Notes about these Notes

This is VERY incomplete.


9.2 Production of Materials
9.3 The Acidic Environment
9.6 Shipwrecks and Salvage

9.2 Production of Materials

V 1. Fossil Fuels provide both energy and raw materials such as ethylene, for the production of other substances
* Construct word and balanced equations of chemical reactions as they are encountered
* Identify the industrial source of ethylene from the cracking of some of the fractions from the refining of petroleum
Ethylene (ethene) comes from distilling crude oil or petroleum into its fractions according to each fraction's boiling point. These parts are 'cracked' through catalytic cracking. This involves zeolites as a catalyst. The saturated hydrocarbons are broken into unsaturated hydrocarbons.
* Identify that ethylene, because of the high reactivity of its double bond, is readily transformed into many useful products
Ethylene has two carbon atoms double bonded to each other and two hydrogens bonded to each carbon. The molecule is unsaturated because the carbon double bond can be taken up by two other atoms. It is reactive. Ethylene can be used to create many different organic compounds.
* Identify that ethylene serves as a monomer from which polymers are made
Ethylene has a double carbon bond so can form bonds with other monomers. It can form in long chains through addition reactions. 2 new atoms are added across the double bond converting it to a single bond. It can also form bonds with other monomers to form a wide range of polymers. Polymerisation is the process of turning monomers into a polymer.
Addition polymerisation is adding functional groups across the bond
Condensation polymerisation is bonding functional groups from the monomers so they are eliminated. They occur commonly between Carboxylic acid (COOH) and either alcohol (OH) or amine (NH2)
* Identify polyethylene as an addition polymer and explain the meaning of this term
Polyethylene is formed through addition reactions. This is where monomers bond with each other by adding two atoms across the double carbon bond to turn it into a single carbon bond. Monomers almost fall into place, into a long polymer chain.
* Outline the steps in the production of polyethylene as an example of a commercially and industrially important polymer
For Low Density Polyethylene (LDPE):
100 - 300 degrees C, high pressure: 1500 - 3000 atmospheres. Organic peroxide is used (a free radicle), it breaks the double bond of the ethene and attaches the radicle to one end and an electron to the other end. Then monomers progressively attach to the main monomer.

For High Density (HDPE):
300 Degrees. Uses aluminuim based metal oxide catalysts.

