A Level Chem - Blog Posts
Covalent and Dative Bonds
Covalent and dative (sometimes called co-ordinate) bonds occur between two or more non-metals, e.g. carbon dioxide, water, methane and even diamond. But what actually are they?
A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. They are found in molecular elements or compounds such as chlorine or sulfur, but also in macromolecular elements and compounds like SiO2 and graphite. Covalent bonds are also found in molecular ions such as NH4+ and HCO3-.
Single covalent bonds have just one shared pair of electrons. Regularly, each atom provides one unpaired electron (the amount of unpaired electrons is usually equal to the number of covalent bonds which can be made) in the bond. Double covalent bonds have two shared pairs of electrons, represented by a double line between atoms, for example, O=C=O (CO2). Triple covalent bonds can also occur such as those in N ≡ N.
Dot and cross diagrams represent the arrangement of electrons in covalently bonded molecules. A shared pair of electrons is represented by a dot and a cross to show that the electrons come from different atoms.
Unpaired electrons are used to form covalent bonds as previously mentioned. The unpaired electrons in orbitals of one atom can be shared with another unpaired electron in an orbital but sometimes atoms can promote electrons into unoccupied orbitals in the same energy level to form more bonds. This does not always occur, however, meaning different compounds can be formed - PCl3 and PCl4 are examples of this.
An example where promotion is used is in sulfur hexafluoride (SF6). The regular configuration of sulfur atoms is 1s2 2s2 2p6 3s2 3p4. It promotes, as shown in the diagram (see excited state), two electrons: one from the 3s electrons to the 3d orbital and one from the 3p to the 3d. Therefore there are 6 unpaired electrons for fluorine atoms to join. It has an octahedral structure.
An atom which has a lone pair (a pair of electrons uninvolved in bonding) of electrons can form a coordinate bond with the empty orbital of another atom. It essentially donates an electron into this orbital which when formed, acts the same as a normal covalent bond. A coordinate bond therefore contains a shared pair of electrons that have come from one atom.
When ammonia reacts with a H+ ion, a coordinate bond is formed between the lone pair on the ammonia molecule and the empty 1s sub-shell in the H+ ion. An arrow represents the dative covalent bond (coordinate bond). Charges on the final ion must be showed.
Summary
A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. They are found in molecular elements or compounds as well as in macromolecular elements and compounds. Also found in molecular ions.
Single covalent bonds have just one shared pair of electrons. Double covalent bonds have two shared pairs of electrons, represented by a double line between atoms. Triple covalent bonds can also occur.
Dot and cross diagrams represent the arrangement of electrons in covalently bonded molecules. A shared pair of electrons is represented by a dot and a cross to show that the electrons come from different atoms.
Unpaired electrons are used to form covalent bonds - they can be shared with another unpaired electron in an orbital but sometimes atoms can promote electrons into unoccupied orbitals in the same energy level to form more bonds. This does not always occur, however, meaning different compounds can be formed.
An example where promotion is used is in sulfur hexafluoride (SF6).
An atom which has a lone pair (a pair of electrons uninvolved in bonding) of electrons can form a coordinate bond with the empty orbital of another atom.
It donates an electron into this orbital which when formed, acts the same as a normal covalent bond. A coordinate bond therefore contains a shared pair of electrons that have come from one atom.
When ammonia reacts with a H+ ion, a coordinate bond is formed between the lone pair on the ammonia molecule and the empty 1s sub-shell in the H+ ion. An arrow represents the dative covalent bond (coordinate bond). Charges on the final ion must be showed.
Structural Isomerism
In one boring history lesson, you and your friend (who both love chemistry) are doodling displayed formulas in the back of your textbook. You both decide to draw C5H12 - however, when you come to name what you’ve drawn, your friend has something completely different. You know what you’ve drawn is pentane and your friend knows what they’ve drawn is 2,3-dimethylpropane. So which one is C5H12?
The answer is both! What you and your friend have hypothetically drawn are structural isomers of C5H12 (another is 2-methylbutane). These are compounds which have the same molecular formula but different structural formulas.
Isomers are two or more compounds with the same formula but a different arrangement of atoms in the molecule and often different properties.
