6.2.1 Crude oil and fractional distillation

Crude oil is a hydrocarbon mixture and a feedstock

1. Crude oil is a thick, dark mixture of many different hydrocarbons, pumped from rocks beneath the land and the seabed.
   1. Most of these hydrocarbons are alkanes, with chains ranging from one carbon atom to more than fifty.
2. Crude oil is the main source of the hydrocarbons used as fuels, such as petrol, diesel and kerosene.
3. It is also a feedstock for the petrochemical industry, which turns its hydrocarbons into new substances.
   1. Plastics, solvents, detergents, lubricants, dyes and many medicines are all made from crude oil hydrocarbons.
4. Crude oil is of little use as it comes out of the ground, because each use needs hydrocarbons with a particular chain length.

Each fraction contains alkanes of similar chain length

1. Fractional distillation separates crude oil into fractions with different boiling point ranges.
2. Each fraction is still a mixture, containing hydrocarbons with similar numbers of carbon atoms and similar boiling points.
3. The fractions are made up largely of alkanes, which form a homologous series.
   1. Each alkane has one more ${CH}_2$ unit than the one before, and all of them fit the general formula: $C_n{H}_{2n+2}$
   2. In the formula, $n$ is the number of carbon atoms, so octane, with $8$ carbon atoms, is $C_8{H}_{18}$.
4. Alkanes contain only single bonds, so each carbon atom is bonded to as many hydrogen atoms as possible.

In fractional distillation, vapours condense at different heights

1. Crude oil is heated to about $350{}^{\circ }C$ in a furnace, so most of it boils and turns to vapour.
2. The hot vapour is piped into the bottom of a tall fractionating column.
3. The column has a temperature gradient, being hottest at the bottom and coolest at the top.
4. The vapours rise up the column through holes in a series of trays, cooling as they go.
   1. Each hydrocarbon condenses to a liquid when it reaches a tray that is cooler than its boiling point, and the liquid is piped off.
5. The smallest molecules never condense, so they leave the top of the column as refinery gases.
6. The largest molecules do not boil at all, so they run out of the bottom as a thick residue called bitumen.

Larger molecules have higher boiling points

1. Hydrocarbon molecules are held to each other by weak intermolecular forces.
2. Longer molecules have more surface in contact with their neighbours, so the intermolecular forces between them are stronger.
3. Stronger forces need more energy to overcome, so larger molecules have higher boiling points.
   1. Molecules with high boiling points condense in the hot lower part of the column, while small molecules with low boiling points rise to the cooler top.
4. The covalent bonds inside the molecules do not break, so fractional distillation separates the hydrocarbons without changing them chemically.
5. Boiling point rises steadily with chain length, from $-162{}^{\circ }C$ for methane to $174{}^{\circ }C$ for decane.

6.2.2 Cracking and dependence on hydrocarbons

Modern life depends on hydrocarbons for fuels and materials

1. Most crude oil is burned as fuel, because hydrocarbons release a lot of energy when they burn.
   1. Petrol and diesel power cars and lorries, kerosene powers aircraft and fuel oil powers ships.
2. Natural gas, which is mostly the hydrocarbon methane, heats most UK homes and generates a large share of the country's electricity.
3. The rest of the crude oil is a feedstock for making plastics, synthetic fibres, solvents, detergents, lubricants and medicines.
   1. Food packaging, polyester clothing, the bitumen in road surfaces and the plastic casing of a phone all come from hydrocarbons.
4. Replacing hydrocarbons would mean changing how people travel, heat buildings and make almost every manufactured product.

Crude oil is finite and forms far too slowly

1. Crude oil formed over millions of years from the remains of tiny sea plants and animals buried in mud.
   1. Heat and high pressure slowly changed these remains into crude oil, which became trapped in porous rock.
2. People are using crude oil up in a tiny fraction of the time it took to form, so it is non-renewable.
3. As the easily reached reserves run out, oil has to come from deeper or more remote places, which costs more and carries more risk.
4. Burning crude oil destroys a feedstock that cannot be replaced, which is one reason to use alternatives such as biofuels, electric vehicles and recycled plastics.

Fractional distillation makes too many long-chain hydrocarbons

1. The amount of each fraction depends on the crude oil used, not on what customers want to buy.
2. Crude oil usually contains more long-chain hydrocarbons, such as fuel oil, than the market needs.
   1. Long-chain hydrocarbons are viscous, hard to ignite and burn with smoky flames, so they make poor fuels for cars.
3. Demand is highest for the short-chain fractions, such as petrol, which crude oil supplies in smaller amounts.
4. Cracking converts the surplus long-chain molecules into shorter, more useful ones, so supply can match demand.