Liquid ethylene (ethylene under high pressure) is heated in the presence of a catalyst to from polyethylene. If the polymer chains are uniform and orderly then it is High Density Polyethylene which is strong and hard. If else, it is Low Density Polyethylene which is softer and is a thermoplastic. Depending on which type of Polyethylene you want you will have to vary the temperature and catalyst. These polymers are used in things such as grocery bags, milk containers, toys, wheely-bins etc.
V Identify the following as commercially significant monomers by both their systematic and common names
* vinyl chloride
Used to make poly vinyl chloride (from the chloroethene monomer), also know as PVC. Rigid and strong when used in conjunction with additives. Used in garden hoses and electrical insulation. It is an ethene with one of the hydrogens as a chlorine atom instead
* styrene
(Vinyl benzene: ethene with the phenyl group instead of a hydrogen) It is used to make styrofoam, insulation, clear plastic casings.
* describe the uses of the polymers made from the above monomers in terms of their properties
Vinyl Chloride is used as electrical insulation and in garden hoses because it does not conduct, resists weathering, fire resistant. It is made into the polymer poly vinyl chloride (PVC).
Polystyrene is rigid and does not conduct, clear or opaque not reactive, low density, resists impacts. So it is used to case things or in cups and foam. It is turned into the polymer polystyrene for use in cups and stuff.
V 2. Some scientists research the extraction of materials from biomass to reduce our dependance on fossil fuels
* discuss the need for alternative sources of the compounds presently obtained from the petrochemical industry
The petrochemical industry relies on fossil fuels such as petroleum and coal. Through their continua use, we are running out of petroleum and petroleum sources. Much of the plastic we create ends up in landfill doing nothing, and they don't break down. It is difficult to incinerate this kind of material as it requires too much energy. For this reason we need to find other sources of plastics.
* explain what is meant by a condensation polymer
A polymer that forms when a functional group of two different monomers reacts and drops a molecule (e.g. water). It this molecule out of the equation the two monomers can now bond together to form a polymer.
* describe the reaction involved when a condensation polymer is formed
Cellulose is made up of lots of glucose monomers. Glucose is C6H12O6 arranged in a ring: 5 carbons and one oxygen in a ring. Off 4 carbons there is a hydrogen atom and a hydroxide molecule. Off one of the carbons is a hydrogen and a CH2OH molecule. The Oxygen in the ring has no attachment.
A reaction occurs and water is eliminated using the attachments to the 1st carbon on the 1st ring and the 4th carbon on the 2nd ring. They all bond together.
* describe the structure of cellulose and identify it as an example of a condensation polymer found as a major component of biomass
Cellulose is made up of glucose which is a ring shape. The glucose are attached to each other by an oxygen.
Biomass is a material that is produced by organisms. It can be plant material or algae or excreta. It is a fuel source that is biodegradable. Fungi can decompose the cellulose that is in biomass.
Acid hydrolysis can convert cellulose into glucose.
* identify that cellulose contains the basic carbon-chain structures needed to build petrochemicals and discuss its potential as a raw material
Cellulose has 5 carbon atoms in it. This can be made into ethane, propane, butane. It can be made into rayon which is used in cellophane. It can also be used to make overhead projector slides.
V 3. Other resources such as ethanol, are readily available from renewable resources such as plants
* describe the dehydration of ethanol to ethylene and identify the need for a catalyst in this process and the catalyst used
Heating ethanol with concentrated sulfuric acid would dehydrate ethanol to give ethene.
C2H5OH -> C2H4 + H2O
The acid attacks the bonds
* describe the addition of water to ethylene resulting in the production of ethanol and identify the need for a catalyst in this process and the catalyst used
To produce ethanol from ethene, water is added with an acid catalyst.
CH2=CH2 + H2O -> CH3CH2OH
This occurs at 300 degrees using sulfuric/phosphoric acid. Acid is used to attack the double bond in the ethene.
* describe and account for the many uses of ethanol as a solvent for polar and non-polar substances
Ethanol has an OH group on the end of it. This enables hydrogen bonding between each molecule. It also means that it can dissolve in water and non-polar substances. Its polar and non-polar properties allow it to dissolve in lots of different liquids.
* outline the use of ethanol as a fuel and explain why it can be called a renewable resource
Ethanol is obtainable from sugar and starch. You can get it from corn and sugar cane and cereals. It comes from plants. Therefore it is renewable.
In the fermentation of glucose to ethanol, carbon dioxide is being produced. It is used as a fuel additive in replacement of lead.
* describe conditions under which fermentation of sugars is promoted
Yeast needs lots of oxygen to reproduce. If you aerate the mixture you will get lots of yeast and then seal it off to stop reproduction of bacteria. It becomes anaerobic. Temperature should remain around 25-30 degrees C. Anything else and the yeast will die. Neutral pH and about 40% sugar is good
* summarise the chemistry of the fermentation process
You begin with glucose (sugar) and water and yeast. It ferments.
C6H12O6 -> 2CH3CH2OH + 2CO2 + heat
* define the molar heat of combustion of a compound and calculate the value for ethanol from first-hand data
∆H = -mC∆T
∆H is the change in heat capacity
m is the mass of...
C is the specific heat capacity of...
∆T is the change in temperature
* assess the potential of ethanol as an alternative fuel and discuss the advantages and disadvantages of its use
Ethanol is a renewable fuel so we wouldn't run out like we would with fossil fuels. It doesn't pollute the air as much and uses up waste products that have no other use.
Unfortunately if we tried to do it on a big scale it would take up large amounts of land and would involve a lot of land clearing. It is also inefficient to make ethanol through fermentation.
* identify the IUPAC nomenclature for straight-chained alkanols from C1 to C8
Carbon prefix: 1 meth-, 2 eth-, 3 prop-, 4 but-, 5 pent-, 6 hex-, 7 hept-, 8 oct-
Location of the double/triple bond is dictated by the starting number.
V 4. Oxidation-reduction reactions are increasingly important as a source of energy
* Explain the displacement of metals from solution in terms of transfer of electrons
Lets take CuS04 for example. This is an ionic solution so the ions are floating around separately in water. They are aqueous. The copper ions have a charge of 2+ and the SO4 has 2-
When two electrons are put into the solution (through an electrode) the Copper ions will take them and change into solid copper.
So when electrons are put into an ionic solution, the metal ion is displaced from the solution as it turns into a solid.

