exsam questions from pp

• give 2 features of a reaction in dynamic equilibrium

  1. forward and reverse reactions proceed at equal rates
  2. concentrations of reactants and products remain constant
• a gas-phase reaction is at equilibrium . when the pressure is increased the yield of products decreases.

  state what can be deduced about the chemical equation for this reaction .

  1. more moles of gaseous products than gaseous reactants
• define the term atomic mass

  • average/mean mass of 1 atom (of an element)
  • -------------------------------------------
  • 1/12 mass of one atom of 12C
• state why isotopes have the same chemical properties

  • same electron configuration
  • same number of electrons
• state how the relative abundance of a ion is determined in a TOF mass spectrometer

  1. at the detector the ions gain an electron
  2. relative abundance depends on the size of the current
• define the term mean bond enthalpy

  • enthalpy change to break 1 mol bonds in gaseous state
  • averaged over a range of compounds
• state how a buffer soultion can be made from solutions of potassium hydroxidide and ethanoic acid

  • add excess ethanoic acid to KOH
• give the equation between potassium hydroxide and ethanoic acid .

  KOH + CH3COOH  CH3COOK + H2O
• how do buffer solutions resists pH change . potassium hydroxide and ethanoic acid

  • CH3COO– (from salt) reacts with (added) acid/H+
• Identify a reagent or test that could be used to distinguish between aqueous solutions of sulfur dioxide and sulfur trioxide with the same concentrations. State the observation in each case.

  • universal indicator
  • SO2(aq) orange
  • red SO3(aq) red
• The mass spectrum of the element phosphorus has a peak at m z = 124 Give the formula of the species responsible for this peak.

  • 31P4 +
• Give an equation for the reaction of phosphorus(V) oxide with sodium hydroxide solution.

  P4O10 + 12NaOH ->4Na3PO4 + 6H2O
• Explain why the melting points of these substances are different. You should refer to the structure of and bonding in each substance ( 6m)

  sodium chloride 1074 chlorine 172 hydrogen chloride 158

  1. Stage structure- NaCl ionic lattice while Cl2 and HCl are simple molecules
  2. forces responsible for melting point - NaCl attractions between + and – ions while Cl2 has vdw forces and HCl has dipole dipole forces
  3. comparison of melting point- ionic bonds stronger than IMF. Cl2 is a bigger molecule and has has more electrons than HCl .stronger IMF in Cl2 than those in HCl
• Give an equation for the reaction of chlorine with water to form a solution containing two acids. Explain, with reference to electrons, why this is a redox reaction

  Cl2 + H2O  HCl + HClO

  chlorine gains electrons to form Cl – and loses electrons to form ClO–
• A student bubbles chlorine gas through a solution of sodium iodide. State the observation the student would make. Give an ionic equation for the reaction.

  brown solution or black solid (forms)

  Cl2 + 2I – → 2Cl – + I2
• The student adds a few drops of concentrated sulfuric acid to a small amount of solid sodium iodide. Two gaseous sulfur-containing products are formed. Give an equation for the formation of each of these sulfur-containing products. State the role of sulfuric acid in the formation of these products.

  • H2SO4 + 2H+ + 2I – → SO2 + 2H2O + I2
  • H2SO4 + 8H+ + 8I – → H2S + 4H2O + 4I2
  • oxidising agent
• The student adds a few drops of acidified silver nitrate solution to a solution of an unknown impure sodium halide. The student observes bubbles of gas and a colourless solution. The student bubbles the gas through calcium hydroxide solution and a white precipitate forms. Deduce the identity of the sodium halide. Suggest the identity of the gas.

  • NaF / sodium fluoride
  • CO2
• Describe and explain the shape of the graph in Figure 1. [3 marks]

  • As T increases rate (1/t ) increases OR time for completion decreases
  • Exponentially
  • Many more particles have E ≥ Ea

  
• Name the mechanism for the reaction of butanone with KCN followed by dilute acid

  Nucleophilic Addition
• Many refrigerators now use pentane as a refrigerant. State the environmental problem that chlorodifluoromethane can cause. Give reasons why pentane does not cause this problem.

