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A level- Chemistry > Hard questions part 2 > Flashcards

Hard questions part 2 Flashcards

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1
Q

Describe why a reaction is first order from its rate to concentration graph

A
  1. A straight line through the origin so is first order
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2
Q

State the difference between fuel cell and electrochemical cell. Write equation for overall reaction in a hydrogen fuel cell. State two ways that hydrogen might be stored as a fuel for cars. Suggest why some people consider that the use of hydrogen as a fuel for cars consumes more energy than petrol or diesel

A
  1. A fuel cell converts energy from reaction of a fuel
    with oxygen into a voltage/electrical energy
  2. 2H2 + O2 → 2H2O
  3. Stored under pressure
  4. Adsorbed on a solid
  5. Adsorbed within solid
  6. Energy is needed to make the hydrogen
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3
Q

Describe why the equilibrium yield of ammonia increased in terms of Kc
N2 + 3H2 ↔ 2NH3

A
  1. Kc does not change
  2. Increased pressure increases concentration terms on bottom of Kc expression more than the top so the system is no longer at equilibrium
  3. Top of Kc expression increases and bottom decreases until Kc is reached
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4
Q

Suggest another process that might be used in the future to obtain hydrogen gas for the Haber process

A
  1. Electrolysis of water
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5
Q

What ions are present in N2Cr2H807

A

NH4+ , Cr2O72-

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6
Q

How do you calculate the percentage molar dissociation from Ka and H+ concentration

A
  1. Divide Ka by [H+] and multiply by 100
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7
Q

Describe why reactions using redox systems might not actually take place

A
  1. Activation energy too high so rate of reaction slow

2. Conditions not standard

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8
Q

State two advantages of vehicles using fuel cells compared with the combustion of conventional fossil fuels

A
  1. Less greenhouse gases

2. Greater efficiency

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9
Q

How do you answer questions about why delta S is positive or negative

A
  1. Talk about more mols of gas produced
  2. Or gas produced from a liquid
  3. Or liquid from gases
    etc
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10
Q

When dissolved in water the enthalpy change of solution of KF is -15kJmol^-1 and RbF is -24 kJmol^1. Explain the difference

A
  1. Ionic radius of K+ is smaller than Rb+
  2. Lattice enthalpy- Lattice enthalpy of KF is more negative than RbF
  3. Hydration enthalpy- delta H(hydration) of K+ is more negative than Rb+
  4. Enthalpy change of solution- Idea that ∆H(solution) is affected more by lattice enthalpy than by hydration enthalpy
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11
Q

Why is the enthalpy change of hydration of chloride ions exothermic

A

Hydration involves bonds forming

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12
Q

Explain why the hydration of Magnesium ions is more exothermic than the enthalpy change of hydration of calcium ions

A
  1. Mg2+ has a higher charge density

2. Mg2+ has a stronger attraction to H2O

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13
Q

Show the reaction where ammonia is heated with oxygen in the air to form NO

A
  1. 4NH3 + 5O2 → 4NO + 6H2O
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14
Q

Explain the relative resistance of bromination of benzene compared to phenol and compared to cyclohexene

A
  1. Benzene electrons are delocalised
  2. Phenol has a lone pair of electrons on the oxygen that is delocalised into the ring
  3. Cyclohexene electrons are delocalised
  4. Benzene has a lower electron density
  5. Benzene can not polarise or induce a dipole into bromine
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15
Q

Write the structural formula of the organic product formed when hydroxyethanal reacts with Tollen’s reagent

A
  1. HOCH2COOH
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16
Q

Write the equation to show the oxidation of hydroxyethanal when heated to reflux with acidified dichromate

A
  1. HOCH2CHO + 3[O] → HOOCCOOH + H2O
17
Q

What reacts with both benzoic acid and phenol
What only reacts with benzoic acid and not phenol
What only reacts with phenol and not benzoic acid

A
  1. NaOH
  2. Na2CO3
  3. Bromine
18
Q

Why is it difficult to determine the lattice enthalpy of sodium carbonate using a normal Born-Haber cycle

A
  1. Cycle needs formation of CO3 2-
19
Q

What is deltaS for this process

NaCl (s) + aq → NaCl (aq)

A

+

20
Q

The exothermic reaction occurs spontaneously at low temperatures but does not occur at very high temperatures. Explain why
2SO2 (g) + O2 (g) → 2SO3 (g)

A
  1. With increasing temperature T∆S is more negative

2. At high temperature T∆S is more negative that ∆H so it is bigger than 0

21
Q

Describe one precipitation reaction and one ligand substitution reaction of copper in the +2 oxidation state

