Chemistry Unit 1

• difference between covalent + dative

  in coordinate, both electrons in the shared pair comes from same atom, whereas in covalent, each atom donates an electron to shared pair.
• strong acid vs highly concentrated acid

  strong: acid fully dissociates in aqueous solution

  concentrated: large quantity of acid in small quantity of water
• All Group 2 carbonates are insoluble and can be precipitated, dried and weighed in experiments similar to that in part (c). However, the strong heating needed to drive off all the water can cause a problem in accurately determining the mass of the carbonate precipitated. Suggest what this problem might be.

  1. some carbonates decompose on heating
  2. results in lower mass recorded
• State two other common sources of error in titration experiments and explain why Elinor's statement cannot be correct. [4]

  (Assume that all the equipment is clean and all chemicals are pure.)

  parallax error - not reading meniscus from bottom

  solution in flask not mixed thoroughly

  if too much acid added moles of [Na2CO3] would be more than expected, not less
• A few students obtained a smaller volume of hydrogen than expected. Suggest two practical reasons why this may have occurred, apart from leaks.

  1. not heated for long enough
  2. sample of [] not pure
• Suggest how Gareth would know when enough calcium chloride solution had been added to react with all of the potassium oxalate present.

  1. test filtrate
  2. add few drops of calcium chloride solution and see if precipitate forms
• Use these figures to comment on the strength of the intermolecular forces between the molecules in each compound suggesting reasons for your answer. [3]

  1. [] has stronger intermolecular forces than []
  2. suggests that O-H bonding is stronger than N-H bonding
  3. O more electronegative than N

  
• State what was seen at the anode to confirm the presence of iodide ions. Explain your answer, including a half-equation. [3]

  1. iodide ions oxidised to form Iodine
  2. 2I- (aq) -> I2 (aq) + 2e-
  3. aqueous iodine produced gives off brown/yellow colour
• filter solution before reaction complete, what effect on titre [2]

  1. increase
  2. fewer HCl moles react with [] therefore more NaOH moles needed to react with excess
• Explain why sodium has a lower melting temperature than aluminium. [1]

  Al has more valence electrons due to greater charge (3+) therefore stronger metallic bonding, and stronger electrostatic forces between positive ions and negative delocalised electrons.
• Use all the data given to choose which isotope is the most suitable to use as a tracer in medicine. Explain your answer. [3]

  NOT:

  1. long half-life - remains too long in body
  2. alpha radiation most ionising
  3. [] most suitable - short half-life and gamma radiation least ionising
• Explain why hydrogen atoms emit only certain definite frequencies of visible light. [2]

  1. electrons fall from higher energy level to lower energy level (n=2)
  2. energy levels are quantised
• State what is meant by a polar covalent bond. [1]

  a covalent bond with unequal electron density
• Suggest why she did not need to measure the volume of water accurately. [1]

  concentration of acid not important, only need moles
• pH formula

  pH = -log ( [H+ ions] x number of H atoms )
• further tests for type of structure [1]

  higher temperatures to melt solid + test molten substance for electrical conductance
• Suggest why it is difficult to identify a material as a metal when it is in powdered form. [1]

  harder for powdered metal to conduct electricity
• electron capture

  electron -> Ve
• bond polarity [2]

  1. unequal electron distribution in bond
  2. because [] is more electronegative than []
• describe emission spectrum of H in visible region + explain origin of its main features [5]

  1. atomic spectrum of H in visible region consists of a series of four coloured lines on a black background
  2. lines converge as frequency increases
  3. lines arise from atom absorbing energy and promoting electron to a higher energy level
  4. fall back down to n=2, emitting energy (EM radiation)
  5. energy levels are quantised
  6. only discrete transitions allowed
• how atomic emission spectrum of H is used to determine IE [2]

  1. convergence limit in Lyman Series
  2. E = hf
• uses of radioisotopes

  1. tracer to be taken up by tissue to be studied - Technetium-99
  2. radiotherapy using gamma radiation - Cobalt-60
  3. plant/animal remains age calculation - Carbon-14 (5570 years)
  4. geological age of rocks - Potassium-40 (1300 million)
  5. electricity production at nuclear power stations - Uranium-235 or Plutonium-239
• Melting temperatures

  1. Simple molecular
  2. metallic
  3. ionic
  4. giant covalent
• transitional metal ions

  4s subshell empties before 3d
• Chromium-24 + Copper-29

  4s1
• VSEPR theory

  1. linear, trigonal planar, tetrahedral, trigonal bipyramid, octahedral + pyramidal, non-linear, square planar
  2. number of bonding pairs + lone pairs
  3. electron pairs arrange themselves around the central atom as far from each other as possible to minimise repulsion between them
• 9:1 peak in mass spectrometry

  1. chlorine has isotopes of Cl-35 and Cl-37 in 3:1 ratio
  2. since peak C due to []+ and peak E due to []+
  3. height of C:E = (3:1)(3:1) = 9:1

  
• why indicator + why rough titration

  1. to show end point
  2. to get rough idea of end point
• mean titre


  use ONLY concordant titres
• Vacuum - mass spectrometry


  Vacuum to reduce chances of ions colliding with molecule therefore less likely for fragmentation to occur
• acid-base reaction

  [] loses H+ ions

  [] gains H+ ions
• State Le Chatelier’s principle. [1]

  If a system at equilibrium is subjected to a change, the position of the equilibrium will shift to minimise that change.

  1. equlibrium shifts towards exo/endothermic DIRECTION
  2. catalyst does not affect position of equilibrium
• why bonds are not purely covalent in [NH3] [2]

  1. due to difference in electronegativity between N and H
  2. there is unequal electron distribution in the bond
• why rinse burette [1]

  to ensure concentration of [] in burette does not change
• why dilute seawater [1]

  Cl- ion concentration is too high so less AgNO3 needed