Sofia A. answered • 07/27/22
AP Chemistry Tutor with a PhD and College Teaching Experience
Let us start by balancing the reaction
ClO3- + La + H+ + H2O ⇄ Cl2 + La(OH)3
Notice that this is a redox reaction
The oxidation state of La changes from 0 to +3, while the oxidation state of Cl changes from +5 to 0
Hence putting 5 in front of La and 3 in front of ClO3- is a good starting point
3 ClO3- + 5 La + H+ + H2O ⇄ Cl2 + La(OH)3
However looking at the right side we notice that the total number of Cl atoms has to be even, so instead of 3 and 5 we should have used 6 and 10.
6 ClO3- + 10 La + H+ + H2O ⇄ Cl2 + La(OH)3
Now let us balance Cl and La on both sides of the equation
6 ClO3- + 10 La + H+ + H2O ⇄ 3 Cl2 + 10 La(OH)3
Now we can balance oxygen (we currently have 19 on the left and 30 on the right) by adding 11 more water molecules.
6 ClO3- + 10 La + H+ + 12 H2O ⇄ 3 Cl2 + 10 La(OH)3
Finally we need to balance hydrogen (we currently have 25 on the left and 30 on the right) by adding 5 more H+ ions.
6 ClO3- + 10 La + 6 H+ + 12 H2O ⇄ 3 Cl2 + 10 La(OH)3
Finally the reaction is fully balanced, so the answer is 12
