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Chemistry AS > AS Chemistry Term 2 > Flashcards

AS Chemistry Term 2 Flashcards

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1
Q

Describe the physical properties of alkanes

A
  • Smaller alkanes (C1-4) are gases, medium sized (C5-16) are liquids and the larger ones are waxy solids.
  • They are non-polar molecules as their charge is evenly spread due to similar electronegativities of H and C. This means they float on top of water as they are less dense. However, they do dissolve in non-polar solvents.
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2
Q

Describe the chemical properties of alkanes

A
  • They are fairly unreactive so their reactions require high temperatures and/or ultra-violet light, or peroxides.
  • Halogenation: Alkanes react with halogens in substitution reactions. Requires light or high temperature.
  • Combustion: Alkanes burn completely to form CO2 + H2O or incompletely to form CO or C + H2O
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3
Q

Describe the mechanism of free radical substitution

A

Initiation: Producing free radicals which is done by homolytic bond breaking. e.g.
Cl-Cl → Cl • + Cl •

Propogation: Free radicals attack normally unreactive alkanes forming a product and another free radical e.g.
CH4 + Cl • → CH3• + HCl
CH3• + Cl2 → CH3Cl + Cl•

Termination: Free radicals react with each other, terminating the propogation and forming a single product e.g.
CH3• + CH3• → CH3CH3
CH3• + Cl• → CH3Cl

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4
Q

Describe how hydrocarbons are derived from crude oil

A
  • Crude oil consists of a mixture of hydrocarbons - alkanes, cycloalkanes (aliphatic hydrocarbons) and aromatic compounds (aromatic hydrocarbons).
  • The hydrocarbons in crude oil are separated by fractional distillation which takes place in a fractionating column.
  • The top of the fractionating column is cooler than the bottom.
  • Crude oil enters as vapour and liquid. The liquid components are tapped off at the bottom while more volatile hydrocarbons move up the column.
  • The gases condense at different levels as the temperature gradually falls and are collected as liquids.
  • The most volatile hydrocarbons leave at the top of the fractionating column as gases.
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5
Q

Describe the process of cracking

A
  • Long alkanes such as kerosene and diesel fractions of crude oil are passed over a strong heating catalyst for a short period of time in the absence of air.
  • This results in hydrocarbon chains producing a mixture of products with shorter hydrocarbon chains, some of which are alkenes.
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6
Q

Describe the different types of addition reactions that occur with alkenes

A
  • Hydrogenation: Uses nickel catalyst at 140°C, forms alkane.
  • Hydration (using steam): Used to make alcohols. Requires concentrated phosphoric acid as a catalyst, a temp. of 330°C and a pressure of 6 MPA.
  • Halogenation: Can be used to test if a molecule is an alkene. If an alkene is added to bromine water, bromine water is decolourised.
  • Addition of hydrogen halides: Halogenoalkane is formed when alkene is bubbled through a concentrated solution of hydrogen halide. With an asymmetric alkene, two products are formed. The major product is the one in which the carbocation intermediate is the most stable.
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7
Q

Describe the oxidation of alkenes

A
  • In a cold, dilute, acidified solution of potassium permanganate, alkenes are converted to diols.
  • In a hot, concentrated acidic solution of potassium manganate(VII), the diol is split into two fragments as the carbon-carbon double bond ruptures and these are further oxidised.
  • if double bond carbon is: CH2= , then oxidised product will be CO2
  • if it is RCH=, then oxidised product will be RCOOH
  • if it is RCR=, then oxidised product will be RCRO
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8
Q

Define addition polymerisation

A

It is the process where unsaturated compounds, called monomers, join together to form a very large molecule called a polymer. The section of a polymer chain inside the brackets is the repeat unit.

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9
Q

Describe the mechanism of electrophilic addition in alkenes

A
  • The double bond consists of sigma and pi bonding which means there is a high electron density around the double bond, making it open to attack by electrophiles.
  • For example, HBr is a polar molecule, so the Br atom carries a partial negative charge and the H atom has a partial positive charge. This means the H atom acts as an electrophile, and accepts a pair of electrons from the C=C bond.
  • Another example is Br2 which is non polar. As the bromine molecule approaches the alkene, the area of high electron density repels the pair of electrons in the Br-Br bond away from the nearer bromine atom making it act as the electrophile.
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10
Q

Describe the inductive effects of alkyl groups on the stability of carbocations

A

Alkyl groups tend to release electrons and have a positive inductive effect on a carbocation. This energetically stabilises the carbocation as the charge is spread over several atoms instead of concentrated over one atom. This means that a tertiary carbocation is the most stable, while secondary is less stable and primary is the least stable.

