8.5

8.5 formation of Binary ionic compounds

Lattice Energy

• The change in energy that takes place when separated gaseous ions are packed together to form an ionic solid
• Often described as the energy released when an ionic solid forms from its ions
• Ex: M+(g) + X-(g) à MX(s)
• Energy is a state function
• Energy change is exothermic(-)

Determining the heat of formation for a substance

• Ex: Use the following data to estimate ∆HfO for sodium chloride
• Na(s) + ½ Cl2(g) --> NaCl(s)
• Lattice energy -786 kJ/mol
• Electron affinity(Cl) -348 kJ/mol
• Ionization energy (Na) 495 kJ/mol
• Bond energy (Cl2) 239 kJ/mol
• Heat sublimation: ∆Hsub Na 108 kJ/mol
• Na(s) à Na(g) is sublimation 108kJ/mol
• Na(g) --> Na+(g) is ionization 495 kJ/mol
• ½ Cl2(g) --> Cl(g) is breaking a bond and only have half a mole so 239/2 kJ/mol
• Cl(g) --> Cl-(g) is electron affinity -348 kJ/mol
• Na+(g) + Cl-(g) --> NaCl(s) is lattice energy -786 kJ/mol
• Sum the heats and reactions:
• Na(s) + 1/2Cl2(g) --> NaCl(s)      -412kJ/mol

Calculating lattice energy

• Calculate the lattice energy of KBr from the following equations
• K(s) + ½ Br2(g) --> KBr(s)
• ∆H sub (K) 90 kJ/mol
• Bond energy (Br2) 112 kJ/mol
• Ionization energy (K) 419 kJ/mol
• Electron affinity (Br) -325 kJ/mol
• ∆HfO KBr(s) -392 kJ/mol
• Do same thing as last example only this time unknown is lattice energy
• K(s) --> K(g) 90 kJ/mol
• K(g) --> K+(g) 419 kJ/mol
• ½ Br2(g) --> Br(g) 112 kJ/mol
• Br(g) --> Br-(g) -325 kJ/mol
• K+(g) + Br-(g) --> KBr(s)  unknown
• K(s) + 1/2Br2(g) --> KBr(s) -392 kJ/mol
• Answer: -632 kJ/mol

Lattice energy continued

• Lattice energy = k(Q1*Q2/r)
• Where k is a proportionality constant that depends on the structure of the solid and the electron configs of the ions
• Q’s are the charges of the ions
• r is the shortest distance between the centers of the cations and anions
• Note: when you have opposite charges the sign is negative as expected since it is an exothermic process
• Process is more exothermic as the ionic charges increase and as the distance between the ions in the solid decrease.