If you put a metal in an ionic solution, sometimes the metal dissolves and the ion turns solid
* Identify the relationship between displacement of metal ions in solution by other metals to the relative activity series of metals
On the activity series of elements, elements that are reactive when put into a solution of a less reactive metal, the metal will displace the ions. e.g. Copper solid in a silver solution will displace the silver ions but in a zinc solution it wont do anything.
* Account for change in the oxidation state of species in terms of their loss or gain of electrons
Oxidation state is used in calculations of redox reactions. It has to do with the number of electrons or the charge of an atom.
Rules for oxidation states:
1. Elements have an OS of 0 e.g. O2 , C, Mg
2. Monatomic ions have their charge as their OS
3. The sum of the OS in a neutral molecule is 0
4. The sum of the OS in a polyatomic ion is the charge of the ion
5. Group I OS = 1
Group II OS = 2
Combined Oxygen OS = -2 (excluding peroxide = -1)
Combined Hydrogen OS = 1 (excluding metal hydrides = -1)

When a atom/ion's oxidation state gets smaller it is reduced and when it is increased it is oxidised.
* Describe and explain galvanic cells in terms of oxidation/reduction reactions
The Cathode of a cell is reduced (gains electrons which are give to the ions in the solution)
The Anode of a cell is oxidised (looses electrons which are transferred to the Cathode. Electrons are taken from the electrode so the ions go into solution)
* Outline the construction of galvanic cells and trace the direction of electron flow
Two beakers: One solution in one and a solution in the other. An electrode usually of the same base atom of each solution is put in each solution. They are connected using a wire. A salt bridge soaked in a solution (usually KNO3) bridges the gap between the two solutions to complete the circuit and balance the charges.
* Define the terms anode, cathode, electrode and electrolyte to describe galvanic cells
Anode: Negative cell; where oxidation occurs
Cathode: Positive cell; where reduction occurs
Electrode: Metal placed in solution in a galvanic cell.
Electrolyte: Solution which electrodes are placed in; ionic solution
V 5. Nuclear chemistry provides a range of materials
* distinguish between stable and radioactive isotopes and describe the conditions under which a nucleus is unstable
Stable isotopes doe not emit radiation while radioactive ones do. There is a graph showing the zone of stability: the ratio of neutrons to protons that give a stable element. For the first 20 elements a rough ratio of 1:1 neutrons to protons gives a stable element. After element 20, there needs to be slightly more neutrons than protons. Elements with an atomic number greater than 83 are generally always unstable as their nucleus is unstable.
* describe how transuranic elements are produced
The creation of transuranic elements involves bombarding the nucleus of one element with another or more simply, just charged particles. Sometimes nuclei are bombarded with just neutrons. Particles are accelerated in cyclotrons to overcome the sub-atomic particle repulsion forces.
* describe how commercial radioisotopes are produced
Medicine uses isotopes to scan people with blood vessel problems. ANTSO uses a nuclear reactor to produce neutrons. These neutrons are absorbed by a nucleus which become commercial isotopes. Neutron deficient isotopes are put in a cyclotron.
* identify instruments and processes that can be used to detect radiation
The first detection of radioactivity came after Antoine-Henri Becquerel stored a photographic plate next to some uranium. Photosensitive items can detect radioactivity. A Geiger counter is another method: It is a tube full of nobel gas. Radiation enters the tube at one end and ionises the gas, causing a current to run and a counter to click. A scintillation counter works by material that gives off light when it comes into contact with radiation.
V identify one use of a named radioisotope:
* in industry
* in medicine
* describe the way in which the above named industrial and medical radioisotopes are used and explain their chemical properties