There are several different kinds of structural isomers: chain, positional and functional group.
Chain isomerism happens when there is more than one way of arranging carbon atoms in the longest chain. If we continue with the example C5H12, it exists as the three chain isomers shown above. Chain isomers have similar chemical properties but different physical properties because more branched isomers have weaker Van der Waals and therefore lower boiling points.
Positional isomers have the same carbon chain and the same functional group but it is attached at different points along the chain.
This is a halogenoalkane. The locant “1″ describes where the chlorine is on the chain. For more on naming organic compounds, check out my nomenclature post.
The final type of isomer you need to know is a functional group isomer. This is a compound with the same molecular formula but a different functional group. For example, C2H6O could be ethanol or methoxymethane.
And surprisingly, that is all you need to know for the AS exam. There are also things called stereoisomers but those will be covered next year. Just make sure you know how to name and draw the three different kinds of structural isomers for the exam. Practice makes perfect!
SUMMARY
Structural isomers are compounds which have the same molecular formula but different structural formulas.
Isomers are two or more compounds with the same formula but a different arrangement of atoms in the molecule and often different properties.
There are several different kinds of structural isomers: chain, positional and functional group.
Chain isomerism happens when there is more than one way of arranging carbon atoms in the longest chain. Chain isomers have similar chemical properties but different physical properties because more branched isomers have weaker Van der Waals and therefore lower boiling points.
Positional isomers have the same carbon chain and the same functional group but it is attached at different points along the chain.
A functional group isomer is a compound with the same molecular formula but a different functional group.
Happy studying!
Combusting Alkanes
If you follow this blog, by now you must be thinking, when will we be done with the alkane chemistry? Well, the answer is never. There is still one more topic to touch on - burning alkanes and the environmental effects. Study up chums!
Alkanes are used as fuels due to how they can combust easily to release large amounts of heat energy. Combustion is essentially burning something in the presence of oxygen. There are two types of combustion: complete and incomplete.
Complete combustion occurs when there is a plentiful supply of air. When an alkane is burned in sufficient oxygen, it produces carbon dioxide and water. How much depends on what is being burnt. For example:
butane + oxygen -> carbon dioxide + water
2C4H10 (g) + 13O2 (g) -> 8CO2 (g) + 10H2O (g)
Remember state symbols in combustion reactions. In addition, this reaction can be halved to balance for 1 mole of butane by using fractions when dealing with the numbers.
C4H10 (g) + 6 ½ O2 (g) -> 4CO2 (g) + 5H2O (g)
Incomplete combustion on the other hand occurs when there is a limited supply of air. There are two kinds of incomplete combustion. The first type produces water and carbon monoxide.
butane + limited oxygen -> carbon monoxide + water
C4H10 (g) + 4 ½ O2 (g) -> 4CO (g) + 5H2O (g)
Carbon monoxide is dangerous because it is toxic and undetectable due to being smell-free and colourless. It reacts with haemoglobin in your blood to reduce their oxygen-carrying ability and can cause drowsiness, nausea, respiratory failure or death. Applicances therefore must be maintained to prevent the formation of the monoxide.
The other kind of incomplete combustion occurs in even less oxygen. It produces water and soot (carbon).
butane + very limited oxygen -> carbon + water
C4H10 (g) + 2 ½ O2 (g) -> 4C (g) + 5H2O (g)
Internal combustion engines work by changing chemical energy to kinetic energy, fuelled by the combustion of alkane fuels in oxygen. When this reaction is undergone, so do other unwanted side reactions due to the high pressure and temperature, e.g. the production of nitrogen oxides.
Nitrogen is regularly unreactive but when combined with oxygen, it produces NO and NO2 molecules:
nitrogen + oxygen -> nitrogen (II) oxide
N2 (g) + O2 (g) -> 2NO (g)
and
nitrogen + oxygen -> nitrogen (II) oxide
N2 (g) + 2O2 (g) -> 2NO2 (g)
Sulfur dioxide (SO2) is sometimes present in the exhaust mixture as impurities from crude oil. It is produced when sulfur reacts with oxygen. Nitrogen oxides, carbon dioxide, carbon monoxide, carbon particles, unburnt hydrocarbons, water vapour and sulfur dioxide are all produced in exhaust fumes and are also pollutants that cause problems you need to be aware of for the exam as well as how to get rid of them.
