Download Year 9 Science revison _15-16_ end of year CHEM

Year 9 Science
Chemistry
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end of year revision
Answers
1] Here is the periodic table box for the metal element Rubidium
a) what is the atomic number of rubidium ?
37
b)
i) How protons, neutrons and electrons does an atom of Rubidium have ?
p = 37, e = 37, n = (85.468 – 37 = ) 48 or 49
The decimal places are due to average atomic mass, so either answer is ok for neutrons.
ii) why do the electrons in atoms of rubidium stay in orbit around the nucleus …and not
just shoot off into space ?
electrons are negatively charged
they are strongly attracted to the positive charge (of the protons) in the
nucleus
c) There are 2 common isotopes of rubidium : Rb-85 (72.2%) and Rb-87 (27.8%).
i) How many neutrons does the Rb-87 isotope have ?
87 is the atomic mass. The atomic number is 37 for all isotopes of rubidium.
So
87 – 37 = 50 neutrons
ii) Re-calcuate the average atomic mass of Rubidium, given the information about the
2 isotopes in part c.
Average atomic mass = (atomic mass isotope 1 x %) + (atomic mass isotope 2 x %)
100
Average atomic mass = (85 x 72.2%) + (87 x 27.8%)
100
=
85.56
d) Rubidium is in the same group of the periodic table as lithium and sodium.
How many electrons does it have in its outer shell ?
1
(all group 1 elements have 1 electron in their outer shell
great rule to learn …saves time !)
e) Rubidium reacts with sulphuric acid to form a compound RbSO4
What is the relative formula mass of this compound ? (you can use a periodic table here)
Use the “big number” in each periodic table box (the atomic mass)
1 x Rb = 1 x 85.468 = 85.468
1 x S = 1 x 32 =
32
4 x O = 4 x 16 =
64
181.468
(there might be slight variations depending on which periodic table that you used)
f) Rubidium is a soft metal element with a melting point of 39.4oc. It’s electrical conductivity is
8.3. It does not dissolve in water. Rubidium forms alloys with metals like gold.
i) what is an alloy ?
2 or more metals mixed together, to gain the best properties of each and
form a more useful metal alloy.
ii) would the Rb-Au alloy have the same melting point as the element rubidium or a
different one ?
Different.
The melting point (a physical property) of a pure substance (eg/ Rubidium) is always different
to the melting point of an impure substance (a mixture in this case).
Hard to say whether it’s higher or lower without testing ! The chemical properties remain the
same.
iii) Is the Rb-Au alloy likely to have a higher or lower electrical conductivity than the
element Rubidium ?
Higher.
Gold is a very good conductor of electricity (the plugs/connectors on expensive music
equipment are gold plated). This should give the mixture of the 2 metals a higher conductivity
than that of just Rubidium. Again, electrical conductivity is a physical property ….can be
changed by alloying.
iv) The compound RbSO4 dissolves in water, but the element Rb doesn’t. Why ?
The compound is made up of ions bonded together. When it dissolves in water, these
ions separate from each other. They are charged (+ and -) and able to attract to
and bond with water. This is why the ionic compound CAN dissolve. The metal element
is uncharged and is not attracted to water. So, it doesn’t dissolve !
g) What would the ionic lattice of solid rubidium sulphate look like ?
(a labelled diagram is a good idea here)
>Labelled ions
>Alternating
>In a roughly 3D cube
……………..something like this with Rb+ ions labelled (where the + ions are on the diagram)
and SO42-/sulphate ions labelled as - . Ions should be close/touching as it’s a solid.
h) Rubidium loses 1 electron when it forms a rubidium ion.
i) why does it lose 1 electron ?
it has 1 electron in the outer shell. It loses that electron (in a chemical reaction) to have a full
outer shell and become “stable” like the noble gases.
You know that it loses 1 electron…..because it’s in group 1.
ii) what is the charge on a rubidium ion ?
1+
It had 37 protons and 37 electrons as an element.
Lost 1 electron.
37+
361+
2] Distillation
a) what would the temperature be on the thermometer ?
100oc - water is boiling off
b) What would be left behind in the round-bottomed flask after carrying out this experiment ?
salt (solid)
c) Does salt water conduct electricity ? Why ?
Yes – the charged ions from the dissolved salt (Na+ and Cl-) are able to carry charge
through the solution. Pure water doesn’t conduct electricity.
3] Thin layer chromatography
A
B
a) Which spot/chemical travelled furthest up the TLC plate – A or B ?
A
b) Which chemical (A or B) was most attracted to the stationary phase (the TLC plate) ?
B – it came out of solution (separated from the solvent) and was left on the TLC plate lower
down/before A.
c) If the solvent front travels 10cm up the plate and spot A travels 8.5cm, calculate the Rf
value for spot A ?
Rf = distance travelled by spot
=
Distance travelled by solvent
8.5
10
= 0.85