chapter 8: energy from electron transfer. electricity – the flow of electrons from a negative...

Chapter 8: Energy from Electron Transfer

Electricity – the flow of electrons from a negative electrode to a positive electrode

Direct current (DC) – electrons only flow in one direction.

Alternating current (AC) – electrons alternately flow in both directions.

Closed circuit – a conductive path is present for flow of electrons.

Open circuit – no complete path for electrons

Voltage – electrical potential of each electron

higher voltage – more energy per electron

Current (amps) – number of electrons flowing per second.

Voltage and amps combined give the power for a particular circuit.

Power plants send out AC current.

Much less power loss over great distances than with DC.

~7% loss is typical

Batteries produce DC current

Useful for portable power.

Computers, cell phones, … need DC current

Energy is lost when converting between AC and DC.

A battery is a system for the direct conversion of chemical energy to electrical energy.

Batteries are found everywhere in today’s society because they are convenient, transportable sources of stored energy.

The “batteries” shown here are more correctly called galvanic cells.

A series of galvanic cells that are wired together constitutes a true battery – like the one in your car.

8.1

When an element forms a compound, or a compound forms an element, electrons are transferred:

Na + Cl → NaCl

Looking at each element independently:

Na → Na+1 + 1e-1

Cl + 1e-1 → Cl-1

Batteries are designed to direct the electron flow through the wire, rather than directly between the elements.

A Laboratory Galvanic Cell

8.1

Oxidation (at anode):Zn(S) Zn2+(aq)

Reduction (at cathode):Cu2+(S) Zn2+(aq)

A galvanic cell is a device that converts the energy released in a spontaneous chemical reaction into electrical energy.

This is accomplished by the transfer of electrons from one substance to another.

Consider a nickel-cadmium (NiCad) battery:

Electrons are transferred from cadmium to nickel.

8.1

Chemical to electrical energy

Oxidation – when an atom loses an e-1

Na → Na+1 + 1e-1

Reduction – when an atom gains an e-1

Cl + 1e-1 → Cl-1

Both oxidation and reduction must occur for a reaction, and to generate electricity.

Oxidation is loss of electrons: Cd Cd2+ + 2 e–

Cd loses electrons

Reduction is gain of electrons: 2 Ni3+ + 2 e– 2 Ni2+

Ni3+ gains electrons

Oil Rig is a useful mnemonic device.

The transfer of electrons through an external circuit produces electricity, the flow of electrons from one region to another that is driven by a difference in potential energy.

8.1

The electron transfer process involves two changes: the cadmium is oxidized, and the nickel is reduced.

Each process is expressed as a half-reaction:

Oxidation half-reaction: Cd Cd2+ + 2 e–

Reduction half-reaction: 2 Ni3+ + 2 e– 2 Ni2+

Overall cell reaction: 2 Ni3+ + Cd 2 Ni2+ + Cd2+

The cadmium releases two electrons, resulting in a 2+ ion.

Two Ni3+ ions accept the two electrons and their respective charges go from 3+ to 2+.

8.1

The overall cell reaction does not have any electrons written in it – they must cancel.

To enable this transfer, electrodes (electrical conductors) are placed in the cell as sites for chemical reactions.

Oxidation occurs at the anode

Reduction occurs at the cathodeThe cathode receives the electrons.

The difference in electrochemical potential between the two electrodes is the voltage (units are in volts).

8.1

Different elements have different attractions for electrons, which results in different voltages between the elements.

Various metals are selected for battery electrodes depending on the desired voltage.

Alkaline AA batteries, for example, have a voltage of about 1.5 V.

8.2

– Mercury batteries had the advantage of being very small – Used in watches, cameras, hearing aids – Toxicity and disposal concerns led to alternatives

Once all of the atoms in an electrode have reacted, the battery is dead (no more electrical chemical potential energy)

Some batteries may be rechargeable, depending on the metals used – applying a potential can reverse the reaction and restore the original elements.

Many AA rechargeable batteries only have a potential of 1.3 V, and will not work in some devices that require the full 1.5 V.

8.2

Lead–Acid Storage BatteriesThis is a true battery as it consists of a series of six cells.