9.3 The Acidic Environment

V 1. Indicators were identified with the observation that the colour of some flowers depends on soil composition
* classify common substances as acidic, basic or neutral
Acidic: Gastric Juice has a pH of 1.2, Wine is pH 3.5, Rain is slightly acidic at pH 5.8
Basic: Sodium Hydroxide pH 13, Baking Soda is pH 8.5, Limewater pH 10.5
Neutral: Milk pH 6.8, Blood pH 7.5
* identify that indicators such as litmus, phenolphthalein, methyl orange and bromothymol blue can be used to determine the acidic or basic nature of a material over a range, and that range is identified by change in indicator colour
Ranges of these indicators:
Methyl Orange: 3-4 Red to Yellow
Universal Indicator: 8-12.5 Red to Purple
Bromothymol Blue: 6 to 7.5 Yellow to Blue
Phenolphthalein: 8-10 Transparent to Pink
Litmus 4.8-8.2 Red to Blue
* identify and describe some everyday uses of indicators including the testing of soil acidity/basicity
Indicators are used to test the pH of soils in which crops can be grown. Having the wrong pH for plants will kill them. It can be used to test the pH of pool water so people don't burn up when they get in. It may also be used to test pH of chemicals to be poured down the sink. If it is too high or low, the piping might get damaged.
V 2. While we usually think of air around us neutral, the atmosphere naturally contains acidic oxides of carbon, nitrogen and sulfur. The concentrations of these acidic oxides have been increasing since the Industrial Revolution
* identify oxides of non-metals which act as acids and describe the conditions under which they act as acids
On the periodic table, oxides of elements increase in acidity from left to right. This is due to electronegativity. Amphoteric oxides can be basic or acidic. An amphoteric oxide is aluminium oxide. Sodium Oxide is an ionic bond substance which is a strong base. Sulfur Trioxide has covalent bonds and reacts with water to change into HSO4- and H+, making it acidic.
* analyse the position of these non-metals in the Periodic Table and outline the relationship between position of elements in the Periodic Table and acidity/bacisity of oxides
Acidic oxides tend to be from the right of the periodic table while basic oxides tend to be from the left. This is because the electronegativity of the atoms involved in the reactions with water dictate how electrons are shared or taken. Atoms on the left will form ionic bonds where the oxygen ion is a strong base and combines with H+. On the right, covalent bonds form and the entire molecule will react with the water to form H+.