  Causes ozone depletion

  Pentane does not have C-Cl bonds and does not produce Cl radicals
• Explain why this is the minor product.

  major product formed via more stable carbocation.

  tertiary carbocation more stable than primary

  Due to electron-releasing character of three alkyl groups as opposed to one

  
• Explain why the strength of the interaction between two cysteine R groups differs from the strength of the interaction between a serine R group and an aspartic acid R group. Use the Data Booklet to help you answer this question.

  Two Cys R groups form a disulfide bridge/link stated or described Ser and Asp R groups form Hydrogen bonds Disulfide bridges are stronger than Hydrogen bonds Because disulfide bridges are covalent bonds

  
• preparation of hexan-2-ol . It is important to wear eye protection and a lab coat when completing this experiment. Suggest, with a reason, one other appropriate safety precaution for this experiment

  Wear gloves Conc phosphoric acid is corrosive
• c After 5 minutes add 10 cm3 of distilled water to the mixture and transfer the boiling tube contents to a separating funnel.

  d Shake the mixture and allow it to settle. e Discard the lower (aqueous) layer.

  f Add a fresh 10 cm3 sample of distilled water and repeat steps d and e

  Give a reason for adding the distilled water in steps c and f?

  To remove water soluble impurities
• This question is about the preparation of hexan-2-ol. Hexan-2-ol does not mix with water and has a boiling point of 140 oC

  dentify the most likely organic impurity, other than hex-1-ene, in the distillate collected j. Suggest one reason why it could be difficult to remove this impurity.

  Impurity: hexan-1-ol

  Reason: It is likely to have a similar boiling point
• Benzene reacts with methanoyl chloride (HCOCl) in the presence of a catalyst . Give an equation for the overall reaction when benzene reacts with methanoyl chloride. Name the organic product.

  C6H6 + HCOCl → C6H5CHO + HCl

  Benzaldehyde
• Benzene reacts with methanoyl chloride (HCOCl) in the presence of a catalyst. Identify the catalyst needed in this reaction. Give an equation to show how the catalyst is used to form the electrophile, [HCO]+

  AlCl3

  HCOCl + AlCl3 → [HCO]++ + [AlCl4]-
• A suitable target titre for the titration is 30.0 cm3 of 0.025 mol dm−3 Br2(aq). Justify why a much smaller target titre would not be appropriate.

  Smaller titre will increase (%) uncertainty / error
• State the reason for inverting the flask several times

  To ensure the solution is homogeneous
• Name the type of compound shown by the formula RCOOK Give one use for this type of compound.

  Carboxylate salt

  Soap
• Suggest why aqueous ethanol is a suitable solvent when heating the coconut oil with KOH. Give a safety precaution used when heating the mixture. Justify your choice.

  Allow to dissolve both oil and KOH Precaution must be linked to heating e.g. Use a water bath for heating mixture Prevents risk of fire / Ethanol is flammable
• The petrol fraction obtained from crude oil can be used as fuel in cars. State the meaning of fraction, as used in the term petrol fraction.

  A group of (hydrocarbons/compounds) with similar boiling points
• Carbon dioxide is formed when petrol is burned. Carbon dioxide acts as a greenhouse gas when it absorbs infrared radiation. Give a reason why carbon dioxide absorbs infrared radiation.

  C=O bonds vibrate at the same frequency as IR
• What is bioethanol used as an alternative to?

  Fossil fuels
• What does the statement about bioethanol being carbon-neutral imply?

  It suggests no net carbon emissions
• What are the stages involved in bioethanol production?

  1. Photosynthesis

  • Plants take CO2 from the air

  1. Fermentation

  • Converts sugars to bioethanol, releasing CO2

  1. Combustion

  • Bioethanol is burned, releasing CO2
• What is the equation for photosynthesis?

  $6CO2 +$$6H2O → C6H12O6 +$$6O2$
• What is the equation for fermentation to produce bioethanol?

  $C6H12O6 → 2C2H5OH +$$2CO2$
• What is the equation for the combustion of bioethanol?

  $2C2H5OH +$$6O2 → 6H2O +$$4CO2$
• What are the environmental impacts of bioethanol production?

  • Deforestation
  • Sacrifice land for food production
  • Loss of biodiversity and habitat
  • Not truly carbon neutral due to fuel use in production and distribution