Greenhouse gases contribute to global warming, an important process where infrared radiation from the sun is prevented from escaping back into space by atmospheric gases. On the one hand, some greenhouse gases need to continue this so that the earth can sustain life as it traps heat, however, we do not want the earth’s temperature to increase that much. Global warming is the term given to the increasing average temperature of the earth, which has seen an increase in the last few years due to human activity - burning fossil fuels like alkanes has produced more gases which trap more heat. Examples of greenhouse gases include carbon dioxide, methane and water vapour.
Another pollution problem the earth faces is acid rain. Rain water is already slightly acidic due to the CO2 present in the atmosphere but acid rain is more acidic than this. Nitrogen oxides contribute to acid rain although sulfur dioxide is the main cause. The equation for sulfur dioxide reacting with water in the air to produce oxidised sulfurous acid and therefore sulphuric acid is:
SO2 (g) + H2O (g) + ½ O2 (g) -> H2SO4 (aq)
Acid rain is a problem because it destroys lakes, buildings and vegetation. It is also a global problem because it can fall far from the original source of the pollution.
Photochemical smog is formed from nitrogen oxides, sulfur dioxide and unburnt hydrocarbons that react with sunlight. It mostly forms in industralised cities and causes health problems such as emphysema.
So what can we do about the pollutants?
A good method of stopping pollution is preventing it in the first place, therefore cars have catalytic converters which reduce the amount of carbon monoxide, nitrogen oxides and unburnt hydrocarbons come into the atmosphere by converting them into less toxic gases. Shaped like a honeycomb for increased SA and therefore rate of conversion, platinum and rhodium coat ceramic and act as catalysts for the reactions that take place in an internal combustion engine.
As they pass over the catalyst, they react with each other to form less pollution:
octane + nitrogen (II) oxide -> carbon dioxide + nitrogen + water
C8H18 (g) + 25NO -> 8CO2 (g) + 12 ½ N2 (g) + 9H2O (g)
nitrogen (II) oxide + carbon monoxide -> carbon dioxide + nitrogen
2NO (g) + 2CO (g) -> 2CO2 (g) + N2 (g)
Finally, sulfur dioxide must be dealt with. The first way it is dealt with is by removing it from petrol before it can be burnt, however, this is often not economically favourable for fuels used in power stations. A process called flue gas desulfurisation is used instead.
In this, gases are passed through a wet semi-solid called a slurry that contains calcium oxide or calcium carbonate. These neutralise the acid, due to being bases, to form calcium sulfate which has little commercial value but can be oxidised to produce a more valuable construction material.
calcium oxide + sulfur dioxide -> calcium sulfite
CaO (s) + SO2 (g) -> CaSO3 (s)
calcium carbonate + sulfur dioxide -> calcium sulfite + carbon dioxide
CaCO3 (s) + SO2 (g) -> CaSO3 (s) + CO2 (g)
calcium sulfite + oxygen -> calcium sulfate
CaSO3 (s) + O -> CaSO4 (s)
SUMMARY
Alkanes are used as fuels due to how they can combust easily to release large amounts of heat energy. Combustion is essentially burning something in the presence of oxygen.
Complete combustion occurs when there is a plentiful supply of air. When an alkane is burned in sufficient oxygen, it produces carbon dioxide and water
Remember state symbols in combustion reactions. In addition, reactions can be halved to balance for 1 mole of compounds by using fractions when dealing with the numbers.
Incomplete combustion occurs when there is a limited supply of air. There are two kinds of incomplete combustion.
The first type produces water and carbon monoxide.
Carbon monoxide is dangerous because it is toxic and undetectable due to being smell-free and colourless. It reacts with haemoglobin in your blood to reduce their oxygen-carrying ability and can cause drowsiness, nausea, respiratory failure or death.
The other kind of incomplete combustion occurs in even less oxygen. It produces water and soot (carbon).
