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Chemistry A2 > A2 Chemistry Term 2 > Flashcards

A2 Chemistry Term 2 Flashcards

1
Q

Describe the differences between Kekule’s structure and benzene

A
  1. Kekule’s structure suggests 3 longer C-C bonds and 3 shorter C=C bonds. Benzene has a perfect, hexagonal arrangement.
  2. Would undergo addition reactions if Kekule’s structure. Does not undergo readily.
  3. enthalpy of hydrogenation is less exothermic meaning benzene is more stable than expected.
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2
Q

Explain why the benzene ring is more stable than expected

A
  • Each carbon atom contributes an electron to a pi bond as the other three electrons form sigma bonds
  • Pi bonds are not localised between pairs of carbon atoms but rather is spread over all six carbon atoms in the hexagonal ring.
  • The electrons are delocalised either above or below the plane of the carbon atoms.
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3
Q

Describe the reaction of benzene/methylbenzene with chlorine or bromine

A
  • Electrophilic substitution
  • AlCl3 catalyst, anhydrous
  • methylbenzene will only react if in the presence of UV light, undergoing free radical subsitution
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4
Q

Explain why the carbon-halogen bond in halogenoarenes is stronger than in halogenoalkanes

A

Lone pairs on the halogen atom overlaps slightly with pi bonding system. This means the bond has a partial double bond character.

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5
Q

Describe the nitration of benzene

A
  • electrophilic substitution
  • electrophile is NO2+

HNO3 + H2SO4 -> NO2+ HSO4 - + H2O

  • reflux + 55°C
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6
Q

Describe the Friedel-Crafts reaction

A
  • electrophilic substitution
  • used to introduce alkyl or aryl groups into a benzene ring.
  • AlCl3 catalyst
  • halogenoalkane is used to form carbocation electrophile.
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7
Q

Describe the oxidation of side-chains in arenes

A
  • forms carboxylic acid
  • reflux with alkaline potassium manganate (VII) and then acidified with dilute sulfuric acid, or acidified potassium dichromate
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8
Q

Explain the relative acidities of phenol and ethanol

A

Phenol: Stronger acid as phenoxide ion formed when phenol dissociates has negative charge spread over whole ion as one of the lone pairs on the oxygen atom overlaps with delocalised pi system. Phenol is less likely to reform.

Ethanol: Weaker acid due to positive inductive effect of alkyl group meaning it is electron-donating. This concentrates more negative charge on the oxygen atom which more readily accepts H+ ion. This means it is less likely to dissociate.

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9
Q

Describe the reactions of phenol

A

Breaking of OH bond:

  • phenol dissolves well in alkali and reacts to form a salt and water.
  • it also reacts with Na to form sodium phenoxide and hydrogen gas.

Substitution into ring of phenol:

  • Overlap of one of lone pairs of oxygen atom with pi bonding system increases electron density of ring meaning it reacts more readily with electrophiles.
  • reacts readily with bromine water
  • reacts readily with dilute nitric acid for nitration
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10
Q

Explain why carboxylic acids are more acidic than phenols and describe the effect of chlorine substituted into the acid on its acidity

A
  • O-H bond is weakened due to electronegativity of O in carbonyl group
  • the carboxylate ion formed is stabilised by the delocalisation of the negative charge on the carboxylate ion reducing its charge density, reducing the likelihood of it binding to H+.
  • These effects are strengthened by the presence of further electron-withdrawing groups such as chlorine, causing the molecule to become more acidic.
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11
Q

Describe the oxidation of carboxylic acids

A

Methanoic acid:

  • stronger reducing agent and will be oxidised by warming with mild oxidising agents such as Tollens and Fehling’s reagent.
  • CO2 + H2O is formed

Ethanedioic acid:

  • acidified potassium manganate is required
  • CO2 + H2O is formed
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12
Q

Describe the different reactions of acyl chlorides

A
  • hydrolysis: water is required and the reaction produces carboxylic acid and HCl
  • alcohols and phenols: Esters and HCl form. Will react vigorously with ethanol. With phenols, the reaction must be warmed and a base must be present in order to form the phenoxide ion which acts as a nucleophile.
  • amines: amide forms. Vigorous reaction.
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13
Q

Explain the relative eases of hydrolysis of acyl chlorides, chloroalkanes, aryl chlorides.

A

Acyl chlorides: Contains strongly electronegative atoms meaning carbon atom has partially positive charge and is open to attack from nucleophiles.

Chloroalkanes: Contains fewer electronegative groups compared to acyl chlorides and so less susceptible to attack from nucleophiles.

Aryl Chlorides: p orbitals from Cl atom tend to overlap with delocalised p electrons in benzene ring causing C-Cl bond to have some double bond character, making it stronger.

