Chemistry Notes

Humans have always exploited their natural environment for all their needs including food, clothing and shelter. As the cultural development of humans continued, they looked for a greater variety of materials to cater for their needs.

The twentieth century saw an explosion in both the use of traditional materials and in the research for development of a wider range of materials to satisfy the specialist needs of space travel and the information technologies. Added to this was a reduction in the availability of the traditional resources to supply the increasing world population.

Chemists and chemical engineers continue to play a pivotal role in the search for new sources of traditional materials such as those from the petrochemical industry. As the fossil organic reserves dwindle, new sources of the organic chemicals presently used have been found. In addition, chemists continually searching for compounds to be used in the design and production of materials to replace those that have been deemed no longer satisfactory for needs.

1. Fossil fuels provide both energy and raw materials such as ethene, for the production of other substances.

· Identify the industrial source of ethene from the catalytic cracking of some of the fractions from the refining of petroleum.

Ethene is obtained from refined petroleum via catalytic cracking (use of catalyst to break larger chains into smaller ones, to meet demand) NOTE that ethene is a by-product of cracking used to produce other alkanes.

(1) When an alkane is cracked,

(2) there are 2 more hydrogens required where bond was broken

therefore, 1 hydrogen moves from one cracked part to the other

(3) the one missing 2 Hs will form a double bond, to make an alkane and an alkene

· Explain that catalytic cracking is an example of an industrial process that involves surface reactions with inorganic catalysts.

The catalysts in the industrial process of catalytically cracking petroleum are structured aluminosilicate called zeolites. This structure causes zeolites to have large surfaces. The cracking reaction occurs on the inner surfaces of the catalysts, thus a surface reaction. Since the zeolites are solid and the reactants are liquid/gas, this reaction is heterogeneously catalysed.

Example:

Decane zedite catalyst ethene + 2-methylheptane

500°C

· Explain the relationship between the properties of alkanes and alkenes and their non-polar nature and weak dispersion forces between molecules.

· Identify data, plan and perform a first-hand investigation to compare the reactiveness of appropriate alkenes with the corresponding alkanes in bromine water in solution.

Since alkanes and alkenes are non-polar and only have weak dispersion forces between their molecules, they have similar (low) boiling points and melting points, as well as similar solubility (in non-polar solvents, not soluble in water). Short chains (1-4) are gases, and higher are liquids.

Alkenes are much more reactive than alkanes. While alkanes are useful since they burn in air, alkenes react much more readily with halogens (and decolourise Cl, Br, I). Alkanes require UV light and undergo a substitution reaction, while alkenes allow addition reactions by easily breaking their double bonds.

C6H14(l) + Br2(soln) --> C6H13Br(soln) + HBr(aq)

Hexane + Bromine --> 1-Bromohexane + Hydrogen bromide

CH3–CH2–CH=CH-CH2–CH3 + HCl --> CH3 –CH2-CH2-CH-CH2-CH3

|

Cl

3-Hexene + Hydrogen chloride --> 3-chlorohexane

Alkanes

· Saturated hydrocarbons of the general formula CnH2n+2 (where n = 1, 2, 3…)

· Commonly called paraffins (Latin: little reactivity) because of their relative unreactivity

· They are non-polar molecules, single covalent bonds linking carbon atoms with hydrocarbon chain. Dispersion forces exist between alkane molecules, strength of this force increases with increasing molar weight, therefore melting and boiling points increase as chain length increases

· Isomers exist because chains may branch

· Dissolve in other non-polar solvents but insoluble in water. Long chain solids will dissolve in liquid alkanes of shorter chain length

· Obtained by fractional distillation of crude oil

Reactions

For alkanes there are only two important reactions:

1. Alkanes burn in air to form carbon dioxide and water. In doing so they liberate large amounts of heat, for example:

For propane: C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)

For octane: C8H18(g) + 121/2O2(g) 8CO2(g) + 9H2O(g)

It is this combustion reaction that makes alkanes useful to us as fuels.

2. Alkanes react with chlorine, bromine and iodine when the mixtures are exposed to ultraviolet (u.v.) light, for example:

For methane: CH4(g) + Cl2(g) CH3Cl(g) + HCl(g)

(chloromethane)

For hexane: C6H14(l) + Br2(soln) C6H13Br(soln) + HBr(aq)

(bromohexane)

A variety of products can be formed by these reactions, depending on conditions used. Alkanes do not react with (decolourise) chlorine, bromine or iodine solutions in the absence of u.v. light but they do react (quite slowly for the liquid ones) in the presence of u.v. light. This contrasts with the behaviour of alkenes.

