Carbon the element

Carbon is element number 6, with six protons and six electrons. It occurs in nature as three isotopes: carbon12, with six neutrons; carbon 13, with seven neutrons, and carbon 14 with eight neutrons. Carbon 14 is radioactive, and slowly changes into nitrogen when one of its neutrons changes into a proton. Small amounts of it are formed in the atmosphere by the action of solar radiation, but any carbon removed from the atmosphere gradually loses its carbon 14 until, after about a hundred thousand years, none is left. This is the basis of carbon dating.

Carbon is the fourth most abundant element in the universe, (after H, He and O) but only the 15th most abundant in the Earth’s crust. The element carbon is formed during the Red Giant stage of a star’s life cycle, then gets spread around when the star explodes in a supernova. Our Sun is formed from the ashes of such an explosion, which is why it contains a relatively large amount of carbon.

Carbon as an element bonds with other carbons. The most common arrangement is that carbon is bonded in hexagons with three other carbons in a flat sheet, using three of its electrons. The fourth electron is ‘half shared’ with the layer above or the layer below. This is called the graphite structure. The term for this fourth ‘half shared’ electron is delocalised. This type of sharing allows the electrons to move around, so it conducts electricity. This arrangement of carbon is called graphite.

We often see carbon as soot e.g. from a candle on the bottom of a plate. This type of carbon is called amorphous carbon, and has a graphite-like structure but in tiny, mixed up bits (amorphous means “without shape”). The tiny 'domains' of graphite are randomly arranged and are poor at conducting electricity because the electrons cannot easily jump from one graphite piece to the next. This form of carbon is very black (the silvery colour of graphite is caused by the electrons moving, the same property that allows it to conduct). Black amorphous carbon occurs in coal, charcoal and soot. Carbon black is a commonly used pigment in printing inks because it is cheap and easy to make by burning hydrocarbons in a low-oxygen flame.

Left: soot from diesel burning. Right: carbon black

Carbon also occurs in a number of 'exotic' structures such as buckyballs, buckytubes and fullerenes. These have the carbon atoms arranged in various specific geometries.

Left: carbon atoms in a buckyball. Right: an example buckytube

Summary note for Carbon the element (cloze exercise PDF)

Carbon Compounds

In this Achievement Standard we look at the chemistry of organic carbon compounds. Inorganic carbon compounds include carbon dioxide, carbon monoxide and a few others. Organic compounds are formed between carbon, hydrogen, oxygen, nitrogen and a few other elements. They get their name because of the fact that they form the key chemicals in living things, including carbohydrates, fats and proteins. There are millions of different chemicals, so scientists use a systematic method of naming them. This is based on:

A: number of carbon atoms, given in the prefix (first part of the name)

1=meth…

2=eth…

3=prop…

4=but…

5=pent…

6=hex…

7=hept…

8=oct…

The end of the name (the suffix) tells you what type of carbon compound it is. For example: the alkane with 1 carbon is methane, the alcohol with one carbon is methanol, the carboxylic acid with one carbon is methanoic acid.

Alkanes

Alkanes are a family of hydrocarbons. Hydrocarbons are carbon compounds containing hydrogen and carbon only. Alkanes are hydrocarbons which are made of chains of carbon atoms joined by single covalent bonds, with remaining bond spaces occupied by hydrogen atoms also held by covalent bonds. Covalent bonds are bonds where electrons are shared – as opposed to ionic bonds where electrons are transferred. Alkanes have the general formula C_nH_2n+2 where “n” is the number of carbon atoms in the compound. A series of compounds with the same general structure and the same general formula like this is called a homologous series.

Carbon compounds in the same homologous series will have the same ending to their name; alkanes have a name ending in “-ane”. The first part of the name tells you how many carbons, so the alkane with six carbons is called hexane and has the formula C₆H₁₄.

Alkanes (and carbon compounds in general) with smaller number of carbons are termed ‘lighter’. Hexane is the lightest alkane that is liquid at room temperature (20°C).

Carbon compounds can be represented by molecular formulae, as shown on the left of the examples below, or structural formulae as shown on the right. A structural formula gives some idea of the way that the atoms are arranged in the molecule but is limited by the 2-dimensional way we draw them. The actual structures are 3D.

