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1.9: Cells at Non-Standard Conditions

  • Page ID
    221444
  • As shown in the section on cell notation, a galvanic cell in shorthand form can be written:

    \[\text{Zn}│\text{Zn}^{2+} ~(1 M)~║~ \text{Cu}^{2+} ~~(1 M)~|~ \text{Cu} \]

    The parenthetical notes of (1 M) are frequently omitted because 1 M is at the standard state. However, cells can be created which use concentrations other than 1 M. In such a case, one must always indicate the concentrations as shown above. In fact, as the reaction for the cell written above takes place (that is, as the cell provides electric current), Cu2+ will be used up while Zn2+ will be generated. The reactant concentrations will decrease and the product concentrations will increase until the solution has reached a state of equilibrium. These equilibrium concentrations are not likely to be at the standard 1 M solutions indicated above.

    The voltage of a cell at non-standard state is modified by the relative concentrations of the reactants and products, indicated as []. The equation for the reaction in the cell is

    \[\text{Zn} + \text{Cu}^{2+} \rightleftharpoons \text{Cu}+\text{Zn}^{2+} \]

    To calculate the electromotive force of a non-standard galvanic cell we use The Nernst Equation

    \[E_{\text{cell}} = E_{\text{cell}}^{\circ} - \dfrac{0.0592}{n}\log {[products]/[reactants]}\] (T = 298 \text{ K})\]

    This equation is valid at 298ºK, which the temperature at which most of the Galvanic cells are used.  Only the soluble species (or gases) affect the cell potential, not the solid electrode. The n value refers to the number of electrons exchanged (in our reaction above, n=2). If you calculate the [product/[reactants] ratio for the standard-state concentration of 1 M, note how the second term on the right-hand-side disappears and you recover \(E_{\text{cell}} = E_{\text{cell}}^{\circ}\).

    Although the Nernst equation is useful to predict the actual voltage of a cell under non-standard conditions, it is frequently more useful to use the measured voltage to detect the concentration of one of the species. For instance, if we use a standard H2/Pt half-cell, the detected voltage of that half-reaction coupled with an unknown concentration of Fe2+ can be used to determine the concentration of Fe2+.

    Example \(\PageIndex{1}\) : Voltage of a Galvanic Cell

    Determine the voltage measured for this galvanic cell. You may need to use the Table of Standard Reduction Potentials.

    \[\text{Sn}^{2+} (aq) (1 M│\text{Sn}║\text{Ag}│\text{Ag}^{+} (0.8 M) \nonumber\]

    Solution:

    The strategy for solving these problems is to first find the standard electromotive force, as shown in another section, and then write the balanced chemical equation, from which you can derive Q and z.

    \[E_{\text{cell}}^{\circ} = 0.7991\text{ V} - (- 0.1375)\text{ V} = 0.9366\text{ V} \nonumber\]

    \[2\text{Ag}^{+}(aq)+\text{Sn(s)} \rightarrow 2\text{Ag}(\text{s}) + \text{Sn}^{2+}(aq) \nonumber\]

    The expression for Q can be derived only from the balanced chemical equation. Thus, using the Nernst Equation, we have,

    \[\begin{align*}E_{\text{cell}} &= E_{\text{cell}}^{\circ} - \dfrac{0.0592 \text{ V}}{n}\log {[product]/[reactants]}\\&= 0.9366\text{ V} - \dfrac{0.0592\text{ V}}{2}\log \dfrac{\left\{\text{Sn}^{2+}(aq)\right\}}{\left\{\text{Ag}^{+}(aq)\right\}^2}\\&= 0.9366\text{ V} - \dfrac{0.0592\text{ V}}{2}\log \dfrac{\left(1\text{ }M\right)}{\left(0.8\text{ }M\right)^2}\\E_{\text{cell}}&=0.9308\text{ V}\end{align*}\]

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