Alkenes

· Unsaturated hydrocarbons of general formula CnH2n (where n = 2, 3, 4…). Molecules are unsaturated because they do not contain maximum number of hydrogen atoms per molecule

· Commonly called olefins (Latin: oil-loving) due to oily nature of liquid alkenes. They are reactive molecules.

· They are non-polar molecules in which one double covalent bond links a pair of carbon atoms. The presence of this double bond has an effect on the dispersion forces between molecules. They tend to have similar or slightly lower boiling points than alkanes of the same chain length

· Isomers exist because chains may branch

· Dissolve in non-polar solvents and are insoluble in water

· Obtained by chemical processes involving the breakdown of products derived from crude oil distillation

Reactions

The presence of the double bond in alkenes makes them very reactive. There are many substances that react with alkenes by opening out the double bond to form two single bonds. These are called addition reactions.

· Identify that ethene, because of the high reactivity of its double bond is readily transformed into many useful products.

Ethene’s double bond allows a high reactivity in addition reactions with various important compounds.

(1) Forming ethanol:

performed industrially at high pressure/temperature, using H3PO4

(2) Ethylene oxide:

reacted with water (H+ catalyst) to form ethylene glycol (1,2-ethanediol), used in polymer manufacture

(3) Vinyl chloride:
(not an addition reaction)

used in making PVC – poly(vinyl chloride)

· Identify that ethene serves as a monomer from which polymers are made.

Ethene is the monomer used in making poly(ethene) , or polyethylene . It is also the basis for other monomers (eg. vinyl chloride) that make up polymers.

· Identify poly(ethene) as an addition polymer and explain the meaning of this term.

Polyethene is called an addition polymer. Like addition reactions, addition polymers are formed by joining molecules, without the loss of any atoms (each double bond opens and joins another).

· Outline the steps in the production of poly(ethene) as an example of a commercially and industrially important polymer.

Polyethene is the systematic name for this substance, but polyethylene is the IUPAC (International Union of Pure and Applied Chemistry) preferred name.

The most important reaction of ethene is its conversion to polyethylene (or polyethene) in a reaction that is called polymerisation.

Polymerisation is a chemical reaction in which many identical small molecules combine together to form one large molecule. The small molecules are called monomers while the large product molecule is called a polymer. These polymers are essentially long alkane molecules: each molecule contains from a few hundred to a few thousand monomer units.

Production of poly(ethene)

branched chains (amorphous)

· gas phase process

· high temperature, high pressure (300C, 2000atm)

· initiator (organic peroxide) used

· causes a lot of chain branching

· uses:

– wrapping materials

– carry bags

– milk bottles, squeeze bottles

linear chains (crystalline)

· Ziegler-Natta process

· moderate temperatures & pressures (60C, ?atm)

· catalysed (TiCl3 & tryalkylaluminium compound)

· causes unbranched polymers, packing in orderly fashion: higher density, more crystalline

· uses:

– kitchen utensils

– toys

– grocery bags

– building materials

Single strands are held together to other strands by strong dispersion forces.

High Density Polyethene (HDPE) is made of closely fitting polyethene chains and is heavier.

Low Density Polyethene (LDPE) has chains with many cross-links and side chains and is more flexible but weaker.

· Identify the following as commercially significant monomers: vinyl chloride, acrylonitrile and styrene (by both there systematic and common names).

· Account for the uses of the polymers made from the above monomers in terms of their properties.

Polymer

Monomer

Properties

Uses

Poly(vinyl chloride)

Vinyl chloride

[chloroethene]

CH2=CH-Cl

C2H3Cl

Resists weathering, fire resistant, cheap to make and process, electrical insulator

Electrical insulation, sewerage pipes, guttering, garden hoses

Polyacrylonitrile

Acrylonitrile

[propenenitrile]

CH2=CH-CN

C3H3N

Forms fibres

Wool substitute in clothing, soft furnishings, carpet

Polystyrene

Styrene

[phenylethene]

CH2=CH-

C8H8

Rigid, electrical insulator, low cost, not chemically reactive, heat and cold insulator, low density, resists high impact

Tool (screwdriver) handles, foam cups, foam packing, containers, insulation, toys, vacuum flasks

Properties of polymers relating to use:

(1) melting point

(2) stability in heat and light

(3) chemical stability

(4) mechanical strength

(5) rigidity

These relate to:

(1) Average molecular weight (chain length)

· number of monomer units in one molecule

· longer chain -> higher melting point -> harder substance

(2) Crystallinity (chain branching)

