Question

How do you write and balance the equation for the complete combustion of ethanol?

Answer 1

Word equation : Ethanol +Oxygen gas - Carbon dioxide gas + water vapour

unbalanced chemical equation : C2H5OH(l) + O2(g) → CO2(g) + H2O(g)
No. C atoms : 2 ≠ 1 C atoms NOT balanced
Need to multiply the number of carbon dioxide molecules by 2 to balance the carbon atoms.
C2H5OH(l) + O2(g) → 2CO2(g) + H2O(g)
No. C atoms : 2 = 2 C atoms balanced
Are hydrogen atoms balanced
No. H atoms : 5+1 = 6 ≠ 2 H atoms NOT balanced
Need to multiply the number of water molecules by 3 to balance the hydrogen atoms.
C2H5OH(l) + O2(g) → 2CO2(g) + 3H2O(g)
No. H atoms : 5+1 = 6 = 3x2 = 6 H atoms balanced
Are oxygen atoms balanced
C2H5OH(l) + O2(g) → 2CO2(g) + 3H2O(g)
No. O atoms : 1 + 2 =3 ≠ 2x2 + 3x1 = 7 O atoms NOT balanced
Need to multiply the number of oxygen molecules (O2) by 3 to balance the oxygen atoms.
C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g)
No. O atoms :1+3×2=7 =2×2+3=7 O atoms balanced

Hence

The balanced chemical equation for the complete combustion of ethanol is:

C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g)