F

Question 11

What needs to be taken into account for Born-Haber cycle of MgO?

Answer 11

Second ionization of energy of Mg is required and second electron affinity of O is also required

Question 12

When gaseous ions are hydrated by surrounding water molecules, what bonds are formed?

Answer 12

Ion-dipole forces are formed

Question 13

Enthalpy of hydration

Answer 13

Enthalpy change when one mole of gaseous ions dissolve in water to form a solution of infinite dilution under standard conditions

Question 14

Infinite dilution

Answer 14

A solution that has excess of water and addition of more water will not cause more heat to be absorbed or released

Question 15

Is enthalpy of hydration positive or negative?

Answer 15

Negative

Question 16

Enthalpy of solution

Answer 16

Enthalpy change when one mole of solute dissolves to form a solution of infinite dilution

Question 17

Equation for enthalpy of solution

Answer 17

Enthalpy of hydration of all ions + Lattice enthalpy

Question 18

What does solubility of an ionic compound depend (to some extent) on?

Answer 18

Sign and magnitude of enthalpy of solution

Question 19

What type of ionic compounds have higher positive enthalpy of solution values and why?

Answer 19

Insoluble ionic compounds as enthalpy of hydration doesn’t compensate for higher lattice enthalpy as dissolving process will be energetically unfavorable

Question 20

Can those with slight positive values still be soluble?

Answer 20

Yes

Question 21

Factors affecting enthalpy of hydration

Answer 21

Ionic radii and charge on ion (charge density)

Question 22

How does ionic radii affect enthalpy of hydration?

Answer 22

Higher the ionic radii, magnitude of enthalpy of hydration decreases as there are weaker ion-dipole forces

Question 23

How does charge density affect enthalpy of hydration?

Answer 23

As it increases, so does magnitude and it also becomes more exothermic as forces are stronger and more energy is thus released

Question 24

What would the equation for enthalpy of solution of MgCl2 look like?

Answer 24

[Lattice enthalpy of MgCl2] + [(Enthalpy of hydration of Mg) + (Enthalpy of hydration of Cl x 2)]