2016- Flashcards
Suggest one reason, other than incomplete combustion or heat transfer to the atmosphere, why the student’s value for the enthalpy of combustion of methanols different from that in a Data Book.
Experiment not done under standard conditions
The student said correctly that using a thermometer with an overall uncertainty for
the rise in temperature of ±0.5 °C was adequate for this experiment.
Explain why this thermometer was adequate for this experiment.
Idea that temperature change/rise is (significantly / much)
bigger than uncertainty/Idea that heat loss is more significant issue
Identify a reagent that could be used in a chemical test to show that oleic acid is unsaturated.
State what would be observed in this test.
bromine water (orange/yellow to) colourless
Explain why determining the precise relative molecular mass of propanal and prop-2-en-1-ol by mass spectrometry could not be used to distinguish between samples of these two compounds.
have the same molecular formula
so have the same relative molecular mass
Predict the relative boiling points of these three compounds from the highest to the lowest boiling points.
Justify this order in terms of intermolecular forces.
Correct order (highest to lowest) =
prop-2-en-1-ol > propanal > butane
Prop-2-en-1-ol has hydrogen bonds
Propanal has (permanent) dipole-dipole forces
Butane has van der Waals’ forces
Strength of intermolecular forces:
hydrogen bonds > dipole-dipole > van der Waals
Draw a labelled diagram to show how you would set up apparatus for refluxing.
flask with condenser vertically above it (without gaps
between flask and condenser)
flask and condenser labelled
Anti-bumping granules are placed in the flask when refluxing.
Suggest why these granules prevent bumping.
form small(er) bubbles or prevent large bubbles
Deduce which of Na+ and Mg2+ is the smaller ion.
Explain your answer.
Mg(2+) or Magnesium
Because Mg2+ has more protons
Write an equation to represent the process that occurs when the first ionisation energy for sodium is measured.
Na(g) → Na+(g) + e−
Explain why the first ionisation energy of sulfur is different from that of phosphorus.
e− paired in (3)p orbital in S
Paired e− repel (so less energy needed to remove)
Name the type of bond formed between N and Al in H3NAlCl3 and explain how this bond is formed.
Dative (covalent) /co-ordinate bond
Shared pair of / both electrons come from the N(H3)
Explain how the value of the Cl-Al-Cl bond angle in AlCl3 changes, if at all, on formation of the compound H3NAlCl3
Aluminium is now surrounded by 4 electron pairs/bonds or is tetrahedral
Therefore Cl-Al-Cl bond angle decreases / changes (from 120° in AlCl3 ) to allow range 107-111° in H3NAlCl3
A solution of sodium chlorate(l) was added to a colourless solution of potassium iodide.
Suggest what is observed.
Explain the reaction that leads to this observation.
Goes brown (or shades of brown)
Due to iodine or I^3−
Because I− oxidised
Write an ionic equation, with state symbols, to show the reaction of calcium with an excess of water.
Metal(s) + H20(l) –> MetalOH(aq) + H2(g)
State the role of water in the reaction with calcium.
Oxidising agent
Write an equation to show the process that occurs when the first ionisation energy of calcium is measured.
Ca(g) Ca+(g) + e–
State and explain the trend in the first ionisation energies of the elements in Group 2 from magnesium to barium.
Decrease
Ions get bigger / more (energy) shells
Weaker attraction of ion to lost electron
Describe how ions are formed in a time of flight (TOF) mass spectrometer.
the sample is bombarded by high energy electrons
the sample molecule loses an electron forming M+
Explain why it is necessary to ionise molecules when measuring their mass in a TOF mass spectrometer.
Ions, not molecules, will interact with and be accelerated by an electric field
Only ions will create a current when hitting the detector
Explain why CF4 has a bond angle of 109.5°.
Around carbon there are 4 bonding pairs of electrons (and no lone pairs)
Therefore, these repel equally and spread as far apart as possible
The student added a few drops of Y(aq) to the equilibrium mixture of X(aq), Y(aq) and Z(aq) in Question 4.3.
Suggest how the colour of the mixture changed. Give a reason for your answer.
Darkened / went more orange
The equilibrium moved to the right
To oppose the increased concentration of Y
The student warmed the equilibrium mixture from Question 4.3.
Predict the colour change, if any, when the equilibrium mixture was warmed.
The orange colour would fade
Sodium bromide reacts with concentrated sulfuric acid in a different way from sodium chloride.
Write an equation for this reaction of sodium bromide and explain why bromide ions react differently from chloride ions.
2NaBr + 2H2SO4 Na2SO4 + Br2 + SO2 + 2H2O
Br– ions are bigger than Cl– ions
Therefore Br– ions more easily oxidised / lose an electron more easily (than Cl– ions)
A colourless solution contains a mixture of sodium chloride and sodium bromide.