Summary exercise for carbon compounds and alkanes (cloze, PDF)

Alkenes

The second homologous series we look at are alkenes. These have a formula C_nH_2n. The simplest alkene is ethene, C₂H₄. Once again, the first part of the name tells you how many carbons; the second part tells you what kind of compound it is.

Alkenes have a double bond between two carbons. This means that two electrons are shared instead of one. We represent this by drawing a double line between the carbons:

Each of these carbons still has four bonds around it; two with one hydrogen each and two more with the adjacent carbon.

The double bond is different in shape and properties from a single bond.

Although hydrocarbon chains can contain more than one double bond, this is not required at Level 1, and you are only required to know about ethene and propene,

The double bond can be broken and further hydrogens or other reactive atoms can be added to the two single bonds that result. For this reason, we term compounds with a double bond unsaturated. Carbon compounds with only single bonds are called saturated because you can't "add any more".

A number of different things can be added in place of the double bond in an alkene; for example, bromine:

This is the basis of the bromine water test for the presence of a double bond in a hydrocarbon:

The video above shows this test, although it uses a higher-carbon alkene (hexene) rather than ethene or propene because those are gases and harder to handle.

The video below gives more of an explanation of what is happening:

Melting and boiling points of alkanes

The melting point and boiling point of alkanes increases with the number of carbons. Methane, ethane, propane and butane gases at room temperature. Butane boils at just -1ºC; propane boils at -70 ºC

• Pentane is a liquid which boils at just 60 ºC. It is sometimes used in applications where a liquid with a low boiling point is needed - for example, it is used in the Ngawha Geothermal Power station to get more heat out of the steam; after the steam condenses, it is piped through pentane which vaporizes and drives more turbines to generate more electricity.

• Hexane, heptane, octane and nonane are liquids and are found in petrol.

• Heavier alkanes – C10 to C14 make up kerosene or light fuel oils. Heavier alkanes than this make up heavy fuel oils. The heaviest alkanes are solids at room temperature and known as paraffin waxes.

• Diesel is a fuel oil made of a mixture of light and moderately heavy fuel oils, its composition varies in different parts of the world (heavier oils can't be used in very cold countries because they solidify in the fuel tank).

• Bitumen (also called asphalt) is a mixture of alkanes with large numbers of carbons and a rather disorganised structure. Although pure alkanes are colourless, bitumen is black because it contains compounds with lots of carbons and few hydrogens (geologists study these a lot). In NZ we call the tarry liquid bitumen and use the word asphalt to mean bitumen combined with sand and grit to make what Americans call asphalt concrete. This useful substance is used extensively on roads; it is heated up to make it soft then rolled onto a surface and compressed. When it cools it becomes a tough, flexible surface less prone to cracking than mineral concretes.

Why do the melting and boiling points vary in this way: The difference in melting and boiling point within the alkane family arises for two reasons:

1. The larger the molecule, the larger its surface area. Since inter-molecular forces (attractive forces between the particles) depend on surface area, this increases the melting and boiling point because the more attractive force, the harder it is to separate the molecules. Separating the molecules is necessary for a change of state to occur.

2. The increase in the mass of the molecules. There is a certain amount of movement required for the molecules to break free of the forces between them to change state and melt or evaporate. At a given temperature, the amount of energy the particles have is (on average) the same. Since the kinetic energy depends on mass and speed, this means that the heavier particles must be moving more slowly. Therefore, it takes a higher temperature to make heavier particles move fast enough to break the forces and change state. If you were to make a molecule of butane using entirely heavy isotopes of carbon and hydrogen, it would have a higher boiling point than normal butane (this is one of the ways geologists work out past temperatures).

The first reason is responsible for much more of the temperature trend than the second.

Where do alkanes come from?

Natural gas and crude oil are mixtures of mostly alkanes. The gas component of wells of both these consists of gases methane through to butane (with carbon dioxide and other gases sometimes as impurities which need to be removed). Many gas fields produce significant quantities of light oil (generally pentane to octane isomers) termed condensate, and most oil fields produce significant amounts of gas.

Gas separation: for natural gas, the butane and propane are further separated (by cooling and pressure) and sold as LPG. This is because of the higher commercial value of these gases (they are valuable because they are easy to liquify and transport in tanks rather than by pipeline). The methane and ethane are sold as natural gas. These gases can be cooled and liquified as LNG, but this requires expensive infrastructure so most is piped to its destination.