A 
Precipitation
1. NaOH (aq)
2. Cu(OH)2 Blue precipitate
3. Cu2+(aq) + 2OH–(aq) → Cu(OH)2(s) 
Substitution
1. NH3(aq)
2. [Cu(NH3)4(H2O)2]2+ darker blue solution
3. [Cu(H2O)6]2+ + 4NH3 → [Cu(NH3)4(H2O)2]2+ + 4H2O
22
Q

A student added some chromium metal to an acidified solution containing copper (II) ions.
Explain in terms of their electrode potentials why chromium is more reactive than copper in this reaction.
Cr 3+ + 3e- ↔ Cr (s) -0.74
Cu2+ + 2e- ↔ Cu(s) +0.34
And explain why bubbles of a gas were observed

A
  1. Electrode potential for chromium (redox system) is more negative/lower/less (than copper redox system)
  2. Chromium system shifts to the left Cr(s) → Cr3+(aq) + 3e–
  3. Copper system shifts to the right Cu2+(aq) + 2e– → Cu(s)
  4. Cr reacts with H+ ions to form H2 gas
23
Q

Suggest two advantages of using methanoic acid as the fuel in a fuel cell rather than hydrogen

A
  1. Methanoic acid is a liquid and so is easier to store/transport
  2. Hydrogen is explosive/more flammable
  3. HCOOH gives a greater cell potential/voltage
  4. HCOOH has more public/political acceptance than
    hydrogen as a fuel
24
Q

The equilibrium position can be shifted by bubbling CO2 gas through the mixture. Suggest how the equilibrium position shifts
3MnO42- + 2H2O ↔ 2MnO4- + MnO2 + 4OH-

A
  1. CO2 reacts with H2O forming an acid- H2CO3
  2. H2CO3 + OH– → H2O + HCO3–
  3. Equilibrium shifts to right to restore OH–
25
Q

Explain why chlorine reacts more readily with C6H5N(CH3)2

A
  1. (lone) pair of electrons on N is (partially) delocalised
    into the ring
  2. Electron density increases/is higher (than in benzene
  3. Cl2/electrophile is (more) polarised
26
Q

Explain with reference to a suitable chemical test how an aldehyde could be identified

A
  1. Add 2,4-dinitrophenylhydrazine and an orange precipitate will form
  2. Take melting point of crystals
  3. Compare to known values
27
Q

How can a student obtain concentration for order of reaction determining experiments

A
  1. Mix 10 cm3 of each of the different components
  2. Dilute each solution in turn with water
  3. Only one solution changed at a time
  4. Total volume kept the same
28
Q

What can be used to reduce Fe3+ to Fe2+ and vice versa

A
  1. KI/I- reduces Fe 3+

2. MnO4- oxidises Fe2+

29
Q

Write half equations and an overall equation for the oxidation of [Co(NH3)]2+ by H2O2

A
  1. [Co(NH3)6]2+ → [Co(NH3)6]3+ + e–
  2. H2O2 + 2e– → 2OH–
  3. 2[Co(NH3)6]2+ + H2O2 → 2[Co(NH3)6]3+ + 2OH–
30
Q

How do you go from CH3CHO to CH3CH2(OH)CH2NH2

A
  1. Step 1: add H2SO4/KCN
    CH3CHO + HCN → CH3CH(OH)CN
  2. Step 2: react with H2/Ni
    CH3CH(OH)CN + 2H2 → CH3CH(OH)CH2NH2
31
Q

What is the name of CH3CH2CH=CHCOOH

A

E-pent-2-enoic acid

32
Q

Ethanedioic acid removes excess dichromate ions Cr2O72-:
3(COOH)2 + Cr2O72– + 8H+ → 6CO2 + 2Cr3+ + 7H2O
Suggest how you could tell when the excess dichromate has completely reacted with the ethanedioic acid

A

Lack of further effervescence

33
Q

Ethanedioic acid removes excess dichromate ions Cr2O72-:
3(COOH)2 + Cr2O72– + 8H+ → 6CO2 + 2Cr3+ + 7H2O
Outline how TLC could be used to monitor the course of this reaction

A
  1. Take samples from reaction mixture at regular intervals
  2. Spot/run on a TLC plate, alongside cyclohexanol (and
    cyclohexanone) controls
34
Q

How do you distinguish between CH3CH2OH and C6H5OH

A
  1. Bromine water
35
Q

How do you get from CH3CH(OH)COOH to CH3CH2(NH2)COOH in two steps

A
  1. NaBr/H2SO4 to make CH3CH(Br)COOH

2. Excess NH3

A level- Chemistry (31 decks)

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