lectrochemistry: Understanding Half-reactions

It Takes Two:

The redox reaction of metallic zinc atoms with aqueous nickel ions is described by the sum of the two half-reactions. These half-reactions occur simultaneously. Let's stepwise add the two half-reactions together to form the complete picture:

Click on each step; Clicking the mouse will reset figures

Figure 2.

Though the oxidation half-reaction was written first in Figure 2, this is not necessary; what is necessary is that the half-reactions be separately balanced, both by mass and charge, before the final addition. As noted previously, charge balance occurs after mass balance and only uses electrons.

On a piece of paper, balance and then add the following half-reactions to yield an overall redox equation. When finished, click on the green checks to reveal the answer for each step. With a chemical equation, you have just described the highly exothermic reaction of potassium metal with water!

Using the half-reaction method for solving redox equations

Equations for redox reactions can be very complex and difficult to balance by inspection. The half-reaction method for balancing redox equations provides a systematic approach. In this method, the overall reaction is broken down into its half-reactions. Each half-reaction is then balanced individually, and then the half-reactions are added back together to form a new, balanced redox equation. Let's dissect an equation!

Chlorine, Cl₂, is a common oxidizing agent. When iron(II) ions are allowed to react with chlorine, a spontaneous transfer of electrons takes place. Here is the unbalanced chemical equation for this reaction (the phases of the substances have be omitted for clarity):

Fe²⁺  +  Cl₂   Fe³⁺  +  Cl^–

Balancing the overall equation by inspection may not be the most efficient way in this case. It would be relatively simple for us to balance the chlorine and iron masses, but what about the charge balance? The easiest way to solve this equation is to break it down into its half-reactions. As we follow along in Figure 3., we should write each step on a piece of paper before checking the solution.

Click on each step; clicking the mouse will reset the figure.

Figure 3.

Before moving to the debriefing you should know how to write and balance an overall redox reacton using its separate half- reactions.

Understanding Half-reactions