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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?

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    $\begingroup$ 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. $\endgroup$ – Yomen Atassi Apr 18 '16 at 4:22
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You are confused because the answer key is wrong. The way to figure it out is to assign oxidation numbers to each species following the rules. Oxidation states are simply a convenient way to keep track of electrons without having to draw out Lewis structures, count electrons pairs and so on. The rules help make that happen.

Notice the position of the sign relative to when you indicate charge. It is not the same.

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.

Recall OIL RIG: Oxidation is loss of electrons and reduction is gain of electrons. Whatever is oxidized increases its oxidation state, and whatever is reduced decreases it.

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  • $\begingroup$ Thanks, I understand now. I believe our teacher told us redox reactions are usually not double replacement and wanted us to eliminate the rest of the answers as a quicker solution than listing oxidation states for each choice. $\endgroup$ – Evan Apr 18 '16 at 4:46
  • $\begingroup$ Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. I have updated your post with chemistry markup. If you want to know more, please have a look here and here. We prefer to not use MathJax in the title field, see here for details. $\endgroup$ – Martin - マーチン Apr 18 '16 at 5:19
  • $\begingroup$ @Martin-マーチン awesome, thanks seems straight forward will work in getting the hang $\endgroup$ – Scient Apr 18 '16 at 5:55
  • $\begingroup$ @Evan - Most acid-base reactions can be rewritten as double displacements as well, so you should be able to rule them out:$$\ce{HCl + KOH -> KCl + HOH}$$ $\endgroup$ – Ben Norris Apr 18 '16 at 11:01

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