Methane also occurs in significant amounts in coal mines, where it can be an explosive hazard - as the Pike Creek Mine disaster showed. Some coal seams (e.g at Huntly) are being exploited for their methane.

Methane is also produced by anaerobic respiration in certain bacteria and is a component of animal digestive gases, bubbles from swamps and similar phenomena. Such biogenic methane which is produced in the deep ocean can be trapped in the form of methane hydrates in very cold water at high pressure. Swamp gas is mostly methane; climate scientists are worried that global warming will cause increased methane production from frozen swamps called tundra, and the lakes in this terrain (Canada, Alaska and Siberia have many of these). Methane is a significant greenhouse gas, and if this happens it could cause a positive feedback pattern with further warming releasing more methane causing more warming and so on.

Petroleum and oil refining:

Crude oil (or petroleum) is a mixture of alkanes from about 6 to over 30 carbons. They are separated by fractional distillation into different length alkanes for different uses. This makes use of a fractionation column as shown below; because alkanes with different numbers of carbons change state (liquid-gas) at different temperatures, the mixture can be separated this way. The mix of different hydrocarbons produced this way is not always what the market requires, so the additional processes of cracking and reforming are sometimes carried out. Cracking involves breaking long chain hydrocarbons down into smaller chain ones, by use of heat and catalysts. Reforming involves changing the molecular makeup of the hydrocarbons. We will cover these two processes in more detail later.

Below is a schematic diagram of a fractionating column for crude oil:

The crude oil feedstock is heated so all hydrocarbons but the bitumen residue evaporate. The mixture feeds into the fractionating column, where the bitumen runs out the bottom. The vapour is made of lighter molecules and rises up the column, which is hotter at the bottom and cooler at the top. At several places up the column are condensers, each one cooler than the one below it. Fuel oil condenses on the bottom condenser and is drawn off while the lighter molecules are still gases and rise further. The condenser above is a bit cooler and diesel condenses out here; the remaining vapour rises and the process is repeated until the lightest molecules escape the top as gases. The gas from the top is often burned in the furnace to provide heat for the whole process.

Cracking and Reforming

As previously stated, crude oil is a mixture of hydrocarbons which can be separated through fractional distillation as described above, However, the proportions of the different hydrocarbons in the crude oil is rarely a perfect match for the market (which also varies in different countries). Two processes can be used to alter the ratio of long to short-chain hydrocarbons in the mix. These processes are cracking, the breaking apart of long molecules, and reforming, the reconstruction of shorter chain molecules into a mixture with more desirable properties.

Cracking

Cracking involves breaking down long hydrocarbon chains. It can be used to produce shorter chain molecules for lighter fuels (such as petrol), and to produce alkenes. Short alkenes are very valuable as the starting material for producing many plastics and industrial chemicals.

During cracking, the long chain hydrocarbon is heated and usually passed over a catalyst. This causes the long chain to break and produce two shorter chains, one of which will contain a double bond (so the overall number of hydrogens remains the same) e.g.

hexane -------(heat, catalyst)--------> butane + ethene

In practice, the starter molecule will usually have more than six carbons. The video below shows a lab demonstration of the cracking of 'paraffin oil' (an alkane with about 18 carbons; 'baby oil' is paraffin with some scent).

Reforming

This is a related process where the mix of hydrocarbons is changed. This is done under pressure with hydrogen gas and a catalyst. You are not expected to know details about it.

Alcohols

Carbon compounds consist of a 'backbone' of carbon atoms, linked together in various ways and surrounded by hydrogens. The hydrogen atoms are joined to the carbons by a covalent bond.

The hydrogen at the end of one of these covalent bonds can be replaced by another atom or group of atoms. This is called a functional group. The type, and position of a functional group profoundly changes the chemistry of the resulting molecule.

The only functional group you need to know about at Level 1 is called the hydroxyl group. This is made of an oxygen and a hydrogen. The oxygen has one of its two covalent bonds joined to the hydrogen, and the other to the carbon.

In text we write this as -OH.

Therefore the chemical formula for methane is CH₄ and if you add an -OH it becomes CH₃OH,

We give it a new name: methanol. The 'meth' part tells you there is one carbon, the 'ol' part tells you it has the -OH group on it.