Cracking breaks long-chain alkanes into shorter alkanes and alkenes

1. Cracking uses heat to break long molecules apart, so it is a type of thermal decomposition.
2. In catalytic cracking, the long-chain alkane is heated to vaporise it and the vapour is passed over a hot catalyst.
   1. The catalyst is usually a powdered zeolite or aluminium oxide at about $500{}^{\circ }C$.
3. In steam cracking, the vapour is mixed with steam and heated above $800{}^{\circ }C$ without a catalyst.
4. The products are usually a mixture of shorter alkanes and alkenes, and there is always at least one alkene.
5. Atoms are conserved, so the carbon and hydrogen atoms in the products add up to those in the starting alkane.
   1. Decane can crack into octane and ethene: $$C_{10}{H}_{22}\to C_8{H}_{18}+C_2{H}_4$$

Cracking products are more useful than long-chain alkanes

1. The shorter alkanes are blended into petrol and other fuels that are in high demand.
2. The alkenes contain a $C=C$ double bond, which makes them more reactive than alkanes.
   1. Alkenes with one double bond have the general formula $C_n{H}_{2n}$, with two fewer hydrogen atoms than the alkane with the same number of carbon atoms.
3. This reactivity makes alkenes a valuable feedstock for the petrochemical industry.
   1. Ethene and propene from cracking are joined into polymers such as poly(ethene) and poly(propene).
   2. Alkenes are also used to make other chemicals, such as ethanol from ethene.

6.2.3 Functional groups and homologous series

Homologous series members share a functional group

1. Members of the same homologous series have the same functional group and the same general formula.
   1. Each member differs from the next by a ${CH}_2$ unit, so the chain grows by one carbon atom at a time.
2. Their physical properties change gradually along the series, so boiling point rises as the chain gets longer.
3. Their chemical properties are similar, because reactions happen at the functional group while the rest of the chain stays the same.
   1. Once you know how ethanol reacts, you can predict how butan-1-ol reacts, because both contain the $-OH$ group.
4. To identify the series a compound belongs to, find its functional group in the name or the displayed formula.

Alkanes have single bonds and names ending in -ane

1. Every carbon atom in an alkane forms four single bonds, so alkanes have the general formula $C_n{H}_{2n+2}$.
2. The prefix of each name gives the number of carbon atoms: meth- is $1$, eth- is $2$, prop- is $3$ and but- is $4$.
   1. Methane, ${CH}_4$: $\begin{array}{ccccc}& & \mathrm{H}& & \\ & & |& & \\ \mathrm{H}& -& \mathrm{C}& -& \mathrm{H}\\ & & |& & \\ & & \mathrm{H}& & \end{array}$
   2. Ethane, $C_2{H}_6$: $\begin{array}{ccccccc}& & \mathrm{H}& & \mathrm{H}& & \\ & & |& & |& & \\ \mathrm{H}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{H}\\ & & |& & |& & \\ & & \mathrm{H}& & \mathrm{H}& & \end{array}$
   3. Propane, $C_3{H}_8$: $\begin{array}{ccccccccc}& & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & \\ & & |& & |& & |& & \\ \mathrm{H}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{H}\\ & & |& & |& & |& & \\ & & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & \end{array}$
   4. Butane, $C_4{H}_{10}$: $\begin{array}{ccccccccccc}& & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & \\ & & |& & |& & |& & |& & \\ \mathrm{H}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{H}\\ & & |& & |& & |& & |& & \\ & & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & \end{array}$

