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STRUCTURE AND BONDING : (Born-Haber Cycles (Describe quantitatively the…
STRUCTURE AND BONDING :
Born-Haber Cycles
Describe quantitatively the energetic factors and characteristics involved in the formation of an ionic bond.
Atomisation (of both cation and anion) The enthalpy change which accompanies the formation of one mole of gaseous atoms from the element under its standard state conditions. Always Δ+.
Ionisation (more than one ionisation energy may be needed depending on the ionic compound formed) Standard enthalpy change when one mole of electrons is lost from a mole of singly charged positive ions. Always Δ+.
Electron Affinity (more than one electron affinity may be needed depending on the ionic compound formed) Standard enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous ions, each with a single negative charge. Always Δ- as it makes the atom more stable
List the properties of ionic crystals, and relate them to the lattice energy.
Conduct Electricity - when in a solid state
Describe the factors that influence the lattice energy.
Whether it is a simple cubic, face-centred cubic or body-centred cubic.
FCC have 6 ions surrounding the centre ion. It has a coordination number of 6.
Explain why lattice energy cannot be measured directly.
They are found indirectly using a Born-Haber cycle to calculate the theoretical values for the bonds which form between oppositel charged ions. Ionic compounds do not usually form directly due to the large endothermic enthalpy of formation. Trying to get the exact volumes of two gaseous ions to form one mole of lattice is impossible to do, and each of the ions will be extremely reactive with anything else, and it makes it virtually impossible to directly measure lattice enthalpy. Separate ions cannot be brought together successfully this way.
Define electron affinity, atomisation and ionization energy.
Electron affinity - Enthalpy change when a mole of electrons is given to a mole of gaseous atoms, each with a single -1 charge to form ions with a two negaative charge.
Atomisation - Energy required to vaporise one mole of atoms from its elements under standard conditions
Ionisation energy - (First) Energy required to remove one mole of electrons from one mole of gaseous atoms under standard conditions.
Why is the formula MgCl 2 and not MgCl?
Each Magnesium ion has a 2+ charge while Cl has a -1 charge. To make the charges equal, two Cl's are needed to make the net chage of the bond 0.
Polarity
Predict whether a compound of two or more elements would be covalent from the position of the elements in their periodic table or from their electronegativity values.
Covalent bonds are usually formed between two nonmetals. They are not usually electronegative however calculating the electronegativity for the compound/molecule may not be reliable for all covalent substances as the dipoles may cancel out.
Predict the relative polarity of bonds based on electronegativity values
Calculate using electronegativity values in the table.
Nonpolar Covalent 0→ 0.4
Polar Covalent 0.5 → 1.9
Ionic 2.0 → 4.0
Describe polar covalent, non-polar covalent, ionic and dative covalent bonds
Dative Covalent - Where both electrons in a covalent bond come from one of the atoms (two electrons from the same atom) such as when ammonia is dissolved in a solution containing hydrogen ions
Polar Covalent - In a molecule such as sodium chloride, there is a difference in attractive forces each atom has. Chlorine attracts more strongly than Sodium and therefore distorting the electron cloud to be more towards the Cl (Cl is electronegative). This distortion creates a distance between the delta positive side and the delta negative side.
Non-Polar Covalent - Where both the atoms in a bond have the same electronegativity, there is therefore no net attraction to either one of the atoms in the bond.
How do we know ions exist?
Solid ionic compounds never conduct electricity, however, do when molten/aqueous. This indicates there is a negative particle which is capable of carrying electric current.
In electron density maps for ionic compounds, there is no single line
representing electron density that surrounds both cations and anions.
What information can be obtained from electron density maps?
They are formed when x rays arepassed through anionic crystal onto photographic film. The elecrons diffract the x rays and produce white lines on the paper, thus proving theexistane of ions. The bigger the ion/atom, the more electrons it has and the brighter the spot produced is.
The charge density can be derived and you can find the amount of electric charge per unit volume (electrons per cubic nanometer). Points of equal density are joined to form a contour-like map.
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