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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!

My friend sent this to her Professor today

#ChemAdvent day 22 features flaming Christmas puddings! 🔥 https://ift.tt/3h7ELcS

Enthalpy - a thermodynamic property

When I first learned about enthalpy, I was shocked - it felt more like a physics lesson than a chemistry lesson. The thought of learning more about thermodynamics than my basic understanding from my many science lessons in lower school made me bored out of my mind. But enthalpy is actually pretty interesting, once you get your head around it…

Reactions which release heat to their surroundings are described to be exothermic. These are reactions like combustion reactions, oxidation reactions and neutralisation reactions. Endothermic reactions take in heat from their surroundings, such as in thermal decomposition. Reversible reactions are endothermic in one direction and exothermic in the other.

These facts are important when you start to look at enthalpy. Enthalpy is basically a thermodynamic property linked to internal energy, represented by a capital H. This is pretty much the energy released in bond breaking and made in bond making. We usually measure a change in enthalpy, represented by ∆H.  ∆H = enthalpy of the products (H1) - enthalpy of the reactants (H2). This is because we cannot measure enthalpy directly.

In exothermic reactions,  ∆H is negative whereas in endothermic reactions,  ∆H is positive.

∆H is always measured under standard conditions of 298K and 100kPa. 

In reversible reactions, the ∆H value is the same numerical value forwards and backwards but the sign is reversed. For example, in a forward exothermic reaction, the  ∆H value would be -ve but in the backwards reaction (endothermic) the  ∆H would be +ve. 

Reaction profiles are diagrams of enthalpy levels of reactants and products in a chemical reaction. X axis is enthalpy rather than ∆H and the Y axis is the progress of reaction, reaction coordinate or extent of reaction. Two horizontal lines show the enthalpy of reactants and products with the reactants on the left and the products on the right. These should be labelled with their names or formulae. 

In an endothermic reaction, product lines are higher enthalpy values than reactants. In an exothermic reaction, product lines are lower enthalpy values than reactants. The difference between product and reactant lines is labelled as  ∆H. Values are measured in kJ mol-1. 

Reaction pathways are shown with lines from the reactants to the products on enthalpy level diagrams. This shows the “journey” that the enthalpy takes during a reaction. They require an input of energy to break bonds before new bonds can form the products. The activation energy is the peak of the pathway above the enthalpy of reactants. It is the minimum amount of energy that reactants must have to react. 

Standard enthalpy values are the ∆H values for enthalpy changes of specific reactions measured under standard conditions, represented by ⊖. There are three of these:

1. Standard enthalpy of reaction ( ΔHr⊖ )

The enthalpy change when substances react under standard conditions in quantities given by the equation for the reaction.

2. Standard enthalpy of formation ( ΔfH⊖ )

The enthalpy change when 1 mole of a compound is formed from its constitutent elements with all reactants and products in standard states under standard conditions.

The enthalpy of formation for an element is zero is it is in it’s standard state for example, O2 enthalpy is zero.

3. Standard enthalpy of combustion ( ΔcH⊖ )

The enthalpy change when 1 mole of a substance is burned completely in excess oxygen with all reactants and products in their standard states under standard conditions.

Values for standard enthalpy of formation and combustion must be kept to per mole of what they refer.

Summary

Reactions which release heat to their surroundings are described to be exothermic. Endothermic reactions take in heat from their surroundings, such as in thermal decomposition. 

Reversible reactions are endothermic in one direction and exothermic in the other.

Enthalpy is a thermodynamic property linked to internal energy, represented by a capital H. We usually measure a change in enthalpy, represented by ∆H. 

 ∆H = enthalpy of the products (H1) - enthalpy of the reactants (H2). We cannot measure enthalpy directly.

In exothermic reactions,  ∆H is negative whereas in endothermic reactions,  ∆H is positive.

∆H is always measured under standard conditions of 298K and 100kPa. 

In reversible reactions, the ∆H value is the same numerical value forwards and backwards but the sign is reversed. 

Reaction profiles are diagrams of enthalpy levels of reactants and products in a chemical reaction. They 

In an endothermic reaction, product lines are higher enthalpy values than reactants. In an exothermic reaction, product lines are lower enthalpy values than reactants.

 The difference between product and reactant lines is labelled as  ∆H. 

Values are measured in kJ mol-1.

Reaction pathways are shown with lines from the reactants to the products on enthalpy level diagrams. They plot enthalpy against reaction progress.

Reactions require an input of energy to break bonds before new bonds can form the products. The activation energy is the peak of the pathway above the enthalpy of reactants. It is the minimum amount of energy that reactants must have to react.

Standard enthalpy values are the ∆H values for enthalpy changes of specific reactions measured under standard conditions, represented by ⊖. 

Standard enthalpy of reaction ( ΔHr⊖ ) is the enthalpy change when substances react under standard conditions in quantities given by the equation for the reaction.

Standard enthalpy of formation ( ΔfH⊖ ) is the enthalpy change when 1 mole of a compound is formed from its constitutent elements with all reactants and products in standard states under standard conditions.

The enthalpy of formation for an element is zero is it is in it’s standard state.

