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CH10134B > Ionic solids > Flashcards

Ionic solids Flashcards

1
Q

How are ionic solids held together?

A

Electrostatic interactions between the cations and anions.

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2
Q

How can the distance between the centres of two ions in an ionic solid can be measured?

A

X-ray crystallography.

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3
Q

What are the 4 trends in ionic radii?

A

Anions are generally bigger than cations, the size increases down the group, as the charge increases the size of the cations decreases and the anions get bigger with increasing charge.

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4
Q

What is a Born-Haber cycle?

A

A cycle that allows the enthalpy of formation of an ionic compound to be broken down into discrete steps, as shown for NaCl.

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5
Q

What is the enthalpy change of formation?

A

The change in enthalpy when one mole of a ionic solid is formed in a reaction between the elements in their standard states.

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6
Q

What is the enthalpy change of atomisation?

A

The change in enthalpy when one mole of an a solid is converted into one mole of a gas.

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7
Q

What is first ionisation enthalpy?

A

The change in enthalpy when one mole of a gas is converted into one mole of cations (gaseous).

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8
Q

What is electron gain enthalpy?

A

The change in enthalpy when a mole of gas is converted into a mole of gaseous anions.

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9
Q

What is the enthalpy change with the largest magnitude in the Born-Haber cycle?

A

The formation of the ionic lattice from the gaseous anions. This is MINUS THE LATTICE ENTHALPY - the lattice enthalpy is the change for the conversion of one mole of the ionic solid into gaseous ions.

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10
Q

Why are Born-Haber cycles useful?

A

Lattice enthalpy values can be determined from experimental data, and they can be found without needing to know other thermodynamic data.

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11
Q

What is deltaH? (enthalpy change)

A

Heat change at constant pressure.

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12
Q

What is deltaU? (internal energy change)

A

Heat change at constant volume.

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13
Q

Despite the differences between enthalpy change and internal energy change, what can usually be assumed when calculating lattice enthalpies?

A

That the difference between the two values is very small and can be ignored.

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14
Q

What is the lattice energy?

A

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

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15
Q

What is the Madelung constant?

A

A term that is used to account for all of the interactions between pairs of ions - repulsive and attractive forces in a crystal.

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16
Q

What does the term e0 stand for?

A

The permittivity of a vacuum, 8.8542 x 10^-12 C^2J^-1m^-1.

17
Q

What is the Born exponent?

A

A part of an equation that accounts for short-range forces between ions in a crystal.

18
Q

How can values for the Born exponent be obtained?

A

Experimentally from the compressibility of the solid, which relates to how the lattice energy changes with applied pressure. An approximate value can be predicted from the electronic configuration of the ions.

19
Q

What are atoms/ions called that are easily distorted called?

A

Polarisable.

20
Q

Why are no compounds completely ionic?

A

The positive charge on the cation slightly distorts the electron clouds of the anion which leads to some degree of covalent character.

21
Q

Why are large anions more polarisable than small anions?

A

The outer electron density is further from the nucleus so is less controlled by it.

22
Q

What cations are the most polarising?

A

Small ones with high charges (high charge density)

23
Q

What is the problem with the Born-Lande equation for compounds with a large covalent character and what does this mean the equation can be used for?

A

It under-estimates the lattice energy and means it can be used for estimating the extent to which a compound is ionic.

24
Q

What are the problems with the Born-Lande equation?

A

It requires the Madelung constant and interionic distance to be known - this needs a detailed knowledge of the structure which may not be available.

25
Q

What did Anatoly Kapunstinskii recognise?

A

When the Madelung constant was divided by the number of ions in the formula unit of the compound, it gave similar values for a wide variety of structures.

26
Q

What does the Kapunstinskii equation allow to be calculated?

A

Lattice energies, ionic radii for polyatomic ions to be calculated e.g. carbonates and sulfates. (thermochemical radii)

27
Q

If a compound has components that both have high electronegativities, what is its likely bonding structure?

A

Covalent.

28
Q

If a compound has components that both have low electronegativities, what is its likely bonding structure?

A

Alloy - metallic bonding.

CH10134B (16 decks)

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