Internal combustion engines work by changing chemical energy to kinetic energy, fuelled by the combustion of alkane fuels in oxygen. When this reaction is undergone, so do other unwanted side reactions due to the high pressure and temperature, e.g. the production of nitrogen oxides.
Nitrogen is regularly unreactive but when combined with oxygen, it produces NO and NO2 molecules:
Sulfur dioxide (SO2) is sometimes present in the exhaust mixture as impurities from crude oil. It is produced when sulfur reacts with oxygen.
Nitrogen oxides, carbon dioxide, carbon monoxide, carbon particles, unburnt hydrocarbons, water vapour and sulfur dioxide are all produced in exhaust fumes and are also pollutants that cause problems you need to be aware of for the exam as well as how to get rid of them.
Greenhouse gases contribute to global warming, an important process where infrared radiation from the sun is prevented from escaping back into space by atmospheric gases. Some greenhouse gases need to continue this so that the earth can sustain life as it traps heat, however, we do not want the earth’s temperature to increase that much. Global warming is the term given to the increasing average temperature of the earth, which has seen an increase in the last few years due to human activity - burning fossil fuels like alkanes has produced more gases which trap more heat.
Another pollution problem the earth faces is acid rain. Nitrogen oxides contribute to acid rain although sulfur dioxide is the main cause.
Acid rain is a problem because it destroys lakes, buildings and vegetation. It is also a global problem because it can fall far from the original source of the pollution.
Photochemical smog is formed from nitrogen oxides, sulfur dioxide and unburnt hydrocarbons that react with sunlight. It mostly forms in industralised cities and causes health problems such as emphysema.
A good method of stopping pollution is preventing it in the first place, therefore cars have catalytic converters which reduce the amount of carbon monoxide, nitrogen oxides and unburnt hydrocarbons come into the atmosphere by converting them into less toxic gases. Shaped like a honeycomb for increased SA and therefore rate of conversion, platinum and rhodium coat ceramic and act as catalysts for the reactions that take place in an internal combustion engine.
As they pass over the catalyst, they react with each other to form less pollution.
octane + nitrogen (II) oxide -> carbon dioxide + nitrogen + water
C8H18 (g) + 25NO -> 8CO2 (g) + 12 ½ N2 (g) + 9H2O (g)
nitrogen (II) oxide + carbon monoxide -> carbon dioxide + nitrogen
2NO (g) + 2CO (g) -> 2CO2 (g) + N2 (g)
Finally, sulfur dioxide must be dealt with. The first way it is dealt with is by removing it from petrol before it can be burnt, however, this is often not economically favourable for fuels used in power stations. A process called flue gas desulfurisation is used instead.
In this, gases are passed through a wet semi-solid called a slurry that contains calcium oxide or calcium carbonate. Since they are bases, these neutralise the acid to form calcium sulfate which has little commercial value but can be oxidised to produce a more valuable construction material.
Happy studying!
Breaking Down Alkanes - isn’t it cracking?
Unfortunately, if you’re sitting your A Level chemistry exam, you need to know a little more than the basic properties of alkanes outlined in my last post. Luckily though, this post takes you through fractional distillation and the two types of cracking - isn’t that convenient?
Crude oil contains carbon compounds formed by the effects of pressure and high temperature on plant and animal remnants. It is viscious, black and found in rocks beneath the earth’s surface. It is a mixture of mainly alkane hydrocarbons which are separated by a process called fractional distillation. Crude oil is essential because it is burned as a fuel and each fraction has different properties e.g. diesel, petrol, jet fuel.
Fractional distillation is the continual evaporation and condensation of a mixture which causes fractions to split due to a difference in boiling point. It is important to note that fractional distillation does not separate crude oil into pure compounds but rather less complex mixtures. Fractions are groups of compounds that have similar boiling points and are removed at the same level of a fractionating column.
The first step in this process is to heat crude oil in a furnace until some changes state from a liquid to a vapour. This mixture goes up a fractionating tower or column which is hotter at the bottom than the top and reaches a layer which is cool enough to condense and be collected. Shorter chain molecules are collected at the top where it is cooler since they have lower boiling points.