Using aqueous silver nitrate and any other reagents of your choice, develop a procedure to prepare a pure sample of silver bromide from this mixture.
Explain each step in the procedure and illustrate your explanations with equations, where appropriate.
Stage 1: formation of precipitates • Add silver nitrate • to form precipitates of AgCl and AgBr • AgNO3 + NaCl → AgCl + NaNO3 • AgNO3 + NaBr → AgBr + NaNO3
Stage 2: selective dissolving of AgCl
• Add excess of dilute ammonia to the mixture of
precipitates
• the silver chloride precipitate dissolves
• AgCl + 2NH3 → Ag(NH3)2+ + Cl–
Stage 3: separation and purification of AgBr
• Filter off the remaining silver bromide precipitate
• Wash to remove soluble compounds
• Dry to remove water
The mass of MgO obtained in this experiment is slightly less than that expected from the mass of Mg(NO3)2 used.
Suggest one practical reason for this.
Some of the solid is lost in weighing product
Explain how the Cahn-Ingold-Prelog (CIP) priority rules can be used to deduce the full IUPAC name of this compound.
Stage 1: consider the groups joined to right hand carbon of the C=C bond
Consider the atomic number of the atoms attached
C has a higher atomic number than H, so CH2OH takes priority
Stage 2: consider the groups joined to LH carbon of the C=C bond
Both groups contain C atoms, so consider atoms one bond further
away C, (H and H) from ethyl group has higher atomic number than H, (H
and H) from methyl group, so ethyl takes priority
Stage 3: conclusion
The highest priority groups, ethyl and CH2OH are on same side of
the C=C bond so the isomer is Z
The rest of the IUPAC name is 3-methylpent-2-en-1-ol
State how your answer to Question 2.2 is likely to differ from the value quoted in
reference sources.
Give one reason for your answer.
Less negative than the reference
Heat loss
A 50.0 g sample of water was used in this experiment.
Explain how you could measure out this mass of water without using a balance.
Water has a known density (of 1.0 g cm–3)
Therefore, a volume of 50.0 cm3 could be measured out
State the meaning of the term structural isomers.
(Compounds with the) same molecular formula but different structural formula
Write an equation for the reaction occurring during fermentation.
C6H12O6 –> 2C2H5OH + 2CO2
In industry, this fermentation reaction is carried out at 35 °C rather than 25 °C.
Suggest one advantage and one disadvantage for industry of carrying out the fermentation at this higher temperature.
Adv: ethanol is produced at a faster rate
Dis: more energy is used / required in the reaction
The method used by the student in Question 4.2 would result in the ethanol being contaminated by ethanoic acid.
How does this contamination occur?
Air gets in / oxidation occurs
Give two differences between the infrared spectrum of a carboxylic acid and that of an alcohol other than in their fingerprint regions.
Alcohol OH absorption in different place (3230–3550 cm–1) from acid OH
absorption (2500–3000 cm–1)
The C=O in acids has an absorption at 1680–1750 cm–1
Reaction 1 is an example of thermal cracking and is carried out at a temperature
of 750 °C.
State one other reaction condition needed.
High Pressure
Reaction 2 is exothermic. A typical compromise temperature of 200 °C is used industrially for this reaction.
Explain the effect of a change of temperature on both the position of equilibrium and the rate of reaction, and justify why a compromise temperature is used industrially
Stage 1: consider effect of higher temperature on
yield
• Le Chatelier’s principle predicts that equilibrium
shifts to oppose any increase in temperature
• Exothermic reaction, so equilibrium shifts in
endothermic direction / to the left
• So a Higher T will reduce yield
Stage 2: consider effect of higher temperature on rate
• At higher Temperature, more high energy
molecules
• more collisions have E>Ea
• So rate of reaction increases/time to reach
equilibrium decreases
Stage 3: conclusion
Industrial conditions chosen to achieve (cost-effective)
balance of suitable yield at reasonable rate
In a trial experiment, the student failed to fill the burette correctly so that the gap between the tap and the tip of the burette still contained air.
Suggest what effect this would have on the measured volume of bromine water in this
trial. Explain your answer.
Measured volume would be greater
Level in burette falls as tap is filled before any liquid is delivered
Other than incorrect use of the burette, suggest a reason for the inconsistency in the student’s results.
Drop sizes vary
Outline how the student could improve this practical procedure to determine the number of C=C double bonds in a molecule of the oil so that more consistent results are obtained.
Use a larger single volume of oil
Dissolve this oil in the organic solvent
Transfer to a conical flask and make up to 250 cm3 with more solvent Titrate (25 cm3) samples from the flask