Alkenes contain a double bond between carbon atoms

1. Every alkene contains a double bond between two carbon atoms, $C=C$, as its functional group.
2. Alkenes with one double bond have the general formula $C_n{H}_{2n}$, with two fewer hydrogen atoms than the alkane with the same chain length.
3. A double bond needs two carbon atoms, so the first four members are ethene, propene, but-1-ene and pent-1-ene.
   1. Ethene, $C_2{H}_4$: $$\begin{array}{ccc}\mathrm{H}& & \mathrm{H}\\ |& & |\\ \mathrm{C}& =& \mathrm{C}\\ |& & |\\ \mathrm{H}& & \mathrm{H}\end{array}$$
   2. Propene, $C_3{H}_6$: $$\begin{array}{ccccccc}\mathrm{H}& & \mathrm{H}& & \mathrm{H}& & \\ |& & |& & |& & \\ \mathrm{C}& =& \mathrm{C}& -& \mathrm{C}& -& \mathrm{H}\\ |& & & & |& & \\ \mathrm{H}& & & & \mathrm{H}& & \end{array}$$
   3. But-1-ene, $C_4{H}_8$: $$\begin{array}{ccccccccc}\mathrm{H}& & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & \\ |& & |& & |& & |& & \\ \mathrm{C}& =& \mathrm{C}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{H}\\ |& & & & |& & |& & \\ \mathrm{H}& & & & \mathrm{H}& & \mathrm{H}& & \end{array}$$
   4. Pent-1-ene, $C_5{H}_{10}$: $$\begin{array}{ccccccccccc}\mathrm{H}& & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & \\ |& & |& & |& & |& & |& & \\ \mathrm{C}& =& \mathrm{C}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{H}\\ |& & & & |& & |& & |& & \\ \mathrm{H}& & & & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & \end{array}$$

Alcohols contain the hydroxyl group and end in -ol

1. Every alcohol contains the hydroxyl group, $-OH$, bonded to a carbon atom, as its functional group.
2. Alcohols with one $-OH$ group have the general formula $C_n{H}_{2n+1}OH$.
3. The -ol ending shows an alcohol, and the number before it shows which carbon atom carries the $-OH$ group.
   1. Methanol, ${CH}_{3}OH$: $\begin{array}{ccccccc}& & \mathrm{H}& & & & \\ & & |& & & & \\ \mathrm{H}& -& \mathrm{C}& -& \mathrm{O}& -& \mathrm{H}\\ & & |& & & & \\ & & \mathrm{H}& & & & \end{array}$
   2. Ethanol, $C_2{H}_{5}OH$: $\begin{array}{ccccccccc}& & \mathrm{H}& & \mathrm{H}& & & & \\ & & |& & |& & & & \\ \mathrm{H}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{O}& -& \mathrm{H}\\ & & |& & |& & & & \\ & & \mathrm{H}& & \mathrm{H}& & & & \end{array}$
   3. Propan-1-ol, $C_3{H}_{7}OH$: $\begin{array}{ccccccccccc}& & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & & & \\ & & |& & |& & |& & & & \\ \mathrm{H}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{O}& -& \mathrm{H}\\ & & |& & |& & |& & & & \\ & & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & & & \end{array}$
   4. Butan-1-ol, $C_4{H}_{9}OH$: $\begin{array}{ccccccccccccc}& & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & & & \\ & & |& & |& & |& & |& & & & \\ \mathrm{H}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{O}& -& \mathrm{H}\\ & & |& & |& & |& & |& & & & \\ & & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & & & \end{array}$

Carboxylic acids contain the carboxyl group

1. Every carboxylic acid contains the carboxyl group, $-COOH$, as its functional group.
   1. In this group, one carbon atom has a double bond to one oxygen atom and a single bond to an $-OH$ group.
2. The carbon atom in $-COOH$ counts in the name, so ethanoic acid has two carbon atoms in total.
   1. Methanoic acid, $HCOOH$: $\begin{array}{ccccccc}& & \mathrm{O}& & & & \\ & & \Vert & & & & \\ \mathrm{H}& -& \mathrm{C}& -& \mathrm{O}& -& \mathrm{H}\end{array}$
   2. Ethanoic acid, ${CH}_{3}COOH$: $\begin{array}{ccccccccc}& & \mathrm{H}& & \mathrm{O}& & & & \\ & & |& & \Vert & & & & \\ \mathrm{H}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{O}& -& \mathrm{H}\\ & & |& & & & & & \\ & & \mathrm{H}& & & & & & \end{array}$
   3. Propanoic acid, $C_2{H}_{5}COOH$: $\begin{array}{ccccccccccc}& & \mathrm{H}& & \mathrm{H}& & \mathrm{O}& & & & \\ & & |& & |& & \Vert & & & & \\ \mathrm{H}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{O}& -& \mathrm{H}\\ & & |& & |& & & & & & \\ & & \mathrm{H}& & \mathrm{H}& & & & & & \end{array}$
   4. Butanoic acid, $C_3{H}_{7}COOH$: $\begin{array}{ccccccccccccc}& & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & \mathrm{O}& & & & \\ & & |& & |& & |& & \Vert & & & & \\ \mathrm{H}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{O}& -& \mathrm{H}\\ & & |& & |& & |& & & & & & \\ & & \mathrm{H}& & \mathrm{H}& & \mathrm{H}& & & & & & \end{array}$

