Chapter 12

What happens when zinc is added to hydrochloric acid?

Zn + 2HCl  ZnCl 2 + H 2 What was the test for hydrogen gas?

What happens when zinc is added to hydrochloric acid? Zn + 2HCl  ZnCl 2 + H 2 We can do better than this!

What happens when zinc is added to hydrochloric acid? Zn + 2HCl  ZnCl 2 + H 2 We can do better than this! Zn + 2H +  Zn 2+ + H 2

What is being reduced? What is being oxidized?

Zn + 2H +  Zn 2+ + H 2 Let’s split the reaction into two parts (called half reactions), the oxidation reaction and the reduction reaction:

Zn + 2H +  Zn 2+ + H 2 Let’s split the reaction into two parts (called half reactions), the oxidation reaction and the reduction reaction: Zn  Zn e - 2H + + 2e -  H 2

Zn  Zn e - 2H + + 2e -  H 2 What do you see now that we have never shown before?

Zn  Zn e - 2H + + 2e -  H 2 What do you see now that we have never shown before? A properly balanced reaction has the same amount of electrons lost in the oxidation half reaction as is gained in the reduction half reaction.

Let’s separate these half reactions into separate beakers. Zn  Zn e - 2H + + 2e -  H 2

Let’s stick the two ends of a wire in. Zn  Zn e - 2H + + 2e -  H 2

What will be traveling through the wire? Zn  Zn e - 2H + + 2e -  H 2

What will be traveling through the wire? Zn  Zn e - 2H + + 2e -  H 2 e-e- e-e- e-e- e-e-

Zn  Zn e - 2H + + 2e -  H 2 e-e- e-e- e-e- e-e- What if we hooked the wire to something, like an iPod? Now we have a battery!

Zn  Zn e - 2H + + 2e -  H 2 e-e- e-e- e-e- e-e- What if we hooked the wire to something, like an iPod? Now we have a battery! This situation makes an electrochemical cell called a voltaic cell or galvanic cell.

A voltaic (or galvanic) cell is a combination of an oxidation and reduction reaction that spontaneously (thermodynamically favorable) produces an electric current (electrons).

There are some basic things necessary to build a voltaic cell:

two electrodes – one is oxidized and called the anode, one is reduced and called the cathode

There are some basic things necessary to build a voltaic cell: two electrodes – one is oxidized and called the anode, one is reduced and called the cathode two ionic solutions – the ions should match the electrodes, and be soluble salts

There are some basic things necessary to build a voltaic cell: two electrodes – one is oxidized and called the anode, one is reduced and called the cathode two ionic solutions – the ions should match the electrodes, and be soluble salts a wire to connect the electrodes, typically with a voltmeter to measure the voltage (electron flow)

There are some basic things necessary to build a voltaic cell: two electrodes – one is oxidized and called the anode, one is reduced and called the cathode two ionic solutions – the ions should match the electrodes, and be soluble salts a wire to connect the electrodes, typically with a voltmeter to measure the voltage (electron flow) a salt bridge – to balance charges so the reaction can continue as long as possible

Consider making a voltaic cell from some copper, silver, copper(II) nitrate, and silver nitrate.

What is the overall spontaneous (thermodynamically favorable) reaction? {Hint: use the shortcut with the standard reduction table.} Don’t forget to balance the electrons!

The Shortcut: using the SRP Table:

1 - find the elements, not ions, and circle

The Shortcut: using the SRP Table: 1 - find the elements, not ions, and circle

The Shortcut: using the SRP Table: 1 - find the elements, not ions, and circle Why pick copper(II) and not copper(I)?

The Shortcut: using the SRP Table: 2 - draw crossing upward arrows

The Shortcut: using the SRP Table: 2 - draw crossing upward arrows

The Shortcut: using the SRP Table: 3 - things on arrow pointing left are reactants

The Shortcut: using the SRP Table: 3 - things on arrow pointing left are reactants Reactants

The Shortcut: using the SRP Table: 4 - things on arrow pointing left are reactants Reactants

The Shortcut: using the SRP Table: 4 - things on arrow pointing right are products Reactants Products

Cu + Ag + + e -  Cu e - + Ag NOT BALANCED! Reactants Products

Cu + Ag + + e -  Cu e - + Ag NOT BALANCED! e - must equal e - gained Reactants Products 2 2 2

Cu + 2Ag + + 2e -  Cu e - + 2Ag Now it is balanced, but not reduced! Reactants Products 2 2 2

Note – if the SRP table elsewhere lists the voltages from negative to positive, then rewrite the two reaction you need in order from positive to negative (or decreasing voltage) before using this method.

