Topic 7 Organic Chemistry Flashcards
What is the definition of a hydrocarbon?
A hydrocarbon is a compound that contains only hydrogen and carbon atoms.
T or F? Increasing the number of carbon atoms in an alkane will increase the viscosity.
True - alkanes with longer carbon chains are more viscous than those with shorter chains.
What is the molecular formula of the alkane that contains two carbon atoms?
C2H6 - the general formula of alkanes is CnH2n+2, which lets you work out the molecular formula of any alkane from the number of carbon (or hydrogen) atoms it contains.
S and J are taking about alkanes. j asks ‘what effect does molecular size have on the boiling point of alkanes?’. ‘It has no effect’ S replies. Is S correct? Explain your answer.
S is wrong. Smaller alkanes have lower boiling points (are more volatile) than larger alkanes, so the size of molecules in an alkane will affect the boiling point.
Under certain conditions pentane can undergo complete combustion. What’s the balanced symbol equation for the complete combustion of pentane (C5H12)?
C5H12 + 8O2 ⇒ 5CO2 + 6H2O
-complete combustion of a hydrocarbon always produces carbon dioxide (CO2) and water (H2O)
A scientist has a sample of butane (C4H10), decane (C10H22) and icosane (C20H42). She ignites the sample using a lighted splint. Which sample will be the hardest to ignite?
Icosane will the hardest to ignite. The ease of ignition of a hydrocarbon depends o its flammability, and the flammability of hydrocarbons decreases with increasing carbon carbon length. Therefore isocane will be the hardest to ignite, as its got the longest carbon chain and is the least flammable.
T or F? As a substance rises up a fractionating column, the temperature of the substance gradually increases?
False. The hottest part of the fractionating column is at the bottom, so substances cool down as the move up the column.
Crude oil is mainly formed from the remains of which type of organism?
Plankton - crude oil is formed from organisms that died millions of years ago and were buried in mud. Over time, high temperature and pressure turned their remains into oil.
Give some examples of fuels that can be produced from crude oil.
petrol, diesel, kerosene, heavy fuel oil, liquified petroleum gas (LPG)
A fractionating column is 50m tall. A certain fraction of crude oil, X, condenses at a height of 30m. Suggest why fraction X condensed at 30m.
The temperature of the fractioning column decreases as you move up the column. A fraction will only condense when it has reached part of the column that has a temperature below its boiling point. The temperature of the column at 30m must have been just below the boiling point of fraction X.
An oil refinery is carrying out fractional distillation of some crude oil. J wants to take a sample of crude oil that contains hydrocarbons with no more than 8 carbon atoms. Where in the fractionating column should J take her sample from? Explain your answer.
J should take her sample from near the top. Hydrocarbons with 8 carbon atoms are relatively short so will have lower boiling points than a lot of hydrocarbons in crude oil. This means that they condense near the top of the column, where its cooler. Hydrocarbons that contain more tan 8 carbon atoms will condense lower down the column, so J’s sample should contain mostly short-chain hydrocarbons if she collects it from near the top of the column.
Give one example of a fuel that is produced by the petrochemical industry, other than fuel.
solvents, lubricants, polymers, detergents
what are the conditions needed for steam cracking?
Steam cracking uses steam and very high temperatures.
3) Nonane (C₂H₂0) can be cracked to produce ethene (C₂H₂) and one other product. What’s the balanced symbol equation for this reaction?
3) C₂H20 → C₂H4 + C7H₁6 - there should be the same number of C and H atoms on both sides of any equations that show cracking. So you can find the formula of the unknown hydrocarbon by finding how many C and H atoms you need to keep both sides balanced. (Hopefully this sounds vaguely familiar it’s basically just the law of conservation of mass, but applied to cracking.)
4) A company wants to use cracking to make petrol. The compounds in petrol contain ~8 carbon atoms.
Look at the table below. How suitable are each of the fractions for cracking to make petrol? Which would be the best choice for the company?
4) The compounds in LPG contain roughly three carbon atoms, so are too short to be cracked to make petrol (which contains compounds with around eight carbon atoms). Kerosene and bitumen both contain hydrocarbons that are long enough to be cracked to make petrol. Bitumen is in lower demand than kerosene, so isn’t as useful. Therefore bitumen is the best choice to be cracked to make petrol.
1) What’s the name of the first member of the alkene homologous series?
1) Ethene — the names of the next three members are propene, butene and pentene.
2) An alkene contains 7 carbon atoms. How many hydrogen atoms will it have?
2) 14 hydrogen atoms the general formula for alkenes is CH₁ so a molecule of any alkene will always contain twice as many H atoms as C atoms.
3)
A scientist finds a bottle of liquid in the lab. Unfortunately, the label on the bottle
is a bit damaged, so the scientist can’t tell if it contains pentane or pentene. The scientist burns the sample in air, and records his observations. How could the scientist use his results to distinguish between the two compounds?
3) If the scientist sees a smoky yellow flame, he can be fairly sure that the compound is pentene. Alkenes burn cleanly in plenty of oxygen, just like other hydrocarbons such as pentane. But in air, where there isn’t quite as much oxygen, alkenes tend to undergo incomplete combustion. This is why a smoky flame is produced.
Mishal is investigating the combustion reactions of alkenes. She starts by burning a sample of propene in a high level of oxygen. She repeats the reaction with a fresh sample of propene, but in a low level of oxygen.
Describe the products that you would expect each reaction to form.
Give reasons for your answer.
4) In high levels of oxygen, propene will combust completely to produce carbon dioxide and water (and release some energy). But when Mishal repeats the reaction in a low level of oxygen, propene will undergo incomplete combustion. This reaction will produce carbon dioxide and water (just like complete combustion), but will also produce at least one of carbon monoxide and carbon/soot.
Use the structures, A-D, on the right to help you answer questions 3-5.
3) which of A-C would be formed if a hydrocarbon containing a C=C bond was reacted with H2?
4) Which alkene would react with Cl₂ to make A?
5) What are the reactants and conditions for converting ethene into D? 6) Genevieve has a sample of hexene (C₂H₁₂).
Describe how she could use bromine water to confirm that the sample is an alkene.
What would be the molecular formula of the organic product formed during the test?
3) B - this compound is ethane, which would form if ethene was reacted with hydrogen.
4) Butene the Cl atoms would add across the C=C double bond in butene. 5) H₂O/steam and a catalyst - these conditions will convert an alkene into an
alcohol.
You can tell that compound D is an alcohol because it contains an -OH functional group.
6) Add bromine water to the sample, and shake. If the solution turns from orange to colourless, the sample contains the alkene functional group (which indicates that the sample could be hexene). The bromine atoms would add across the double bond in hexene, to form a product with the molecular formula C₂H₁₂Br₂.
5) The formula of a polymer is shown on the right.
Give the name of the monomer that forms this polymer.
5) Ethene - you can work out the formula of a monomer by counting the number of atoms of each type in the repeating unit. There are 2 C atoms and 4 H atoms in the repeating unit of this polymer, so the monomer that made it has the formula C₂H₂, which is ethene.