· less branching -> crystallinity -> higher density -> harder substance

· more branching -> amorphous -> flexible

· eg. HD polyethylene ‘crackles’, and LD is softer and ‘clingy’

(3) Chain stiffening

· chain stiffening is putting a side group into a linear chain to reduce flexibility (eg replacing H with CH3, polyethylene -> polypropylene)

· causes rigidity and stiffness

· [PVC is plasticised to soften it]

(4) Crosslinking

· linear chains connected to form 2D-network

· more common in condensation polymers than addition polymers

· increases rigidity, or causes elasticity (used in rubber/elastic)

Stability:

· PVC: C-Cl bonds are weaker than C-H and break with UV light.
Additives are used to protect PVC from sunlight
In high temperatures, forms HCl, extremely corrosive

· Acrylics: when heated strongly, forms hydrogen cyanide, extremely poisonous gas

Some scientists continue to research the extraction of materials and energy from biomass to reduce our dependence on fossil fuels

· Discuss the need for alternative sources of the compounds presently obtained from the petrochemical industry.

At society’s current rate of consumption of crude oil-based products (mostly fuels for transportation), there is concern that it will all be used up in a few decades and cost will increase. While alternative fuels are being researched, this does not help the industries based on by-products of crude oil refining, such as the petrochemical industry.

As crude oil diminishes and costs increase, alternative sources are required for the products we gain from petrochemicals. Thus we need a new source of ethene.

Cellulose is the important biopolymer (naturally occurring polymer) that can be used and is readily available in most countries. Ethanol is the prime candidate for an alternative source of ethene. Ethanol can be produced by fermentation from a variety of agricultural crops and it can be easily converted to ethene.

· Explain what is meant by a condensation polymer and describe the reaction involved when a condensation polymer is formed.

A condensation polymer is a polymer formed by eliminating a small molecule, usually water, when monomers join together – outer parts of the molecules combine to form the eliminated substance, while the rest joins to become a polymer.

· Describe the structure of cellulose and identify it as an example of a condensation polymer found as a major component of biomass.

Glucose condenses into cellulose by the elimination of water:

HO—C6H10O4—OH HO—C6H10O4—OH HO—C6H10O4—OH HO—C6H10O4—OH

Glucose:

Cellulose:

Note glucose is inverted to bond, and linearity of cellulose

Cellulose is the major component of plant material (which is biomass), which makes it readily available to be used.

In starch the a - glucose units add from C1 to C4 to form a helix structure. Strength + Shape + Fibre (in diet), energy source enzymes present in humans for digestion.

In cellulose the b - glucose units add from C1 to C4 to form a linear structure. No enzymes in humans for digestion. Enzymes do occur in termites and bacteria (cow guts).

· Identify that cellulose contains the basic carbon-chain structure needed to build petrochemicals and discuss its potential as a raw material.

Cellulose (already used in cotton, paper, cardboard) contains glucose’s carbon chain, meaning it could possibly be used to derive a source for ethene, propene or butene, yet currently there is no simple way to break it down into glucose (while small organisms do this, they break it down further).

Starch is more useful, but there is much more cellulose in biomass and it can be obtained from wastes, which would make it a good source of chemicals. Cellulose is a potential source of petrochemicals because of its carbon chain structure.

Bacteria that live in the intestine of termites and ruminants produce enzymes to break down cellulose into glucose monomers. Enzymic breakdown of cellulose is the first step in the production of petrochemicals. The glucose can be fermented to form ethanol, which can be readily dehydrated to form ethene. From ethene a wide variety of petrochemicals (eg. Halogenated hydrocarbons and polymers) are currently manufactured.

· Assess current developments in the use of biopolymers and describe a process currently used industrially to produce polymers.

· Use available evidence to gather, process and present data from secondary sources and analyse progress in the development and use of a named biopolymer. This analysis should name the specific enzyme(s) used or organism used to synthesise the material and an evaluation of the use or potential use of the polymer produced related to its properties.

Partially synthetic biopolymers have been in use for decades, but their use has been declining as petroleum based polymers have been taking over.

Eg. rayon, viscose rayon, viscose, cellophane, cellulose nitrate (film, explosive), cellulose acetate (projector slides)

Biopolymers, though, unlike petroleum-based polymers are biodegradable.

PHAs [poly(β-hydroxy-alkanoate)] can be formed from certain bacteria in specific conditions, producing a polymer similar to polypropylene, except that they are biodegradable.

Biopolymers are polymers that are made totally or in large part by living organisms. Originally the word meant biologically synthesised polymers such as cellulose, starch, proteins and nucleic acids. However the word now includes polymers that are chemically modified versions of natural polymers and polymers that can be produced by ‘manipulating’ biological organisms.

