Hydrocarbons Flashcards

1
Q

What are the two methods by which alkanes can be produced?

A

Alkanes are hydrocarbons that can be produced by the addition reaction of hydrogen to an alkene or by cracking of longer alkane chains.

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2
Q

How can alkanes be produced from hydrogenation?

A

The addition reaction of alkanes with hydrogen is called hydrogenation. When hydrogen gas and an alkene are heated and passed over a finely divided Pt/Ni catalyst the addition reaction produces an alkane. The Pt/Ni catalyst is finely divided to increase its surface area and therefore increase rates of reaction.

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3
Q

How can alkanes be produced from cracking?

A

In cracking large less useful hydrocarbon molecules found in crude oil are broken down into smaller more useful molecules. The large hydrocarbon molecules are fed into a steel chamber and heated to a high temperature (500 degrees) celsius under crystals of aluminum silicate (Al2Si2O) moderately low pressure and then passed over an aluminum oxide (Al2O3) catalyst. The chamber does not contain oxygen to prevent combustion of hydrocarbon to water and carbon dioxide.

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4
Q

Are the reactions to produce alkanes endothermic or exothermic?

A

Cracking is endothermic and hydrogenation is exothermic.

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5
Q

What are the possible outcomes of cracking?

A

1.alkane + alkene + hydrogen
2. Alkane + C2H4
3. C2H4 + H2

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6
Q

What are the uses of cracking?

A

To meet supply and demand. To produce ethene which is an important compound used in the petrochemical industry.

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7
Q

Describe complete combustion of alkanes.

A

When alkanes are burnt in excess of oxygen, complete combustion will take place and carbon and hydrogen will be oxidized to carbon dioxide and water respectively.

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8
Q

Describe the incomplete combustion of alkanes

A

When alkanes are burnt only in a limited supply of oxygen, incomplete combustion will take place and not all the carbon is fully oxidized. The partially oxidized carbon will form carbon monoxide.

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9
Q

Describe carbon monoxide gas.

A

Carbon monoxide is a toxic gas as it will bind to hemoglobin in the blood which can then no longer bind to oxygen. As no oxygen can be transported around the body, The victims feel dizzy, lose consciousness and if not removed from carbon monoxide they can die. It is extremely dangerous as it is odorless and will not be noticed.

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10
Q

Which harmful pollution caused by burning hydrocarbons can take place in a car engine?

A

Incomplete combustion often takes place inside a car engine due to a limited supply of oxygen present. As well as releasing carbon monoxide road traffic also releases acidic nitrogen oxides. Cars also release unburned hydrocarbons often referred to as VOCs.

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11
Q

What are the effects of acidic nitrogen oxides?

A

Road traffic releases acidic nitrogen oxides mainly NO and NO2. These contribute to the problem of acidic rain. Acid rain can kill trees and aquatic animals in lakes. It also erodes limestone buildings and statues as well as corroding metals such as iron.

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12
Q

What are the effects of unburned hydrocarbons being released?

A

Cars also released unburned hydrocarbons often referred to as VOCs into the air. Some of these are carcinogens (can cause cancer) and can form PAN a contributor to photochemical smoke.

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13
Q

How can we reduce traffic emissions?

A

Motor vehicles can now be fitted with catalytic converters to reduce traffic emissions. This causes:
Oxidation of carbon monoxide to form carbon dioxide. Carbon dioxide is not toxic but a pollutant gas.
Reduction of nitrogen oxides to form nitrogen gas which takes place on the surface of the precious metal catalyst in catalytic converters. (2CO+2NO→2CO2 +N2).
Oxidation of unburnt hydrocarbons to form CO2 + H2O.

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14
Q

What type of substituition reactions do alkanes undergo?

A

Alkanes can undergo free radicle substituition in which hydrogen atoms get substitute by a halogen (Chlorine/Bromine). Ultraviolet light (sunlight) is needed for this reaction.

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15
Q

Describe the first step of alkane free radical substituiton.

A

Initiation step:The Cl-Cl or Br-Br bond is broken by energy from the uv light. This produces two radicles in a homolytic fission reaction.

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16
Q

Describe the second step of the free radicle substituion of alkanes.

A

Propogation step:Free radicles are very reactive and will attack the unreactive alkanes.
A C-H bond breaks homolytically
An alkyle free radicle is formed
This can then attack another chlorine/bromine molecule to form the halogenoalkane and regenerate the chlorine/bromine free radicle.
The free radicle now formed can go back and react with the alkane in step 1. This reaction will continue until the termination step.

17
Q

What is the final step in free radicle substituition reaction of alkanes?

A

8Termination step: The reaction stops as two free radicles react together and form a single unreactive molecule.
Cl+Cl=Cl2
CH2CH3+Ch2+CH2CH3=CH3CH2CH2CH3
CH2CH3+Cl=ClCH2CH3

18
Q

Explain the unreactivity of alkanes.

A

The electronegativity of the carbon and hydrogen atoms in alkanes are almost the same. As a result alkanes are nonpolar molecules. therefore do not react with any polar reagents. They have no polar deficient areas to attract nucleophiles. They also lack electron rich areas to attract electrophiles.

19
Q

In what ways can alkenes be produced?

A

Cracking
Dehydration
Elimination

20
Q

How can alkenes be produced from elimination reactions?

A

Alkanes can be produced from the elimination reaction of a halogenoalkane. An elimination reaction is one in which a small molecule is lost, in case of halogenoalkanes, the small molecule that is eliminated is a hydrogen halide. The halogenoalkane is heated with ethanolic sodium hydroxide to produce the alkene+water+NaX where X is the halogen.

Look At example in notes

21
Q

eHow can alkanes be produced from dehydration reaction?

A

Alkenes can also be produces from the elimination reaction in which a water molecule is lost. Alchol vapour is passed over a hot catalyst of aluminium oxide powder (al2)3) acid, perices of porous pot or pumice can also be used as a catalyst.
Alcohol + Al2O3 →alkenes+ water.
Ethanol loses an H and OH group so H2O is eliminated.

22
Q

How can small alkenes be collected?

A

The samller alkenes (such as ethene, propene and butene) are all gases at room temperature and can be collected over water.

23
Q

How can alkenes be produced from cracking?

A

Alkenes can also be produced from the cracking of long hydroarbon molecules in crude oil.
Aluminium oxide is used a s a catalysr and high temperatures are used to speed up this reaction.

24
Q

Why do we produce alkenes from cracking?

A

The low molecuar alkenes are more lractive than alaknes as they have an electron rich bond. They can therefore be used for making new products. These include most plastics.

25
Q

What are the reactions of alkenes?

A

Electrophillic addition reaction of hydrogen halides
Hydrogenation reaction
Electrophillic addition reaction of halogens

26
Q

What is electrophillic addition reaction?

A

Electrophullic addition reaction is a the addition of an electrophile to a double bond. The c-c double bond is broken and a new single bond is formed between the two carbon atoms.

27
Q

Describe the hydrogenation reaction of alkenes.

A

When hydrogen gas and an alkene are heated and passed over a finely divided platinum/nickel catalyst, the addition reaction produces an alkane. Hydrogenation is used in the manufacture of of maragrine.

28
Q

Decribe the electrophillic addition reaction of hydrogen halides.

A

When an alkene is bubbled through a concentrated solution of a hydrogen halide (HF, HCl, HBr or HI) at room temperature, the product is a halogenoalkane. With asymmetric alkenes, there are two possible poroducts which can be formed. The major product is the one that has the halogen atom bonded to the c=c carbon atoms with the highest number of alkyl groups bonded to it.

29
Q
A