Standard enthalpy of combustion ( ΔcH⊖ ) is the enthalpy change when 1 mole of a substance is burned completely in excess oxygen with all reactants and products in their standard states under standard conditions.

Values for standard enthalpy of formation and combustion must be kept to per mole of what they refer.

Happy studying!

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

Halogenoalkanes

Halogenoalkanes are a homologous series of saturated carbon compounds that contain one or more halogen atoms. They are used as refrigerants, solvents, flame retardants, anaesthetics and pharmaceuticals but their use has been restricted in recent years due to their link to pollution and the destruction of the ozone layer.  

They contain the functional group C-X where X represents a halogen atom, F,Cl, Br or I. The general formula of the series is CnH2n+1X.

The C-X bond is polar because the halogen atom is more electronegative than the C atom. The electronegativity decreases as you go down group 7 therefore the bond becomes less polar. Flourine has a 4.0 EN whereas iodine has a 2.5 EN meaning it is almost non-polar.

The two types of intermolecular forces between halogenoalkane molecules are Van Der Waals and permanent dipole-dipole interactions. As the carbon chain length increases, the intermolecular forces (due to VDWs) increase as the relative atomic mass increases due to more electrons creating induced dipoles. Therefore the boiling point of the halogenoalkanes increases since more forces must be broken.  

Branched chains have lower boiling points than chains of the same length and halogen because the VDWs are working across a greater distance and are therefore weaker.

When the carbon chain length is kept the same, but the halogen atom is changed, despite the effect of the changing polar bond on the permanent dipole-dipole interactions, the changing VDWs have a greater effect on the boiling point. Therefore as RAM increases, the boiling point increases meaning an iodoalkane has a greater boiling point than a bromoalkane if they have the same carbon chain length.  

Halogenoalkanes are insoluble or only slightly soluable in water despite their polar nature. They are soluble in organic solvents such as ethanol and can be used as dry cleaning agents because they can mix with other hydrocarbons.

Summary

Halogenoalkanes are saturated carbon compounds with one or more halogen atoms. Their general formula is CnH2n+1X, where X is a halogen. Their functional group is therefore C-X.

They are used as refrigerants, solvents, pharmaceuticals and anaesthetics but have been restricted due to their link to the depletion of the ozone layer.

C-X bonds are polar due to the halogen being more electronegative than the carbon. The polarity of the bond decreases down group 7.

Van der Waals and permanent dipole-dipole interactions are the intermolecular forces in halogenoalkanes.

When carbon chain length increases, boiling points increase due to RAM increasing and the number of Van Der Waals increasing too.

In branched halogenoalkanes, Van Der Waals are working across a greater distance therefore attraction is weaker and boiling points are lower than an identical unbranched chain.

When the halogen is changed, the boiling point increases down the group due to the effect of a greater RAM  - more VDWs mean more intermolecular forces to break.  

Halogenoalkanes are insoluble in water but soluble in organic solvents like ethanol.

Bonus: free radical substitution reactions in the ozone layer

Ozone, O3, is an allotrope of oxygen that is usually found in the stratosphere above the surface of the Earth. The ozone layer prevents harmful rays of ultraviolet light from reaching the Earth by enhancing the absorption of UV light by nitrogen and oxygen. UV light causes sunburn, cataracts and skin cancer but is also essential in vitamin D production. Scientists have observed a depletion in the ozone layer protecting us and have linked it to photochemical chain reactions by halogen free radicals, sourced from halogenoalkanes which were used a solvents, propellants and refrigerants at the time.  

CFCs cause the greatest destruction due to their chlorine free radicals. CFCs – chloroflouroalkanes – were once valued for their lack of toxicity and their non-flammability. This stability means that they do not degrade and instead diffuse into the stratosphere where UV light breaks down the C-Cl bond and produces chlorine free radicals.

RCF2Cl UV light —> RCF2● + Cl●

Chlorine free radicals then react with ozone, decomposing it to form oxygen.

Cl● + O3 —> ClO● + O2

Chlorine radical is then reformed by reacting with more ozone molecules.

ClO● + O3 —-> 2O2 + Cl●

It is estimated that one chlorine free radical can decompose 100 000 molecules of ozone. The overall equation is:

2O3 —-> 3O2

200 countries pledged to phase of the production of ozone depleting agents in Montreal, leading to a search for alternatives. Chemists have developed and synthesised alternative chlorine-free compounds that do not deplete the ozone layer such as hydroflurocarbons (HFCs) like trifluromethane, CHF3.

SUMMARY

Ozone, found in the stratosphere, protects us from harmful UV light which can cause cataracts, skin cancer and sunburn.  

Ozone depletion has been linked to the use of halogenoalkanes due to their halogen free radicals.  

CFCs were good chemicals to use because they have low toxicity and were non-flammable. The fact they don’t degrade means they diffuse into the stratosphere. 

Chlorine free radicals are made when CFCs are broken down by UV light.

These go on to react with ozone to produce oxygen.

Chlorine free radicals are then reformed by reacting with more ozone.

It is a chain reaction that can deplete over 100 000 molecules of ozone.

There is a 200 country ban on their use and scientists have developed alternatives like hydrofluorocarbons to replace them

Happy studying!

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