9.6 Shipwrecks and Salvage (Option)

9.6 Shipwrecks, Corrosion and Conservation
V 1. The chemical composition of the ocean implies its potential as an electrolyte
V identify the origins of the minerals in oceans as:
The ocean is a steady state system: the minerals in it are in constant balance. Sedimentation and sea spray are ways that the ocean loses minerals, while these are the ways that the ocean gains minerals:
* leaching by rainwater from terrestrial environments
The rain that falls on land filter through rock and soil, dissolving salts and minerals such as calcium carbonate from limestone and chloride salts. These minerals are taken to rivers which find their way to the ocean. The dissolved minerals are brought to the ocean with the water.
* hydrothermal vents in mid-ocean ridges
Hydrothermal vents spout magma into the ocean which quickly hardens and cracks. Water seeps into these cracks and dissolves the minerals there. This water then returns to the ocean. There are sulfide minerals in it.
* outline the role of electron transfer in oxidation-reduction reactions
Oxidation is the loss of electrons, reduction is the gain of electrons (OIL RIG). When a substance is oxidised, it transfers electrons to another substance which is reduced.
* identify that oxidation-reduction reactions can occur when ions are free to move in liquid electrolytes
An electrolyte is a usually liquid substance that can conduct electricity. A better definition os a substance that when molten or dissolved, can conduct an electric current and is decomposed by this current (into ions). Sometimes when molecules are dissolved they break into ions of separate/opposite charges.
In an electrolyte, ions are free to move around in the solution. When a current is applied, the ions move towards the corresponding electrode in the solution which is how the charge is carried. No free ions means no reaction.
* describe the work of Galvani, Volta, Davy and Faraday in increasing understanding of electron transfer reactions
Galvani experimented with frog legs in the 1780's. He investigated static charges on muscles and nerves in frog legs. The legs twitched when touched with charge. If 2 different metals touched on the leg at the same time, they would twitch too. He thought there was animal electricity being produced in the muscles. He was the first to investigate this type of thing and led the way for others to do so. He realised a closed circuit was needed to conduct electricity.
Volta worked with Galvani but disagreed with some of his ideas. He thought it was the metals contacting that provided the electricity. He invented the Voltaic Pile, a zinc disk with a copper disk with a salt solution between them repeated over and over produced electricity like a battery. He incorrectly concluded that it was the metals doing this but it was actually the salt solution. Proved that electricity could be made without animals present.
Davy corrected Volta's hypothesis and found out how to do electrolysis by passing a current through a compound. This decomposed the compound. He realised that current could be produced if the electrolyte could oxidise one of the metals. Discovered the relationship between metal reactivity and voltage produced. Formed the first samples of potassium, sodium, magnesium etc. First to do cathodic protection of ships.
Faraday related amount of oxygen produced at an electrode and the amount of electricity in the cell. Created the laws of electrolysis. He produced the terminology we use today.
V 2. Ships have been made of metals or alloys of metals
* account for the differences in corrosion of active and passivating metals
Corrosion is the oxidation of metals or other substances by oxygen or water usually, which can destroy the substance slowly. An active metal will readily react with the environment of oxygen and water. Iron is an active metal that will form a porous layer on its surface. This layer is red brown and is iron(III) oxide. It allows water and oxygen through which causes further destruction of the metal underneath. Passivating metals form a protective layer on the surface which is inert, non-porous and adhesive to the surface of the metal. Water and oxygen cannot get through this layer so the metal is protected from further corrosion. Metals such as aluminium and chromium do this.
* identify iron and steel as the main metals used in ships
Iron and steel are the main metals used in ships. Back in the day there were wooden ships by they rotted. Iron first became used in the 1860's in ships. They are used because the metals are quite hard, strong, can be welded and can be moulded well.
* identify the composition of steel and explain how the percentage composition of steel can determine its properties
To make steel you get pig iron and turn it into cast iron. From here it can become wrought iron or steel. Steel always has some carbon in it along with other metals to give it special properties. With more carbon in the lattice, the steel becomes harder and stronger but more brittle and less ductile. Nickel, chromium, manganese can all be added for strength. Cast iron has silicon in it. Stainless steel has chromium in it to resist corrosion. Wrought iron can resist corrosion and can hold protective coatings better than low carbon steel. Cobalt steel has better magnetic properties due to cobalt presence. Added copper increases corrosion resistance and strength.
* describe the conditions under which rusting of iron occurs and explain the process of rusting
Rusting occurs when the surface of the metal can become anodic. An impurity should be present on the metals surface to provide a cathode area. Electrons should flow through the metal and there should be an electrolyte present. Iron rusts with water and oxygen and this is accelerated in acidic environments, higher temperatures, more impurities, contact with a less active metal or there is stress in the metal. Iron is oxidised at the anode. At the cathode (somewhere on the surface of the metal): O2(g) + 2H2O(l) + 4e- -> 4OH-(aq)
The iron ions bond with the hydroxide ions to form iron(II) hydroxide which is further oxidised to iron(III) hydroxide by reduction of oxygen.
V 3. Electrolytic cells involve oxidation-reduction reactions
* describe, using half equations, what happens at the anode and cathode during electrolysis of selected aqueous solutions
During the electrolysis of water into oxygen and hydrogen:
At the cathode (here it is the negative electrode) reduction occurs:
2H2O + 2e- -> H2 + OH-
At the anode (here it is the positive electrode) oxidation occurs:
2H2O -> O2 + 4H+ + 4e-
V describe the factors that affect an electrolysis reaction