As you go down the fractionating column, bear in mind that: the column temperature increases, the boiling point increases, the number of carbon atoms increases and the strength of the Van der Waals’ between molecules increases.
Different fractions have different usefulnesses and often, it is the fractions with lower boiling points and shorter chains which are much more purposeful. Therefore there needs to be a process to getting shorter chains because they are the least abundant in crude oil samples. To meet demand, long chain molecules that are less useful are broken down into shorter chain molecules. This is done by cracking.
Cracking is a process where long chain hydrocarbon molecules are broken down into shorter chain molecules which are in high demand. This can be done one of two ways - thermal or catalytic.
Thermal cracking involves heating long chain alkanes to high temperatures - usually between 1000 - 1200K. It also uses high pressures up to 70atm and takes just one second. It only needs a second because the conditions could decompose the molecule completely to produce carbon and hydrogen instead. The conditions produce shorter chain alkanes and mostly alkenes.
A typical equation for this:
Decane -> octane + ethene
C10H22 -> C8H18 + C2H4
Catalytic cracking also breaks down long alkanes by heat under pressure using the presence of a zeolite catalyst. Temperature used is approx. 800-1000K and the pressure is often between 1-2 atm. Zeolite is an acidic mineral with a honeycomb structure, made from aluminium oxide and silicion dioxide. The honeycomb structure gives the catalyst a larger surface area which increases ROR. Factories which catalytically crack are often operated continuously for around 3 years at a time and produce branched alkanes, cycloalkanes and aromatic compounds.
You need to be able to compare the conditions of catalytic and thermal cracking for the A Level exam. Know that thermal cracking has a high temperature and pressure, a short duration, no catalyst and produces a high percentage of alkenes and some short chain alkanes. Catalytic uses a catalyst, a high temperature, a low pressure and produces aromatic hydrocarbons and motor fuels.
SUMMARY
Crude oil contains carbon compounds formed by the effects of pressure and high temperature on plant and animal remnants. I It is a mixture of mainly alkane hydrocarbons which are separated by a process called fractional distillation.
Fractional distillation is the continual evaporation and condensation of a mixture which causes fractions to split due to a difference in boiling point.
It is important to note that fractional distillation does not separate crude oil into pure compounds but rather less complex mixtures.
Fractions are groups of compounds that have similar boiling points and are removed at the same level of a fractionating column.
The first step in this process is to heat crude oil in a furnace until some changes state from a liquid to a vapour. This mixture goes up a fractionating tower or column which is hotter at the bottom than the top and reaches a layer which is cool enough to condense and be collected. Shorter chain molecules are collected at the top where it is cooler since they have lower boiling points.
As you go down the fractionating column, bear in mind that: the column temperature increases, the boiling point increases, the number of carbon atoms increases and the strength of the Van der Waals’ between molecules increases.
Fractions with lower boiling points and shorter chains are much more purposeful but are the least abundant in crude oil samples. To meet demand, long chain molecules that are less useful are broken down into shorter chain molecules.
Cracking is a process where long chain hydrocarbon molecules are broken down into shorter chain molecules which are in high demand.
Thermal cracking involves heating long chain alkanes to high temperatures - usually between 1000 - 1200K. It also uses high pressures up to 70atm and takes just one second. It only needs a second because the conditions could decompose the molecule completely to produce carbon and hydrogen instead. The conditions produce shorter chain alkanes and mostly alkenes.
Catalytic cracking also breaks down long alkanes by heat under pressure using the presence of a zeolite catalyst. Temperature used is approx. 800-1000K and the pressure is often between 1-2 atm. Zeolite is an acidic mineral with a honeycomb structure, made from aluminium oxide and silicion dioxide. The honeycomb structure gives the catalyst a larger surface area which increases ROR.
You need to be able to compare the conditions of catalytic and thermal cracking for the A Level exam. Know that thermal cracking has a high temperature and pressure, a short duration, no catalyst and produces a high percentage of alkenes and some short chain alkanes. Catalytic uses a catalyst, a high temperature, a low pressure and produces aromatic hydrocarbons and motor fuels.
Happy studying!