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11
Q

Describe the uses of polythene and polyvinylchloride

A

Polythene (poly(ethene)): Used as an insulator and packaging.

Polyvinylchloride (poly(chloroethene)): Used as electrical insulation, plastic bottles, clothing.

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12
Q

Describe how to deduce the repeat unit of a given monomer, as well as how to identify the monomer present in a section of a polymer

A

Deducing repeat unit: Turn the double bond into a single bond and show the bonds on either side of the two carbon atoms.

Identifying monomers present: Split the polymer into repeat units and put the C=C double bond back into the monomer.

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13
Q

Why is it difficult to dispose of polyalkenes

A
  • Their lack of reactivity mean that they are resistant to chemical attack and they take hundreds of years to decompose, causing visual pollution.
  • A solution to this is to burn the polyalkenes and using the energy released to generate electricity. However, this adds CO2, meaning it would not help to combat global warming. CO may also be released from incomplete combustion of hydrocarbons.
  • Another problem is the difficulty of separating plastic waste as if poly(chloroethene) is burnt, HCl is released as well as toxic dioxins.
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14
Q

State the colours of the halogens, describe the trend in volatility and explain this trend.

A

Fluorine: A pale, yellow gas with the lowest boiling point and melting point of all the halogens.

Chlorine: Pale green gas with a higher boiling point than fluorine.

Bromine: Orange/Brown Liquid

Iodine: Grey/Black Solid

  • Volatility of the halogens decrease down the group. The boiling point values of all the halogens are relatively low because there are only weak Van Der Waals forces between their molecules. As the number of electrons increase, the greater the opportunities for instantaneous dipoles arising within molecules, and for induced dipoles on neighbouring molecules. Hence, iodine has the greatest boiling point as it has the greatest number of electrons as the largest molecule.
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15
Q

Explain why halogens are strong oxidising agents, with fluorine being the strongest oxidising agent

A
  • Halogen atoms gain an electron to achieve a stable electron configuration meaning they oxidise other substances, becoming oxidising agents and reducing themselves.
  • Due to the small atomic radii of fluorine, an electron entering its outer shell will be much closer to the nucleus and will experience less shielding meaning that fluorine has the highest electronegativity.
  • This makes fluorine the strongest oxidising agent
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16
Q

Describe the reactions of halogens with hydrogen

A
  • The halogens all form hydrogen halides with hydrogen. However, fluorine reacts explosively, even in cool, dark conditions, while iodine forms an equilibrium mixture when heated.
  • Chlorine reacts explosively in sunlight, while bromine reacts slowly on heating.
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17
Q

Explain the relative thermal stabilities of hydrides in terms of bond energies

A
  • HCl and HF do not decompose in temperatures up to 1500°C.
  • HBr decomposes slightly more readily than HCl and HF, however, HI is the least thermally stable and thermally decomposes readily.
  • This trend in thermal stability is due to the relative bond energies of the hydrides, with the H-I bond having the lowest bond energy and H-F having the highest.
  • This is because iodine is the largest atom meaning the overlap of its outer shell with a hydrogen atom gives a longer bond length, meaning it is weaker and requires less energy to break it.
18
Q

Describe the reactions of halide ions with silver ions followed by NH3

A
  • All silver halides, except for silver fluoride, forms a precipitate, meaning that adding silver ions to a halide solution can be used to distinguish which halide is present.
  • AgCl is a white precipitate, AgBr is a creamy white precipitate, and AgI is a pale yellow precipitate.
  • AgCl dissolves in dilute ammonia, AgBr dissolves in concentrated ammonia, and AgI does not dissolve.
19
Q

Describe the reactions of sodium or hydrogen halides with concentrated H2SO4

A
  • NaCl(s) + H2SO4(l) → NaHSO4(s) + HCl(g)
  • NaBr(s) + H2SO4(l) → NaHSO4(s) + HBr(g)
    HBr is a strong enough reducing agent to reduce H2SO4
  • 2HBr(g) + H2SO4(l) → SO2(g) + Br2(g) + 2H2O(l)
  • NaI(s)+ H2SO4(l) → NaHSO4 + HI(g)
    HI is a strong reducing agent
  • HI(g) + H2SO4(l) → SO2(g) + I2(g) + 2H2O(l)
    HI also reduces SO2
  • 6HI(g) + SO2(g) → H2S(g) + 3I2 + 2H2O(l)
20
Q

