Question

Consider the KF molecule, which has an ionic bond. The bond length is 2.17 x 1010 m (a) Calculate the energy required to dissociate the KF molecule into the ions K* and F |(b) The energy required to dissociate KF into neutral atoms is 498 kJ/mol. Given that the first ionization energy for K is 418 kJ/mol, calculate the electron affinity (in kJ/mol) for F. Show your work for all calculations 2. (a) +6.40 x 10 kJ mol1 or +1.06 x 1018 J (b) +276 kJ mol

Answer 1

a) +640 kJ/mol or +1.06x10⁻¹⁸ J

b) +276 kJ/mol

Step-by-step explanation:

To dissociate the molecule, the bond must be broken, thus, it's necessary energy equal to the energy of the bond, which can be calculated by:

E = (Q1*Q2)/(4*π*ε*r)

Where Q is the charge of the ions, ε is a constant (8.854x10⁻¹²C²J ⁻¹ m⁻¹), and r is the bond length. Each one of the ions has a charge equal to 1. The elementary charge is 1.602x10⁻¹⁹C, which will be the charge of them.

1 mol has 6.022x10²³ molecules (Avogadros' number), so the energy of 1 mol is the energy of 1 molecule multiplied by it:

E = 6.022x10²³ *(1.602x10⁻¹⁹)²/(4π*8.854x10⁻¹²*2.17x10⁻¹⁰)

E = +640113 J/mol

E = +640 kJ/mol

Or at 1 molecule: E =640/6.022x10²³ = +1.06x10⁻²¹ kJ = +1.06x10⁻¹⁸ J

b) The energy variation to dissociate the molecule at its neutral atoms is the energy of dissociation less the difference of the ionization energy of K and the electron affinity of F (EA):

498 = 640 - (418 - EA)

640 -418 + EA = 498

222 + EA = 498

EA = +276 kJ/mol