AQA Electrolysis

Electrolysis
Electrolysis is the decomposition of a liquid by passing an electric current through it. The liquid that is decomposed (broken down) is known as the electrolyte. The electrolyte must have ions that are free to move so that it is able to conduct electricity.

The negative electrode is called the cathode. Positively charged ions called cations move towards it.

The positive electrode is called the anode. Negatively charged ions called anions move towards it.

Try to remember that opposites attract and the acronym PANCAKE;

  • Positive
  • Anode
  • Negative
  • Cathode

The voltage across the two electrodes causes the ions to move and so carry charge through the liquid. When the ions reach the oppositely charged electrodes they either gain or lose electrons;

  • Cations (positively charged ions) are attracted to the cathode where they gain electrons.
  • Anions (negatively charged ions) are attracted to the anode where they lose electrons.

Water contains hydrogen ions (H+) and hydroxide ions (OH-). The hydrogen ions are attracted to the cathode, where they gain electrons and form hydrogen gas (H2). The hydroxide ions are attracted to the anode, where they lose electrons and oxygen gas is formed (O2).

This can be shown by the use of half equations;

At the cathode: 2H+ (aq) + 2e- H2(g)

At the anode: 4OH- (aq) – 4e- O2(g) + 2H2O (l)

Notice that both the number of atoms and the charges must be balanced in each half equation.

Molten electrolytes:

When ionic substances are solid, their ions are in fixed positions and therefore cannot move, this means they cannot carry a charge so can not be electrolysed. However, if the substance is heated until it melts, the ions are then free to move and the substance can now be electrolysed.

During electrolysis of molten ionic substances the metal ions move to the negative electrode. When molten aluminium oxide is electrolysed, aluminium forms at the negative electrode and oxygen at the positive.

The half equations for this reaction would be;

Al3+(l) + 3e- Al(l)

2O2-(l) – 4e- O2(g)

Note that the state symbols are l (liquid) because the solid has been melted into a liquid and not dissolved as in elements with the “aq” state symbol.

Electrolysis of aqueous copper (II) sulfate:

Copper (II) sulfate, CuSO4, is an ionic compound that dissolves in water to form a blue aqueous solution. During this electrolysis, copper is formed at the cathode, which becomes coated with copper. Oxygen is formed at the anode and bubbles of gas can be seen being released to the surface at the anode.

Copper is less reactive that hydrogen, so it is produced at the cathode during electrolysis of aqueous copper (II) sulfate. However, if the metal in solution is more reactive than hydrogen (such as potassium or sodium), or there is no metal in solution, hydrogen gas is produced instead. (Don’t forget hydrogen is formed from the water present in aqueous electrolytes).

If the aqueous solution contains chloride ions, chlorine gas is produced at the anode. If not, oxygen gas is produced instead. This table shows some examples;

Electrolyte / Produced at cathode / Produced at anode
CuCl2 (aq) / copper / chlorine
CuSO4 (aq) / copper / oxygen
KNO3 (aq) / hydrogen / oxygen
NaOH (aq) / hydrogen / oxygen
H2SO4 (aq) / hydrogen / oxygen

Energy transfers – fuel cells
Hydrogen is a highly flammable gas that will react explosively with oxygen when ignited with a spark or a flame and produce water vapour. The reaction is exothermic, meaning that is releases energy to the surroundings. It is difficult to use hydrogen as a fuel in this way. Some cars use hydrogen instead of petrol in their engines, but this is less efficient than using a fuel cell.

The hydrogen – oxygen fuel cell.

A fuel cell produces electrical energy efficiently from an exothermic reaction. When oxygen or air is supplied into the hydrogen – oxygen fuel cell, the hydrogen and oxygen react and the energy released is used to produce a potential difference or voltage. This causes a current to flow when the fuel cell is connected in a circuit.