Anode = PbCathode = PbO2

Pb(s) + PbO2(s) + 2 H2SO4(aq) 2 PbSO4(s) + 2 H2O(l)discharging

recharging

Rechargeable:

Hybrid vehicles use a combination of electric motors and a gasoline generator for power.

The generator is used to charge up the batteries, which power the motor.

With standard cars, when applying the brakes the kinetic energy is lost as thermal energy.

With hybrids, brakes turn the kinetic into magnetic, and then into electrical energy, which is used to recharge the batteries. This is why hybrids are best in local driving, with shorter trips and more stops/starts.

For hybrid vehicles, the energy source is still gasoline.

For fully electric vehicles, the coal or nuclear power plant supplies the energy.

Alternate power sources, such as fuel cells based on H

2 have also been proposed.

Fuel cells operate similarly to galvanic cells, but need a constant supply of fuel.

This fuel, usually hydrogen, is highly reactive, and requires energy to produce.

Solar energy has been proposed to split water into hydrogen and oxygen for fuel cells.

Hydrogen can be adsorbed onto metal substrates to reduce its potential hazards.

8.5

Honda FCX Powered by Fuel Cells

8.5

A fuel cell is a galvanic cell that produces electricity by converting the chemical energy of a fuel directly into electricity without burning the fuel.

Both fuel and oxidizer must constantly flow into the cell to continue the chemical reaction.

Chemical Changes in a Fuel CellH2 + ½ O2 H2O

Anode reaction: (Oxidation half-reaction)H2(g) 2 H+(aq) + 2 e–

Cathode reaction: (Reduction half-reaction) ½ O2(g) + 2 H+(aq) + 2 e– H2O(l)

H2(g) + ½ O2(g) + 2 H+(aq) + 2 e– 2 H+(aq) + 2 e– + H2O(l)

Overall (sum of the half-reactions)

H2(g) + ½ O2(g) H2O(l)

Net equation:

8.5

8.5

Comparing Combustion with Fuel Cell Technology

8.5

Methanol or Methane Gas Could be Converted to Hydrogen Gas Using a Reformer

8.6

High Pressure Hydrogen Storage

Hydrogen Storage in Metal Hydrides

Refueling a Honda FCX Clarity with hydrogen gas

Hydrogen is absorbed onto metal hydrides and released with increased temperature or decreased

pressure.

8.6

If the “Hydrogen Economy” becomes reality, where will we get the H2?

One potential source is fossil fuels:

165 kJ + CH4(g) + 2 H2O(g) → 4 H2(g) + CO2(g)

247 kJ + CO2(g) + CH4(g) → 2 H2(g) + CO(g)

But these reactions produce carbon dioxide and carbon monoxide.

Electrolysis of water could also be used:

249 kJ + H2O(g) → H2(g) + ½ O2(g)

8.6

Hydrogen (and oxygen) gas produced by the electrolysis of water.

Electrolysis of Water:

But this process requires energy.

8.6

• Using fossil fuels or electrolysis uses a lot of energy.

• Instead, use energy from the sun for this process.

Use Energy from the Sun to Split Water:

Photovoltaic Cells

8.7

Energy of the future? An awesome source of energy.

Our society has been using photovoltaic cells (solar cells) at a minimum level, but will we all begin picking up on this natural form of energy as a way to conserve our natural coal and petroleum supply?

8.7

Other countries making use of solar energy

Solar Park Gut Erlasee in Bavaria. At peak capacity, it can generate 12 MW.

Harnessing the energy of the sun for pumping water

It’s time to make important decisions and advances in alternative energy technologyand new sources of renewable energy.

8.7

How does a photovoltaic cell generate electricity?

• A Semiconductor can be used (usually made of crystalline silicon). Semiconductors have a limited ability to conduct electricity.

• Two layers of semiconducting materials are placed in direct contact to induce a voltage in the PV cell.

• n-type semiconductor – has an abundance of electrons• p-type semiconductor – deficient in electrons

Arsenic-doped n-type silicon semiconductor

Gallium-doped p-type silicon semiconductor

8.7

Schematic diagram of a solar cell:

8.8

Other Renewable Energy Sources: Solar Thermal

Aerial view of the Solar Millennium Andasol project in

Spain

Close-up of a portion of the mirrored array

Other renewable energy sources include wind, water, and geothermal energy.