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Aqueous Reactions Study Guide (page 3)

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Updated on Sep 24, 2011

Example 1:

Assign the oxidation state for each element in the following molecules: C2H6, KMnO4, and SO42–.

C2H6: H is +1 (covalent compound), so 6(+1) + 2C = 0 (compounds are 0), solving for C = +3 KMnO4: K is +1 (monatomic ion) and oxygen is –2, so (+1) + Mn + 4(–2) = 0 (compounds are 0), solving for Mn = +7 SO42–: O is –2, so S + 4(–2) = –2 (polyatomic ion = charge), solving for S = +6

An oxidation-reduction reaction, or commonly called a redox reaction, is characterized by a transfer of electrons. This process is significantly different than the previous two aqueous reactions. In a redox reaction, an electron transfer between the oxidizing agent (oxidizes another by accepting its electrons) and the reducing agent (reduces another by donating electrons) takes place. Oxidation corresponds to a loss of electrons, and reduction corresponds to a gain of electrons. Sound contradictory? Let us explain using the combustion of methane gas: CH4(g) + O2(g) → H2O(g) + CO2(g). First, the oxidation states must be assigned:

 

oxidation states: (for each atom)

Next, the substances oxidized or reduced can be identified:

  • The species being oxidized is carbon. Carbon is losing electrons, thus increasing in charge.
  • The species being reduced is oxygen. Oxygen is gaining electrons, thus decreasing in charge.
  • The oxidizing agent is O2. The reactant oxygen gas is the oxidizing agent because it contains the species being reduced.
  • The reducing agent is CH4. The reactant methane gas is the reducing agent because it contains the species being oxidized.

The reducing agent helps another compound to be reduced, hence being oxidized itself. Likewise, the oxidizing agent helps another compound to be oxidized. Your travel agent helps you travel, but he or she does not travel him- or herself.

Example 2:

Identify the species being oxidized, the species being reduced, the oxidizing agent, and the reducing agent in the oxidation of methanol (CH4O) to formaldehyde (CH2O) with chromic acid:

C2H6O + H2CrO4 → C2H4O + CrO2 Assign the oxidation states:

oxidation states: (for each atom)

Next, the substances oxidized or reduced can be identified:

  • The species being reduced is chromium. Cr is going from a +6 down to a +4 oxidation state.
  • The species being oxidized is carbon. C is going from a –2 up to a 0 oxidation state.
  • The oxidizing agent is chromic acid (H2CrO4), which contains Cr, the species being reduced.
  • The reducing agent is methanol (CH4O). Methanol contains C, the species being oxidized.

Note

The redox reaction does not need to be balanced.

OIL RIG: Oxidation involves loss; reduction involves gain.

Practice problems for these concepts can be found at - Aqueous Reactions Practice Questions

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