Cu + 2Ag +  Cu Ag If you do it right, the electrons will ALWAYS cancel out. This is a great way to check your work.

Cu + 2Ag +  Cu Ag If you do it right, the electrons will ALWAYS cancel out. This is a great way to check your work. What are the oxidation and reduction half reactions?

Cu + 2Ag +  Cu Ag If you do it right, the electrons will ALWAYS cancel out. This is a great way to check your work. What are the oxidation and reduction half reactions? red: Ag + + e -  Ag

Cu + 2Ag +  Cu Ag If you do it right, the electrons will ALWAYS cancel out. This is a great way to check your work. What are the oxidation and reduction half reactions? red: Ag + + e -  Ag ox: Cu  Cu e -

What is the cell potential? Note: voltage is an intensive property so does not get multiplied. red: Ag + + e -  Ag0.80 V - ox: Cu  Cu e V

What is the cell potential? Note: voltage is an intensive property so does not get multiplied. red: Ag + + e -  Ag0.80 V - ox: Cu  Cu e V Where did these numbers come from?

What is the cell potential? Note: voltage is an intensive property so does not get multiplied. red: Ag + + e -  Ag0.80 V - ox: Cu  Cu e V 0.46 V

Although often on the AP test pictures of voltaic cells appear, it is burdensome to use a picture each time it is talked about. So we have a shortcut to write the parts of a voltaic cell on paper. This cell notation is written anode to cathode and takes this form: Anode Metal  Anode Ion (and concentration)  Cathode Ion (and concentration)  Cathode Metal Filling in our Cu and Ag voltaic cell: Cu  Cu 2+ (1M)  Ag + (1M)  Ag

Try this: As a child I found catalogs that offered to sell a “potato powered” clock, which consisted of a digital clock, a copper electrode and a zinc electrode, and a stand for holding a potato. What is the overall reaction?

Try this: As a child I found catalogs that offered to sell a “potato powered” clock, which consisted of a digital clock, a copper electrode and a zinc electrode, and a stand for holding a potato. What is the overall reaction? Zn + Cu 2+  Cu + Zn 2+

What are the half reactions?

Zn + Cu 2+  Cu + Zn 2+ What are the half reactions? red: Cu e -  Cu

Zn + Cu 2+  Cu + Zn 2+ What are the half reactions? red: Cu e -  Cu ox: Zn  Zn e -

Zn + Cu 2+  Cu + Zn 2+ What are the half reactions? red: Cu e -  Cu ox: Zn  Zn e - Contrast copper with zinc to copper with silver.

What is the cell potential? red: Cu e -  Cu 0.34 V ox: Zn  Zn e V

What is the cell potential? red: Cu e -  Cu 0.34 V ox: Zn  Zn e V 1.10 V

Sketch out the voltaic cell and show the movement of electrons through the clock.

Where is the real source of electricity for the clock? How is the potato involved?

This clock could also be used with a lemon. Lemons contain citric acid, and all acid is a good source of hydrogen ions. Find hydrogen in the standard reduction potentials list. Hydrogen ions are listed before copper ions, so which will be reduced first?

What is the overall reaction?

What are the oxidation and reduction half reactions?

What is the overall reaction? What are the oxidation and reduction half reactions? What is the cell potential?

Sketch out the voltaic cell and show the movement of electrons through the clock.

How is the lemon involved? If the toy clock where made to work with either a potato or a lemon, what is the minimum voltage the clock needs to work with?

Try these: What would be the two best electrodes to use if you wanted to make the best “Bagdad” battery possible? What is the overall reaction? What are the oxidation and reduction half reactions? What is the cell potential?

Sketch out the voltaic cell with a salt bridge and electrolytes of your choice, and show the movement of electrons through a wire.

Sketch out the cell made of two nestled porous clay pots (no salt bridge needed), and show the movement of electrons through an idol (which would then somehow emit a “power of god” if touched.)

If you had to make a Baghdad battery out of things you could find at a hardware store, what would be the best choices of electrodes? What is the overall reaction? What are the oxidation and reduction half reactions? What is the cell potential?