The overall reaction that takes place in the hydrogen – oxygen fuel cell is the same one that happens when hydrogen explodes in air, although in a fuel cell it happens in a much more controlled way;

hydrogen + oxygen  water

2H2 (g) + O2 (g)  2H2O (g)

At the electrodes:

Hydrogen molecules lose electrons at the anode and become hydrogen ions:

H2(g) – 2e- 2H+ (aq)

This is an oxidation reaction because hydrogen has lost electrons. These electrons then pass through the electrical circuit and the hydrogen ions pass through a special artificial membrane, which will not allow hydrogen or oxygen gas to pass through.

When the hydrogen ions reach the cathode, they combine with oxygen and electrons from the electrical circuit:

4H+(aq) + O2(g) + 4e- 2H2O(g)

This is a reduction reaction because electrons are gained. The overall reaction is an example of a redox reaction. Reduction happens at the cathode while oxidation happens at the anode.

Energy level diagrams:

The energy involved in a chemical reaction can be represented using an energy level diagram. The energy level diagram for the exothermic reaction found in a hydrogen-oxygen fuel cell is shown below;

Fuel cells in space:

Spacecraft rely on hydrogen – oxygen fuel cells for their electricity. There are several advantages that make them particularly suited for space;

  • They are lightweight.
  • They are compact.
  • They have no moving parts.
  • They provide drinking water for the astronauts.

Fuels cells on Earth:

There is also lots of interest from the car industry. Most vehicles currently use petrol or diesel which are both hydrocarbons made from crude oil. Crude oil is not only a non – renewableresource, combustion of it also produces carbon dioxide, which is a green house gas that has been linked to global warming and climate change. The advantages of using hydrogen – oxygen fuel cells instead of petrol or diesel include;

  • Water vapour is the only waste product.
  • There are no emissions of carbon dioxide from the car.
  • There is a large potential source of hydrogen (as water can be decomposed by electrolysis).

However it is important to remember that the use of hydrogen-oxygen fuel cells will still produce some pollution. There are two main sources;

  • Fuel cells contain poisonous catalysts that need to be disposed of at the end of the life time of the fuel cell.
  • Production of the hydrogen and oxygen required will involve the use of energy which will most likely have come from the burning of fossil fuels.

Redox reactions

A redox reaction is one in which both oxidation and reduction happen at the same time. Reduction can be described as the removal of oxygen from a substance or the gaining of electrons and oxidation can be described as either the gain of oxygen by a substance, the reaction of a substance with oxygen or the lose of electrons.

An oxidising agent is a substance that can remove electrons from other substances, oxidising them.

A reducing agent is a substance that can give electrons to other substances, reducing them. Look at this example of a redox reaction involving electrons;

Cl2 (g) + 2Fe2+ (aq)  2Cl- (aq) + 2Fe3+ (aq)

chlorine + Iron(II)  chloride + Iron(III)

reduced oxidised
oxidising agent reducing agent

Chlorine gains electrons and is reduced to chloride ions. While iron(II) ions loses electrons and are oxidised to iron(III) ions.

Displacement reactions:

When a strip of zinc foil is dipped into copper (II) sulfate solution, it becomes coated in copper and the blue colour of the solution gradually fades. This is because of a type of redox reactions called a displacement reaction.

zinc + copper (II) sulfate zinc sulfate + copper

Displacement reactions occur because a more reactive metal can displace a less reactive metal from its compound. In this example, zinc is more reactive that copper so displaces copper from copper (II) sulfate. The sulfate swaps places from the less reactive metal to the more reactive one. Displacement reactions will only occur if the most reactive metal is separate from the compound.

Diagram showing reactivity series of 5 common metals

Rusting:

Iron and steel both rust when they come into contact with both oxygen and water. Rusting is a redox reaction because the iron atoms are oxidised to iron (III) ions and the oxygen atoms are reduced to oxide ions. Rust is hydrated iron (III) oxide;

Iron + oxygen + water  hydrated iron (III) oxide

Preventing rust:

Many methods of rust prevention rely on stopping air and water reaching the surface of the iron. These include;

  • Coating the metal part with oil or grease.
  • Painting the surface of the metal part.
  • Plating the surface with zinc (galvanising).
  • Plating the surface with tin.

Some iron alloys are resistant to rusting. Stainless steel contains chromium. This oxidises to chromium oxide when exposed to air, forming a thin film on the surface of the steel. The layer stops air and water reaching the metal below.