6.2.4 Reactions of alkanes, alkenes and alcohols

Complete combustion of alkanes, alkenes and alcohols

1. Alkanes contain only strong single bonds, so apart from combustion they are fairly unreactive.
2. In complete combustion, every carbon atom ends up in carbon dioxide and every hydrogen atom ends up in water.
3. The same two products form from any hydrocarbon or alcohol that burns in plenty of oxygen.
   1. For methane: $${CH}_4+2O_2\to {CO}_2+2H_{2}O$$
   2. For ethene: $$C_2{H}_4+3O_2\to 2{CO}_2+2H_{2}O$$
   3. For ethanol: $$C_2{H}_{5}OH+3O_2\to 2{CO}_2+3H_{2}O$$
4. To balance a combustion equation, balance carbon first, then hydrogen, and oxygen last.

Alkenes react by addition across the double bond

1. The $C=C$ double bond makes alkenes unsaturated and much more reactive than alkanes.
2. In an addition reaction, one bond of the double bond breaks and a new atom joins each of the two carbon atoms.
   1. Two molecules join to make one product, which is saturated.
3. With hydrogen and a nickel catalyst at about $150{}^{\circ }C$, an alkene forms the alkane with the same carbon chain: $$C_2{H}_4+H_2\to C_2{H}_6$$
4. With bromine, an alkene forms a compound with a bromine atom on each carbon of the old double bond: $$C_2{H}_4+{Br}_2\to C_2{H}_4{Br}_2$$
   1. The product from ethene is 1,2-dibromoethane: $\begin{array}{ccccccc}& & \mathrm{H}& & \mathrm{H}& & \\ & & |& & |& & \\ \mathrm{H}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{H}\\ & & |& & |& & \\ & & \mathrm{Br}& & \mathrm{Br}& & \end{array}$
5. Shaking an alkene with bromine water turns it from orange to colourless, while an alkane leaves it orange.
   1. Acidified potassium manganate(VII) also turns from purple to colourless with an alkene, but it reacts with alcohols too, so bromine water is the clearer test for a $C=C$ bond.

Steam adds to alkenes to form alcohols

1. Ethene reacts with steam at about $300{}^{\circ }C$ and high pressure, with a phosphoric acid catalyst, to form ethanol: $$C_2{H}_4+H_{2}O\to C_2{H}_{5}OH$$
   1. A hydrogen atom joins one carbon of the double bond and the $-OH$ group joins the other.
2. For any alkene, keep the whole carbon chain and change only the double bond to a single bond.
3. Then add the atoms from the other reactant, one to each carbon atom that was in the double bond.
4. In general formulae, an alkene $C_n{H}_{2n}$ gives $C_n{H}_{2n+2}$ with hydrogen, $C_n{H}_{2n}{Br}_2$ with bromine and $C_n{H}_{2n+1}OH$ with steam.
   1. Propene with hydrogen gives propane, $C_3{H}_8$.
   2. Propene with steam gives propanol, $C_3{H}_{7}OH$.

Alcohols burn and are oxidised to carboxylic acids

1. Alcohols burn in plenty of oxygen to form carbon dioxide and water, which is why ethanol is blended into UK petrol.
   1. For methanol: $$2{CH}_{3}OH+3O_2\to 2{CO}_2+4H_{2}O$$
2. Warming an alcohol with acidified potassium manganate(VII) oxidises it to a carboxylic acid.
   1. The solution changes from purple to colourless as the manganate(VII) reacts.
3. The $-{CH}_{2}OH$ end of the alcohol becomes $-COOH$, so the carbon chain keeps the same length.
   1. Ethanol is oxidised to ethanoic acid: $$\begin{array}{ccccccccc}& & \mathrm{H}& & \mathrm{H}& & & & \\ & & |& & |& & & & \\ \mathrm{H}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{O}& -& \mathrm{H}\\ & & |& & |& & & & \\ & & \mathrm{H}& & \mathrm{H}& & & & \end{array}\to \begin{array}{ccccccccc}& & \mathrm{H}& & \mathrm{O}& & & & \\ & & |& & \Vert & & & & \\ \mathrm{H}& -& \mathrm{C}& -& \mathrm{C}& -& \mathrm{O}& -& \mathrm{H}\\ & & |& & & & & & \\ & & \mathrm{H}& & & & & & \end{array}$$
4. Wine or beer left open to the air turns to vinegar, because bacteria oxidise its ethanol to ethanoic acid.

