C3 - Quantitative Chemistry

• Give two advantages and two disadvantages of using fermentation to produce ethanol.

  adv

  (fermentation) low energy usage

  (fermentation) uses renewable raw materials

  disadv

  (fermentation) produces impure ethanol

  (fermentation) slow rate of reaction
• why high atom economies are used in economy?

  for sustainable development

  • for economic reasons

  • to produce a high(er) percentage of useful product
• why is aluminium extracted by electrolysis and not by carbon?

  aluminium is more reactive than carbon

  carbon cannot displace aluminium.
• Suggest how the scientist could carry out Step 6 safely. 'evaporate off the ethanol'

  heat with a water bath

  heat with an electric heater

  crystallise at room temperature
• Explain why the scientist adds excess zinc rather than excess iodine.

  to make sure that all the iodine reacts

  (as) excess iodine would remain in solution

  (so) iodine could not be filtered off
• Describe a safe method for making pure crystals of copper sulfate from

  copper carbonate and dilute sulfuric acid.

  sulfuric acid in beaker (or similar)

  • add copper carbonate one spatula at a time

  • until copper carbonate is in excess or until no more

  effervescence occurs *

  • filter using filter paper and funnel

  • filter excess copper carbonate

  • pour solution into evaporating basin / dish

  • heat using Bunsen burner

  • leave to crystallise / leave for water to evaporate / boil off water

  • decant solution

  • pat dry (using filter paper)

  • wear safety spectacles / goggles
• Describe how a sample of copper chloride crystals could be made from copper carbonate and dilute hydrochloric acid. (4 marks)

  add excess copper carbonate (to dilute hydrochloric acid)

  filter (to remove excess copper carbonate)

  heat filtrate to evaporate some water or heat to point of crystallisation

  leave to cool (so crystals form)
• Approximately 1% of ethanoic acid molecules ionise in water.

  A solution is made by dissolving 1 g of hydrogen chloride in 1 dm3 of water.

  a dilute solution of a strong acid
• This is part of the method the student used for the titration. Describe how the student would complete the titration.

  1. Measure 25.0 cm3 of the sodium hydroxide solution into a conical flask using a pipette.

  2. Add a few drops of indicator to the flask.

  3. Fill a burette with citric acid solution.

  4) add the citric acid (to the flask) until there is a (permanent) colour change

  5) measure / record the volume (of citric acid) added

  6) swirl, use a white tile, add the citric acid dropwise (near the end-point), repeat and calculate a mean
• Give two reasons why a burette is used for the citric acid solution.

  can add (the citric acid) in small increments

  • can measure variable volumes

  • more accurate than a measuring cylinder
• Dilute hydrochloric acid is a strong acid. Explain why an acid can be described as both strong and dilute

  (strong because) completely ionised (in aqueous solution)

  (dilute because) small amount of acid per unit volume
• Explain why the student should use a pipette to measure the dilute sulfuric acid and a burette to measure the sodium hydroxide solution.

  pipette measures a fixed volume (accurately)

  (but) burette measures variable volume
• Citric acid is a weak acid. Explain what is meant by a weak acid.

  produces H+

  hydrogen ions in aqueous solution

  (but is) only partially / slightly ionised
• Write the ionic equation for a neutralisation reaction. Include state symbols

  H+ (aq) + OH− (aq) → H2O(l)
• A student used a pipette to add 25.0 cm3 of sodium hydroxide of unknown concentration to a conical flask. Describe how the student would complete the titration

  • adds indicator, eg phenolpthalein / methyl orange / litmus added to the sodium hydroxide (in the conical flask)
  • (adds the acid from a) burette
  • with swirling or dropwise towards the end point or until the indicator just changes colour
  • until the indicator changes from pink to colourless (for phenolphthalein) or yellow to red (for methyl orange) or blue to red (for litmus
• Give two advantages of using hydrogen fuel cells instead of using rechargeable cells to power cars.

  no toxic chemicals to dispose of at the end of the cell's life

  • take less time to refuel (than to recharge rechargeable cells)

  • travel further before refuelling (than before recharging rechargeable cells)

  • no loss of efficiency (over time)
• Reactions occur at the positive electrode and at the negative electrode in a hydrogen fuel cell.


  H2 + 2 OH- ⟶ 2 H2O + 2 e

  O2 + 2 H2O + 4 e- ⟶ 4 OH
• Suggest two reasons why wood is more sustainable than natural gas as a fuel for central heating boilers.

  wood is renewable

  or (natural) gas is finite

  (burning) wood produces the same amount of carbon dioxide as the

  trees absorbed

  wood is carbon-neutral allow

  wood does not add to global warming

  (burning natural) gas increases the amount of carbon dioxide (in the

  atmosphere)
• The three experiments gave different results for the amount of copper produced. This was caused by experimental error. Suggest two causes of experimental error in these experiment

  different balances used or faulty balance

  reading / using the balance incorrectly

  spilling copper oxide/copper

  not all of the copper oxide was reduced/converted to copper

  not enough / different amounts of methane used
• Explain why aluminium forms at the negative electrode during electrolysis

  (because) aluminium ions are positive

  and are attracted / move / go to the negative electrode / cathode

  where they gain electrons / are reduced / Al3+ + 3e−

  → Al
• How well do the student’s results support her hypothesis?

  The more carbon atoms there are in a molecule of any fuel, the better the fuel is.


  (supports hypothesis) because when the fuel contained more carbon the temperature of the water went up more / faster (in 2 minutes)

  does not support hypothesis as) temperature change per gram decreases as the number of carbons increases

  (does not support hypothesis) because the more carbon in the fuel the more smoke or the dirtier / sootier it is

  only tested hydrocarbons / alkanes / fuels with between 5 and 12 carbon atoms - valid, justified, conclusion
• Suggest two causes of random error in the experiment.

  incorrect reading of thermometer / temperature

  • incorrect measurement of volume of acid

  • incorrect measurement of volume of alkali (burette).
• The higher pressure gives a greater yield of methanol and an increased rate of reaction. Explain why. 


  (yield)

  equilibrium position moves to the product side

  (because) fewer molecules / moles / particles on product side

  moles / particles on the right

  (rate)

  more collisions per unit time

  energetic / forceful collisions

  (because) more molecules / particles per unit volume
• Suggest one improvement that could be made to the apparatus used that would give more accurate results.

  use a pipette / burette to measure the acid because it is more accurate volume than a measuring cylinder so no gas escapes when bung removed