Partially synthetic biopolymers based on cellulose have been used commercially for nearly a century. Rayon, or in recent decades viscose rayon or just viscose, is a reconstituted cellulose as is cellophane. Cellulose nitrate is a synthetically modified cellulose that was widely used for photographic and movie film early this century: it was also used as an explosive (gun cotton) and as a plastic called celluloid. Unfortunately it was highly flammable and has been largely replaced by other plastics. Cellulose acetate, which is much less flammable, is still widely used, for example for overhead projector slides.

The major problem with petroleum-based polymers is that they are not biodegradable. This means that they do not decompose when discarded into the environment or put into rubbish dumps. Carelessly discarded synthetic plastics are causing harm and often death to marine and bird life and remain almost indefinitely as eyesores in landscapes.

Polylactic acid (PLA)

Bacteria are fed starch which forms lactic acid (CH3CHOHCOOH) that is them polymerised. At present, the use of PLA polymers is limited to medical implants. Compatibility of PLA homopolymers with other biopolymers is being examined for direct application in packaging and extrusion processing.

Development work

Research is also being focused on developing low cost biodegradable starch-based packaging material (starch biopolymers) as alternatives to conventional plastic packaging. Industry worldwide is examining new ways to reduce plastic waste and the use of non-renewable resources, namely, petroleum-based products, from which plastics are made. A starch-based biodegradable alternative to polyethylene film, including a black plastic now used as mulch flim, is being developed.

A major obstacle to the use of starch biopolymers is the need to enhance moisture stability while controlling processing and biodegradability. These problems are also the basis of current research programs.

· Solve problems and perform a first-hand investigation to identify cellulose and analyse information to examine the effect of decay bacteria on cellulose.

Decay bacteria on cellulose: many bacteria, fungi and insects can decompose cellulose into simple molecules (like glucose) and yet continue to form carbon dioxide and water. Eg. Rotting timber, decaying compost.

· Process data from secondary sources to analyse the efficiency of conversion of plant material to ethanol.

Ethanol can be obtained by fermenting starches & sugars with yeast. Glucose is broken down by the enzymes in yeast to ethanol and CO2. Enzymes first break down the more complex sugars, and others break down the simple sugars into the products.

Yeast can produce ethanol up to 14%, before killing the yeast. It is necessary to distil the alcohol for higher concentrations.

It is inefficient to grow plants for the purpose of producing ethanol. Molasses (leftover sugar from milling), leftover starchy grains could be used to increase efficiency of the entire process by using by-products.

Fermentation also produces heat, which causes less efficient energy change.

Other resources, such as ethanol, are readily available from plants. Such renewable resources will be of increasing importance as fossil fuel reserves are depleted

· Describe the dehydration of ethanol to ethene and identify the need for a catalyst in this process and the catalyst used.

Dehydration of ethanol is removal of water from the compound, to create ethene and water. Ethanol is heated with concentrated sulfuric or phosphoric acid catalyst.

Dehydration of ethanol:

· Describe the addition of water to ethene resulting in the production of ethanol and identify the need for a catalyst in this process and the catalyst used.

The reverse, hydration, requires heat and a catalyst, dilute H2SO4

Hydration of ethene:

· Describe the uses of ethanol as a solvent and relate this to the polar nature of the ethanol molecule.

· Plan, choose equipment and perform a first-hand investigation to gather information about the range of substances which can be dissolved by ethanol.

Ethanol is the 2nd most important solvent after water and is used in:

· Cosmetics (eg. Perfumes and deodorants)

· Food colourings and flavouring (eg. Vanilla essence)

· Medicinal preparations (eg. Antiseptics)

· Some cleaning agents

It is biodegradable in sunlight and is also widely used as a solvent in industry.

It is a good solvent since it is very polar, the O is much more electronegative than H or C (ie. In the COH bond, O is d-, and the others are d+). It is therefore a good solvent for polar substances. It also can form hydrogen bonds with other substances (and is miscible with water).

· Outline the use of ethanol as a fuel and explain why it can be called a renewable resource.

Ethanol has been promoted as a fuel on the grounds that it is a renewable resource. Ethanol readily burns, and as a liquid is easily transportable, making it a useful fuel. 10-20% ethanol can be used in normal car engines, but pure ethanol would require modification of engines.

It is a renewable resource in that it can be obtained from glucose, which is create by plants from CO2, H2O, and sunlight, which are basically the products of using ethanol as a fuel. Therefore it is a cycle.

· Describe conditions under which fermentation of sugars is promoted.

