9: Battery applications

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Redox Applications: Building a Battery

Model 1: Mixing Everything in One Container

When a piece of zinc (Zn) metal is placed in aqueous Cu2+ the following reaction occurs spontaneously:

Zn(s) + Cu²⁺(aq) -> Zn²⁺(aq) + Cu(s)

Over time Cu(s) will deposit on the Zn strip and some of the solid zinc strip will dissolve.

Model 2: Separating the Half-Cells

A more useful approach is to separate the half-cells and put something in the middle like a light bulb or an iPhone. This is called a battery or voltaic cell. An example is:

The same reaction occurs in Models 1 and 2 above:

Zn(s) + Cu²⁺(aq) -> Zn²⁺(aq) + Cu(s)

• Assuming that these reactions occur spontaneously, what is the sign of ΔG for the reaction?

• Write the half-reaction for the zinc electrode in Model 2 directly under the electrode in the figure. Is it an oxidation or a reduction?

• Write the half-reaction for the copper electrode in Model 2 directly under the electrode in the figure. Is it an oxidation or a reduction?

• Would the oxidation and reduction half-reactions you wrote for questions 2 and 3 be any different for Model 1?

• Model 2 is called a voltaic cell (a fancy term for battery), what advantage does it have over Model 1?

• Label the following parts or indicate the following on Model 2.
  1. Label the anode – the site of oxidation.

  2. Label the cathode – the site of reduction.

  3. Use an arrow to show which way electrons flow through the wire.

  4. Label the negative pole with a negative sign. (Hint: Electrons flow from negative to positive.)

  5. Label the positive pole with a positive sign. (Memory trick: Think of the ‘t’ in cathode as a plus sign.)

Voltaic cells are always set up the same way in terms of the anode and cathode.

• Fill in the blanks below:

  The anode is always on the __________ (left, right) and that is where ___________ (oxidation, reduction) occurs. The anode is also the ____________ (positive, negative) pole of a battery.

The salt bridge in Model 2 is very important since solutions cannot be charged.

• For example, what ion is being made at the anode in Model 2?

• Will the water near the anode become positive or negative?

• If the salt bridge contained KCl(aq), which of the ions in KCl would flow to the anode to balance the charge that is being made?

• Draw arrows on the salt bridge showing which direction the K⁺ and Cl^- move.

Model 3: Shorthand Notation

Drawing out voltaic cells like Model 2 can be cumbersome, so a shorthand notation has been developed. The shorthand notation example below is a compact version of the voltaic cell shown in Model 2.

• Write the chemical equation that correlates to:

  Na(s)|Na⁺(aq)||Ag⁺(aq)|Ag(s)

• Write the shorthand notation for the following reaction. Note that the equation does not need to be balanced to do the shorthand notation.

  H⁺(aq) + Al(s) -> Al³⁺(aq) + H₂(g)

Batteries We Use

Car Battery

A car battery utilizes two reactions that involve lead (Pb).

One unique aspect is that one of the products, PbSO4, is the same for each reaction.

• What happens to the charge on lead during the anode reaction?

• What happens to the charge on lead during the cathode reaction?

• Use E°cell = E°cathode – E°anode to determine the voltage for a car battery.

• Calculate the voltage for a car battery using this approach:
  • Add the reduction potentials for the two half-reactions.

    

• A typical car battery is a series of cells with a total of 12 Volts. How many cells are needed to get close to 12 Volts?

Application Problems

1. The energizer battery uses this following set of reactions:

   

   • Determine the voltage for an Energizer.

   • Label the anode, the cathode, and use an arrow to show which way the electrons flow in the somewhat cheesy diagram.

     

2. A “dental voltaic cell” is created when a person chews on aluminum foil and it comes into contact with a metal filling. Pain results if the flow of electrons encounters a nerve ending. If you have chewed a piece of aluminum foil you know the sensation.

   

   • Write the overall balanced equation and identify the species that is oxidized and the one that is reduced.

   • Use the table of reduction potentials in your text to determine the voltage that is transferred to a person’s nerve ending.

   • Which would cause a bigger jolt of pain: aluminum foil or tin foil? Briefly explain your answer. (Hint: Consult a table of reduction potentials.)

3. A voltaic cell is made as shown by the shorthand notation below.
   • Balance the reaction in aqueous acidic solution.

     MnO_2(s)|MnO₄^-_(aq)||F_2(g)|F^-_(aq)

4. In a battery, an oxidation half-reaction is physically separated from a reduction half-reaction so that the electrons are transferred “through the wires” to do useful work instead of causing a direct reaction.
   • Using the table of Standard Reduction Potentials, what should be the

     maximum voltage obtainable from a single battery cell? What would be the overall reaction occurring in this battery? Why do you suppose there are no practical applications for this battery?

   • A lithium ion battery has a voltage of about 3V. It can be shown that ΔG^o = - nFE^o, where n is the number of electrons transferred in the balanced equations (this is one in a lithium battery), F is the Faraday constant (96,500 joules/Volt). Calculate ΔG^o for the reaction in kJ. For a chemical reaction is this free energy change relatively large or small? As equilibrium will it be mostly products, reactants or a mixture of both?

   • What, chemically, happens when a battery is “dead”? What, chemically, happens when a battery is recharged?