Enthalpies

Question 1

What is Hess’ law of constant heat summation?

Answer 1

It states that enthalpy changes are independent of the reaction pathway - as enthalpy is a state function.

Reactants and products must have same conditions

Question 2

What is the enthalpy change of formation?

Answer 2

It’s the enthalpy change that accompanies the formation of 1 mol of compound in its standard state from its constituent elements in their standard states.

Question 3

What’s the enthalpy change of formation of an element?

Answer 3

0 KJ/mol

Question 4

How do you calculate the enthalpy change of reaction?

Answer 4

∆Enthalpy = sum of products - sum of reactants

(Sum of enthalpy change)

Question 5

Give an example of the enthalpy change of vaporisation

Answer 5

H2O(l) —> H2O(g)

Question 6

Give an example of the enthalpy change of sublimation

Answer 6

H2O(s) —> H2O(g)

Question 7

Give an example of the enthalpy change of bond dissociation

Answer 7

Cl2(g) —> 2Cl(g)

Question 8

Give an example of the enthalpy change of atomisation

Answer 8

Na(s) —> Na(g)

Question 9

Give an example of the enthalpy change of ionisation

Answer 9

Na(g) —> Na+(g) + e-

Question 10

Give an example of the enthalpy change of hydration

Answer 10

Na+(g) —> Na+(aq)

Question 11

Give an example of the enthalpy change of lattice dissociation

Answer 11

NaCl(s) —> Na+(g) + Cl-(g)

Question 12

Define lattice dissociation enthalpy

Answer 12

Is the enthalpy change that accompanies the dissociation of an ionic solid into gaseous ions

AB(s) —> A+(g) + B-(g)

Question 13

Is lattice dissociation enthalpy exothermic or endothermic?

Answer 13

Lattice dissociation enthalpy is ALWAYS endothermic
- and hence always positive

Question 14

What’s the reverse of lattice dissociation?

Answer 14

Lattice formation - hence always exothermic

Question 15

How do you find the value of lattice dissociation enthalpy?

Answer 15

Construct a Hess cycle that separates ionic solid into its elements

Assuming ∆Enthalpy are known, the sum will be equal to the lattice dissociation enthalpy.

Question 16

What is d0 in the Coulomb energy (Vc) equation?

Answer 16

d0 = equilibrium bond length

Question 17

What is d* in the Coulomb energy (Vc) equation?

Answer 17

d* = 34.5pm

(1pm = 1 x 10^12)

Question 18

Why does NaCl form an ionic solid and not a diatomic molecule?

Answer 18

Because ionic bonding is non-directional - opposite ions attract and surround each other as much as possible, the inverse is the same for same charge ions

This gives a crystalline compound with a regular periodic structure

Question 19

What is the Madelung constant and what does it mean?

Answer 19

The Madelung constant A, expresses the relative stability of an ionic lattice of a given structure type to that of a single ion pair

Question 20

What is the relative stability of a single ion pair AB in terms of the Madelung constant?

Answer 20

1

Question 21

What is the equilibrium distance equal to the sum of?

Answer 21

d0 = the sum of ionic radii

d0 = rA + rB

Question 22

What’s the benefit of using the Born-Mayer equation instead of Born-Haber cycles to find enthalpies?

Answer 22

The Born-Mayer equation gives a more accurate result since it is based on a simple ionic model that uses tabulated radii

Born-Haber cycles use enthalpy changes which are prone to experimental errors - less accurate

Question 23

What do larger/smaller ions result in?

Answer 23

Larger ions result in lower enthalpy

Smaller ions result in higher enthalpy

Question 24

What also increases the energy released when the ionic structure is broken?

Answer 24

A greater difference in electronegativity = a greater energy released

(Greater energy required to separate the strongly bound ions)

Question 25

What lattice enthalpy relationship can be used to drive chemical reactions?

Answer 25

The inverse relationship between lattice enthalpy and the sum of the ionic radii means that combinations of small ions have disproportionally higher lattice enthalpies

Question 26

Why does the thermal stability of carbonates increase down group 2?

Answer 26

Because the metal oxides corresponding with the metal carbonates (MgCO3 —> MgO) are much are stable due to the ions being much smaller

Thermal stability of carbonates increases because the inverse is true as well - less stable oxide = more stable carbonate