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
