1 THERMODYNAMICS The enthalpy change when an electron is removed from a full electron shell will always be much higher than the enthalpy change when an electron is removed from a partially full electron shell. Evidence from successive ionisation enthalpies can therefore be used to identify the group of the Periodic Table that an element is in.
two delocalised electrons in the metallic lattice of magnesium more strongly. Aluminium has an even greater ionic charge (Al3+) than magnesium, so its enthalpy of atomisation is even greater. Potassium has larger atoms than sodium and has more inner electron shells to screen the nucleus from the delocalised electrons. So the force of attraction is weaker and the enthalpy of atomisation is less than that for sodium.
QUESTIONS 4. a. Why is there such a difference in Ei (ionisation energy) between Mg2+ and Mg3+? b. Explain why removing an electron from an atom is an endothermic process. 5. The successive ionisation enthalpies in kJ mol−1 for elements X, Y and Z are:
X: 801, 2427, 3660, 25 026, 32 828
Y: 1086, 2353, 4621, 6223, 37 832, 47 278
Z: 1314, 3388, 5301, 7469, 10 989, 13 327, 71 337, 84 080.
Giving your reasons, say which group of the Periodic Table each of the three elements is in.
Enthalpy of atomisation The standard enthalpy of atomisation is the enthalpy change when one mole of gaseous atoms is formed from an element in its standard state. Atomisation is always an endothermic process (ΔHϴ is positive) since chemical bonds are broken. The symbol ΔatHϴ is used. For example, Na(s) → Na(g)
ΔatHϴ = +107 kJ mol−1
1 Cl (g) 2 2
ΔatHϴ = +121 kJ mol−1
→ Cl(g)
The enthalpy of atomisation is related to the forces of attraction between atoms. In metals these forces are metallic bonds (electrostatic attraction of metal ions for delocalised electrons). In nonmetals they are covalent bonds (mutual electrostatic attraction of two nuclei of shared electrons). Magnesium has a greater enthalpy of atomisation than sodium (Table 2) because the greater ionic charge (Mg2+ compared with Na+) attracts the
Element
∆atH p/ kJ mol−1
Element
∆atH p/ kJ mol−1
lithium
+159
chlorine
+121
sodium
+107
bromine
+112
iodine
+107
potassium
+89
magnesium
+149
oxygen
+249
aluminium
+326
nitrogen
+472
Table 2 Enthalpies of atomisation
The enthalpies of atomisation for the halogens are quite similar because they all have single covalent bonds that are broken. The values decrease as the halogen atoms get larger, since the forces holding the atoms together are weaker. The values for oxygen and nitrogen are larger than for any of the halogens because atomising oxygen involves breaking a double bond and for nitrogen a triple bond.
QUESTIONS 6. Why is atomisation an endothermic process? 7. Explain why potassium has a higher ΔatHϴ than rubidium.
Bond enthalpies of elements The bond enthalpy is the enthalpy change when one mole of covalent bonds in a gaseous element is broken. It is often called the bond energy. These are not the same, but for our purposes they are interchangeable and you will see both terms being used. Energy is transferred from the surroundings to the gaseous element when covalent bonds are broken and, therefore, all bond enthalpies are endothermic.
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