Ion Electron Method

Ion Electron Method Ch 20

Write an oxidation and a reduction half reaction. Sn2+→ Sn 4+ Hg 2+ + Cl-1→ Hg2Cl2

Balance each half reaction in terms of atoms. Sn2+→ Sn 4+ 2Hg 2+ + 2Cl-1→ Hg2Cl2

Balance charges on opposite sides of each half-reaction equation by adding electrons to the appropriate side. Sn2+→ Sn 4+ + 2e- 2e- + 2Hg 2+ + 2Cl-1→ Hg2Cl2

The number of electrons lost in the oxidation half reaction must equal the number of electrons gained in the reduction half reaction. • If necessary, multiply each half reaction equation by a stoichiometric coefficient to equalize the number of electrons transferred. Sn 2+→ Sn 4+ + 2e- 2e- + 2Hg 2+ + 2Cl-1→ Hg2Cl2

Add the resulting half-reactions to obtain the balanced net ionic equation. Sn 2+→ Sn 4+ + 2e- 2e- + 2Hg 2+ + 2Cl- → Hg2Cl2 2e- + Sn 2+ + 2Hg 2+ + 2Cl-1→ Sn 4+ + Hg2Cl2 + 2e-

Cancel out any species that is the same on both sides of the reaction. Sn 2+ + 2Hg 2+ + 2Cl-1→ Sn 4+ + Hg2Cl2 Note: Both atoms and charges are balanced.

* In many oxidation-reduction reactions that take place in aqueous solution, water plays an active role. * Any aqueous solution contains the species H20, H+, and OH-. * In acidic solutions the predominant species are H20 and H+; * In basic solutions they are H20 and OH-. * When balancing half reactions that occur in solution, we can use these species to achieve material balance.

If the reaction occurs in acidic solution … Cr2O7 2- + H2S → Cr 3+ + S Write a half reaction for sulfur: H2S → S Balance the H+ : H2S → S + 2H+

Balance the charge by adding electrons: H2S → S + 2H+ + 2e- Now write the chromium half reaction: Cr2O72-→ Cr 3+ Balance the chromium atoms: Cr2O72-→ 2Cr 3+

Balance the oxygens on the left by adding water to the right side of the equation: Cr2O72-→ 2Cr 3+ + H2O Now add H+1 to the left: H+1 + Cr2O72-→ 2Cr 3+ + H2O Balance the H’s and O’s: 14H+1 + Cr2O72-→ 2Cr 3+ + 7H2O

Now add electrons to balance the charge: 14H+1 + Cr2O72-→ 2Cr 3+ + 7H2O There is 14+ and 2- on the left (12+) There is 6+ on the right Therefore, add 6e- to the left to balance the charge. 6e- + 14H+1 + Cr2O72-→ 2Cr 3+ + 7H2O

Add the 2 half reactions together 3 (H2S → S + 2H+ + 2e-) 6e- + 14H+ + Cr2O72-→ 2Cr 3+ + 7H20 3H2S + 14H+ + Cr2O72- + 6e- → 3S + 6H+ + 2Cr 3+ + 7H20 + 6e- Cancel out anything that is the same on both sides: 3H2S + 8H+ + Cr2O72-→ 3S + 2Cr 3+ + 7H20 Note: notice how some of the H+ ions cancel out.

In summary, when balancing half-reactions in acid solution: • (a) To balance a hydrogen atom we add a hydrogen ion, H+, to the other side of the equation. • (b) To balance an oxygen atom we add a water molecule to the side deficient in oxygen and • then two H+ ions to the opposite side to remove the hydrogen imbalance.

If the reaction occurs in basic solution … • Although you can use H2O and OH- directly, the simplest technique is to first balance the reaction as if it occurred in acidic solution, and then perform the "conversion" (described on the next slide) to adjust it to conform to conditions in basic solution.

Balance the Reaction in a Basic Solution Pb → PbO • First we balance it as if it occurred in an acidic solution. H20 + Pb → PbO + 2H+ + 2e- Add water to balance the oxygens, add H+ to balance the H’s then add e- to balance the charge.

The conversion to basic solution follows these three steps: • Step 1 • For each H+ that must be eliminated from the equation, add an OH- to both sides of the equation. • In this example, we have to eliminate 2H+, so we add 2OH- to each side. H20 + Pb + 2OH- → PbO + 2H+ + 2OH- + 2e-

Step 2 • Combine H+ and OH- to form H20. • We have 2H+ and 2OH- on the right, which give 2H20. H20 + Pb + 2OH- → PbO + 2H2O + 2e- 2H+ + 2OH

Step 3 • Cancel any H20 that are the same on both sides. • We can cancel one H20 from each side. • The final balanced half-reaction in basic solution is: • Pb + 2OH- → PbO + H2O + 2e-

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Ion Electron Method