From Wikibooks, open books for an open world
Jump to navigation Jump to search

Electrolysis involves an electric field and the motion of electrons through a liquid.

GCSE Science/Electricity

Electrolysis is the decomposition of certain types of substance using electricity. The types of substance that can be split are ionic substances. This just means that they are made of charged ions rather than neutral atoms. {Remember that an ion is just an atom that has either a positive or negative charge}. An example of an ionic substance is common table salt—sodium chloride. The sodium ion has a positive charge, the chlorine ion has a negative charge. It is usually written as Na+Cl-.

Q1) Check in a periodic table, what is the symbol for sodium—Na or Cl?

As you may already know if you've studied the Metals module, a salt is any substance made by combining an acid with an alkali. Acids, alkalis, and therefore all salts are ionic.

Q2) Which of the following substances can be broken up by electricity Sodium chloride, iron sulphate, copper nitrate?

Basic Experimental Setup

[edit | edit source]
The liquid electrolyte is connected
to a battery or power supply.
The electrodes must not touch!

Most ionic compounds are not liquid at room temperature. This is a problem because the ions need to be able to move for the electric current to be able to flow. This can be achieved by melting. Look at the electrical setup shown on the right. The electrodes are just two carbon rods connected to a battery. The one connected to the positive electrode is called the anode. The one connected to the negative electrode is called the cathode.

Consider for example the compound lead bromide. This compound is a solid at room temperature but can be molten over a Bunsen flame. So what you would do is put some lead bromide into a beaker. Put the beaker on a tripod over a Bunsen flame. Melt the lead bromide, then put in the electrodes and turn the power supply on at a setting of say 2V. What you would see happening is the anode, is a silvery coating of pure lead forming, and bromine forming at the cathode. The current would continue to flow until all the lead bromide was turned into lead and bromine.

Q3) It takes energy to split up a compound like lead bromide. Where does this energy come from ?

Q4) Predict what products you would get at the anode and cathode if copper chloride was the electrolyte.

What happens at the anode

[edit | edit source]
The bromine atoms (blue)
give up 2 electrons each (green) to the anode.

The anode is the positive electrode. It attracts negatively charged ions, because unlike charges attract. The bromine ions move through the melt until they reach the anode. Once they get there, they give up their two extra electrons to become bromine atoms.

Br2- --> Br + 2e-

The electrons flow up the anode to the positive terminal of the battery.

What happens at the cathode

[edit | edit source]

The cathode is the negative electrode. It attracts the positively charged ions. Metal ions are always positive and so the lead ions flow through the melt until they get to the cathode. Once they get there they take two electrons each from the cathode. The electrons flow down the cathode from the negatively terminal of the battery and onto the lead ions.

Pb2+ + 2e- --> Pb

Q5) Solid ionic substances do not conduct electricity and are not split up by it. Why do you think that is?

Quantity calculations (higher tier only)

[edit | edit source]

In the experiment with lead bromide, you saw that lead was deposited at the cathode. If you actually do the experiment you will see that the lead coats the cathode. In this section we will look at how much metal will coat a cathode in a given time.

A scientist performed the following experiment.

  • A copper cathode was carefully cleaned and accurately weighed.
  • It was placed along with an abnode into a solution of copper sulphate.
  • It was connected via a ammeter to a variable power supply.
  • A current was left to run for a given time then the cathode was removed and weighed again.

His results were:

Current /A Time /s Mass of copper deposited /g
1 3000 1.0
2 3000 2.0
2 1500 1.0
1 1500 1.0

You can see from the results that the total amount of copper deposited depends on both the current and the time it flows. This is because the number of copper atoms that can be made from ions depends on the total amount of charge that flows. The unit of charge is the coulomb.

One coulomb is the amount of charge when one Amp flows for one second (see GCSE Science/Measuring electricity

Q6)Look at the results table above. How much copper is deposited when 1A flows for 3000 seconds?

Q7)How much copper do you predict would be doposited if 1A were to flow for 6000 seconds.

Q8)What about if 2A were to flow for 12000 seconds ?

Electrolysis of Aqueos solutions (Advanced)

[edit | edit source]

Before studying this section check with your teacher to see if you need to.

Earlier on in this module you've learned that ions must be able to move in order for electrolysis to work. If the ions are held rigid {such as in a solid}, they can't move and no electricity will flow. We've looked at how the freeing up of ions can occur by melting the electrolyte. Another way to achieve this is by dissolving th electrolyte in water. The trouble with this method is, there will be mor than one type of ion present.

water partially splits up into ions {this is why it's such a good solvent for ionic compounds}. It splits into hydrogen ions and hydroxide ions.