What conclusion can you make about the electrodes’ position in the list and the cell potential? Sketch out the voltaic cell with a salt bridge and electrolytes of your choice and show the movement of electrons through a voltmeter.

Section 2

So far we have been calculating the cell potential under standard conditions (normal standard conditions, plus one molarity of each solution). But what happens to the voltage as you use a battery? Specifically what happens to the electrodes and what happens to the concentration of the ions in the solution?

Experience with batteries tell us to expect the voltage to change as we “use up” a battery, so we need a way to figure out what the new voltage will be. Fortunately Hermann Nernst determined a way to calculate the non-standard voltage of a voltaic cell, whether it is being used up or just not at standard conditions to start with.

The Nernst Equation: where n = number of moles of e - transferred between the redox reactions (must be a whole number). This equation even works for the standard conditions of 1 M, try it now! E = E° ln [product ions] n[reactant ions]

Section 3

How to make quantitative measurements of voltaic cells: We have calculated cell potentials, and it would be easy enough to measure a cell potential and know one electrode and solve for the identity of the other electrode.

For example: A voltaic cell is made from a copper cathode (Cu 2+ version) and an unknown metal electrode. The initial voltage is measured to be 0.47 volts. What is the identity of the second electrode?

But we can do more! There is a way to calculate how much of the electrode would be oxidized or reduced, which we could then turn into grams. So first, some equalities from some famous dead scientists: A volt is a joule per coulomb. A coulomb is achieved with a current of 1 amp run for 1 second. 96,485 coulombs is the charge of 1 mole of electrons (also called Faraday’s constant).

Try this: A voltaic cell of copper (copper(II) version)and silver produces a current of 3.4 amps for 34 minutes. What are the changes in masses of copper and of silver?

So far we have only considered redox reactions that occur spontaneously (thermodynamically favorable), but many reactions that we want are the reverse of ones that occur spontaneously. So if a spontaneous reaction produces electricity, then what would it take to force an nonspontaneous (not thermodymaically favorable) reaction to occur? These types of electrochemical cells are called electrolytic cells.

Molten sodium chloride can be separated into sodium and chlorine using electricity. What is the overall reaction of this nonspontaneous reaction? (Warning, you cannot use the SRP!)

Molten sodium chloride can be separated into sodium and chlorine using electricity. What is the overall reaction of this nonspontaneous reaction? Na + + 2Cl -  Na + Cl 2

What are the oxidation and reduction half reactions? (Make the reactions in the SRP match the overall reaction.)

What are the oxidation and reduction half reactions? (Make the reactions in the SRP match the overall reaction.) red: Na + + e -  Na-2.71 V

What are the oxidation and reduction half reactions? (Make the reactions in the SRP match the overall reaction.) red: Na + + e -  Na-2.71 V ox: 2Cl -  Cl 2 + 2e V

What are the oxidation and reduction half reactions? (Make the reactions in the SRP match the overall reaction.) red: 2Na + + 2e -  2Na V ox: 2Cl -  Cl 2 + 2e V Don’t forget to balance electrons gained and lost.

red: 2Na + + 2e -  2Na V ox: 2Cl -  Cl 2 + 2e V 2Na + + 2Cl -  2Na + Cl V What is different about voltage?

Sketch this cell, with a battery instead of a voltmeter and show the movement of both the electrons and the ions.

If a current of 5.4 amps was used for 2.5 hours, what mass of pure sodium is produced and what volume of chlorine is produced?

What if the sodium chloride was dissolved in water instead of melted? Then would solid sodium and chlorine gas still be the products, or could water have an effect? Explain your answer using the standard reduction table. (Note – for electrolytic cells read from top down!)

One final thought. Throughout the year when we discuss thermochemistry we will be referencing something called “Gibbs Free Energy”, which is basically the energy available from a reaction. (Why is it not possible to get 100% of the reactants energy out of a reaction?) This is no different for electrochemistry.

For electrochemistry we can calculate the Gibbs Free Energy from the equation ΔG = -n F E°, where n is the number of moles of electrons transferred, F is faraday’s constant (what is that?), and E° is the standard cell potential.

In review: Type of reactionsign of Gsign of E°cell spontaneous -+ (will occur as written – products favored) nonspontaneous +- (will occur opposite as written – reactants favored)