· Summarise the chemistry of the fermentation process.

· Present information from secondary sources by writing a balanced equation for the fermentation of glucose to ethanol.

Fermentation is a process in which glucose is broken down to ethanol and carbon dioxide by the action of enzymes present in yeast.

For fermentation:

· Suitable grain or fruit is mashed up with water

· Yeast is added

· Air is excluded, and

· The mixture is kept at about blood temperature, 37°C.

Enzymes (biological catalysts) in the mixture first convert any starch or sucrose in the mixture into glucose and/or fructose, the other enzymes convert glucose or fructose into ethanol and carbon dioxide:

C6H12O6(aq) yeast 2CH3-CH2-OH(aq) + 2CO2(g)

Bubbles of carbon dioxide are slowly given off: hence the name, fermentation.

Yeast can produce ethanol contents up to about 15%. Alcohol concentrations above this level kill the yeast and stop further fermentation. To produce higher alcohol contents it is necessary to distil the liquid.

If the aqueous mixture from a fermentation process is subjected to fractional distillation, 95% ethanol can be obtained. This is common industrial or laboratory alcohol. To obtain 100% ethanol more elaborate distillation procedures are needed.

Fermentation and distillation have been used to produce ethanol for centuries. Ethanol was one of the earliest organic compounds to be synthesised and obtained in nearly pure form.

‘Suitable grain or fruit’ for fermentation is plant material that has a high content of starch or simple sugars (sucrose, glucose, fructose). Starchy grains (such as wheat, maize or corn, barley), tubers (such as potatoes, cassava or tapioca), and fruits (such as grapes, apples) with high simple sugar contents are commonly fermented to ethanol.

Molasses, the leftover syrup from sugar milling, is also widely used. Sucrose has a very high solubility in water; this means that the mother liquor left over after sucrose has been crystallised out still has a high sucrose content, so ethanol can be obtained from an otherwise useless waste product of sugar production.

· Define the molar heat of combustion of a compound and calculate the value for ethanol from first-hand data.

The molar heat of combustion of a substance is the heat liberated when one mole of the substance undergoes complete combustion with oxygen at a constant pressure of exactly one atmosphere with the final products being carbon dioxide gas and liquid water.

The molar heat of combustion is minus the enthalpy change for the combustion process (because enthalpy change, DH, is always energy absorbed).

In practise, when we burn a fuel the water generally forms a gas. However the values we tabulate are the ones we would get if the water formed as a liquid. In particular for ethanol the molar heat of combustion is 1360 kJ/mol. This means that for:

C2H5OH(l) + 3O2(g) ---à 2CO2(g) + 3H2O(l)

DH = -1360 kJ/mol

· Assess the potential of ethanol as an alternative fuel and discuss the advantages and disadvantages of its use.

Ethanol as a car fuel alternative

Advantages

Disadvantages

· Theoretically lower greenhouse emissions / more energy efficient (if solar energy used)

· Different fuel properties

· Works as a solvent to clean engine

· Cleaner air (less prone to incomplete combustion)

· Less dependence on fossil fuels (renewable unlike crude oil)

· Petrol engines are designed for octane

· Industry already developed

· Inefficiency of crops to produce ethanol

· Land required for crops – soil salinity, erosion, deforestation problems

· Distillation & fermentation require energy -> waste

· Requires water removal from end product

· Process information from secondary sources to summarise the processes involved in the industrial production of ethanol from sugar cane.

Industrially, ethanol is fermented from sugar cane:

C12H22O11 + H2O à 2C6H12O6 à 4C2H5OH + 4CO2

As above, this is inefficient if crops are grown for the purpose, and molasses or leftover starchy grains should be used.

Ethanol is then used as a fuel, solvent, alcoholic beverage, production of ethene.

· Process information from secondary sources to summarise the use of ethanol as an alternative car fuel, evaluating the success of current usage.

Current usage & successes

· Used as a fuel extender in some petrol mixes (10-20% without needing engine modifications)

· Success depends on support of government, cost / benefit analysis

· In the 80s in Brasil, sugar cane crops were grown for the purpose of producing ethanol for fuel – not economically viable: abandoned

· No successful application as a fuel replacement

Electrochemical methods are important in producing materials and making energy available

Explain the displacement of metals from solution in terms of transfer of electrons.

Recall:

Oxidation is the gain of oxygen and/or loss of hydrogen, also the loss of electrons.

Reduction is the loss of oxygen and/or gain of hydrogen, also the gain of electrons.

