7.02 – Voltaic Cells & EMF

Today, we will put redox to work.  Batteries are an elegant way to use the different abilities of materials to cause electrons to flow.  Those electrons can then be used for practical things, like using your cell phone.


Learning Objectives

  • Sketch a voltaic cell and identify its cathode, anode, and the directions in which electrons and ions move. (20.3)
  • Calculate standard emfs (cell potentials), Ecell° from standard reduction potentials. (20.4)
  • Use reduction potentials to predict whether a redox reaction is spontaneous. (20.4)

To become familiar with the topics presented in this mission, view the slides below and take note of the key ideas.  These are from sections 20.3-20.4 of your text.

Now work through practice problems 20.4-20.8, and post your work to OneNote.

Work through these mastery problems and post your work to OneNote.  The key is available on OneNote also.

20.25  A voltaic cell similar to that shown in Figure 20.5 is constructed. One half-cell consists of an aluminum strip placed in a solution of Al(NO3)3, and the other has a nickel strip placed in a solution of NiSO4. The overall cell reaction is 2 Al(s) + 3 Ni2+(aq) → 2 Al3+(aq) + 3 Ni(s)

(a) What is being oxidized, and what is being reduced?

(b) Write the half-reactions that occur in the two half-cells.

(c) Which electrode is the anode, and which is the cathode?

(d) Indicate the signs of the electrodes.

(e) Do electrons flow from the aluminum electrode to the nickel electrode or from the nickel to the aluminum?

(f) In which directions do the cations and anions migrate through the solution? Assume the Al is not coated with its oxide.


20.38  Given the following half-reactions and associated standard reduction potentials:

AuBr4(aq) + 3e→ Au(s) + 4 Br(aq)                      E°red=−0.858 V

Eu3+(aq) + e→ Eu2+(aq)                                          E°red=−0.43 V

IO(aq) + H2O(l) +  2e→  I(aq) +2OH(aq)        E°red=+0.49 V

(a) Write the equation for the combination of these half-cell reactions that leads to the largest positive emf and calculate the value.

(b) Write the equation for the combination of half-cell reactions that leads to the smallest positive emf and calculate that value.


20.96  Predict whether the following reactions will be spontaneous in acidic solution under standard conditions:

(a) oxidation of Sn to Sn2+ by I2(to form I)

(b) reduction of Ni2+ to Ni by I (to form I2)

(c) reduction of Ce4+ to Ce3+ by H2O2

(d) reduction of Cu2+ to Cu by Sn2+ (to form Sn4+)


20.97  Gold exists in two common positive oxidation states, +1 and +3. The standard reduction potentials for these oxidation states are

Au+(aq) + e→ Au(s)            E°red=+1.69 V

Au3+(aq) + 3 e→ Au(s)        E°red=+1.50 V

(a) Can you use these data to explain why gold does not tarnish in the air?

(b) Suggest several substances that should be strong enough oxidizing agents to oxidize gold metal.

(c) Miners obtain gold by soaking gold-containing ores in an aqueous solution of sodium cyanide. A very soluble complex ion of gold forms in the aqueous solution because of the redox reaction

4 Au(s) + 8 NaCN(aq) + 2 H2O(l) + O2(g) → 4 Na[Au(CN)2](aq) + 4 NaOH(aq)

What is being oxidized and what is being reduced in this reaction?

(d) Gold miners then react the basic aqueous product solution from part (c) with Zn dust to get gold metal. Write a balanced redox reaction for this process. What is being oxidized, and what is being reduced?


2003b FRQ#6





Understanding how cells function and their voltages is a huge part of AP Chem.  Good work getting through it.

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