Define the term disproportionation and describe the reaction of chlorine with hot and cold NaOH

A
  • When an atom is both oxidised and reduced in a reaction
    In cold alkali (15°C):
    Cl2 (aq) + 2NaOH (aq) → NaCl(aq) + NaClO(aq) + H2O(l)
    In hot alkali (70°C):
    3Cl2(aq) + 6NaOH(aq) → 5NaCl(aq) + NaClO3(aq) + 3H2O(l)
21
Q

Discuss the uses of chlorine

A
  • Chlorine is used as a water purifier as the reaction with water forms HClO, or chloric(I) acid, which decomposes slowly in solution. This releases reactive oxygen atoms that kill bacteria in water.
  • Chlorine is used in bleach as NaCl and NaClO which bleaches colours and stains because oxygen atoms oxidise dye and other coloured molecules.
  • Chlorine is found in organic compounds which are used as solvents, refrigerants and aerosols.
22
Q

Describe the substitution reactions of halogenoalkanes

A
  • Are all nucleophilic substitution
    Hydrolysis:
  • Reagent is NaOH(aq). Conditions are reflux in aqueous solution. OH- is the nucleophile. Water will also hydrolyse the haloalkane but more slowly as water is a poor nucleophile.

Formation of Nitriles:
- Reagent is alcoholic potassium cyanide. Reflux in alcoholic solution. Nucleophile is CN-

Formation of primary amines by reaction with ammonia:

  • Reagent is alcoholic ammonia. Reflux in alcoholic solution under pressure. Product is an amine. Nucleophile is NH3
  • Amine produced is also a nucleophile and can attack another haloalkane forming a secondary amine, then a tertiary amine and finally as ionic quaternary ammonium salt.
23
Q

Describe elimination reactions in halogenoalkanes

A
  • elimination reactions convert halogenoalkanes to alkenes. It involves the loss of a small molecule from the original organic molecule.
  • The reagent used is ethanolic sodium hydroxide.
  • If aqueous NaOH is used, a nucleophilic substitution reaction occurs instead.
24
Q

Describe the different mechanisms of nucleophilic substitution in halogenoalkanes

A

SN2: S stands for substitution, N for nucleophilic and 2 is the rate of reaction.
- A nucleophile approaches the haloalkane and donates a pair of electrons to the slightly positive carbon atom. At the same time the halogen-C bond is breaking and the halogen takes both electrons in the bond (heterolytic fission) and leaves as a halide ion.

SN1: The halogen-C bond breaks and a halide ion leaves forming a carbocation. A nucleophile bonds to the carbocation.

  • Tertiary halogenoalkanes favour SN1 as their carbocations are more stable due to the inductive effect of alkyl groups. Primary halogenoalkanes favour SN2 as their carbocations are less stable. Secondary halogenoalkanes exhibit both SN1 and SN2 fairly equally.
25
Q

Why is the use of CFC’s concerning?

A
  • CFC’s are very chemically unreactive and were used as aerosol propellants, solvents and refrigerants.
  • CFC’s, however, are broken down by UV rays into chlorine free radicals which react with ozone molecules. This has created a hole in the ozone layer which can lead to skin cancer and sunburn.
26
Q

Describe the combustion of alcohols

A

Alcohols react with oxygen in the air and form CO2 and H2O if combusted completely. Ethanol can be used in biofuels which are a more environmentally-friendly alternative to petrol or diesel.

27
Q

Describe substitution reactions of alcohols to form halogenoalkanes

A
  • C is partially positive due to the electronegativity of the O. This leaves it open to nucleophilic attack.
  • ROH(l) + PCl5(l) → RCl(l) + POCl3(l) + HCl(g)
  • 3ROH(l) + PCl3(l) → 3RCl(l) + H3PO3
  • ROH + SOCl2(l) → RCl(l) + SO2(g) + HCl(g)
  • ROH + HCl(aq) → RCl(l) + H2O(l) : ZnCl2 catalyst, Lucas test used to distinguish between water soluble alcohols. Tertiary alcohols go cloudy immediately, secondary in a few minutes and primary will have no reaction.
28
Q

Describe the reaction of alcohols with sodium

A

C2H5OH + Na → C2H5O-Na+ + H2

- hydrogen and alkoxide ions are produced.