Carbon compounds with a hydroxyl added onto the basic hydrocarbon structure are called alcohols. The alcohol in wine and other beverages is ethanol. You can tell from its name that it has two carbon atoms. Methanol (shown above, with one carbon) and ethanol are the only two alcohols you need to know about. This is because life starts getting more complicated for alcohols with 3 or more carbons because there start to be alternative positions for the -OH on the carbon chains.

Differences from alkanes

Methanol and ethanol are both liquids at room temperature. This contrasts with methane and ethane, which are both gases. The lightest liquid alkane is pentane, which is three times heavier per molecule than methanol.

Methanol and ethanol are also both miscible (that is, mix freely) with water. This is in contrast with liquid alkanes, which will float in a liquid layer over the top of the water and can't mix with it. The reasons for these two properties are related.

Although oxygen forms covalent bonds - that is, sharing electrons - with carbon and hydrogen it isn't an 'equal' sharing. Oxygen attracts electrons much more strongly than either carbon and hydrogen (in chemistry you would say that it is more electronegative).

This means that when when the electrons 'whizz around' the two atoms in the bond, they are more attracted to the oxygen. This makes the oxygen atom more negative and the other end of the bond relatively positive. This creates a 'dipole' which can be attracted to an electric field (recall the picking up of pieces of paper by a charged rod in the electricity section).

Production of alcohol by fermentation

Ethanol can be produced industrially from ethene (see lower in the page). It can also be produced by fermentation, a process using microbes. The commonest form of this is the fermentation by yeast of glucose:

The process happens because the yeasts are using the glucose in oxygen-free conditions as a source of energy i.e. anaerobic respiration. The ethanol and carbon dioxide are waste products as far as the yeasts are concerned.

Note that other alcohols can be made by fermentation. Methanol can be produced as an unintended product if fermentation conditions aren't controlled properly, and often turns up as an impurity in 'home-made' spirits. This can be a particular problem in some 'cheap' tourist destinations in developing countries, where people have been poisoned and have died as a result of consuming these local home-brew drinks (it is less of a problem when the homebrew is not distilled into spirits; methanol has a lower boiling point than ethanol so is preferentially concentrated into the products of home-distilled moonshines).

The carbon dioxide produced in this reaction gives us the bubbles in beer and sparkling wine, and is also the rising agent in yeast breads.

Although the reaction above is specific for glucose, many carbonhydrates can be used as starters for fermentation becuase they are readily altered to glucose by other enzymes. It is also enzymes in the yeast that carry out the chemical reaction of fermentation.

Production of methanol from natural gas

New Zealand produces over a million tonnes of methanol from natural gas at the Motunui plant in Taranaki. The process of making methanol from methane is a multi-step process:

1. Methane is reacted with steam using a catalyst to produce carbon monoxide and hydrogen:

CH₄ + H₂O --> CO + 3H₂

2. The hydrogen and carbon monoxide are reacted to produce methanol:

CO + 2H₂ --> CH₃OH

Because the first reaction produces more hydrogen than is needed for the second reaction, a way of dealing with the extra hydrogen is required. This is done by reacting it with carbon dioxide:

3. CO₂ + 3H₂ --> CH₃OH + H₂O

By carefully controlling reaction conditions the water can be recycled into the first reaction.

Uses of alcohols

Alcohols can be used as fuels - methanol is used in some racing cars. Some countries add 10% or more ethanol to petrol, both as a 'green' fuel and to increase the octane rating. Petrol engines can run on pure ethanol with some modification (ethanol can corrode some engine components in a standard engine). Small ethanol burners are often used by caterers to keep food warm.

Alcohols are good solvents and make a good 'half-way' between being a solvent for things that dissolve in water and things that dissolve in oils. Therefore alcohols can be used for cleaning. Ethanol is used as a carrier for perfume oils e.g. the Lynx that boys so love to spray around is mostly ethanol, with various scent oils dissolved in it (usually only about 5%).

Combustion

Combustion means burning. Many carbon compounds are burned as fuels. There are two types of combustion – complete or incomplete. Complete combustion: produces carbon dioxide and water only and requires sufficient oxygen You need to be able to write word and formula equations for complete combustion.