Sacrificial protection – galvanising:

Galvanising involves coating the surface of the iron or steel object with a layer of zinc. The zinc coating stops air and water reaching the metal below but it also does something else. Zinc is more reactive than iron, so it is more likely to be oxidised. It sacrifices itself to protect the iron below. Galvanising is used to protect car body panels before painting and to protect metalwork that will be left outside. Sacrificial protection will also work with other metals as long as they are more reactive than the iron. Because Tin is less reactive than iron, if it were used the iron would rust even faster if the tin layer were broken or scratched.

Alcohols

Alcoholic drinks contain the alcohol, ethanol. All alcohols contain carbon, hydrogen and oxygen bonded together. Alcohols often burn well and as such ethanol is used as a fuel for cars. It is also used as a solvent in perfumes and aftershaves.

Making alcohol:

Fermentation is one method of making ethanol. Fermentation requires the use of yeast as a catalyst. Yeast contains enzymes that can convert glucose to carbon dioxide and ethanol, as long as oxygen is kept out (oxygen must be kept out to prevent the ethanol being oxidised to ethanoic acid):

Glucose  carbon dioxide + ethanol

C6H12O6 2CO2 + 2C2H5OH

Below is a picture of the equipment required to carry out fermentation in the school lab. The delivery tube is placed in limewater to allow the carbon dioxide being produced to escape and also prevent any oxygen from getting into the conical flask. The optimum temperature for this reaction is between 25oC and 50oC. Any lower and the enzymes will not work and any higher will cause them to denature.

Fermentation is a slow process and a few days are needed to make a sufficient amount of ethanol to be able to test. The ethanol will eventually kill the yeast and this sinks to the bottom of the conical flask. Ethanol is easily separated from the mixture by fractional distillation as water boils at 100oC but ethanol boils at only 78oC.

Hydration of ethane:

Ethanol can also be made by passing ethene with steam over a hot phosphoric acid catalysis and causing a hydration reaction;

ethene + water  ethanol

C2H4 + H2O  C2H5OH

This method is used to produce ethanol for industrial use, whereas fermentation is used for the ethanol used to produce alcoholic drinks. Each of these methods have their advantages and disadvantages and a table of these is shown below;

Feature / Fermentation of sugars / Hydration of ethene
Conditions / Low temperatures & normal pressure / High temperature and high pressure
Raw materials / Sugar from plants (renewable) / Ethene from crude oil (non renewable)
Purity of product / Low – needs filtering & distillation / High – no by products are made
Percentage yield / Low – approx 15% / High -100%
Atom economy / 51% / 100%
Type of process / Batch / Continuous

More alcohols

No. of carbon atoms / Molecular formula / Displayed formula / Name
1 / CH3OH / / methanol
2 / C2H5OH / / ethanol
3 / C3H7OH / propanol
4 / C4H9OH / butanol
5 / C5H11OH / pentanol

Notice the general formula for alcohols is CnH2n+1OH.

Depletion of the ozone layer

Ozone is a form of oxygen with the formula O3 as opposed to the more usual O2. O3 is a pale blue gas that has a smell similar to bleach. Ozone is found throughout the atmosphere, but it is most concentrated in the stratosphere (the part of the atmosphere 10 – 50km above the Earth’s surface – also called the ozone layer).

Sunlight contains ultraviolet (UV) light, as well as visible light. UV light causes O2 to react to form O3. This happens in two stages, but overall three O2 molecules, produce two O3 molecules;

3O2  2O3

The ozone layer absorbs most of the UV light in sunlight, stopping it reaching the Earth’s surface.

CFCs.

