Ionic Bonding and s-Block Chemistry- Approaches to Ionic Bonding Flashcards
What holds together ionic solids
- By electrostatic interactions between cations and anions
2. The ions are assumed to be hard spheres with fixed sizes
How can the distance between the centres of two ions in an ionic solid be meastures
- Using X-ray crystallography
Why is it difficult to determine the values of individual ionic radii from internuclear distance
- It is hard to tell at exactly what point the electron density of the cation stops and that of the anion starts
Describe the trends in ionic radii
- Anions generally bigger than cations
- Ionic radii increase down a group with increasing PQN
- Cations get smaller with increasing charge
- Anions get bigger with increasing charge
What can Born-Haber cycle be used for
- Allows the enthalpy of formation of an ionic compound to be broken down into discrete steps
Define Lattice enthalpy of formation of NaCl
- ΔfH°(NaCl) is the enthalpy change of formation of NaCl
- It is the change in enthalpy when one mole of sodium chloride is formed in a reaction between the elements in their standard states
Define enthalpy change of atomisation of sodium
- ΔaH°(Na) is the enthalpy change of atomisation of sodium
2. It is the change in enthalpy when one mole of sodium metal is converted into one mole of sodium gas
Define the first ionisation enthalpy for sodium
- ΔiH(1)°(Na) is the first ionisation enthalpy for sodium
2. It is the change in enthalpy when one mole of sodium gas is converted into one mole of sodium cations
Define enthalpy change of atomisation of chlorine
- ΔaH°(Cl) is the enthalpy change of atomisation of chlorine
- It is the change in enthalpy when half a mole of Cl2 is converted into one mole of Cl gas.
- ΔaH°(Cl) is half the Cl-Cl bond dissociation enthalpy
Define the electron gain enthalpy of chlorine
- ΔegH°(Cl)
- It is the change in enthalpy when a mole of Cl gas is converted into one mole of Cl- ions
- ΔegH°(Cl) is minus the electrons affinity of chlorine
Define lattice enthalpy
- The enthalpy change for the conversion of one mole of the ionic solid into the gaseous ions
- They are always positive
How can you work out a missing value in a born haber cycle
- Lattice= atomisation + ionisation + atomisation + electron gain - formation
How can a theoretical value for lattice energy be obtained for ionic models
- The energy change when the gaseous ions come together to form an ionic solid comes from the electrostatic interactions between the ions
- By considering all the electrostatic interactions present in an ionic solid, you can obtain a theoretical value for the lattice energy
What is the difference between lattice enthalpy and energy
- Calculations based on the interactions between ions give internal energy changes DU rather than enthalpy changes DH
- DH is the heat change at constant pressure whereas DU is the heat change at constant volume
- DH = DU + pV
- The difference is normally relatively small and can usually be ignored- only lose a bit of accuracy
What does each part of this equation for lattice energy represent DU=(-Z+Z-e^2)/4piE0r and what can this be used for
- A= Madelung constant
- Z+ and Z- are positive integers equal to the charges on the ions
- r= distance between the ions (metres)
- E0= is the permittivity of a vacuum - constant
- Used if only one cation + anion interacting
- Lattice energy is negative of the potential energy from this equation
What is the Madelung constant
- In a crystal there are millions are interactions between pairs of ions.
- Those between cations and anions are attractive but those between ions of the same charge are repulsive
- Madelung constant, A, is introduced to account for these interactions
What is the Born exponent
- Looks at repulsions between overlapping electron clouds in its equation.
- Erep= B/r^n
- n= Born exponent
- B and n are constants
What is the Born Lande eqaution
- Takes into account repulsive energy and adds in avogadros number
- DlattU=(ANA-Z+Z-e^2)/4piE0r *(1-1/n)
- 1-1/n scales down lattice energy and includes repulsive forces
How can you find n the born exponent
- Can be obtained experimentally from the compressibility of the solid which relates to how the lattice energy changes with applied pressure
- Can be predicted from the electronic configurations of ions
- Take an average value of cation and anion
What are key things to remember when using the Born Lande equation
- Around 4sf
2. Need to convert r to metres
How do you calculate the Madelung constant for NaCl
- The nearest neighbours to the central Na+ ion are the 6 chloride ions, which lie at a distance r away.
- The Interactions with the central Na+ ion give an attractive energy DU1=6((-11e^2)/4piE0r= -6 * e^2/4piE0r
- The second nearest neighbours are the twelve sodium ions which lie at a distance root2r away- repulsive energy so positive
- DU2= 12((+11e^2)/4piE02^1/2 *r)
- Carry on
- Total energy DU is given by the sum of all the terms
Can a compound be completely ionic
- No
- e.g. positive charge on the Na+ cations distorts the spherical electron clouds of the Cl- anions, leading to some degree of covalent character
What is the name for atoms or ions that are easily distortable
- Polarisable
How does the size of anions affect their polarisability
- Large anions are more polarisable than small anions
2. The outer electron density is further from the nucleus and so less controlled by it.
How does the size of cations affect their polarisability
- Small cations with high charges (Be2+/Al3+) are strongly polarising and tend to induce distortions in anions- high charge density
Which compounds tend to have the highest degrees of polarisation and what does this mean
- Compounds containing small highly charged cations and large polarisable anions
- Leads to high covalent character
If ionic compounds have a large covalent character what does this mean for the Born-Lande equation value
- Tends to under-estimate the lattice energy
- The actual values are considerably higher
- Closer the value from the Born-lande equation is to that from the Born-Haber cycle, the more ionic the compound is
What is a problem with the Born-Lande equation
- The Madelung constant and interionic distance require a detailed knowledge of structure which may not be available
What can be used instead of the Born-Lande equation
- The Kapustinskii equation
- Uses an average value of the Born exponent,n
- Combines all the constants to give a single constant,l k
- Assume that the interionic distance is the sum of the two ionic radii
What is the meaning of each part of the Kapustinskii equation DlattU= kvz+z-/r+ + r-
- z+ / z- are positive integers equal to the charges on the ions
- V is the number of ions in the formula unit
- K is the constant
- r+ / r- are the ionic radii in pm
What else can the Kapustinskii equation be used for
- Allows the ionic radii for polyatomic ions such as carbonates and sulfates to be estimated from the lattice energies of their compounds
- These values are often called thermochemical radii
How can you predict bond types for binary compounds
- The greater the difference in electronegativity between the elements, the greater the ionic character of the bond
- When the electronegativities of the atoms are both high- compound tends to be covalent
- When the electronegativities of the atoms are both low, the substance will be an alloy with metallic bonding
What is a van Arkel-Ketelaar triangle
- Summarises bond type and electronegativity
- 3 corners represent the extremes of metallic, ionic and covalent bonding
- The average electronegativity of the elements, X, is plotted on the x-axis and the difference in electronegativity is plotted on the y-axis.
