Redox reactions can involve various substances that sometimes act in combination to promote the oxidation or reduction of other substances.

What if you were given a mixture of reactants and asked to predict the balanced redox reaction? Would you be able to do this?

The following five-step method enables you to predict the most likely redox reaction that will occur in solution.

1. List all entities initially present as they exist in an aqueous environment. Refer to Table 6 on page 575 of the textbook and classify each substance as an oxidizing agent, reducing agent, or both.
   
   
   • Remember to dissociate soluble ionic compounds and list H₂O(l) as one of your entities.
   • Recall that some substances can act as both oxidizing agent (OA) and reducing agent (RA), such as H₂O(l), Fe²⁺(aq), Sn²⁺(aq).
   • Consider that some oxidizing and reducing agents occur in combination. For example, MnO₄^- only acts as an oxidizing agent in acidic conditions (MnO₄^-/H+). Unless you are advised specifically that acid is present, you cannot assume that H+ ions are present!
   • Assume that any substances not present in the table are spectator ions and, therefore, can be ignored.
   
   

2. From the list of potential oxidizing agents that you have generated, identify the strongest oxidizing agent and write its half-reaction from the redox table. Remember that oxidizing agents are listed on the left-hand side of the table with the strongest OA located at the top of the table.

   
   

3. From the list of potential reducing agents that you have listed, identify the strongest reducing agent and write its half-reaction from the redox table. Remember that reducing agents are listed on the right-hand side of the table with the strongest RA located at the bottom of the table.

   
   

4. Balance the number of electrons transferred by multiplying one or both half-reactions by a coefficient.

   
   

5. Add the half-reactions to obtain a net ionic reaction and identify the reaction as spontaneous or non-spontaneous.

   
   

Examples

The following two examples illustrate how the above steps can be used to predict the most likely redox reaction occurring in solution.

1. A strip of zinc is placed in a chromium(III) chloride solution.
   
   

   Species List	\( \mathrm { Zn(s),~Cr^{3+}(aq),~Cl^-(aq),~H_2O(l) } \)
   Oxidizing Agents	\( \mathrm { Cr^{3+}(aq),~H_2O(l) } \) \( \mathrm { SOA= Cr^{3+}(aq) } \)
   Reducing Agents	\( \mathrm { Zn(s), ~Cl^-(aq),~H_2O(l),~Cl^-(aq)~and~H_2O(l) } \) \( \mathrm { SRA= Zn(s) } \)
   Reduction Half-Reaction	\( \mathrm { 2 (Cr^{3+}(aq)+ e^- \rightarrow Cr^{2+}(aq)) } \)
   Oxidation Half-Reaction	\( \mathrm { Zn(s) \rightarrow Zn^{2+}(aq) + 2e^- } \)
   Net Ionic Equation	\( \mathrm { 2 Cr^{3+}(aq) + Zn(s) \rightarrow 2 Cr^{2+}(aq) + Zn^{2+}(aq) } \)
   The reaction is spontaneous
   Empirical Evidence	Solid zinc disintegrates. Green colour of chromium(III) ions fades. Blue colour of chromium(II) ions appears.

   
   
   
2. An acidified tin(II) bromide solution is mixed with a solution of potassium dichromate.
   
   

   Species List	\( \mathrm { Sn^{2+}(aq),~ Br^-(aq),~ H^+(aq),~ K^+(aq),~ Cr_2O_7^{2-}(aq),~ H_2O(l) } \)
   Oxidizing Agents	\( \mathrm { Sn^{2+}(aq),~ H^+(aq),~ K^+(aq), Cr_2O_7^{2-}(aq)~ and~ H+(aq),~ H_2O(l) } \) \( \mathrm { SOA= Cr_2O_7^{2-}(aq) ~ and~ H^+(aq) } \)
   Reducing Agents	\( \mathrm { Sn^{2+}(aq),~ Br^-(aq),~ H_2O(l) } \) \( \mathrm { SRA = Sn^{2+}(aq) } \)
   Reduction Half-Reaction	\( \mathrm { Cr_2O_7^{2-}(aq) + 14 H^+(aq) + 6 e^- \rightarrow 2 Cr^{3+}(aq) + 7 H_2O(l) } \)
   Oxidation Half-Reaction	\( \mathrm { 3 (Sn^{2+}(aq) \rightarrow Sn^{4+}(aq) + 2e^-) } \)
   Net Ionic Equation	\( \mathrm { Cr_2O_7^{2-}(aq) + 14 H^+(aq) + 3 Sn^{2+}(aq) \rightarrow 3 Sn^{4+}(aq) + 2 Cr^{3+}(aq) + 7 H_2O(l) } \)
   The reaction is spontaneous.
   Empirical Evidence	pH increases as H+(aq) is consumed. Orange colour of dichromate ions fades. Green colour of chromium(III) ions appears.

---

 Read from page 575 to the end of "Communication example 1" on page 577 of the textbook. A summary of the five-step method described in this section appears on page 578.

---

Check Your Understanding

Go to the textbook and Complete "Practice" Question 26 on page 579 and Section 13.2 Question 9 on page 582. Consult Table 1 on page 805 of the text for a list of some standard diagnostic tests.