Ionic bonding & s - block chemistry

Question 1

What are Ionic solids?

Answer 1

Ionic solids are held together by electrostatic interactions between the cations and anions.

Question 2

In the ionic model, what are the ions are assumed to be?

Answer 2

In the ionic model, the ions are assumed to be hard spheres with fixed sizes.

Question 3

How can we measure the distance between the centres of two ions in an ionic solid?

Answer 3

X-ray crystallography

Question 4

What are the trends in ionic radii?

Answer 4

Anions are generally bigger than cations.
Ionic radii increase down a group with increasing principal quantum number.
Cations get smaller with increasing charge.
Anions get bigger with increasing charge

Question 5

What is the enthalpy change of formation?

Answer 5

It is the change in enthalpy when one mole of the product is formed in a reaction between the elements in their standard states

Question 6

What is the enthalpy change of atomisation?

Answer 6

the enthalpy change when 1 mol of an element in its standard state is atomised to produce 1 mol of gaseous atoms.

Question 7

What is the first ionisation enthalpy?

Answer 7

the energy required to remove one mole of the most loosely held electrons from one mole of gaseous atoms to produce 1 mole of gaseous ions each with a charge of 1+.

Question 8

What is the electron gain enthalpy?

Answer 8

the energy released when a neutral isolated gaseous atom accepts an extra electron to form the gaseous negative Ion i.e. anion.

Question 9

What is Lattice enthalpy?

Answer 9

the lattice enthalpy is the enthalpy change for the conversion of one mole of the ionic solid into the gaseous ions

Question 10

Are lattice enthalpies always positive or negative?

Answer 10

positive.

Question 11

What is lattice energy?

Answer 11

The lattice energy is the difference in potential energy between the ions in the solid lattice and the ions widely separated as a gas.

Question 12

Equation describing the change in internal energy (at 0 K) when two ions, of charges +z+ and −z−, are brought from an infinite distance to a distance r

Answer 12

ΔU = -(Z+ x Z- x e^2) / (4 x π x ε0 x r)

ΔU  = internal energy 
Z+ = charge of positive ion
Z- = charge of negative ion
e = charge of the electron (1.6022 x 10^-19 C)
ε0 = permittivity of a vacuum (8.8542 x 10^-12 C^2 J^-1 m^-1)

Question 13

Why do we use the Madelung constant, A?

Answer 13

To account for all of the many millions of interactions between pairs of ions.

Question 14

What is the new equation involving the Madelung constant and Avogadro’s constant?

Answer 14

ΔU = (A x Na x Z+ x Z- x e^2) / (4 x π x ε0 x r)

Question 15

What is the Born-Landé equation?

Answer 15

ΔlattU = ((A x Na x Z+ x Z- x e^2) / (4 x π x ε0 x r)) - (1 - 1/n)

Question 16

How can The Born exponent n can be obtained experimentally?

Answer 16

The Born exponent n can be obtained experimentally from the compressibility of the solid, which relates to how the lattice energy changes with applied pressure.

Question 17

Are large or small anions more polarisable and why?

Answer 17

Large anions are more polarisable than small anions as the outer electron density is further from the nucleus and so less controlled by it.

Question 18

When does the Born–Landé equation under-estimate the lattice energy?

Answer 18

For ionic compounds with a large covalent character, the Born–Landé equation under-estimates the lattice energy, and the actual values obtained from a Born–Haber cycle are considerably higher.

Question 19

How can you use the Born–Landé equation to estimate the extent to which a compound is ionic?

Answer 19

the Born–Landé equation can be used as a way of estimating the extent to which a compound is ionic. The closer the value from the Born–Landé equation is to that from the Born–Haber cycle, the more ionic the compound is.

Question 20

Why was The Kapustinskii equation created? And how does it differ from the Born-Lande equation?

Answer 20

The Born–Landé equation gives good predictions of lattice energy for many ionic compounds.
There is, however, a drawback in using it, as both the Madelung constant and interionic distance, r, require a detailed knowledge of the structure, which may not be available.
Kapustinskii recognised that, when the Madelung constant A was divided by the number of ions in the formula unit of the compound (ν), it gave very similar values for a wide variety of structures.
By using an average value of the Born exponent, n, combining together all of the constants to give a single constant k, and assuming that the interionic distance is the sum of the two ionic radii (r+ + r−), the Born–Landé equation can be simplified.

Question 21

What is The Kapustinskii equation?

Answer 21

ΔlattU = (k x v x Z+ x Z-) / (r+ + r-)

k = a constant
v = number of ions in the formula unit
r+ and r- = ionic radii in pm

Question 22

What is The Kapustinskii constant?

Answer 22

107 900 pm Kj/mol

Question 23

What does the Kapustinskii equation help us to d for polyatomic ions?

Answer 23

the Kapustinskii equation allows the ionic radii for polyatomic ions such as carbonate and sulfate to be estimated from the lattice energies of their compounds.
These values are often called thermochemical radii:

Question 24

What type of character does a larger difference in electronegativities between elements give?

Answer 24

For a binary compound, the greater the difference in electronegativity between the elements, the greater the ionic character of the bond.