Oxidation and reduction always occur together

Redox reactions are very important since they are involved in things such as:

· Corrosion of metals

· Combustion reactions (burning)

· Production of metals from ores

· Cells and batteries

· Electrolysis to produce some metals and compounds

These all involve the loss and gain of electrons.

A displacement reaction is a reaction in which a metal converts the ion of another metal to the neutral atom.

Oxidation-reduction reactions are also called redox reactions and electron transfer reactions.

When a more reactive metal (M) is added to a solution in which a less reactive metal (L) is ionised, M is oxidised and loses its valence electrons to L, which is reduced. Thus L is displaced from solution, and M is dissolved.

· Identify the relationship between displacement of metal ions in solution by other metals to the relative reactivity of metals.

{Na, K} > {Li, Ca, Ba} > {Mg, Al, Fe, Zn} > {Sn, Pb} > Cu > {Ag, Au, Pt}

(Species within curly brackets have equal activities).

Only a more reactive metal will displace the metal ions of a less reactive metal. See activity series.

Once it has been established we can use this activity series to decide which metal (of a given pair) displaces which from solution:

The metal further to the left will displace the other metal from a solution of its ions.

It other words, the metal further to the left loses electrons more easily or is more easily oxidised. Oxidation and reduction or electron transfer has some implications for the way we name certain species.

· Account for changes in the oxidation state of species in terms of their loss or gain of electrons.

Recall: the valence of various metal ions and of the need with some of these ions to state the valence (in capital Roman numerals) in the name: tin(II) oxide, iron(III) chloride, copper(I) sulfide and the like. What we referred to there as valence is also called the oxidation state of the element.

For positive monatomic ions the oxidation state is the charge on the ion.

The oxidation state:

· of copper in the oxide Cu2O (2Cu+ O2-) is +1

· of iron in the sulfide FeS (Fe2+ S2-) is +2

· of titanium in the chloride TiCl3 (Ti3+ 3Cl-) is +3

The oxidation state of a species increases with its loss of electrons, thus the species is oxidised. The oxidation state decreases when electrons are gained; the species is reduced.

· Describe and explain galvanic cells in terms of oxidation/reduction reactions.

By reduction and oxidation within a system occurring at different locations, charged particles pass through an attached wire, causing a current. This is a galvanic cell.

It requires a difference in potential between the redox reactions occurring at either electrode, in order for there to be any reaction. Electrons are removed from the anode and pass through the circuit to the cathode, whose ions in solution are then reduced.

· Outline the construction of galvanic cells and trace the direction of electron flow.

The salt bridge is required in order for ions to be transferred/migrate, and thus for the electrolytes. Positive ions from the anode solution migrate to the cathode across the salt bridge, pushing the positive ions in the salt bridge. Equally, negative ions migrate in the opposite in order to equal out the charges on either side of the cell.

It is important to use a solution in the salt bridge that does not precipitate with other ions in the cell. This is why KNO3 is used (neither precipitates with any ions).

A cell can be described by A | A+ || C+ | C, where the left is based on the oxidation half-equation, and the right is the reduction half-equation. The | represents a change in phase, and the || represents a salt bridge.

· Identify the use the terms anode, cathode, electrode and electrolyte to describe galvanic cells.

The anode is the electrode at which electrons are released, ie oxidation occurs / negative electrode

The cathode is where reduction occurs (ie. Positive electrode)

The electrolytes are the solutions or molten substances used within the cell that conduct electricity.

In summary:

A cell consists of:

2 electrodes:

· An anode (-) where oxidation occurs

· A cathode (+) where reduction occurs

And an electrolyte which is a conduction paste or solution contain mobile ions.

Electrons are carried by external wires and a salt bridge, which is paper or gel which contains ions, usually K+, Na+ and NO3-.

A battery is two or more cells joined together.

A common cell in use consists of two metal half-cells in which a metal electrode is a solution of the same metal ions.

We can represent cells using a short hand notation:

Anode / electrolyte /Salt bridge/ electrolyte / Cathode

Eg. Fe(s) / Fe2+(aq) // Sn2+(aq) / Sn(s)

Redox reactions involve transfer of electrons from one reactant to another.

· Describe the industrial use of electrolysis for refining a named metal identifying: oxidant, reductant, electrolyte used, and conditions under which the electrolysis must occur and the reasons for these conditions.

Electrolysis is the causing of a (redox) reaction to occur by supplying a voltage across the reaction (reverse of galvanic/voltaic cell).

Copper can be purified by electrolysis, whereby impure (blister copper) is electro-refined into pure copper.

