# How to identify a redox reaction?

Which balanced equation represents a redox reaction? \begin{align} &\mathrm{A.} &\ce{AgNO3 + NaCl &-> AgCl + NaNO3} \\ &\mathrm{B.} &\ce{BaCl2 + K2CO3 &-> BaCO3 + 2KCl} \\ &\mathrm{C.} &\ce{CuO + CO &-> Cu + CO2} \\ &\mathrm{D.} &\ce{HCl + KOH &-> KCl + H2O} \end{align}

I don't understand how to identify a redox reaction. The answer key shows the correct answer as D, but I'm confused as to why.

D shows an acid-base reaction. Are all acid-base reactions redox reactions and why aren't the other answer choices correct?

• The correct answer should be C. As you can clearly see the copper ion is reduced, while carbon in carbon monoxide is oxidized into carbon dioxide. Remember redox equations involve electron exchange. – Yomen Atassi Apr 18 '16 at 4:22

The correct answer is C though: $$\ce{CuO + CO -> Cu + CO2}$$ with oxidation states $$\ce{\overset{(+2)}{Cu}\overset{(-2)}{O} + \overset{(+2)}{C}\overset{(-2)}{O} -> \overset{(0)}{Cu} + \overset{(+4)}{C}\overset{(2\times(-2))}{O2}}$$ Notice Cu went from +2 to 0. It gained 2 electrons to do that, thus it was reduced. What gave up the electrons? The other carbon, because it went from +2 to +4: its oxidation state increased by 2, thus it lost 2 electrons.
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• @Evan - Most acid-base reactions can be rewritten as double displacements as well, so you should be able to rule them out:$$\ce{HCl + KOH -> KCl + HOH}$$ – Ben Norris Apr 18 '16 at 11:01