29
Q

Describe oxidation reactions of alcohols

A
  • Alcohols can be oxidised by acidified potassium dichromate (VI) solution, which is acidified by dilute sulphuric acid.
  • Potassium Dichromate (VI) turns from orange to green after oxidation of the alcohol.

Primary Alcohols: First oxidised to an aldehyde, which has a low boiling point due to no hydrogen bonding, and so must be distilled off quickly to be obtained. It is further oxidised to a carboxylic acid, which can be obtained by reflux.

Secondary Alcohols: Oxidised to ketones

Tertiary Alcohols: No reaction.

30
Q

Describe dehydration reactions in alcohols

A
  • Water is lost and alkenes are formed.
  • An acid catalyst, such as sulphuric acid, or hot Al2O3 powder is required.
  • Dehydration occurs most readily with tertiary alcohols and least readily with primary alcohols.
31
Q

Describe esterification reactions

A
  • a reaction between carboxylic acids and alcohols in which an ester and water is formed. It is an equilibrium reaction so it is reversible.
  • a strong acid catalyst usually concentrated H2SO4 is heated under reflux.
  • Esters have sweet, fruity smells.
  • The first part of the name comes from the alcohol and the second from the carboxylic acid. The O that is released in H2O comes from the acid and not the alcohol.
32
Q

Describe the tri-iodomethane or iodoform reaction

A

If the hydroxy or carbonyl group is on the second carbon, the compound will pass the iodoform test and a yellow, crystalline solid will be seen.

33
Q

Describe the reduction of ketones and aldehydes to form alcohols

A

Ketones are reduced to form secondary alcohols while aldehydes are reduced to form primary alcohols. Reduction of carbonyl compounds can be done in two ways:

  • Warming the aldehyde or ketone with an aqueous alkaline solution of sodium tetrahydridoborate
  • Adding lithium tetrahydridoaluminate dissolved in dry ether at room temperature. Very strong reducing agent so cannot use water as it reacts vigorously with water.
34
Q

Describe the reaction of carbonyl compounds with CN-

A
  • It is a nucleophilic addition reaction in which the partially positive carbon atom in the C=O bond is attacked by the CN- nucleophile.
  • An intermediate called an oxyanion is formed which has a negative O, which quickly reacts with an H+ ion from HCN forming 2-hydroxynitriles.
35
Q

Describe the reactions of carbonyl compounds with 2,4-dinitrophenylhydrazine

A
  • Carbonyl groups can be tested for using 2,4-DNPH forming a deep orange precipitate.
  • The precipitate can be purified by recrystallisation and its melting point can be measure experimentally to determine the specific aldehyde or ketone.
  • It is a condensation reaction
36
Q

Describe the testing of aldehydes and ketones with Tollen’s and Fehling’s solution.

A
  • Tollen’s reagent is an aqueous solution of silver nitrate in excess ammonia solution (ammoniacal silver nitrate solution). When warmed, the Ag+ ions will oxidise the aldehyde, forming carboxylate ions. Ag+ ions are reduced forming silver atoms forming a mirror on the side of the tube. Ketones will have no reaction.
  • Fehling’s solution is an alkaline solution containing Cu2+ ions which are reduced to form Cu+ ions while also oxidising the aldehyde to carboxylate ion. The solution turns from blue to brick-red. Ketones are not oxidised and so Fehling’s remains blue.
37
Q

Describe the hydrolysis of esters

A
  • Can be hydrolysed in an equilibrium reaction when esters are heated in reflux of dilute acid. The acid catalyses the reaction in which the original reactants are reformed (alcohol and carboxylic acid)
  • Can also be hydrolysed by alkali in a non-reversible, full reaction. All ester is broken down by excess alkali. Alcohol and sodium salt of carboxylic acid are formed with aqueous sodium hydroxide.
38
Q

Describe the formation of carboxylic acids

A
  • Can be formed by the oxidation of primary alcohols.
  • Can be formed by the hydrolysis of nitriles in a reflux of dilute hydrochloric acid causing C≡N group to be converted to COOH group.
39
Q

Describe the reactions of carboxylic acids as acids

A
  • disassociates in water into COO- ions and H+ ions in an equilibrium reaction.
  • Reacts with alkalis to form salt and water
  • Reacts with metals to form salt and hydrogen gas
  • Reacts with carbonates to form salt, water and carbon dioxide gas.
40
Q

Describe the reduction of carboxylic acids

A
  • can be reduced to primary alcohols using reducing agent LiAlH4 in dry ether at room temperature.

Chemistry AS (3 decks)

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