Electrochemical Conventions
- Page ID
- 9316
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)I. General Oxidation-Reduction (Redox)
- Oxidation = loss of electrons. (Losing electrons increases the charge (oxid. state).)
- Reduction = gain of electrons. (Gaining electrons reduces the charge (oxid. state).)
Can add LEO says GER and OIL RIG etc.
- Oxidizing Agent:
- causes an oxidation
- takes electrons
- is itself reduced
- Reducing Agent:
- causes a reduction
- gives electrons
- is itself oxidized
II. Electrochemistry
a) Electrodes = Where the oxidation/reduction (redox) reactions occur
- Anode = Where oxidation half reaction occurs. (begin with vowels).
- Cathode = Where reduction half reaction occurs. (begin with consonants).
b) In the external circuit (ie, the wire) the electrons always flow from the oxidation site to the reduction site.
- Therefore, electrons in the external circuit always flow from the anode to the cathode.
c) Voltaic (Galvanic) Cells- an electrochemical cell in which a spontaneous reaction produces electricity.
- The cell potential is always positive. (If you got a problem where the voltage was negative, it means the cell is spontaneous in the reverse direction.)
- Electrons move in the external circuit from the negative electrode to the positive electrode.
- Thus, in the voltaic cell, anode = negative and cathode = positive
d) Electrolytic Cells- an electrochemical cell in which a non-spontaneous reaction is carried out by electrolysis.
- (Electrolysis: the decomposition of a substance (in a molten state or in an electrolytic solution) by an electrical current.) The voltage is always negative and a metal is plated out or a gas is evolved.
- Electrons are forced to move in the external circuit from the positive electrode to the negative electrode.
- Thus, in the electrolytic cell, anode = positive and cathode = negative
e) Drawing cells (both voltaic and electrolytic)
The negative electrode (not necessarily the anode) is shown on the LEFT I don’t think this is necessarily true.
Thus, the left-hand electrode is the:
- anode if voltaic cell
- cathode if electrolytic cell
f) Line Diagrams (Cell Diagrams)
- the left hand electrode is the anode (where oxidation occurs)
- The sign on the left hand electrode is negative
- a boundary between different phases is represented by a single line (÷).
- reactants are at the left of the double line (||) which represents the boundary between the half cells (usually a salt bridge); products are at the right of |.
- Different species in the same solution half cell compartment are separated by commas. | Pt(s) ê Cl2(g) êCl-(aq) || Pb2+(aq), H+(aq) ê PbO2(s)|
- (some professors do not care about the order of the ions presented in a line diagram, while others say that, ions can be ordered in each half cell so that increasingly more positive ions are nearest the double lines. Example for a voltaic cell:
anode electrode(s) ÷ A-1 (aq, B+1(aq) ÷÷ C+2(aq), D-1(aq)÷ cathode electrode(s))
g) Standard Reduction Potentials (or voltages)
- voltages of a half cell in which all gases are 1 atm and all solutions are 1 M (25oC)
- half cells are listed as reduction processes.
- the more positive the voltage, the more easily reduced.
- thus, the strongest oxidizing agents are found at the top left (as F2 (g)) and the strongest reducing agents are found at the bottom right (as Li(s))
- a useful memory aid is “upper left reacts with lower right.”