Note Blister copper anode (+ve Cu --> Cu2+ + 2e disintegrates in reaction

Pure copper sheet cathode Cu2+ + 2e– --> Cu deposits on cathode

Concentrated Cu2+ electrolyte, acidified slightly for increased conductivity

[Too acidic could cause H+ to reduce, alkali and Cu(OH)2 precipitates]

Voltage (PD) across cell is controlled such that it is enough to reduce Cu2+, not to oxidise less active metals (anode mud: Ag, Au, etc) and give a good rate of deposition of copper, not to reduce more active metal ions (Fe2+, Zn2+, etc).

· Identify an example of the use of electroplating and explain why the process is used.

Electroplating is used in order to coat a metal object evenly with a thin film of another. The metal to be coated is used as the cathode, in order to attract metal ions from the anode (and electrolyte with plating ions). An example of its use is in coating cutlery with silver or jewellery with gold (cheaper coated with more expensive, or chroming to protect soft metals with harder [eg car parts, taps]).

· Gather, process and present information on the structure and chemistry of a dry cell or lead-acid cell and evaluate it in comparison to: button cell, mercury, vanadium redox cell, lithium, liquid junction photovoltaic device (eg the Gratzel cell) in terms of: chemistry, cost and practicality, impact on society, and environmental impact.

Standard batteries compared:

(1) Dry cell

· Chemistry

– Zn | ZnCl2, NH4Cl | MnO2, C

– O: Zn --> Zn2+ + 2e–

– R: NH4+ + MnO2 + H2O + e– --> Mn(OH)3 + NH3

· Cost & Practicality

– Relatively cheap

– Useful when small currents are needed

– Easy to store and use, standard

– Bad: low current for its size, can leak

· Impact on society

– First commercialised battery

– Therefore made portable items (torches, radios, clocks) possible

· Environmental impact

– Minimal: Mn3+ oxidised in the environment, small amounts of Zn not problematic, NH4 salts, C are harmless

– Not rechargeable

(2) Button cell

· Chemistry

– Zn, ZnO | KOH (paste) | Ag2O, Ag

– O: Zn + 2OH– --> ZnO + H2O + 2e–

– R: Ag2O + H2O + 2e– --> 2Ag + 2OH–

· Cost & Practicality

– Very small

– Can produce considerable amounts of electricity

– Constant voltage (1.6V)

– Relatively long lifespan

– More expensive than dry cell

· Impact on society

– Allows for miniature electrical appliances (watches, hand-held calculators, hearing aids)

– Non-toxic (so possible inside the body)

· Environmental impact

– Not rechargeable

(3) Mercury battery

· Chemistry

– Button style

– O: Zn + 2OH– --> ZnO + H2O + 2e–

– R: HgO + H2O + 2e– --> 2Ag + 2OH–

· Cost & Practicality

– Small

– Steady output (1.6V)

– Lasts longer on a steady voltage than dry cell

– Mercury ores not abundant

· Impact on society

– Useful where compactness is required

· Environmental impact

– Not rechargeable

– Toxic mercury released

– Waste is poisonous, causes landfill

(4) Vanadium redox battery

· Chemistry

–

– V2+ <--> V3++e–

– VO2++2H+ <--> VO2++H2O

· Cost & Practicality

– Easy maintenance

– Flexible voltage

– Fully rechargeable, quickly (readily reversed, 2V can recharge 100V)

– Vanadium is plentiful

– Safe (electrolytes are not dangerous together)

· Impact on society

– Large power supply

– Can be used as solar/wind energy storage

– Underground storage possible

· Environmental impact

– Little waste, doesn’t need to be thrown away

– Easy recharge

– Independent of fossil fuels

– Can be used to replace fossil fuels -> improved air quality

(5) Lithium battery

· Chemistry

– O: Li --> Li+ + e–

– R: I2 + 2e– --> 2I–

· Cost & Practicality

– Small, button

– Higher voltage than dry cell (2.8V)

– More expensive than dry cell

· Impact on society

– Small

– Higher output useful for new applications

· Environmental impact

– Not rechargeable

(6) Liquid junction photovoltaic device (Gratzel cell)

· Chemistry

– Converts light energy to chemical

– Light is absorbed by a dye, D

– O: D --> D+ + e–

– Electron goes to mediator 2D+ + 3I– + e– --> 2D + I3–

– R: I3– + 2e– --> 3I–

· Cost & Practicality

– More expensive than dry cell – different application

– One-off cost

– Easier to produce & cheaper than other photovoltaic cells

– Durable

– Titanium oxide, and other metal oxides used to absorb light are easily obtainable

· Impact on society

– Cheaper solar power may increase the use of solar power

· Environmental impact

– Clean source of energy

– Materials are non-toxic

Nuclear chemistry provides a range of materials to assist in tracing and thus better understanding complex chemical reactions

· Distinguish between stable and radioactive isotopes and describe the conditions under which a nucleus is unstable.

