Did collegeboard made an error in this question?

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<p><a href="http://www.collegeboard.com/prod_downloads/ap/students/chemistry/ap06_sg_chemistry_form_b.pdf%5B/url%5D"&gt;http://www.collegeboard.com/prod_downloads/ap/students/chemistry/ap06_sg_chemistry_form_b.pdf&lt;/a&gt;&lt;/p>

<p>Go to question 2a, ii) It asked for E, but why don't they switch the signs for 1.66V? Shouldn't it be -0.14 - 1.66V? I know that means it is not spontaneous, but why??</p>

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<p>The equation is meant to use a negative sign, so you do not change any signs. Use the reduction potentials as they are given.</p>

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<p>But when you swtich the reaction, shouldn’t it be negative??</p>

4

<p>It is Al 3+/Al = -1.66 V. It is flipped so then it would be Al/Al 3+ = +1.66 V. Then this would be added to the -.14V (Sn 2+/Sn). Thus, the standard potential is 1.52 V.</p>