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14
Q

Describe the ways in which amines are formed

A
  1. Halogenoalkane undergoes nucleophilic substitution with ammonia using excess, hot ethanolic ammonia.
  2. Reduction of nitriles using hydrogen gas and nickel catalyst. LiAlH4 can be used in dry ether. Nitrile formed by heat+ reflux of halogenoalkane with ethanolic KCN
  3. LiAlH4 in dry ether to reduce amides to amines.
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15
Q

Describe the relative basicities of ethylamine, ammonia and phenylamine

A
  • Basicity is based on availability of lone pair of electrons to which the H+ can bind to.

Ethylamine: positive inductive effect releases electrons to N atom making the lone pair more readily available.

Phenylamine: Lone pair of N atom is delocalised into the benzene ring making it less available to pair with H+

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16
Q

Describe the formation of phenylamine

A
  • made by reducing nitrobenzene

- must be heated with tin and concentrated hydrochloric acid

17
Q

Describe the reactions of phenylamine

A

Aqueous bromine:

  • White ppt forms
  • lone pair on N is delocalised into benzene ring, increasing electron density, making it more readily attacked by electrophiles.
  • bromine attaches to the 2,4,6 positions

Diazotisation:

  • add NaNO2 or HNO2 and dilute HCl to form benzenediazonium chloride. This reaction occurs at less than 10°C
  • the diazonium ion reacts with an alkaline solution of phenol to form an azo dye.
18
Q

Define a zwitterion

A

an ion formed from amino acids containing two charges meaning overall it has no charge

19
Q

Describe how amides are formed

A
  • reacting an acyl chloride with concentrated ammonia solution or a primary amine.
  • these reactions occur at room temperature releasing white fumes of HCl
20
Q

Describe the hydrolysis of amides

A

Refluxed with acid: carboxylic acid and primary amine are formed. Amine will react with excess acid

Refluxed with alkali: salt of carboxylic acid and primary amine are formed.

21
Q

Describe electrophoresis

A
  • ions are placed into an electric field between electrodes.
  • Attracted to opposite charges
  • image formed is a series of bands called electropherogram
  • size and charge of ions affect lines formed as smaller, higher charged ions move further
22
Q

Define standard electrode potential

A

The voltage measured under standard conditions (298K, ions at 1.00 mol/dm^3, 1 atm) with a standard hydrogen electrode as the other half-cell

23
Q

Describe the standard hydrogen electrode

A
  • consists of hydrogen gas at 1 atm, H+ ions in solution, platinum electrode
  • platinum electrode allows close contact of hydrogen gas and H+ ions so equilibrium is established quickly
  • The electrode potential is 0 and so allows for comparison
24
Q

Describe components required to connect two half cells

A
  • to form the electrochemical cell wires and salt bridge is required.

Wires: Connect metal rods in each half-cell to a high-resistance voltmeter and allows for flow of electrons

Salt bridge: Completes electrical circuit allowing movement of ions between two-half cells.

25
Q

Describe how E values can be used to deduce the direction of electron flow

A
  • lower E means it is more difficult to reduce
  • flow of electrons is from the more negative to the more positive pole
  • this means the electrons move from the lower E reaction to the higher one
26
Q

Describe how changes in ion concentration affects the value of E

A

If conc. increases: position of equilibrium shifts to the right meaning more reduction reaction occurs and so E increases.

If conc. decreases: position of equilibrium shifts to the left meaning more oxidation reactions occur and so E decreases.

27
Q

State the Nernst equation

A

Ecell = E + RT/zF ln [oxidised form]/[reduced form]

28
Q

Describe how we can predict the identity of the substance liberated during electrolysis

A
  • ease of discharge of cations at cathode increases with increasing electrode potential as the substance that is most easily reduced goes to the cathode
  • ease of discharge of anions at anode increases with increasing ease of oxidation
  • which substance is discharged is also based on the concentration of each ion
29
Q

Describe rechargeable cells and car batteries

A
  • can be recharged by passing current through allowing products to change back into reactants so cell can function again
  • Consists of plates of lead and lead oxide immersed in sulfuric acid. They contain 6 cells to provide 12V.
  • Battery is recharged by alternator while car engine is running
  • Lead-acid batteries are heavy but cheap to manufacture.
30
Q

Describe solid state cells

A
  • Primary cells that have improved voltage and smaller size.
  • Also give constant voltage over time
  • No liquid or paste so do not leak
31
Q

Describe hydrogen-oxygen fuel cells

A
  • hydrogen and oxygen gas are bubbled through two porous platinum-coated electrodes where the half reactions take place.
  • electrons flow through external circuit from the negative to positive pole.
  • Their energy is used to drive an electric motor
  • H+ ions diffuse through membrane to positive electrode where they react with oxygen and electrons to form water.
32
Q

State advantages and disadvantages of using hydrogen-oxygen fuel cells

A

Advantages:

  • Water only waste product
  • Produce more energy per gram of fuel burnt than petrol
  • Very efficient, no moving parts in which energy is lost as heat

Disadvantages:

  • High cost
  • Manufacturing of fuel cells produce toxic by-products
  • High pressure tanks required
  • Hydrogen can only be produced cheaply by using fossil fuels
  • Do not work well at lower temperatures.

Chemistry A2 (3 decks)

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