H2O --> H+ +OH-

So at the cathode there will be two ions present. The Metal ion and the hydrogen ion from the water. Which element is actually produced at the cathode depends on how reactive the metal is. If the metal is very reactive, such as potassium, or sodium, then hydrogen will be produced. If the metal is unreactive such as silver, the metal will be produced.{I hope you learned your displacement reactions in lower school}. To work out which ion wins, the metal or the hydrogen, compare their reactivities in a the reactivity series. The one that is most reactive, will not be produced at the cathode.

A similar situation occurs at the anode. Hydroxide ions {from the water} are usually discharged at the anode ultimately producing oxygen.

OH- --> OH + e-

4OH --> 2H2O + O2.

This is true for most things but if the ions are bromine, chlorine, or fluorine {i.e. a halogen} then the halogen will be produced. {You are unlikely to need to remember this, only understand it}

Q9) Sodium chloride is dissolved in water and subjected to electrolysis. Explain what you see at each of the electrodes.

Only ionic substances can be split up by electrolysis
The ionic compounds need to be melted or dissolved in water so the ions can move
Positive ions are attracted to the cathode, where they pick up electrons from the electrode
Negative ions are attracted to the anode where they give electrons to the electrode

Answers | <<Advanced static electricity | Circuits>>

A fuel cell is an electrochemical device similar to a battery, but differing from the latter in that it is designed for continuous replenishment of the reactants consumed; i.e. it produces electricity from an external fuel supply as opposed to the limited internal energy storage capacity of a battery.

Typical reactants are hydrogen on the anode side and oxygen on the cathode side. In contrast, conventional batteries consume solid reactants and, once these reactants are depleted, must be discarded, recharged with electricity by running the chemical reaction backwards, or, at least in theory, having their electrodes replaced. Typically in fuel cells, reactants flow in and reaction products flow out, and continuous long-term operation is feasible virtually as long as these flows are maintained.

Fuel cells are also attractive in some applications for their high efficiency and low pollution. Some applications that have been suggested include

  • baseload utility power plants,
  • emergency backup power,
  • off-grid power storage,
  • portable electronics, and
  • electrically-powered vehicles.

Types of fuel cells

[edit | edit source]

There are five generally recognised types of fuel cells, of which two are the main subject of intensive research.

PEM fuel cells have a disputed acronym, meaning either proton-exchange membrane or polymer-electrolyte membrane, which both are in fact a good description. In this fuel cell, hydrogen is split at the membrane surface in protons, that travel through the membrane, and electrons, that travel through our external electric circuit, and provide our power. The hydrogen ions travel through the water that is entrained in the membrane to the other side, where they are combined with oxygen to form water. Unfortunately, while the splitting of the hydrogen molecule is relatively easy, splitting the stronger oxygen molecule is more difficult, and this causes significant losses that result in a sharp decrease of performance of the fuel cell. The PEM fuel cell is a prime candidate for vehicle and other mobile applications of all sizes down to mobile phones, because of its compactness. However, the water-entraining membrane is crucial to performance: too much water will flood the membrane, too little will dry it; in both cases, power output will drop; water management is a very difficult subject in PEM fuel cells. Furthermore, the platinum catalyst on the membrane is easily poisoned by CO.

PEM systems that use reformed methanol were proposed, as in Daimler Chrysler Necar 5; reforming methanol, i.e. making it react to obtain hydrogen, is however a very complicated process, that requires also purification from the CO the reaction produces. A platinum-ruthenium catalyst is necessary as some CO will unavoidably reach the membrane. The level should not exceed 10 parts per million.

A subcategory of PEM is the DMFC, or direct methanol fuel cell; here, the methanol is not reformed, but fed directly to the fuel cell. One does not need complicated reforming, and storage of methanol is much easier than that of hydrogen. However, efficiency is low, due to the high permeation of methanol through the membrane, and the dynamic behaviour is sluggish.

The main manufacturer of PEM is Ballard Power Systems of Vancouver, Canada. Efficiencies of PEM are in the range of 40-50%.

Solid oxide fuel cells, or SOFC, are intended mainly for stationary applications (power plants). They work at very high temperatures (some at 1000°C), and their off-gases can be used to fire a secondary gas turbine to improve efficiency. Efficiency could reach as much as 70% in these hybrid systems. This time, it's oxygen being transferred through a solid oxide at high temperature to react with hydrogen on the other side. SOFC have such a high temperature that they can be fed (provided some modifications) with natural gas, that will react to give hydrogen in the fuel cell itself. SOFC are very resistant to poisoning, and can indeed be run on CO, which is a poison for PEM.

