2010 BT Examiner's report

Topics: Atom, Ammonia, Covalent bond Pages: 28 (5146 words) Published: February 4, 2014
H2 Chemistry 9647 Term 3 Block Test 2010 – Examiner’s report

H2 CHEMISTRY 9647
Paper 2 (Section A)
Structured Questions

Question 1 – Atomic Structure and Redox Titration
Most elements exist in nature as mixture of isotopes. At present, there are about 1600 known isotopes, of which only one-sixth are stable. The unstable isotopes lose energy by emitting ionising particles or radiation through the process of radioactive decay. Ernest Rutherford and later Paul Villard discovered the three types of radiation: -radiation (nucleus of helium atom); -radiation (electron); and -radiation. (a) (i) Define the term isotope.

Answer:
Isotopes are atoms having the same number of protons / proton number / atomic number / of the same element but have different number of neutrons / different mass numbers / different number of nucleons

Comments:
Although this is a simple recall question, less than half of the students gave the correct definition. This is unacceptable, and students must make a more serious attempt to memorise (with understanding) all the definitions for each topic.

Many students left out the word “atoms” which was crucial to the definition, and resulted in the loss of the mark.
(ii) A stream of -particles, -particles and -rays is subjected to an electric field as shown below. The path taken by the -rays has already been drawn. Sketch on the diagram, how the -particles and -particles are affected by the electric field, given that they are travelling at the same speed. Label your answers clearly.

+
-particles

source

-rays



-particles

Comments:
This question is only slightly modified from the tutorial question, and it required students to realise that -particles (helium nucleus) are heavier and positively charged, while -particles (electrons) are lighter and negatively charged. Therefore, it is a pity that many students did not manage to score the fully two marks for the question.

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H2 Chemistry 9647 Term 3 Block Test 2010 – Examiner’s report

(b) Some radioactive nuclides can have different paths of decay. For example, approximately 36 % of bismuth-212 will undergo decay to form thallium-208, and 64 % of bismuth-212 will undergo decay to form polonium-212. Both products will decay subsequently to the stable lead-208.

The two paths of decay are summarised in the decay scheme below: 212
83 Bi

36 %

l +

81 T

radiation X

64 %
212
84 Po

+

208
82

Pb

radiation Y
(i) State the number of neutrons present in the nuclei of bismuth-212 and thallium-208. Answer:
bismuth-212: 129, thallium-208:127
Comments:
Some students did not attempt this part presumably because they did not read the question carefully, or did not understand the question as a whole. Students are advised to read the question carefully, and they would have realised that this part simply required them to take the difference between the mass number (given in the question) and the atomic number (can be found from the periodic table) of the isotope.

(ii) From the information given above, deduce the identity of radiation X and Y. Briefly explain how you arrive at one of your answer.
Answer:
radiation X: -particles / nuclei of He atoms,
radiation Y: -particles / electrons
Conversion of Bi to Tl: mass number decrease by 4 units, proton number decrease by 2 units which corresponds to the loss of helium nuclei (-particles) Or
Conversion of Bi to Po: proton number increase by 1 units which corresponds to the loss of electron (-particles) − a neutron converted into a proton and an electron Comments:
This question required students to interpret the information given in the question, and apply the concepts learned in an unfamiliar context (which is to say that although the working is very simple, it required deeper understanding of the underlying concepts). It was expected that many students would have problems answering the question.

Some students seemed to have the...
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