Radioactive isotopes (as opposed to stable) emit radiation from their nuclei to become stable. Nuclei are most stable at a certain ratio (1-1.5) of neutrons to protons for the isotope.

 decay is  decay is

· Describe how transuranic elements are produced in nuclear reactors.

Elements past U, ie, atomic numbers 92 to 118 are transuranic, and don’t occur naturally. Natural elements are bombarded with neutrons (or other particles) in nuclear reactors to produce the higher elements.

· Describe how commercial radioisotopes are produced in nuclear reactors.

Commercial (industry & medicine) radioisotopes are produced similarly in nuclear reactors, specifically made to obtain a certain radioisotope.

· Identify instruments and processes that can be used to detect radiation.

Instruments for the detection of radiation

Photographic film

· darkening is quantitative measure of radiation

Cloud chamber

· air-tight, with supersaturated water/alcohol vapour

· radiation ionises air, vapour condenses around ions

· this is visible

alpha gives straight, dense trails

beta gives lighter zigzags

gamma gives fainter tracks

Geiger-Mueller counter

· metal tube with thin micra film window (to allow a and b particles to enter)

·

· filled with argon gas

· radiation ionises Ar, ions go to terminals, e– causes more ionisation

· causes pulse, collected by counter

· can detect small doses of radiation (one particle)

Scintillation counter

· phosphors emit flashes of light when radiation strikes

· these can be amplified and electronically counted

· Identify one use of a named radioisotope: in industry, in medicine, and to determine and/or verify reaction mechanisms in chemistry.

· Describe the way in which the above named radioisotopes are used and relate this to the chemical properties of the radioisotopes described.

Examples of commercial isotopes:

Industry:

· 60Co (half-life 5.27y, -->  stable 60Ni, neutron-bombard 59Co)

o means can be reused, and is not very harmful once disposed

o ideal intensity not to make subject radioactive, but to destroy what is required

· food irradiation

· radiation breaks chemical bonds and ionises particles

· damages DNA

· slows ripening, sprouting, spoilage; kills some bugs

· problems: can create new, unknown chemicals in food, destroys thiamine, some foods unsuitable, radiation exposure due to negligence

Medicine:

· 60Cobalt

o beta is soft and can be filtered out

o gamma is constant and allows for accuracy

o cheap and easy to manufacture

· rods of 60Co are placed in a teletherapy unit

· directs gamma rays to specific area of body

· destroys DNA in carcinoma, and thus reduces or eradicates them

Reaction mechanisms:

· Since radioactive isotopes react chemically the same, they are used to determine positions of broken bonds, etc in reactions

· Known as isotopic tracers, since their movement is observable as a member of a chemical reaction

· Isotopic labelling is where one element in the reactants is radioactive and then is traced in the end of the reaction to determine which reactants were involved in producing the product

18O shows where the OH comes from in the hydrolysis of methyl acetate *methyl acetate + water --> acetic acid + methanol

· Process and analyse information from secondary sources by using a flow diagram to trace the sequence of fission products released during the decay of uranium.

Fission is where a radioisotope divides into two roughly equal parts, releasing enormous amounts of energy and neutrons. These neutrons are either absorbed or cause fission with another of the original element. For controlled fission, all but one of the neutrons are allowed to continue (others absorbed).
Eg

· Process information from secondary sources to describe recent discoveries of elements.

Recent discoveries of elements: element 118 (Uuo Ununoctiom) produced from fusion of [krypton ion beam accelerated by electrical and magnetic fields into lead target by team at Berkley national lab June '99]. Six alpha decays 106Sg (Seaborgium).

· Use available evidence to analyse benefits and problems associated with the use of radioactive isotopes in identified industries and medicine.

Benefits and problems of the use of radioisotopes

Benefits

Problems

· allows treatment and tracing of medical illnesses and problems

· allows development in chemistry

· allows safer, and longer-lasting foods

· production of materials (with thickness gauges)

· leak detection tracing

· medical sterilisation

· smoke alarms

· requires nuclear reactors: accidents of operation

· emissions produced and disposed radioactive waste

· can cause damage to organisms if not used and stored safely (infection, cancer, tissue damage)

· in medical diagnosis, waste needs to be removed quickly, due to chemicals formed

	Created by OllieJackson almost 11 years ago

Year 12 Chemistry Note on Untitled, created by OllieJackson on 26/07/2013.

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