Rules when balancing combustion reactions

these only apply to COMPLETE combustion as only they have a single correct answer for balancing

1. Add enough waters to use up all the hydrogen e.g. if it was heptane C₇H₁₆ with 16 hydrogen in the formula you would need to add eight water on the right hand side i.e. 8H₂O

2. Add the correct number of carbons dioxides to use up all the carbons; in this case it would be seven to use up all the seven carbons i.e. 7CO₂

3. Now count up the oxygens on the right side: seven carbon dioxide with two oxygen each gives fourteen, plus another 8 for the eight water gives a total of 22 oxygen atoms.

4. Have this number to get the total number of oxygens on the left at two per molecule; in this case it would be 11

5. For heptane this then gives C₇H₁₆ + 11O₂ --> 7CO₂ + 8H₂O

Note: if the number of oxygens on the right was odd e.g. if you did the steps above for octane you would get 9 waters and 8 carbon dioxide giving 25 oxygen atoms; you can either write a fractional number of O2 molecules (12.5)

i.e. C₈H₁₈ + 12.5 O₂ --> 8CO₂ + 9H₂O

OR you can double every number in the whole equation

e.g. 2C₈H₁₈ + 25O₂ --> 16CO₂ + 18H₂O

Both the fractional and non fractional answer would be marked correct at Level 1 in NCEA (they are fussier in level 2 and require non fractional answers).

You can’t write a balanced equation easily unless you are told which products are present, but if you are told one of these things is present you know combustion is incomplete. Some carbon dioxide may also be produced during incomplete combustion. If you are given one of these equations, you will have to work out how to balance it on the basis of what you are told. In practice, the products of incomplete combustion are a mixture and so the amounts don't matter (the reaction is non-stoichiometric). Nevertheless, if you are told to write a formula reaction for incomplete combustion you will need to balance it.

An example equation would be

C₆H₁₄ + 5O₂ --> 3CO + 3C + 7H₂O

In this example reaction I have produced equal amounts of carbon monoxide and soot. Notice all the hydrogens react with the available oxygen.

Methanol and ethanol will, as far as you are concerned, only undergo complete combustion.

Some racing cars use methanol as a fuel; a hazard is that the flame is nearly invisible in daylight so that it is hard to tell in a crash whether the car is on fire.

Carbon monoxide is highly poisonous. It prevents the hemoglobin in the blood from transporting oxygen. For this reason, it is important not to use appliances that burn hydrocarbons without adequate ventilation. However, carbon monoxide also has numerous industrial applications and is manufactured for these using a variety of methods (often from coal but sometimes from methane).

Appliances such as LPG heaters, portable barbeques or any others that rely on combustion must be used with adequate ventilation, both to ensure adequate oxygen supply for combustion and to allow any CO produced to safely disperse. Fatalities from the failure to observe this precaution are not uncommon.

Some reactions of alkenes

Alkenes are highly useful chemicals because the double bond can easily be broken. This allows a range of useful products to be made. This is why cracking, which always produces alkenes, is commonly done at oil refineries.

Ethene reacts with water to produce ethanol:

Names: poly-ethene is more commonly known as polythene or polyethylene. Poly-propene is known as polypropylene.

Conditions for polymerisation: Ethene and propene are gases, so they will tend to polymerise under very high pressure because this takes less space. Heat and a catalyst are also used.

Comparing these polymers: the extra CH₃ group on the carbon backbone of polypropylene gives it extra surface area and extra strength along its length (because the adjacent CH₃ groups attract each other. Therefore, polythene has a lower melting point and is more flexible than polypropylene.

• polyethylene is used for things like milk bottles, cling-wrap, wrapping hay bales etc. It has high chemical stability (resists water and many chemicals) but relatively low resistance to heat and stretching.

• polypropylene has is more resistant to stretching and heat. It is used a lot for clothing (e.g. thermal underwear, because it is also very insulating), rope and similar applications. It is denser than polyethylene and doesn't always float in water.

In practice, polymers of alkenes can be mixed in their liquid state to produce a plastic of the qualities needed (and other ingredients also added). Also, the length of the carbon chain can be manipulated to optimise the properties of the plastic for a particular application.

Revision resources:

Note that many of these will not work unless you are logged into SHC Domain as they link to resources in Google Drive.

Carbon SciPad answers

NZQA 2020 Exam paper Video on answering: Question 1 Question 2 Question 3

Cloze 1 Cloze 2 (more coming)

Carbon revision note template