(2)
(ii) Draw the shape of this molecule.
1. (a) Calculate the number of
a and the number of b particles emitted
in the change
23290 Th to 22488
Ra.
Briefly explain your answer.
.......2 alpha particles
and 2 beta particles. .................
........Two alpha particles
have a mass of 8 (232 - 224).........
......... Loss of 2 alpha
particles decreases atomic number by 4 but loss of 2 beta particles increases
atomic number by 2 so there is an overall decreases of 2 from 90 to 88.............
(3) ......
(b) A sample of 22488 Ra reduces
to one-eighth of its original activity after 10.92 days. Calculate its
half-life.
.....Reducing to 1/8 of activity takes 3 half-lives....................
......so half-life = 10.92/3 days = 3.64 days..........................................
(2)
(c) Radioactive decay of 22488
Ra results in 22086 Rn.
(i) Write a nuclear equation for this reaction.
.......... 22488 Ra ------>
22086
Rn + 42He ................................................
(ii) 22086 Rn is radioactive but unreactive chemically.
Explain this difference.
...............Rn has an unstable nucleus so is radioactive..............
...............Rn has a complete outer shell so is
chemically unreactive...................
(iii) Radon gas is given off naturally by some building materials.
Given that
22086 Rn is an a emitter,
comment on why this isotope is considered to be a health hazard.
.......Radon is a gas so can be inhaled.........
......Alpha radiation is highly
ionising and short range so is trapped in the body and does a lot of damaged..............
(5)
(d) From the position of radium in the Periodic table,
predict the following:
(i) the formula of radium carbonate;
...................RaCO3........................................................................................................
(ii) the equation for the thermal decomposition of radium
carbonate;
...................RaCO3(s)
------> RaO(s) + CO2(g)
(iii) how the decomposition temperature required in (d)(ii)
would compare with that required for magnesium carbonate.
......The decomposition temperature
would be higher for Radium carbonate than for magnesium carbonate.......................................................................................................
(4)
(e) If the gas from (d)(ii) is passed into dilute aqueous
calcium hydroxide, a white precipitate forms which shows no radioactivity.
Explain this observation.
.....The white precipitate is calcium
carbonate formed from carbon dioxide.............
......Radium is radioactive but
carbon is not.........................................................
.......................................................................................................................
(2)
Total 16 marks
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(2)
(ii) Draw the shape of this molecule.
tetrahedral
Explain your answer in terms of the Electron Pair Repulsion
Theory.
...... The molecule has 4 bonding
pairs of electrons..................................................
....... A tetrahedral shape gives
the maximum separation for 4 electron pairs................
(iii) State the shape of a molecule of AlCl3 and
explain why it is different from that of SiCl4.
...... Trigonal planar ...................
........ AlCl3 has 3
bonding pairs of electrons ...........................................
(6) ......
(d) (i) Give an equation for the reaction of SiCl4
with cold water.
.................... SiCl4
+ H2O -------> SiO2 + 4HCl ................................
(ii) How does the behaviour of carbon tetrachloride with
cold water compare with this? Explain any differences.
........... Carbon tetrachloride
does not react with water .....................
........... Carbon does not have
vacant d orbitals like silicon ........................................
........... so carbon cannot accept
a lone pair from an oxygen atom in water................ (4)
Total 14 marks
(b) Outline, giving the reagent, the essential conditions
and the equation, how a sample of anhydrous iron(II) chloride could be
made from iron.
... Pass dry HCl gas over heated
iron. .............................
.... Fe + 2HCl -----> FeCl2
+H2...........................
........................................................................
(3)
(c) Give the formula and name of the metal-containing
ion formed when iron(II) chloride is dissolved in water. State the
shape of this ion.
Formula: ...................... [Fe(H2O)6]2+
..............................................................................
Name: .......................
hexaaquairon(II) ....................................................
Shape: ........................
octahedral ........................... (3)
(d) (i) Describe what you would observe if dilute sodium
hydroxide is added to the solution in (c).
............................ A
green precipitate forms which is insoluble in excess NaOH ..................
...................................................................................................................................
(ii) Give an equation for this reaction and state the
type of reaction occurring.
Equation: ................ [Fe(H2O)6]2+(aq)
+ 2OH-(aq) ----->[Fe(H2O)4(OH)2]2+(s)
+ 2H2O(l) ....
Type of reaction: ............................. deprotonation
............................................................. (4)
(e) (i) What further change would you observe if the product
from (d)(i) was allowed to stand in air?
............................... The
colour would slowly change to red-brown ..................................
(ii) What type of reaction is occurring in the change?
...................... Redox
...............................................................................
(iii) Explain briefly, in the light of your answer to
(a), why you might expect this reaction to occur.
............ Iron
(III) is more stable than iron (II) because iron (III) has a stable 3d5
configuration. ...
.....................................................................................................................
(4)
Total 16 marks
(iv) A hydrated form of the acid also exists, H2X.yH2O.
A solution containing 6.30 gdm-3 of the hydrated acid has the
same (molar) concentration as the solution of the anhydrous acid, H2X,
originally used. Using this information and your answer from (b),
calculate the value of y.
Mass of anhydrous H2X
in 1 dm3 of original solution = 2*2.25g = 4.5g
amount of H2X in 1 dm3
of original solution = 4.5g/90gmol-1 = 0.05mol
molar mass of H2X.yH2O
= mass in 1dm3/amount in 1 dm3 = 6.30/0.05 = 126
gmol-1
molar mass of H2X.yH2O
= 90 +18y = 126
so y=2
(8)
(c) The presence of sulphur dioxide in the atmosphere
is the main cause of acid rain. Outline a method which could be used
to estimate quantitatively the concentration of sulphur dioxide in a sample
of air.
...... Pass a known volume of air
through a solution of sodium hydroxide of know concentration until all
of the SO2 is completely absorbed. Titrate the remaining sodium hydroxide
with acid of know concentration. Calculate the amount of acid used,
the amount of NaOH remaining and so the amount of SO2 absorbed. Carbon
dioxide or other acidic gases may interfere with the procedure..........................................
..................................................................................
(5)
Total 14 marks