The heat of formation of CaCI2 is -795 kJ/mol. The energy needed to change one m

Question

The heat of formation of CaCI2 is -795 kJ/mol. The energy needed to change one mole of Ca(s) into one mole of Ca(g) is +192 kJ/mol. The first and second ionization energies of Ca are +590 kJ/mol and +1146 kJ/mol, respectively. The bond energy of Cl_2 is 238 kJ per mole of CI-CI bonds. The electron affinity of CI is -348 kJ/mol. Set up a Born-Haber cycle and calculate the lattice energy (in kJ/mol) of CaCI_2 in kJ/mol. Six steps are needed in the Born-Haber cycle to calculate the lattice energy of CaCI_2. Each of the steps is given below. Set up the Born-Haber cycle by ordering the steps from 1 to 6 in the order they would occur in the cycle. Then assign the energy change (with the appropriate sign) to each step making sure to simplify your values. Lastly, determine the lattice energy in kJ/mol.

Explanation / Answer

steps in born haber cycle for CaCl2                   Energy involved

step 1 : Ca(s) + Cl2(g) ---> Ca(s) + 2Cl(g)              238 kJ/mol

step 2 : Ca(s) + Cl2(g) ---> Ca(g) + 2Cl(g)              192 kJ/mol

step 3 : Ca(s) + 2Cl(g) ---> Ca+(g) + 2Cl(g)            590 kJ/mol

step 4 : Ca+(g) + 2Cl(g) --> Ca2+(g) + 2Cl(g)       1146 kJ/mol

step 5 : Ca2+(g) + 2Cl(g) --> Ca2+(g) + 2Cl-(g)     -696 kJ/mol

step 6 : Ca2+(g) + 2Cl-(g) --> CaCl2(s)                  -795 kJ/mol

The lattice energy of CaCl2 is = -795 - 238 - 192 - 590 - 1146 + 696 = -2265 kJ/mol

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