Shapes of Molecules
his post is more information than trying to explain something - the truth is, you just need to learn shapes of molecules like you do with anything. I’ve got a physical chemistry mock tomorrow that I’m dreading since I’ve done zero revision. The fact that I run a study blog yet don’t revise myself is odd, but what else can I do? Oh, wait … revise. So here it is, my last minute revision for myself and you too - I present, shapes of molecules!
VSEPR stands for valence shell electron pair repulsion theory. If you’ve ever seen a moly-mod or a diagram of a molecule in 3D space, you may wonder how they decided it was that shape. Well, VSEPR answers all.
The theory essentially states that electron pairs are arranged to minimise repulsions between themselves - which makes sense, since electrons carry the same charge and therefore try to repel each other. Of course, there are different types of electron pairs, lone and bonding. The strongest repulsions happen between lone pair - lone pair followed by lone pair - bonding pair and finally, bonding pair - bonding pair have the least repulsion.
Since the repulsion governs the shape of the molecule, to work out a molecule’s shape you must look at dot and cross diagrams or electron configurations to see how a molecule is bonded. There are many methods to do this, but the bottom line is that you must work out how many bonding pairs of electrons and how many lone pairs are involved.
The easiest shape to learn is linear. This has two bonding pairs and no lone pairs at an angle of 180 degrees, since that is the furthest the two can get away from each other. Examples of linear molecules include carbon dioxide and beryllium chloride.
Next up is trigonal planar. This has three bonding pairs and no lone pairs, each at the angle of 120 degrees. Trigonal means three and planar means on one plane, this should help you in identifying the molecules since after a fourth pair of electrons, the shape becomes 3D. Examples of trigonal planar molecules include boron trifluoride and sulfur trioxide.
What if you were to have two bonding pairs and two lone pairs? Well, then you’d have a bent molecule. Water is a good example of a bent molecule. Since it has two lone pairs that repel the other two bonding pairs more than they repel each other, the bond angle is 104.5. I’d be careful though, as in many textbooks it shows a bent molecule to have one lone pair and a different bond angle.
Another variation of the bent molecule I’ve seen is the one with two bonding pairs and one lone pair. It is deemed as bent with a bond angle of 109 or sometimes less than 120 degrees.
Tetrahedral molecules have four bonding pairs and no lone pairs. The bond angle is 109.5 degrees. Examples of this include the ammonium ion, methane and the phosphate ion. A good thing to note here is how these molecules are drawn. To demonstrate the 3D shape, where the molecule moves onto a plane, it is represented with a dashed line and triangular line along with a regular straight line.
Trigonal pyramidal, sometimes just called pyramidal, is where there are three bonding pairs and a lone pair. Bond angles are roughly 107 degrees due to the repulsion from the lone pairs. An example of a trigonal pyramidal molecule is ammonia, which has a lone pair on the nitrogen.
Having five bonding pairs gives a trigonal bipyramidal structure. I guess the three bonding pairs on the trigonal plane accounts for that part of the name, where the rest comes from the position of the remaining two. These molecules have no lone pairs and have a bond angle of 90 degrees between the vertical elements and 120 degrees around the plane. Diagrams below are much clearer than my description! Examples of this include phosphorus pentachloride.
Six bonding pairs is an octahedral structure. I know this is confusing because octahedral should mean 8 but it’s one of those things we get over, like the fact sulfur isn’t spelt with a ph anymore. It’s actually to do with connecting the planes to form an octahedral shape.There are no lone pairs and each bond angle is a nice 90 degrees. Common examples include sulfur hexafluoride.
Square planar shapes occur when there are six bonding pairs and two lone pairs. All bond angles are 90 degrees! They take up this shape to minimise repulsions between electrons - examples include xenon tetrafluoride.
The final one to know is T-shape. This has three bonding pairs and two lone pairs. These molecules have bond angles of (less than) 90 degrees, usually a halogen trifluoride like chlorine trifluoride.
There are plenty more variations and things you could know about molecular geometry, but the truth is, there won’t be an extensive section on it. It’s a small part of a big topic!
I’m not going to do a summary today since I’d just be repeating the same information (I tried to keep it concise for you guys) so instead I’ll just leave you with,
Happy studying!