Identifying the Elements Oxidized and Reduced

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Oxidation-reduction reactions (redox reactions) are reactions in which electrons are lost by an atom or ion in one reactant and gained by an atom or ion in another reactant. Although electrons are gained and lost in these reactions, the balanced equation for a redox reaction does not show the electrons that are being transferred. In order to tell whether a redox reaction has occurred or not, we need a way to keep track of electrons. The best way to do so is by assigning oxidation numbers to the atoms or ions involved in a chemical reaction.

Now that you know how to assign oxidation numbers to the atoms or ions present in a particular substance, how do you use those oxidation numbers to determine whether a redox reaction has occurred? How do you know which element has been oxidized and which one has been reduced?

When oxidation occurs, an element loses electrons and its oxidation number increases (becomes more positive). When reduction occurs, an element gains electrons and its oxidation number decreases or is reduced (becomes more negative).

To determine if a redox reaction has occurred and to identify the element oxidized and the element reduced:

  1. Assign oxidation numbers to all atoms in the equation. (NOTE: Ignore the coefficients in the equation. They are not important when determining the oxidation numbers.)
  2. Compare oxidation numbers from the reactant side to the product side of the equation.
    • If a redox reaction has occurred, you will find that the oxidation numbers of two (no more/no less) elements has changed from the reactant side to the product side.
  3. The element oxidized is the one whose oxidation number increased.
  4. The element reduced is the one whose oxidation number decreased.

Example: Determine if a redox reaction has occurred. If so, identify the element oxidized and the element reduced.

I2O5 (s) + 5 CO (g) --> I2 (s) + 5 CO2 (g)

Solution:

Assign oxidation numbers to each element:

I2O5 (s)
+
5 CO (g)
-->
I2 (s)
+
5 CO2 (g)
+5
-2
+2
-2
0
+4
-2

Compare oxidation numbers from side to side:

  • The oxidation number of I decreased from +5 in I2O5 to 0 in I2.
  • The oxidation number of C increased from +2 in CO to +4 in CO2.
    • Since the oxidation numbers of two elements changed from side to side, a redox reaction DID occur.

Since the oxidation number of I decreased from +5 to 0, I was reduced.

Since the oxidation number of C increased from +2 to +4, C was oxidized.

 

Chem 1115

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Practice Problems

For each of the following equations, determine if a redox reaction occurred. If so, identify the element oxidized and the element reduced.

  1. PBr3 (l) + 3 H2O (l) --> H3PO3 (aq) + HBr (aq)

  2. 3 SO2 (g) + 2 HNO3 (aq) + 2 H2O (l) --> 3 H2SO4 (aq) + 2 NO (g)

  3. H2SO4 (aq) + 2 KOH (aq) --> K2SO4 (aq) + 2 H2O (l)

 

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