Electrochemistry

• Electrochemical Cell – an apparatus that uses redox reactions to produce electrical energy.

• Voltaic Cell – a type of electrochemical cell that converts chemical energy to electrical energy with a spontaneous redox reaction.

Set Up

• We will use the reaction:

Zn(s) + Cu2+ Zn2+ + Cu(s)

• We will place 1M ZnSO4 and a strip of zinc on one beaker.

• We will place 1 M CuSO4 and a strip of copper in the other beaker.

Set Up

• In order for electrons to be exchanged, a wire is connected to each metal piece.

• Since zinc in the strip is being oxidized, there will be a build up of positive ions (Zn+2) in solution as electrons leave.

• The zinc strip loses mass over time.

Set Up

• In the copper beaker, since electrons are entering, there will be a build up of negative ions in the solution. (Cu+2 in solution becomes Cu)

• The copper strip gains mass over time.

Set Up

• A salt bridge is used to allow ions to flow from one side to the other.

• It contains a soluble salt that is contained by a “plug” such as agar gel.

• Ions can move through plug, but solutions do not mix.

• Anions enter the cell where oxidation occurs.

• Cations enter the cell where reduction occurs.

Voltaic Cell

• The electrode where oxidation takes place is the anode.

• The electrode where reduction takes place is the cathode.

• Which metal becomes oxidized or reduced is determined by it’s reduction potential.

• Reduction potential is the tendency of an substance to gain electrons. This is measured in the units of volts.

• The more negative the number the more likely it will be oxidized, the more positive the more likely it will be reduced.

Reduction Potentials

Calculating Cell Potential

• All reduction potentials are measured against hydrogen (which can be oxidized or reduced by the substance.)

• To measure the cell we created we first look at the copper.

Calculating Cell Potential

• Now to measure what is happening at the zinc electrode.

• Since copper has the more positive value, it will be reduced and the zinc will be oxidized.

Calculating Cell Potential

• To calculate the overall potential: E0

cell = E0

red – E0

ox

• Therefore for our cell:

0.342V-(-0.762V) = +1.104 V

• A positive number indicates it occurs spontaneously

Practice Problems

• Calculate the cell potential of the following:

• A. Chromium in Cr+3

solution and copper in Cu+2 solution.

• B. Tin in Sn+2 solution and iodine in I- solution.

• A. E° = +1.086 V• B. E° = +0.6730 V

Battery

• Battery- can contain a single cell or packages of several cells

• Small batteries (household) contain a single cell.

• Large batteries (car) contains many cells that can conduct more current. These are lead-acid batteries.

Types of Batteries

• Primary batteries – produce electricity by using a redox reaction that is not reversible.

• Once reactants are gone, battery is thrown out.• Typical alkaline batteries oxidize powdered zinc.• Secondary batteries – are rechargeable because

redox reactions are reversible.• These batteries are usually made of nickel and

cadmium (NiCad).

Types of Batteries

• Lithium batteries – use lithium because it is lightest in mass of all metals and has lowest standard reduction potential.

• This allows the battery to last much longer.

Some Vocabulary

• Electroplating: using electrical current to cause ions in solution to form a metal layer on a surface.

• One Ampere: (A) amount of electrical current equal to 1 coulomb of charge per sec.

• One Coulomb: (C) amount of charge that passes a point when 1 amp of current flows for 1 sec.

(1A = 1C/sec)• One Faraday: (F) amount of electricity that passes

per 1 mole of electrons (1F=96485C or 1 mole e- =96485C)

Let’s try some math

• A current of 2.50 amps is passed through a solution of Ni(NO3)2 for 2.00 hours. What mass of nickel is deposited at the cathode?

• Ans: 5.48 g

Another Problem

• If you wish to convert 1.00 grams of Au+3 ions to solid Au, how long must you electrolyze the solution if the current passing through the circuit is 2.00 amps?

• Ans: 735 sec

Let’s do one more!

• After 9.50 minutes at 5.50 amps, all of the Fe+3 ions were plated out of 600.0 mL of solution. What was the original concentration of Fe+3 in solution?

• Ans: 0.0181 M