* effect of concentration
If the concentration of electrolyte in an electrolysis reaction is increased then the ions in solution will more likely be reduced than water
* nature of electrolyte
Molten liquid salts allow for the most oxidation-reduction reactions to occur as only the ion is present to be reduced. Aqueous solutions may have the water oxidised or reduced depending on the activity of the substance.
* nature of electrodes
Sometimes electrodes can react. Metals do not accept electrons so usually the anode will be oxidised if it is a reactive metal.
V 4. Iron and steel corrode quickly in a marine environment and must be protected
V identify ways in which a metal hull may be protected including:
* corrosion resistant metals
You can coat stuff in tin which adheres tightly to the surface of a metal to protect it. It is less active than iron so it will protect it. Unfortunately is water can get into a scratch or crack, the tin acts as a cathode in a galvanic cell where the hull of a ship is the anode. This increases corrosion of the hull. It is better to use a metal such as Zinc which galvanises the hull. It is more reactive than iron so if it is scratched the zinc will corrode instead of the iron. Instead of coating, you can attach a block of zinc to the hull and this acts as a sacrificial anode.
* development of surface alloys
Certain steel alloys can be made that create a passivating layer for themselves to protect itself.
* new paints
Paint can protect a hull by not allowing water to contact the iron. Enamel coatings are stronger than normal paints. It can prevent biofouling by replacing sacrificial anode technology.
* predict the metal which corrodes when two metals form an electrochemical cell using a list of standard potentials
You know that list of standard potentials you get in the HSC?
For GALVANIC CELLS: When faced with two metals, the ionic equation with the most negative emf will corrode more easily. That means the higher on the table, the better it will oxidise or corrode. (Make sure the equation is pointing the right way, if it isn't, reverse the negative sign on the emf)
* outline the process of cathodic protection, describing examples of its use in both marine and wet terrestrial environments
Cathodic protection involves turning a metal that would usually be an anode into a cathode. There are many ways of doing this.
Sacrificial anodes: you can stick a block of magnesium on an iron hull and it will corrode in preference to the iron. This is because it is higher on the standard potentials table (more reactive). The magnesium is oxidised while water is reduced at the cathode which is iron. If Iron ions do form, they are reduced.
Impressed current: An electric current forces electrons into the the metal that would have been the anode but is now the cathode due to the current. An inert electrode can be used for the anode so it isn't oxidised, water is instead. Water is reduced at the cathode too.
Both methods can be used on pipes buried in the ground or on ships.
* describe the process of cathodic protection in selected examples in terms of the oxidation/reduction chemistry involved
In a sacrificial anode (magnesium) bolted to an iron hull, the magnesium (the anode), is oxidised and the electrons it loses are taken to the cathode (the iron). If there are iron ions here, they will be reduced. If not, water will be reduced.
In an impressed current situation, an inert electrode becomes the anode. Because it is inert, it does not get oxidised, instead, water is. Water is also reduced at the cathode.
V 5. When a ship sinks, the rate of decay and corrosion may be dependent on the final depth of the wreck
V outline the effect of these on the solubility of gases and salts:
* temperature
As the temperature of the solvent increases, the solubility of gases decreases
As the temperature of the solvent increases, the solubility of salts increases
Think of it this way: cold fizzy drinks are better because they are more fizzy: they have more gasses dissolved, it is easier to dissolve salt or milo or something like that is the water/milk is hot.
* pressure
As pressure increases, the solubility of gases increases.
Pressure has no effect on salt solubility.
* identify that gases are normally dissolved in the oceans and compare their concentrations in the oceans to their concentrations in the atmosphere
There are gasses dissolved in the ocean, but are much less concentrated there than in the atmosphere. Carbon dioxide is relatively more abundant in the sea than in the atmosphere because it creates carbonic acid when dissolved.
* compare and explain the solubility of selected gases at increasing depths in the oceans
There are bigger concentrations of gases at the surface of the ocean. Wind whips the surface allowing greater contact between the two spheres. Towards the middle of the ocean oxygen concentrations decrease due to fish respiration and bacterial action. After 1km depth, oxygen begins to increase due to ocean currents
* predict the effect of low temperatures at great depths on the rate of corrosion of a metal
Deep down in the ocean, oxygen concentration is really low. This reduces the corrosion of iron a lot because it needs oxygen to keep going. Lower temperatures also inhibit the reaction as reaction rates are slower at lower temperatures. So ships should corrode slower deep down than at the surface right? Of course, all of this is theoretical.
V 6. Predictions of slow corrosion at great depths were apparently incorrect
* explain that ship wrecks at great depths are corroded by electrochemical reactions and by anaerobic bacteria
Anaerobic bacteria a a bacteria type that thrive on little oxygen. They 'eat' sulfate ions. They eat by oxidising sulfate ions to sulfur and then sulfite (sulfur ions). Where do the bacteria get the electrons to make this happen? By oxidising the iron hull of the ship. So corrosion of the ship still happens because of bacteria.
* describe the action of sulfate reducing bacteria around deep wrecks
The sulfate reducing bacteria oxidise the iron of the ship hull to form sulfate ions. These react with the iron ions to form black FeS. In non-acidic environments they do this:
SO42- + 4H2O + 8e- -> S + 8OH-
SO42- + 5H2O + 8e- -> HS- + 9OH-
* explain that acidic environments accelerate corrosion in non-passivating metals
Sometimes the environment a ship is might be acidic, or in the action of sulfate reducing bacteria H+ ions are created.
SO42- + 8H+ + 6e- -> S + 4H2O
The electrons are here provided by the iron. The S bonds with 2H+ ions to form H2S which reacts with metals to cause damage to them.