Question

For the reaction Si(s) + O2(g) --> SiO2(s):

Write the balanced half cell redox reactions that could be used to measure the Ecell and Delta G. Label the oxidation and reduction half cell reactions

Answer 1

the potentials available

SiO2(s) + 4 H+ + 4 e− ⇌ Si(s) + 2 H2O −0.91

O2(g) + 2 H2O + 4 e− ⇌ 4 OH−(aq) +0.401

add both equations:

invert (1)

Si(s) + 2 H2O ⇌SiO2(s) + 4 H+ + 4 e− E = 0.91 this is going to OXIDIZE

O2(g) + 2 H2O + 4 e− ⇌ 4 OH−(aq) +0.401 This is going to REDUCE

Add all

O2(g) + 2 H2O + 4 e− + Si(s) + 2 H2O ⇌SiO2(s) + 4 H+ + 4 e− + 4 OH−(aq) Etotal = 0.91 + 0.401 = 1.311

cancel common terms

O2(g) + 4H2O + Si(s) ⇌SiO2(s) + 4H2O(l) Etotal = 0.91 + 0.401 = 1.311

cancel water

O2(g) + Si(s) ⇌SiO2(s) Etotal = 1.311

for dG:

dG = -nF*E°cell

we know E|cell = 1.311

dG = -nF*E°cell

n = 4 elecxtrons transfer , F = 96500,

dG = -4*96500 * 1.311 = -506046 J/mol