Galvanic Cells From Chemistry to Electricity. Electrolytic Cells From Chemistry to Electricity... And back again!

Galvanic Cells

From Chemistry to Electricity

Electrolytic Cells

From Chemistry to Electricity . . . And back again!

Recall: the Galvanic Cell

Cu2+

Cu2+

Zn(s)

Zn2+

Zn2+

Cu(s)

Cu2+

2e-Zn2+

Recall: the Galvanic Cell

Cu2+

Cu(s)

Zn(s)

Conductor

Zn2+

Zn2+

Cu(s)

Cu2+

Current →

Load

Oxidation happens here Reduction happens here

AnodeCathode

Electrolyte

Zn2+

Salt Bridge

Questions:

• Which way did the electrons go?

• Why?

• What happens when we use a different pair of metals?

Voltage

• Voltage is also known as electromotive force and potential difference

• It is a measure of how much energy electrons have to get them moving

• It is related to the distance between two metals are on the activity series

AnalogyLithium

Potassium

Barium

Calcium

Sodium

Magnesium

Aluminium

Zinc

Iron

Nickel

Lead

(Hydrogen)

Copper

Silver

Gold

e-

e-

Potential Difference

This activity series is the inverse of the reduction table

Question:

• Can we push the electrons back up again?

The Electrolytic Cell

+-

Na+ Na+

Cl-

Cl-

Anode

Cathode

Electrolyte: eg. NaCl

External Voltage

The Electrolytic Cell

+-

Na(s)Na(s)

Cl2(g)

Anode

Cathode

External Voltage

Electrolyte: eg. NaCl

Similarities and Differences

Galvanic Cells• Oxidation occurs at the

anode• Reduction occurs at the

cathode• Anode is negative• Cathode is positive• Does not require external

voltage source• Changes chemical

reactions into electrical energy

Electrolytic Cells• Oxidation occurs at the

anode• Reduction occurs at the

cathode• Anode is positive• Cathode is negative• Requires external voltage

source• Changes electrical

energy into chemical reactions

Etymology

• Electrolysis comes from two Greek words: electron and lysis (meaning to break.) Therefore, the word means “breaking apart using electrons

Water Reaction

2 H2O(l) → O2(g) + 4 H+(aq) + 4 e-

2 H2O(l) + 2 e- → H2(g) + 2 OH-(aq)

Water Reaction

Oxidation at the Anode: 2 H2O(l) → O2(g) + 4 H+(aq) + 4 e-

Reduction at the Cathode: 4 H2O(l) + 4 e- → 2 H2(g) + 4 OH-(aq)

6 H2O(l) → 2 H2(g) + O2(g) + 4 H+ + 4 OH-(aq)

The Electrolytic Cell

+-

Anode

Cathode

Electrolyte

H2(g)O2(g)

External Voltage

Practice

• Where would the hydrolysis reaction be useful?

• Draw an electrolytic cell for AgBr(l)?

• Draw a galvanic cell for the reaction

Au+3 (aq) + Ag(s) Ag+1 (aq) + Au (s)

Review of Batteries

• Define the following terms:

• Primary Cell

• Secondary Cell

• Power Density

• Memory Effect

Nickel-Cadmium

Advantages• Can be recharged 1000 times

or more• Low cost/cycle• Tough, stands up to abuseDisadvantages• Low energy density• Memory effect• Contains toxic metalsPopular Uses• Two-way radios, power tools,

medical equipment

Lithium-Ion

Advantages• High energy density• No memory effectDisadvantages• More expensive than Ni-Cd• Not fully mature, technology is

still evolvingPopular Uses• Cell-phones, iPods, laptop

computers

Lead-Acid

Advantages• Mature technology, well

understood• Cheap and easy to manufacture• No memory effectDisadvantages• Very low energy density; most

applications require huge batteries

• Limited number of full discharge cycles

• Environmental concernsPopular Uses• Electric cars, golf carts,

scooters

Reusable Alkaline

Advantages• Cheap to manufacture• More economical than primary

alkaline cellsDisadvantages• Limited current, cannot be

made on large scale• Limited cycle life (about 10

cycles); fully discharging shortens life

Popular Uses• Personal CD players, radios,

flashlights

Fuel Cells

Behind the hype

The Limit of Batteries

• A battery is a fancy type of galvanic cell. It changes chemicals into electricity.

• Eventually, all the chemical are reacted and the battery goes dead.

• If the battery is a secondary cell, you can recharge it, but this takes time and energy. Also, there is a limited number of times you can do this.

• Wouldn’t it be nice to be able to just open up a battery, and pour in some more chemicals, like refueling a car?

Fuel Cells: The Ultimate Battery

• A fuel cell is a type of galvanic cell that allows you to add fresh chemicals continuously. It will continue to run as long as you keep adding fuel.

Fuel Cell Comparisons

Battery Fuel Cell Engine

Energy Conversion

Chemical>

Electrical

Chemical>

Electrical

Chemical>

Mechanical>

Electrical

Fuel Zinc or other metal

H2, Methane, Propane, Methanol, etc.