Since SOFC are made of ceramic materials, they tend to be brittle; they are therefore unsuited for mobile applications. Furthermore, thermal expansion demands a uniform and slow heating process at startup, that will cause very long startup times: typically, 8 hours are to be expected. Research is going now in the direction of lower-temperature SOFC (600°C), which will enable the use of metallic materials with better mechanical properties and heat conductivity.

Molten-carbonate fuel cells (MCFC) are also high-temperature, but in the range of 600°C. Their main problem is corrosion, and the need to operate a high-temperature liquid rather than a solid as in the SOFC.

Phosphoric-acid fuel cells (PAFC) are a mature technology that is commercially available. Unfortunately, the phosphoric acid solidifies under 40°C, making startup very difficult. They have been used for stationary applications with an efficiency of about 40%, and many believe they do not offer much potential for further development.

The alkaline fuel cell (AFC) is the cell that brought the Man to the Moon. Used in Apollo-series missions and on the Space Shuttle, it is a very good fuel cell but for the fact that it is poisoned by CO2. This means that the cell will require pure oxygen, or at least purified air. As this process is relatively expensive, not much development is being done on AFC. NASA has decided they will shift to PEM for the next generation of Space Shuttles.


[edit | edit source]

Fuel cells are electrochemical devices, so they are not constrained by the maximum thermal (Carnot) efficiency as combustion engines are. Consequently, they can have very high efficiencies in converting chemical energy to electrical energy.

In the archetypal example of a hydrogen/oxygen polymer electrolyte membrane (PEM) fuel cell, a proton-conducting polymer membrane separates the anode ("fuel") and cathode sides. Each side has an electrode, typically carbon paper coated with platinum catalyst.

On the anode side, hydrogen diffuses to the anode catalyst where it dissociates into protons and electrons. The protons are conducted through the membrane to the cathode, but the electrons are forced to travel in an external circuit (supplying power) because the membrane is electronically insulating.

On the cathode catalyst, oxygen molecules react with the electrons (which have travelled through the external circuit) and protons to form water.

In this example, the only waste product is water vapor.

Also, there is the possible use of fuel cells at home, to store energy at the cheap off-peak electricity rates and used at peak-use hours. It may even be profitable to sell back some of the energy to the power company, like they do with windmill electric power. Peak power production reaches twice the normal level, which means that the very expensive powerplant capacity is sized for levels used for a short period of time. Also, power plants are most efficient at only one production rate and their efficiency drops off significantly at off-peak rates.


[edit | edit source]

The first fuel cell was developed in the 19th century by British scientist Sir William Grove. A sketch was published in 1843. But fuel cells did not see practical application until the 1960s, where they were used in the U.S. space program to supply electricity and drinking water (hydrogen and oxygen being readily available from the spacecraft tanks). Extremely expensive materials were used and the fuel cells required very pure hydrogen and oxygen. Early fuel cells tended to require inconveniently high operating temperatures that were a problem in many applications.

Further technological advances in the 1980s and 1990s, like the use of Nafion as the electrolyte, and reductions in the quantity of expensive platinum catalyst required, have made the prospect of fuel cells in consumer applications such as automobiles more realistic.

The fuel cell industry

[edit | edit source]

Ballard Power Systems is a major manufacturer of fuel cells and leads the world in automotive fuel cell technology. Ford Motor Company and Chrysler are major investors in Ballard. As of 2003, the only major automobile companies pursuing internal development of fuel cells for automotive use are General Motors and Toyota; most others are customers of Ballard.

United Technologies (UTX) is a major manufacturer of large, stationary fuel cells used as co-generation power plants in hospitals and large office buildings. The company has also developed bus fleets that are powered by fuel cells.

Pros and cons of fuel cells in various applications

[edit | edit source]

Their use is controversial in some applications. The hydrogen typically used as a fuel isn't a primary source of energy. It is usually only a source of stored energy that must be manufactured using energy from other sources. Some critics of the current stages of this technology argue that the energy needed to create the fuel in the first place may reduce the ultimate energy efficiency of the system to below that of highly efficient gasoline internal-combustion engines; this is especially true if the hydrogen is generated from electrolysis of water by electricity. On the other hand, hydrogen can be generated from methane (the primary component of natural gas) with approximately 80% efficiency. The methane conversion method releases greenhouse gases, however, and the ideal environmental system would be to use renewable energy sources to generate hydrogen through electrolysis. Other types of fuel cells don't face this problem. For example, biological fuel cells take glucose and methanol from food scraps and convert it into hydrogen and food for the bacteria.