Predicting products from the functional group

1. Start by finding the functional group: only single bonds for an alkane, $C=C$ for an alkene or $-OH$ for an alcohol.
2. Then pick the reaction: combustion for all three, addition for an alkene and oxidation for an alcohol.
3. Copy the carbon chain unchanged, then change only the atoms at the functional group.
   1. Pentan-1-ol, $C_5{H}_{11}OH$, is oxidised to pentanoic acid, $C_4{H}_{9}COOH$.
   2. Hex-1-ene, $C_6{H}_{12}$, adds hydrogen to form hexane, $C_6{H}_{14}$.
4. Finish by checking that the product has the same number of carbon atoms as the starting compound, unless it has burned.

6.2.5 Addition polymerisation and repeat units

Addition polymerisation joins alkene monomers through double bonds

1. Addition polymerisation needs monomers with a $C=C$ double bond, which is the functional group of the alkenes.
2. At high pressure and with a catalyst, one bond in each double bond breaks.
   1. Each opened monomer then forms new single bonds to the monomers on either side of it.
3. Thousands of monomers join in this way to make one very long polymer chain.
4. The polymer is the only product, so every atom from the monomers ends up in the polymer.
5. The polymer is named by putting the monomer's name in brackets after poly, so ethene forms poly(ethene).

Repeating units have the monomer's atoms but single bonds

1. In poly(ethene), the unit that repeats contains two carbon atoms and four hydrogen atoms, like one ethene molecule.
2. The double bond of the monomer becomes a single bond in the repeating unit.
3. A bond sticks out from each end carbon atom through the brackets, to show that the unit joins to identical units on either side.
4. The letter $n$ after the brackets stands for a large number of repeating units.
5. The equation for making poly(ethene) is: $$n{CH}_2={CH}_2\to {\left[{CH}_2-{CH}_2\right]}_n$$
   1. In displayed formulae: $$n\begin{array}{ccc}\mathrm{H}& & \mathrm{H}\\ |& & |\\ \mathrm{C}& =& \mathrm{C}\\ |& & |\\ \mathrm{H}& & \mathrm{H}\end{array}\to {\begin{array}{ccccc}& \mathrm{H}& & \mathrm{H}& \\ & |& & |& \\ -& \mathrm{C}& -& \mathrm{C}& -\\ & |& & |& \\ & \mathrm{H}& & \mathrm{H}& \end{array}}_n$$
6. The repeating unit and the monomer have the same molecular formula, $C_2{H}_4$ for ethene.

Drawing a polymer from its monomer

1. Draw the monomer with its two double-bonded carbon atoms side by side and the other groups above and below them.
2. Change the double bond to a single bond.
3. Add a bond sticking out from each of the two carbon atoms, then draw brackets across these bonds and write $n$ after them.
4. Keep every side group, such as ${CH}_3$ or $Cl$, on the same carbon atom as in the monomer.
5. Chloroethene, ${CH}_2=CHCl$, forms poly(chloroethene), or PVC, which is used for window frames and pipes.
6. Tetrafluoroethene, ${CF}_2={CF}_2$, forms PTFE, the non-stick coating on frying pans.
   1. In displayed formulae: $$n\begin{array}{ccc}\mathrm{F}& & \mathrm{F}\\ |& & |\\ \mathrm{C}& =& \mathrm{C}\\ |& & |\\ \mathrm{F}& & \mathrm{F}\end{array}\to {\begin{array}{ccccc}& \mathrm{F}& & \mathrm{F}& \\ & |& & |& \\ -& \mathrm{C}& -& \mathrm{C}& -\\ & |& & |& \\ & \mathrm{F}& & \mathrm{F}& \end{array}}_n$$

Finding the monomer from a polymer

1. Find the repeating unit, which in a simple addition polymer has two carbon atoms in the main chain.
2. Remove the two bonds that stick out through the brackets.
3. Put a double bond between the two carbon atoms, keeping every other atom and group where it was.
4. Name the monomer by removing poly and the brackets from the name of the polymer, so poly(propene) comes from propene.
   1. Poly(chloroethene) has the repeating unit ${\begin{array}{ccccc}& \mathrm{H}& & \mathrm{H}& \\ & |& & |& \\ -& \mathrm{C}& -& \mathrm{C}& -\\ & |& & |& \\ & \mathrm{H}& & \mathrm{Cl}& \end{array}}_n$, so its monomer is chloroethene: $$\begin{array}{ccc}\mathrm{H}& & \mathrm{H}\\ |& & |\\ \mathrm{C}& =& \mathrm{C}\\ |& & |\\ \mathrm{H}& & \mathrm{Cl}\end{array}$$
5. In a long section of chain, the pattern repeats every two carbon atoms, so a chain of six carbon atoms contains three repeating units.

