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Predict whether the following reactions would occur spontaneously in aqueous solution at $25^{\circ} \mathrm{C}$. Assume that the initial concentrations of dissolved species are all $1.0 \mathrm{M}.$(a) $\mathrm{Ca}(s)+\mathrm{Cd}^{2+}(a q) \longrightarrow \mathrm{Ca}^{2+}(a q)+\mathrm{Cd}(s)$(b) $2 \mathrm{Br}^{-}(a q)+\mathrm{Sn}^{2+}(a q) \longrightarrow \mathrm{Br}_{2}(l)+\mathrm{Sn}(s)$(c) $2 \mathrm{Ag}(s)+\mathrm{Ni}^{2+}(a q) \longrightarrow 2 \mathrm{Ag}^{+}(a q)+\mathrm{Ni}(s)$(d) $\mathrm{Cu}^{+}(a q)+\mathrm{Fe}^{3+}(a q) \longrightarrow$$\mathrm{Cu}^{2+}(a q)+\mathrm{Fe}^{2+}(a q)$

Only (a) and (d) are spontaneous.

Chemistry 102

Chapter 18

Electrochemistry

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University of Toronto

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00:44

In chemistry, electrochemistry is a branch of physical chemistry that studies chemical reactions involving electrical energy, either directly or indirectly. The name derives from the Greek words for amber (electron) and chemistry (chemeia). Electrochemistry deals with the electron transfer reactions that take place at the interface of two electrodes in contact with an electrolyte. The study of these reactions is associated with the study of electrolysis. The process of electrolysis is used to generate a potential difference (voltage) between two electrodes, which is used to drive an electrical current through the electrolytic solution.

03:11

In chemistry, a redox reaction is a chemical reaction in which the oxidation states of the atoms of the participating elements are changed. Any such reaction involves both a reduction process and a complementary oxidation process, two key concepts involved with electron transfer processes. A redox reaction may be written with either an oxidation sign or a reduction sign.

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in each part of this problem were given an overall chemical reaction, and we need to determine whether or not this reaction will proceed spontaneously. We know that in order for a reaction to be spontaneous, that value for it's standard cell potential has to be positive in order for that value to be positive. That means that the species that undergoes reduction has to have 1/2 reaction with a greater value of its standard reduction potential than the species that undergoes oxidation. So we can begin by looking at the reaction part and determining which species undergoes oxidation. Which one undergoes reduction. We see that calcium is oxidized from its solid state to its acquiesced to plus state by losing two electrons. And if we look up the value for the standard reduction potential for the half reaction of see a two plus plus two electrons going to see a solid, then we see that it's standing reduction potential comes out to negative 2.87 volts, and now we can see that CD two plus is reduced to CD solid by adding two electrons, and that half reaction has a standard reduction potential of negative 0.40 volts. And now we see that CD, which is reduced, has a larger or less negative value for its standard reduction potential. Then for the reduction of see a two plus, which means that this reaction will proceed spontaneously and again. This is because when we find the overall cell potential, we take the standard reduction potential of the species that is reduced an added TV reverse of the standard reduction potential of the species that is oxidized. And when we do that, that would come out to a positive value. If her positive values for the cell potential, this reaction would be spontaneous. So that is a reasoning that we used to work through the rest of this problem on part B. We see that BR minus is oxidized to be are too, and that has a standard reduction potential of 1.7 volts and s and two plus is reduced to us and solid, and that has a standard reduction potential of negative 0.14 volts. So now we see that the species that undergoes reduction has a smaller value for its standard reduction potential than the species undergoes oxidation. So That means when we go out to you calculate the overall cell potential, it will end up being negative, meaning that this reaction will be non spontaneous. And now, in part C, we can see that silver is oxidized from its solid state to its acquis plus one state, and that has a standard reduction potential of zero 0.80 volts. And we can also see that on I is reduced from two plus two. It's solid state, and that has a student reduction potential of negative 0.25 volts. So, just like in part B, we see that the standard reduction potential for the reaction that undergoes reduction is smaller than for the species that undergoes oxidation. So that means overall cell potential will end up being negative, making this reaction non spontaneous. And now, in party, we see that we have see you plus being oxidized to see you two plus by losing an electron and that has a standard reduction potential of 0.15 volts. We also see that F E is reduced from three plus 22 plus, and that has a standard reduction potential of 0.77 volts and this time we see that the species that undergoes reduction as a larger value for its standard reduction potential than the species that undergoes oxidation. And so when we flip the sign of this value and added to this value, that will come out to a positive overall value for the standard cell potential, making this reaction spontaneous.

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