Question

Balance in basic solution; H2O2(aq) + Cl2O7(aq) = ClO2-(aq) + O2(g)

Answer 1

H2O2(aq) + Cl2O7(aq) = 2ClO2-(aq) + O2(g)

Step 1: Separate in half reactions.
Step 2: In each half reaction, balance all elements except O & H.
Step 3: Balance O by adding H2O.
Step 4: Balance H by adding H+.
Step 5: Balance charges by adding e-.
Step 6: Multiply all coefficients in 1 or both half reactions by an integer to get the number of e- in the two half reactions equal.
Step 7: Add the 2 half reactions together & cancel out any species that appear on both sides of net reaction.
Step 8: Check that charges and atoms are balanced.

So,
H2O2(aq) ---> O2(g) + 2H+ + 2e-
Cl2O7(aq) + 6H+ + 8e- ---> 2ClO2^-(aq) + 3H2O
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4H2O2(aq) + Cl2O7(aq) ---> 2ClO2^-(aq) + 4O2(g) + 2H+ + 3H2O

Since it is in basic solution, add 2 OH^- to both sides, and consolidate the waters:

4H2O2(aq) + Cl2O7(aq) + 2OH^- ---> 2ClO2^-(aq) + 4O2(g) + 5H2O