Introductory Chemistry

Quiz: Redox reactions.

Cracolice 2/e Ch 19.

Answer key
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Quiz is "closed book" except for PT & calculator (which is not really needed here).

In each question here, a redox (oxidation-reduction) reaction is described. The general instruction is to write a balanced equation for the reaction. More specific instructions may be given with the questions.

Some parts specify that you should use a particular method. In the real world, you can choose whichever method you find most suitable. However, in order to make that choice, you need to learn the various methods.

I have written the equations with the smallest possible set of integers as coefficients. Any multiple of that is also balanced, though some people prefer "smallest integers".

You should show the phase for each chemical. I consider this a routine part of learning chemistry.


1. Consider the oxidation of iron metal by molecular oxygen, to form iron(III) oxide. The goal here is to write an equation for this reaction, and balance it. For practice, it would be good to try two different methods for balancing. One of them may be "overkill" for this case, but it is good practice to learn the method.

a. Write and balance an equation for this reaction, "by inspection". That is, use ordinary balancing methods, without taking redox into account.

b. Write and balance an equation for this reaction, using half cells.

2. Consider the decomposition of hydrogen peroxide, H2O2, to produce water and molecular oxygen. Write a balanced chemical equation for this reaction. As with #1, try to do this by two methods. However, if you have trouble with part a, go on to part b.

a. Write and balance an equation for this reaction, "by inspection". That is, use ordinary balancing methods, without taking redox into account.

b. Write and balance an equation for this reaction, using half cells. Note that the peroxide has an unusual oxidation state for the O.

3. Ferrous ion (Fe2+) can be oxidized to ferric ion (Fe3+) by dichromate (Cr2O72-). The Cr becomes Cr3+. The reaction occurs in acidic aqueous solution. The reaction is useful for measuring Fe2+ because of the distinct color change, from orange to green, of the Cr. Write a balanced equation for this reaction, using the half-cell method.


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Last update: October 30, 2019