Chapter 13 - Further Energetics
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What is standard lattice formation enthalpy?
Where one mole of an ionic solid is formed from its gaseous ions in standard conditions
What factors affect standard lattice enthalpy?
The charges on the ions The radius of the ions
What is the relationship between standard lattice enthalpy and the magnitude of charges on the ions?
The greater the magnitude of charges on the ions, the more negative the value for the lattice enthalpy
What is the relationship between standard lattice enthalpy and the size of the ions?
The smaller the ions (the smaller the inter-ionic distance), the more negative the value for the lattice enthalpy
What is the inter-ionic distance?
The distance between the centre of two ions
What is standard enthalpy change of atomisation?
The enthalpy change when one mole of gaseous atoms are formed from an element in its standard state at standard conditions
Give an example of the atomisation of hydrogen
1/2 H2(g) -> H(g)
What is first electron affinity?
The enthalpy change when each atom in one mole of atoms in the gaseous state gain an electron to form one mole of -1 ions
Give an example of the first electron affinity of oxygen
O(g) + e- -> O-(g)
Is first electron affinity exothermic or endothermic?
Exothermic (negative)
What is the exception to first electron affinity being exothermic?
Noble gases first electron affinity is not exothermic
Is second electron affinity endothermic or exothermic?
Endothermic (positive)
Why is second electron affinity endothermic?
Because the electron is added to an ion which is already negative therefore it must overcome the repulsion
What assumptions are made when finding theoretical lattice energy?
• The ions are in contact with one another • The ions are perfectly spherical • The charge on each ion is evenly distributed around
What causes differences between theoretical and experimental lattice energy?
Covalent character in the bonding of an ionic lattice
What causes covalent character?
The polarisation of the anion by the cation
What does the polarisation of the anion by the cation mean?
There is a distortion in the electron density within the anion, resulting in a higher electron density near the cation i.e. there is some electron density shared between the two ions (covalent character)
Why does the cation polarise the anion?
The positive cation will attract the negative electrons of the anion
What two factors affect the extent of covalent character?
The size of the ions The charge of the ions
What will increase the extent of polarisation?
• High charge and small cation (i.e high charge density) - High charge and large anion
What is the ability of a cation to attract electrons from the anion known as?
Polarising power
What can be calculated to give an approximate value of the polarising power of the cation?
Charge density (charge/r^2)
What will be the effect on the difference between theoretical and experimental lattice energy of a cation with high polarising power?
High polarising power = greater covalent character = greater difference
What factors affect how easily an anion is polarised?
The larger the charge and the larger the size of the anion, the more easily it is polarised