CFC is short for chlorofluorocarbon. CFCs contain carbon atoms with chlorine and fluorine atoms attached. There are many different types of CFC (used in refrigerators and aerosols in the past) but they all cause damage to the ozone layer. CFC molecules spread out in the atmosphere after release. The UV light in the stratosphere breaks the carbon-chlorine bonds in the CFC molecules;

CCl3F  CCl2F + Cl

Highly reactive chlorine atoms called free radicals are released and these react with ozone molecules and break them down to form oxygen, seen below;

Step 1: ∙Cl + O3∙ClO + O2

This reaction sets up a chain reaction that goes on to use up more ozone, therefore letting more UV light react the surface of the Earth (causing it to become warmer) and produce yet more chlorine free radicals meaning the reaction can continue and destroy more of the ozone layer;

Step 2: ∙ClO + O3 2O2 + ∙Cl

Steps 1 and 2 can be combined to give the overall equation;

2O3 3O2

Increased levels of UV light can cause a number of medical problems, including;

  • Skin cancer
  • Increased risk of sunburn
  • Increased risk of eye cataracts
  • Faster ageing of skin

Although there are many different types of CFC they all have similar properties, including;

  • Low boiling points (they are usually gases)
  • They are insoluble in water
  • They are chemically inert, so they don’t react easily with other substances.

The Montreal protocol:
Due to their lack of reactivity CFCs can only be removed from the stratosphere very slowly. Scientists first realised in the 1970s that CFCs were depleting the ozone layer so in a meeting in Montreal in 1987, 24 countries signed a treaty, restricting the use of CFCs. Now nearly all countries have signed this treaty and the ozone is starting to show signs of recovery.

Natural fats and oils

Whether a substance is classified as an oil or fat depends on its state at room temperature;

  • Fats are solid at room temperature
  • Oils are liquid at room temperature.

Fats and oils are esters. These are compounds that form when a carboxylic acid reacts with an alcohol. Some esters are used in perfumes as solvents, these are simple esters such as ethyl ethanoate, made from ethanol and ethanoic acid. Fats and oils are more complex. They consist of fatty acids chemically joined to glycerol, which is an alcohol. Fatty acids have long chains of carbon atoms. Glycerol has three hydroxyl groups (–OH), whereas most alcohols only have one.

Saturated or unsaturated?

Fats and oils can be saturated or unsaturated:

  • In a saturated fat or oil, all the carbon-carbon bonds in the fatty acid parts are single covalent bonds.
  • In an unsaturated fat or oil, one or more of the carbon-carbons bonds in the fatty acid part is a double covalent bond.

This is similar to alkenes and alkanes. Unsaturation is alkens, fats and oils can be shown using bromine water;

  • Bromine water remains orange when mixed with saturated samples.
  • Bromine water turns from orange to colourless when mixed with unsaturated samples.

The bromine reacts with the carbon-carbon double bond in an addition reaction, producing a dibromo compound. The reaction is shown below;

Fats and oils are an important part of a healthy diet, but too much of them can cause health problems and make us overweight. Saturated fats, mainly found in meat and dairy products, can raise the levels of cholesterol in the blood, which can lead to blocked arties and heart disease.
Unsaturated fats and oils, mainly found in vegetables, fruits and nuts, tend to be more healthy and some are thought to help reduce the risk of heart disease.

Emulsions:

If vegetable oil and water are poured into a beaker, the vegetable oil forms a layer on top of the water. This is because the oil is less dense than the water and the two liquids are immiscible (they do not dissolve into each other). However, if they are vigorously shaken together, they form a mixture called an emulsion.

In an emulsion, tiny droplets of one of the liquids are dispersed throughout the other liquid. The two liquids will eventually settle out into separate layers, unless they are stabilised by an emulsifier.

There are two different types of emulsion depending on which liquid forms droplets and which surrounds the droplets;

  • Oil in water emulsions consist of tiny droplets of oil dispersed in water (such as milk).
  • Water in oil emulsions consist of tiny droplets of water dispersed in oil (such as butter).

Saponification:
Soap is made when oils or fats react with hot sodium hydroxide solution. The reaction splits the glycerol and sodium salts of the fatty acids. These sodium salts are the soap and the process is known as saponification, it is an example of a hydrolysis reaction;

Fat + sodium hydroxide  soap + glycerol

C6h – Detergents

Detergents are used in washing up liquids and washing powders. They have the ability to surround fat or oil molecules in stains and remove them from the clothes or plates, etc.