There are practical problems to be overcome as well. Although the use of fuel cells for consumer products is probable in the near future, most current designs won't work if oriented upside down. They currently cannot be scaled to the small size needed by portable devices such as cell phones. Current designs require venting and therefore cannot operate under water. They may not be usable on aircraft because of the risk of fuel leaks through the vents. Technologies for safe refueling of consumer fuel cells are not yet in place.

Among the controversies in the use of hydrogen are: First, the energy used to produce the hydrogen is comparable to the energy in the hydrogen, it is inefficient therefore too expensive. If conventional powerplants were used to produce the hydrogen, at best, there would be no gains in current pollution rates. Second, some have suggested that this is a "stalking horse" to bring back nuclear power, which may be the only commercially viable way to make hydrogen and reduce pollution. Finally, the need to provide the very long, costly and vulnerable thousands of miles of gas lines makes hydrogen too costly without government help.

There are several advantages to hydrogen as well. Clean, renewable energy sources like solar and wind power are non-continuous and unreliable through the course of a day. So power from these sources is not always available at the time it is needed. The electricity produced from solar panels or wind generators could be stored in large battery complexes but this can be expensive and batteries have a limited storage capacity and lifetime. If the electricity is used to produce hydrogen however, the energy can be stored more easily. As a gas hydrogen is not hard to store until it is needed.

[edit | edit source]


[edit | edit source]

What is a battery?

[edit | edit source]

Batteries in circuits

[edit | edit source]

Batteries in series can increase or decrease the combined voltage.

Toys use batteries, for example, and it is important to note from the associated drawing(s) which way the batteries are to be inserted into the toy because their voltages can add or subtract as the case may be. To add voltages the + of one battery is connected to the - of the next battery. Some damage may be caused by inserting batteries in the wrong direction.

Batteries in parallel are not usually used; at a given rated voltage each battery is able to provide only a certain maximum current, but to increase the maximum available output current several batteries could be connected in parallel, meaning that all the + terminals are connected together, and so are all of the - terminals.

The life of a battery depends approximately on the value of (current times time), but the chemistry involved can permit a battery that seems to have been finished its lifetime to regain some of its life after some time, after the chemistry was able to do its work.

Many different kinds of batteries exist. They are used for different purposes, using different materials, sizes, rated voltages.

See also

[edit | edit source]

Types of batteries

[edit | edit source]

Reversible, rechargeable, irreversible.

Alkaline = Basic

[edit | edit source]

All batteries, except the 9V variety, are 1.5V. As alkaline batteries lose their charge, or in Layman's terms "juice", the voltage outputted slowly decreases. This is what causes flashlights to dim and remote controls to lose their effectiveness whilst using the last of the power left in the battery.


[edit | edit source]


[edit | edit source]

Lithium batteries are available in rechargeable and "normal" forms. Rechargeable lithium-ion batteries are typically found in small to medium electronics, such as laptops, music players, some cordless mice etc. Lithium batteries have little/no proven research of memory effect. Lithium batteries are also available as single decharge cycle batteries. These batteries are marketed as using lithium to increase life span but offer little/no fiscal advantage over standard alkaline batteries.

Nickel Cadmium (NiCd)

[edit | edit source]

Nickel Cadmium batteries are typically not used in the present but were one of the first easily obtainable cheap rechargeable batteries besides the battery. Nickel Cadmium (AKA NiCd) batteries can typically be found in radio controlled cars, older rechargeable "standard" batteries (AA,AAA, etc.), some wireless mice and many other applications. There is a myth that Nickel-Cadmium batteries have a memory effect, where if they are recharged when still somewhat full, they will lose capacity. laboratory research has proven this false in modern batteries (it does happen, to a small extent in old (pre-1950) sintered-plate NiCds). NiCds do suffer from voltage depression, which is similar to the memory effect, but only occurs when the batteries are overcharged.

Nickel Metal Hydride (NiMH)

[edit | edit source]

NiMH batteries can be looked at as a newer version of NiCd batteries. NiMH batteries are often used in the same applications as NiCd batteries. However, they last longer and don't have voltage depression as severe as NiCd batteries.

Lead Acid

[edit | edit source]

Car and boat batteries are the most common forms of lead acid batteries. In both uses they are typically rated for 12V. They have a high current output and can severely injure a person who was to short the circuit.

Memory effect

[edit | edit source]

The memory effect is a myth that states when the battery is not fully charged before it is recharged, it loses capacity, it "forgets" that it has the capacity. The myth also states that the memory effect can sometimes be reversed by using a conditioner. A battery conditioner almost completely discharges a battery (down to 1 volt in NiCds) and are designed to reverse voltage depression, not memory effect. the memory effect does not exist except in very old (1950 or earlier) sintered plate NiCds. There is no need to use a battery conditioner on any battery to reverse memory effect.

For further reading

[edit | edit source]