6.2.6 Naturally occurring polymers

Natural polymers are long chains of small monomers

1. A natural polymer is made inside living things from many small monomer molecules joined by covalent bonds.
2. The monomers join end to end into long chains, which can contain thousands of units.
3. Natural polymers fall into three families according to their monomers.
   1. DNA is built from nucleotides.
   2. Starch and cellulose are built from sugars.
   3. Proteins are built from amino acids.

DNA is a polymer made from four different nucleotides

1. DNA, or deoxyribonucleic acid, is found in the cells of every living organism and carries its genetic code.
2. Its monomers are nucleotides, and there are four different types.
   1. Each nucleotide contains a sugar, a phosphate group and one of four bases.
   2. The bases are adenine, thymine, cytosine and guanine, so the four nucleotides are often labelled A, T, C and G after their four bases.
3. A DNA molecule contains millions of nucleotides joined in two strands, which twist into a double helix.
4. The order of the nucleotides along the chain stores the information for making proteins.

Starch, cellulose and glycogen are polymers of sugars

1. Plants make the sugar glucose by photosynthesis.
2. Plants join glucose molecules into long chains of starch, which stores energy in foods such as potatoes, rice and wheat.
3. Glucose also forms cellulose, the tough polymer in plant cell walls.
   1. Cotton and paper are mostly cellulose, so a cotton T-shirt is made of a natural polymer.
4. Animals store glucose as glycogen, a polymer found in the liver and muscles.
5. Starch and cellulose are both made from glucose, but the glucose units are linked differently, which gives them very different properties.

Proteins are natural polymers built from amino acids

1. Proteins are long chains of amino acids joined together.
2. There are about 20 different amino acids in the proteins of living things.
   1. The order of the amino acids decides the shape of each protein and so the job it does.
3. Enzymes are proteins that act as biological catalysts, such as amylase in saliva.
4. Hair and nails are made of the protein keratin, and silk and wool are also proteins.
5. The body digests proteins in food back into amino acids, then joins them in new orders to build its own proteins.

6.2.7 Chemical cells and fuel cells

Chemical cells work until their reactants run out

1. Inside a cell, a chemical reaction pushes electrons around the external circuit, producing a potential difference.
2. An everyday cell, such as an AA battery, contains a fixed amount of reactants sealed inside it.
   1. An alkaline AA cell reacts zinc with manganese(IV) oxide to give about $1.5V$.
3. As the cell is used, its reactants are used up, and once one runs out the potential difference falls to zero and the cell is flat.
4. A rechargeable cell can be recharged by passing a current through it, which reverses the reaction and remakes the reactants.

Fuel cells run while fuel and oxygen are supplied

1. Fuel and oxygen, usually from the air, are fed into a fuel cell continuously from outside.
   1. The products are removed as they form, so the cell does not run flat while the supply continues.
2. In a hydrogen/oxygen fuel cell, hydrogen reacts with oxygen to form water: $$2H_2+O_2\to 2H_{2}O$$
3. This is the same overall reaction as burning hydrogen, but the energy is transferred directly as electrical energy rather than as heat in a flame.
   1. Fuel cells therefore waste less energy as heat, so they are more efficient than engines that burn fuel.

Hydrogen fuel cells give off only water

1. The only product is water, so no carbon dioxide is released at the point of use.
   1. They also release no nitrogen oxides or soot particles, which petrol and diesel engines give out.
2. Fuel cells convert more of the energy in their fuel into electrical energy than a petrol or diesel engine does.
3. They have no moving parts, so they are quiet and need less maintenance.
4. A hydrogen car can be refuelled in a few minutes, much faster than most battery cars can be recharged.
5. A hydrogen tank and fuel cell are lighter than batteries storing the same energy, which suits long journeys and heavy vehicles.

