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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!

Happy #StPatricksDay2021! Ever wondered why the bubbles in Guinness appear to fall rather than rise, and what causes its dark colour? This graphic in C&EN has the answers! https://ift.tt/2WfNI8j https://ift.tt/3rY7DIW

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!

Follow @productive-tips for more tips and content like this posted daily! Handpicked and curated with love :)

me when tetravalence

Good news! You CAN rewire your brain. Through the same mechanism as forming habits, you can change your automatic thought patterns. When you catch yourself with unhealthy thoughts, STOP, and make a conscious effort to replace that thought pattern with a more beneficial/healthy thought pattern. Keep doing that whenever those thoughts occur. Slowly but surely your mind’s default voice will match what you’re teaching it.

Follow @productive-tips for more tips and content like this posted daily! Handpicked and curated with love :)

Around a year ago, scientists determined the structure of the SARS-CoV-2 spike protein. Here’s a look at how it was done and how it helped the fight against #COVID19 in the latest edition of #ChemVsCOVID with the Royal Society of Chemistry: https://ift.tt/3pZiZe9 https://ift.tt/3002NPh

finally, some content! this was a quick info graphic I drew up on Procreate to revise for my ochem test tomorrow. disclaimer: I used information from this source (https://www.masterorganicchemistry.com/2010/05/24/imines-and-enamines/) since my own notes are based off lectures I received at my university that I’m not really allowed to share without heavy modification.

general post disclaimer: I’m an undergraduate student studying biochemistry and genetics. Posts are made for the purposes of education, revision and aesthetics. Not all the content I produce can be taken as entirely accurate and I do not take responsibility for errors made as a result of using this resource. Always consult course textbooks and lectures to aid in your specific learning outcomes. Do not repost without the original caption citing any extra references I used to make this post or remove my watermark. Other posts can be found on my blog as-studypeach@tumblr.com. Any problems, feel free to get in touch via my messages.

Master Organic Chemistry

Imines and Enamines

Imines are the nitrogen analogues of aldehydes and ketones, containing a C=N bond instead of a C=O bond. They are formed through the dehydra

What comes to mind when you think of alcohol? Probably alcoholic drinks like beer or wine. But in organic chemistry alcohols are an important and versatile family of compounds. In this episode of Crash Course Organic Chemistry, we’ll use alcohols as a starting point to get other types of compounds like ethers, epoxides, and more!