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When concentrated hydrochloric acid (HCl) and nitric acid (HNO3) are mixed with gold, oxidation and reduction reactions produce gaseous nitrogen monoxide (NO) and soluble, stable AuCl4– ions. You will learn how to write and balance chemical equations for redox reactions such as this one.
If the oxidation number of an element in a reacting species changes, then that element has undergone either oxidation or reduction. Therefore, the reaction as a whole must be a redox reaction.
Balancing Redox Equations > Two Ways to Balance Redox Equations
In a balanced redox equation, the total increase in oxidation number of the species oxidized must be balanced by the total decrease in the oxidation number of the species reduced.
Balancing Redox Equations > Two Ways to Balance Redox Equations
Using Half-Reactions
• A half-reaction is an equation showing just the oxidation or just the reduction that takes place in a redox reaction.
• In the half-reaction method, you write and balance the oxidation and reduction half-reactions separately before combining them into a balanced redox equation.
Balancing Redox Equations > Two Ways to Balance Redox Equations
To balance a redox reaction using half-reactions, write separate half-reactions for the oxidation and the reduction. After you balance atoms in each half-reaction, balance electrons gained in the reduction with electrons lost in the oxidation.
Balancing Redox Equations > Choosing a Balancing Method
Choosing a Balancing Method
• Balancing by oxidation number change usually works well if the oxidized and reduced species appear only once on each side of the equation.
• Balancing by the half-reaction method works best in redox reactions where the same element is both oxidized and reduced and in reactions that take place in acidic or alkaline solution.
1. Choose the correct words for the spaces. One method used to balance redox reactions compares changes in ______ numbers. When an equation is balanced, the total number of _________ lost must be equal to the total number gained.