Hydrogen is hard to store and often fossil-based

1. Hydrogen is a very flammable gas, so leaks can cause explosions.
2. Hydrogen gas takes up a large volume, so it is stored in heavy high-pressure tanks.
3. There are few refuelling stations, so hydrogen vehicles cannot yet travel everywhere.
4. Most hydrogen is made from natural gas or other hydrocarbons, a process that releases carbon dioxide.
5. Hydrogen can also be made by the electrolysis of water, but this needs a lot of electricity, which may come from burning fossil fuels.
   1. Only hydrogen made using renewable electricity, such as wind or solar power, gives a fuel cell very low carbon emissions overall.
6. Fuel cells are expensive, partly because they use a platinum catalyst.

Weighing benefits and drawbacks when choosing a fuel cell

1. Other fuel cells use fuels such as methanol, a liquid alcohol.
   1. Methanol is easier to store and carry than hydrogen, because it is a liquid at room temperature.
   2. Methanol contains carbon, so a methanol fuel cell releases carbon dioxide as well as water: $$2{CH}_{3}OH+3O_2\to 2{CO}_2+4H_{2}O$$
   3. Methanol is toxic and flammable, so its cartridges must be sealed and handled safely.
2. For a given use, compare the emissions, efficiency, fuel storage, cost and safety of each option.
3. A hydrogen fuel cell suits a city bus that returns to one depot each night, where a single hydrogen supply can refuel the whole fleet.
4. A methanol fuel cell suits a remote sensor or a camping power pack, where a small liquid cartridge is easier to carry than a gas cylinder.

6.2.8 Condensation polymerisation and how to make one

Condensation polymerisation releases a small molecule per link

1. Each monomer has two functional groups, one at each end of the molecule.
   1. The group at one end reacts with a monomer on one side and the group at the other end reacts with a monomer on the other side, so a chain grows.
2. Most condensation polymers are made from two different monomers that join alternately.
3. Each time two functional groups react, a new link forms and a small molecule, usually water, is released.
4. Condensation polymerisation therefore gives two products, the polymer and the small molecule.
5. Addition polymerisation, in contrast, uses monomers with a $C=C$ double bond and makes the polymer as its only product.

A diol and dicarboxylic acid form a polyester

1. A diol has two hydroxyl groups, $-OH$, and a dicarboxylic acid has two carboxyl groups, $-COOH$.
2. The $-COOH$ group of the acid reacts with the $-OH$ group of the diol to form an ester link, $-COO-$, and a water molecule.
   1. The water forms from the $-OH$ of the acid group and the hydrogen atom of the alcohol group.
3. Each monomer reacts at both ends, so the chain grows with acid and diol units alternating.
4. Using a box and a circle for the carbon chains between the functional groups, the equation is: $$nHOOC-\square -COOH+nHO-◯-OH\to {\left[-OC-\square -COO-◯-O-\right]}_n+2n{H}_{2}O$$
5. PET, made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid, is the polyester used for drinks bottles and polyester clothing.

A diamine and dicarboxylic acid form a polyamide

1. A diamine has an amine group, $-{NH}_2$, at each end of its molecule.
2. The $-COOH$ group reacts with the $-{NH}_2$ group to form an amide link, $-CONH-$, and a water molecule.
3. The polymer formed is a polyamide, and nylon is the best-known example.
   1. Nylon-6,6 forms from hexane-1,6-diamine and hexanedioic acid, which each have six carbon atoms.
   2. Nylon is strong and hard-wearing, so it is used for ropes, fishing line and clothing.
4. If the acid is replaced by an acid chloride such as hexanedioyl dichloride, with a $-COCl$ group at each end, the small molecule released is hydrogen chloride, $HCl$.
5. Proteins and DNA are also condensation polymers, made in living things from amino acids and nucleotides.

Making nylon where two solutions meet

1. One monomer, hexane-1,6-diamine, is dissolved in water.
2. The other, hexanedioyl dichloride, is dissolved in an organic solvent such as cyclohexane, which does not mix with water.
3. The two solutions are immiscible, so they form two layers, with the less dense cyclohexane solution on top.
   1. The monomers can only meet at the boundary between the layers, so a thin film of nylon forms there.
4. As the film is pulled out, fresh monomers meet at the boundary and more nylon forms, so a continuous thread can be drawn out.
5. The reaction releases hydrogen chloride, so the diamine solution often contains a little sodium hydroxide to neutralise it.