Structural Isomerism

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.

Slice of Life

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!

Polarity, Resonance, and Electron Pushing: Crash Course Organic Chemistry #10:

We’ve all heard the phrase “opposites attract.” It may or may not be true for people, but it’s definitely true in organic chemistry. In this episode of Crash Course Organic Chemistry, we’re learning about electronegativity, polarity, resonance structures, and resonance hybrids. We’ll practice a very important skill for this course that will help us avoid a lot of memorization in the future: electron pushing. It’ll be a lot of trial and error at first, but we all start somewhere!

Just a meme

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!

Alkanes: Saturated Hydrocarbons

So you want to be an organic chemist? Well, learning about hydrocarbons such as alkanes is a good place to start…

Alkanes are a homologous series of hydrocarbons, meaning that each of the series differs by -CH2 and that the compounds contain carbon and hydrogen atoms only. Carbon atoms in alkanes have four bonds which is the maximum a carbon atom can have - this is why the molecule is described to be saturated. Saturated hydrocarbons have only single bonds between the carbon atoms.

The general formula of an alkane is CnH2n+2 where n is the number of carbons. For example, if n = 3, the hydrocarbon formula would be C3H8 or propane. Naming alkanes comes from the number of carbons in the chain structure.

Here are the first three alkanes. Each one differs by -CH2.

Shorter chain alkanes are gases at room temperature, medium ones are liquids and the longer chain alkanes are waxy solids.

Alkanes have these physical properties:

1. They are non-polar due to the tiny difference in electronegativity between the carbon and hydrogen atoms.

2. Only Van der Waals intermolecular forces exist between alkane molecules. The strength of these increase as relative molecular mass increases therefore so does the melting/boiling point.

3. Branched chain alkanes have lower melting and boiling points than straight chain isomers with the same number of carbons. Since atoms are further apart due to a smaller surface area in contact with each other, the strength of the VDWs is decreased.

4. Alkanes are insoluble in water but can dissolve in non-polar liquids like hexane and cyclopentane. Mixtures are separated by fractional distillation or a separating funnel.

The fractional distillation of crude oil, cracking and the combustion equations of the alkanes will be in the next post.

SUMMARY

Alkanes are a homologous series of hydrocarbons. Carbon atoms in alkanes have four bonds which is the maximum a carbon atom can have - this is why the molecule is described to be saturated. Saturated hydrocarbons have only single bonds between the carbon atoms.

The general formula of an alkane is CnH2n+2 where n is the number of carbons.

Shorter chain alkanes are gases at room temperature, medium ones are liquids and the longer chain alkanes are waxy solids.

They are non-polar.

Only Van der Waals intermolecular forces exist between alkane molecules. The strength of these increase as relative molecular mass increases therefore so does the melting/boiling point.

Branched chain alkanes have lower melting and boiling points than straight chain isomers with the same number of carbons.

Alkanes are insoluble in water but can dissolve in non-polar liquids like hexane. Mixtures are separated by fractional distillation or a separating funnel.

The Name’s Bond ... Ionic Bond.

This is the first in my short series of the three main types of bond - ionic, metallic and covalent. In this, you’ll learn about the properties of the compounds, which atoms they’re found between and how the bonds are formed. Enjoy!

When electrons are transferred from a metal to a non-metal, an ionic compound is formed. Metals usually lose electrons and non-metals usually gain them to get to a noble gas configuration. Transition metals do not always achieve this.

Charged particles that have either lost or gained electrons are called ions and are no longer neutral - metal atoms lose electrons to become positive ions (cations) whereas non-metals gain electrons to become negative ions (anions).

The formation of these ions is usually shown using electron configurations. Make sure you know that the transfer of electrons is not the bond but how the ions are formed. 

An ionic bond is the electrostatic attraction between oppositely charged ions.

You need to know how to explain how atoms react with other atoms and for this the electron configurations are needed. You can use dot and cross diagrams for this. 

Ionic solids hold ions in 3D structures called ionic lattices. A lattice is a repeating 3D pattern in a crystalline solid. For example, NaCl has a 6:6 arrangement - each Na+ ion is surrounded by 6 Cl- and vice versa. 

Ionic solids have many strong electrostatic attractions between their ions. The crystalline shape can be decrepitated (cracked) on heating. Ionic Lattices have high melting and boiling points since they need more energy to break because atoms are held together by lots of strong electrostatic attractions between positive and negative ions. The boiling point of an ionic compound depends on the size of the atomic radius and the charge of the ion. The smaller the ion and the higher the charge, the stronger attraction.

Look at this diagram. It shows how atomic radius decreases across a period regularly. This is not the case with the ions. Positive ions are usually smaller than the atoms they came from because metal atoms lose electrons meaning the nuclear charge increases which draws the electrons closer to the nucleus. For negative ions, they become larger because repulsion between electrons moves them further away - nuclear charge also decreases as more electrons to the same number of protons.

Ionic substances can conduct electricity through the movement of charged particles when molten or dissolved (aqueous). This is because when they are like this, electrons are free to move and carry a charge. Ionic solids cannot conduct electricity.

Ionic compounds are usually soluble in water. This is because the polar water molecules cluster around ions which have broken off the lattice and so separate them from each other. Some substances like aluminium oxide have too strong electrostatic attractions so water cannot break up the lattice - it is insoluble in water.

Molecular ions such as sulfate, nitrate, ammonium or carbonate can exist within ionic compounds. These compounds may have covalent bonds within the ions but overall they are ionic and exhibit thee properties described above.

SUMMARY

When electrons are transferred from a metal to a non-metal, an ionic compound is formed.

Charged particles that have either lost or gained electrons are called ions and are no longer neutral - metal atoms lose electrons to become positive ions (cations) whereas non-metals gain electrons to become negative ions (anions).

The formation of these ions is usually shown using electron configurations. The transfer of electrons is not the bond but how the ions are formed.

An ionic bond is the electrostatic attraction between oppositely charged ions.

Ionic solids hold ions in 3D structures called ionic lattices. A lattice is a repeating 3D pattern in a crystalline solid.

Ionic solids have many strong electrostatic attractions between their ions. The crystalline shape can be decrepitated (cracked) on heating. 

Ionic Lattices have high melting and boiling points since they need more energy to break because atoms are held together by lots of strong electrostatic attractions between positive and negative ions.

The boiling point of an ionic compound depends on the size of the atomic radius and the charge of the ion. The smaller the ion and the higher the charge, the stronger attraction.

 Positive ions are usually smaller than the atoms they came from because metal atoms lose electrons meaning the nuclear charge increases which draws the electrons closer to the nucleus. Negative ions become larger because repulsion between electrons moves them further away - nuclear charge also decreases as more electrons to the same number of protons.

Ionic substances can conduct electricity through the movement of charged particles when molten or dissolved (aqueous). This is because when they are like this, electrons are free to move and carry a charge. Ionic solids cannot conduct electricity.

Ionic compounds are usually soluble in water because the polar water molecules cluster around ions which have broken off the lattice and so separate them from each other.

 Some substances like aluminium oxide have too strong electrostatic attractions so water cannot break up the lattice - it is insoluble in water.

Molecular ions such as sulfate, nitrate, ammonium or carbonate can exist within ionic compounds. These compounds may have covalent bonds within the ions but overall they are ionic and exhibit thee properties described above.

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