Transcript half-reactions - University of South Carolina

Chapt. 18
Electrochemistry
Sec. 2
Balancing Redox Reactions
© University of South Carolina Board of Trustees
Balancing Redox Reactions
Copper dissolves in nitric acid to form Cu2+.
NO(g) is released at the same time. Find the
balanced chemical equation.
aCu(s) + bNO3−(aq) + cH+(aq)  dCu2+(aq) + eNO(g) + fH2O(l)
© University of South Carolina Board of Trustees
Balancing Redox Reactions
Acid Solution
1. Assign oxidation states.
2. Identify reduction and oxidation
half-reactions.
aCu(s) + bNO3−(aq) + cH+(aq)  dCu2+(aq) + eNO(g) + fH2O(l)
© University of South Carolina Board of Trustees
Half-Reactions
2a. Write the reduction and oxidation
portions of the reaction separately.
2b. Use e− to balance oxidation states in
each half-reaction.
● e− will cancel out in full reaction
© University of South Carolina Board of Trustees
Balancing Redox Reactions
Acid Solution
1. Assign oxidation states.
2. Identify reduction and oxidation
half-reactions.
3. Balance the electrons, then balance atoms
except H and O.
4. Combine half-reactions so no free e− appear.
5. Add H2O to balance the O.
6. Add H+ to balance H.
7. Double check charge balance.
© University of South Carolina Board of Trustees
Student Example
Balance the equation (acid solution)
Cr2O72− + C2H5OH  Cr3+ + CO2
© University of South Carolina Board of Trustees
Balancing Redox Reactions
Basic Solution
Balance the equation
Cu(s) + NO3-(aq) Cu2+(aq) + NO(g)
if the reaction occurs in basic solution
(H2O and OH− readily available)
© University of South Carolina Board of Trustees
Balancing Redox Reactions
Basic Solution
Balance the equation
Cu(s) + NO3-(aq) Cu2+(aq) + NO(g)
if the reaction occurs in basic solution
(H2O and OH− readily available)
1. Solve the problem in acidic solution.
© University of South Carolina Board of Trustees
Balancing Redox Reactions
Basic Solution
Balance the equation
Cu(s) + NO3-(aq) Cu2+(aq) + NO(g)
if the reaction occurs in basic solution
(H2O and OH− readily available)
1. Solve the problem in acidic solution.
2. Add OH- to each side to eliminate H+.
© University of South Carolina Board of Trustees
Balancing Redox Reactions
Basic Solution
Balance the equation
Cu(s) + NO3-(aq) Cu2+(aq) + NO(g)
if the reaction occurs in basic solution
(H2O and OH− readily available)
1. Solve the problem in acidic solution.
2. Add OH− to each side to eliminate H+.
3. Cancel extra H2O formed.
© University of South Carolina Board of Trustees
Student Example
Balance the reaction
Zn + ClO−  Zn(OH)42− + Cl−
in basic solution.
© University of South Carolina Board of Trustees
Final Example
When O2 and H2 are present in reaction they
are involved in the redox chemistry
Ag + CN- + O2 Ag(CN)2In this case you must add H2O to the reaction.
Ag + CN- + O2 Ag(CN)2- + 2H2O
Similar with H2
H2 + Pt+ Pt
Must add H+ to reaction to be able to solve
© University of South Carolina Board of Trustees
H2 + Pt+ Pt + 2H+