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Oxidation and Reduction Reaction Example Problem

Which Atom is Oxidized or Reduced?

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You can use oxidation states to help determine whether an atom is oxidized or reduced in a reaction.

You can use oxidation states to help determine whether an atom is oxidized or reduced in a reaction.

David Freund, Getty Images

In an oxidation-reduction or redox reaction, it is often confusing to identify which molecule is oxidized in the reaction and which molecule is reduced. This example problem shows how to correctly identify which atoms undergo oxidation or reduction and their corresponding redox agents.

Problem:

For the reaction:

2 AgCl(s) + H2(g) → 2 H+(aq) + 2 Ag(s) + 2 Cl-

identify the atoms that undergo oxidation or reduction and list the oxidizing and reducing agents.

Solution:

The first step is to assign oxidation states to each atom in the reaction.

For review:
Rules for Assigning Oxidation States | Assigning Oxidation States Example Problem
 

  • AgCl:
    Ag has a +1 oxidation state
    Cl has a -1 oxidation state
  • H2 has an oxidation state of zero
  • H+ has a +1 oxidation state
  • Ag has an oxidation state of zero.
  • Cl- has a -1 oxidation state.
The next step is to check what happened to each element in the reaction.
 
  • Ag went from +1 in AgCl(s) to 0 in Ag(s). The silver atom gained an electron.
  • H went from 0 in H2(g) to +1 in H+(aq). The hydrogen atom lost an electron.
  • Cl kept its oxidation state constant at -1 throughout the reaction.
Oxidation involves the loss of electrons and reduction involves the gain of electrons.

For review:
Difference Between Oxidation and Reduction

Silver gained an electron. This means the silver was reduced. Its oxidation state was 'reduced' by one.

To identify the reduction agent, we must identify the source of the electron. The electron was supplied by either the chlorine atom or the hydrogen gas. Chlorine's oxidation state was unchanged throughout the reaction and hydrogen lost an electron. The electron came from the H2 gas, making it the reduction agent.

Hydrogen lost an electron. This means the hydrogen gas was oxidized. Its oxidation state was increased by one.

The oxidation agent is found by finding where the electron went in the reaction. We've already seen how hydrogen gave an electron to silver, so the oxidation agent is the silver chloride.

Answer:

For this reaction, hydrogen gas was oxidized with the oxidizing agent being silver chloride.
Silver was reduced with the reducing agent being H2 gas.

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