Thermodynamics

Question 1

Enthalpy of formation

Answer 1

• Enthalpy change 1 mole of a compound is formed from its constituent elements
• Under standard conditions, all reactants and products in their standard states
• Na (s) + ½Cl₂ (g) NaCl (s) [Hf = - 411.2 kJ mol-1]

Question 2

Lattice enthalpy of dissociation

Answer 2

• Enthalpy change when 1 mol of an ionic substance is dissociated into its component gaseous ions
• Under standard conditions
• NaCl (s) ⇒ Na⁺ (g) + Cl^- (g)

Question 3

Enthalpy of atomistation

Answer 3

• Enthalpy change when 1 mole of gaseous atoms are made from its element in its standard state
• Na (s) ⇒ Na(g)
• ½ O₂ (g) ⇒ O (g)

Question 4

First ionisation energy

Answer 4

• Enthalpy change when 1 mole of gaseous atoms is converted into 1 mole of gaseous ions with a single positive charge
• Mg (g) ⇒ Mg+ (g) + e-

Question 5

First electron affinity

Answer 5

• Enthalpy change when 1 mole of a gaseous atoms is converted to 1 mole of gaseous ions, with a single negative charge
• By the addition of 1 electron
• O (g) + e- ⇒ O- (g)

Question 6

Second electron affinity

Answer 6

• Enthalpy change when 1 mole of electrons is added to 1 mole of gaseous ions with 1- charge to form gasous ions with 2-
• O^- (g) + e- ⇒O₂- (g)

Question 7

Lattice formation energy

Answer 7

• Enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions
• Na+ (g) + Cl- (g) ⇒ NaCl (s)

Question 8

Enthalpy of hydration

Answer 8

• Enthalpy change when 1 mole of a gaseous ion is converted to an aqueous ion
• X⁺(g) + aq ⇒ X⁺
• X^- (g) + aq ⇒ X^- (aq)

Question 9

Enthalpy of solution

Answer 9

• Enthalpy change when 1 mole of a solute dissolves completely** in **enough solvent to form a solution which the molecules or ions are far enough apart to not interact with each other
• NaCl (s) + aq ⇒ Na⁺ (aq) + Cl^-(aq)

Question 10

Mean bond dissociation enthalpy

Answer 10

• Mean enthalpy change when 1 mole of covalent bond is broken homolytically, forming 2 gaseous atoms
• This is averaged over a range of compounds

Question 11

Even though G would be more positive, why do we increase the temperature

Answer 11

• To speed up the reaction as Reaction would proceed too slowly
• Make sure the energy of the molecule exceeds that of the Ea

Question 12

Hess’s law

Answer 12

The enthalpy change of a chemical reaction is the same irrespective of the route taken

Question 13

Why is the second electron affinity of ‘S’ endothermic

Answer 13

• The negative S– ion repels the electron being added
• Energy must be supplied to overcome the repulsion

Question 14

Why is the second ionisation energy of Ca bigger than the first?

Answer 14

• Electron being removed from a +ve ion
• The electron is closer** and more strongly **attracted to the nucleus

Question 15

Why is the entropy of anything 0 at 0k

Answer 15

• Molecules are stationary
• No disorder
• entropy is zero at 0 k by definition

Question 16

Why is the entropy change bigger when a liquid turns to gas than ice to water?

Answer 16

The increase in disorder is greater when a gas is formed from a liquid

Question 17

Why is the ionisation energy of Na (top) greater than Cs (bottom) of group 1?

Answer 17

• Na is a smaller atom, so it has less shells so less shielding
• The atomic radius is also smaller
• Stronger attraction between the nucleus and outer electrons so more energy required

Question 18

Feasible

Answer 18

G is less than or equal to 0

Question 19

Why would there be a decrease in entropy?

Answer 19

• Less number of moles
• Fewer gas molecules
• Solid instead of solution
• All of this means there is more order in a system

Question 20

Enthalpy change

Answer 20

• Heat energy change
• Measured at constant pressure

Question 21

Why does the electron affinity of fluorine has a negative value.

Answer 21

• There is an attraction between the nucleus and the added electron
• Energy is released when the electron is gained so exothermic
• F- more stable than F

Question 22

Explain why the theoretical enthalpy of lattice dissociation is different from the experimental value

Answer 22

• Experimental lattice enthalpy value includes covalent interaction
• Theoretical lattice enthalpy value assumes there’s only ionic interaction, evenly distrubuted charges and perfect spheres
• Forces in the actual lattice are stronger** than pure ionic attractions so **more energy is needed to break them

Question 23

The theoretical enthalpy of lattice dissociation for AgCl is +770 but AgF is +995. Explain the difference

Answer 23

• Cl- is bigger than F- so F- has a bigger charge density
• Attraction between Ag+ and F– is stronger

Question 24

Why is the hydration of the chloride ion an exothermic process

Answer 24

• Water is polar, as it has H^δ+
• Cl^- attracts the H in water molecules
• energy is released

Question 25

2 properties of ions that influence the value of a lattice enthalpy calculated using a perfect ionic model.

Answer 25

• Ionic radius as this affects the distance between ions
• Charge density of ions

Question 26

Explain why the ΔG for the dissolving of KCl in water can negative, even though the ΔH is positive.

Answer 26

• The entropy change must be positive meaning there’s an increase in disorder
• Because no of particles increases as 1 mol (solid) → 2 mol (aqueous ions)
• Therefore T∆S > ∆H

Question 27

Why is there a positive correlation between temperature and entropy?

Answer 27

• As T increases, there is an increased kinetic energy
• particles start vibrate more
• Disorder and randomness increases

Question 28

Why is the bond enthalpy for Cl bigger than Br?

Answer 28

• In Cl₂ the bonding pair closer to nucleus and there’s less shielding due to less electron shells
• So attraction between nucleus and bond pair is stronger

Question 29

Why is the bond enthalpy of ClF different to that of ClF3

Answer 29

Cl–F bond in ClF is different from that in ClF3

Question 30

Explain why the lattice dissociation enthalpy of MgCl₂ is greater than that of CaCl₂.

Answer 30

• The Mg²⁺ is smaller so it has a greater charge density
• The attraction between the Cl- is stronger
• Stronger ionic bonds are formed

Question 31

Explain why the lattice dissociation enthalpy of MgO is greater than that of MgCl₂

Answer 31

• The O^2- has a greater charge so the charge density is greater than the Cl-
• So it attracts the Mg²⁺ more strongly

Question 32

why can’t we obtain the enthalpy of solution of magnesium oxide by experiment

Answer 32

Allow MgO does not dissolve in water as it is insoluble

Question 34

Why does N₂ have a higher entropy than C?

Answer 34

• N₂ is a gas whereas C is a solid
• N₂ has a greater kinetic energy so there’s more vibration
• There’s more disorder

Question 35

Why is turning ice into water vapour endothermic

Answer 35

• Hydrogen bonds between water molecules
• Energy needed to break them

Question 36

Why does pure ice look pale blue when illuminated by white light.

Answer 36

• Complementary colour to blue light in visible spectrum absorbed by ice
• Blue light transmitted and observed

Question 37

Why is the hydration enthalpy of the fluoride ion more negative than that of the chloride ion.

Answer 37

• Fluoride ions smaller than chloride so greater charge density
• Water is polar, it has Hδ+
• This means negative charge attracts Hδ+ in water more strongly
• More energy is released (exothermic)

Question 39

Why does Mg²⁺ attract water during hydration

Answer 39

• H₂O is polar as O is delta -ve
• Mg²⁺ attracts the lone pair on O