Propane, Methane, Gasoline

Powered By Electrochemical Reaction

Electrochemical reaction

Combustion Reaction

Power Output Low Variable High

Advantages of Fuel Cells

• Extremely versatile – can power everything from cell phones to buses

• Can run on a variety of fuels• More environmentally friendly than combustion

Disadvantages of Fuel Cells

• Technology is still somewhat unreliable

• Some types still produce greenhouse gas emissions

• EXPENSIVE

The Promise of Hydrogen

• Many Fuel Cells are emission-free because they run on hydrogen

H2(g)

Anode CathodeElectrolyte

2 H+

O2(g)

2 e-

The Promise of Hydrogen

• Many Fuel Cells are emission-free because they run on hydrogen

H2(g)

Anode CathodeElectrolyte

H2O(l)

Overall Reaction: H2 + O2 → H2O

Question: Where does hydrogen come from?

Types of Fuel Cells

• Proton Exchange Membrane (PEM)

• Solid Oxide Fuel Cells (SOFC)

• Alkaline Fuel Cells (AFC)

• Direct Methanol Fuel Cells (DMFC)

• There are many more, but we won’t get into them here

Your Task

• You will work in groups of about 5• Half of each group will argue “for” a

particular type of fuel cell the other half will argue “against”

• Each half-group will prepare an extremely short (60 s) presentation to convince the audience of their stance

• The class will vote on who was most convincing

What if there was a chemical reaction that:

• Turned vehicles and buildings into dust

• Caused billions of dollars worth of damage per year

• Was virtually unstoppable

• Had the potential to destroy an entire planet’s atmosphere

Rust

The Silent Killer

Why does rust happen?

• Iron, like most metals, is a strong reducing agent

• Earth’s atmosphere is 21% O2, which is a powerful oxidizing agent

• Galvanic cells are easy to set up, and can be as simple as a drop of water

The Rust Galvanic Cell

Fe(s)

2 H2O(l)Particle

O2(g)

Oxidation: Fe(s) Fe2+ + 2 e-

Reduction: O2(g) + 2H2O + 4e- 4 OH-

The Rust Galvanic Cell

Fe(s)

2 H2O(l)Particle

O2(g)

Oxidation: Fe(s) Fe2+ + 2 e-

Reduction: O2(g) + 2H2O + 4e- 4 OH-

2 e-Fe2+

The Rust Galvanic Cell

Fe(s)

Particle

Oxidation: Fe(s) Fe2+ + 2 e-

Reduction: O2(g) + 2H2O + 4e- 4 OH-

Fe2+4 OH-

2 Fe(s) + O2(g) + 2H2O 2 Fe2+ + 4 OH-

The Rust Galvanic Cell

Fe(s)

Particle

2 Fe2+4 OH-

The Rust Galvanic Cell

Fe(s)

Particle2 Fe(OH)2

2 Fe2+ + 4 OH- 2Fe(OH)2

The Rust Galvanic Cell

Fe(s)

Particle4 Fe(OH)24 e-

O2(g)

2 H2O(l)

4 Fe(OH)2 + O2(g) + 2 H2O(l) 4 Fe(OH)3

The Rust Galvanic Cell

Fe(s)

Particle4 Fe(OH)2

4 Fe(OH)2 + O2(g) + 2 H2O(l) 4 Fe(OH)3

4 OH-

The Rust Galvanic Cell

Fe(s)

Particle4 Fe(OH)3

4 Fe(OH)2 + O2(g) + 2 H2O(l) 4 Fe(OH)3

The Rust Galvanic Cell

Fe(s)

4 Fe2O3 · 3 H2O

4 Fe(OH)2 + O2(g) + 2 H2O(l) 4 Fe(OH)3

Fe(OH)3 Fe2O3·3 H2O

Questions

• How did the water become an electrolyte?• What was the anode?• What was the cathode?• Would this happen for other metals? Which

ones? How would it be different?• Corrosion costs billions of dollars a year in

damage as boats, cars, trains, building, etc. all gradually turn to dust. What can we do to prevent rusting from causing so much damage?

Rust Prevention: Protective layer

• Adding a protective layer of paint, plastic, or glass prevents the iron from coming in contact with the electrolyte

• What happens if the protective layer develops a scratch?

Rust Prevention: Galvanizing

• If you coat iron in a thin layer of zinc, it is called galvanization. The layer both protects the iron and will act as the anode if a scratch develops

• What happens when all the zinc is oxidized?

Fe2O3 + 3 Zn 3 ZnO + 2 Fe

Rust Prevention: Sacrificial Anode

• Some ships and gas pipelines are protected by putting a block of zinc, aluminum, or magnesium on them. The more reactive metal is oxidized and the iron stays intact.

• Who pays to replace the sacrificial anode every year?

Rust on Mars

• The surface of mars is completely covered in rust. Scientists think that Mars might once have had an atmosphere like earth’s, but all of that oxygen is now tied up in Fe2O3.

• Question: why hasn’t this happened on Earth?

Practice Questions

• In 2000, Transport Canada recalled thousands of cars with corroded engine mounts in Nova Scotia, New Brunswick, and PEI. Why was corrosion such a problem in these provinces?

• A small scratch in a car door can quickly develop into a major rust spot. Why does this happen?

• Does acid rain promote or prevent corrosion? Explain?