CAPE Chemistry June 2021 Unit 1 Paper 2 - Past Paper Solutions

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Notes relevant to each question follow the solutions.

Question 1 Notes:

1(e)    It could be that CXC has obtained permission to use past exam questions from         

          different examining bodies. Here is the source of this         

          one: https://capechemistry.blogspot.com/2021/12/source-of-cape-chemistry-june-2021-u1.html

Source Of CAPE Chemistry June 2021 U1 P2 Q1(e)

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The question below was seen before, in a 2005 paper from another examining body.

Its mark scheme follows. The only significant difference is "NaoH" in part (iv) of the CAPE exam vs. "NaOH" in the corresponding part of the original question.

CAPE Chemistry June 2009 U1 P1 - Answers And Explanations

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1. B

2. D

3. A

4. D

5. C

6. D

7. B

8. C

9. C

10. B

11. C

12. D

13. C

14. B

15. D

16. A

17. B

18. C

19. B

20. A

21. D

22. B

23. A

24. C

25. B

26. A

27. D

28. C

29. B

30. B - Detailed solution below.

31. A

32. B

33. D

34. B

35. C

36. A

37. D

38. C

39. D

40. A

41. D

42. A

43. D

44. C

45. B

Question # 30 - Detailed Solution

Hydrogen Emission Spectrum

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Theory for this syllabus section, U1 M1 SS 1.7 is broken up into 6 small, and easily manageable parts. After going through them, you will be shown how to use each part to answer the only three CAPE past paper questions on the topic. They are:

1. 2019 U1 P2 Q1(a) - Link to paper
2. 2009 U1 P2 Q1(a) - Link to paper
3. 1999 U1 P2 Q4(a)

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Part 1

Each element has a characteristic 'fingerprint' line emission spectrum due to its unique arrangement of electrons/electronic configuration. The simplest of these is produced by hydrogen.

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Part 2

• When sufficient energy is supplied to the atoms of hydrogen, an electron is promoted (excited) from a lower energy level to a higher energy level where it becomes unstable.
• The unstable electron will emit the excess energy as radiation and drop back to the lower energy level. Each line in the emission spectrum occurs due to the transitions of electrons moving from a higher energy level to a lower energy level.
• The energy difference between the higher energy level and lower energy level is fixed. The fixed frequencies of each line provide evidence for discrete energy levels.
• Each line represents emitted radiation of a specific wavelength and frequency. Electronic transitions from higher levels to the level, n = 2 give rise to a series of lines known as the Balmer series.

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Part 3

In summary,

• Electrical or thermal energy is passed through and absorbed by a sample of the element.
• Radiation is emitted at certain wavelengths and frequencies.
• The emission spectrum consists of separate lines at the particular frequencies.

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Part 4

Electron transitions occurring between energy levels associated with the hydrogen emission spectrum are shown in the diagram below.

These transitions produce the hydrogen spectrum which follows.

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Part 5

The three main regions of the hydrogen emission spectrum, as related to series of lines are:

Lyman series < 400 nm - ultraviolet region.

Balmer series 400 nm to 700 nm - visible region.

Paschen series > 700 nm - infrared region.

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Part 6

To calculate the energy, E, of a quantum of radiation with a frequency (ν) of 4.57 × 10¹⁴ Hz where h = 4 ×10^-13 kJ s mol^-1, we use the formula E = hν as illustrated next.

E = h × ν

   = 4 ×10^-13 kJ s mol^-1 × 4.57 × 10¹⁴ Hz

   = 182.8 kJ mol^-1

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How To Answer The Past Paper Questions

2019 U1 P2 Q1(a)(i) - Each point in Part 3 is one mark each.

2019 U1 P2 Q1(a)(ii) - Part 5 says it is the Balmer series.

2019 U1 P2 Q1(a)(iii) - First three points and first sentence in 4th point of Part 2 are one mark each.

2019 U1 P2 Q1(a)(iv) - Lines in Balmer series, only, of Part 4 for full marks.

2009 U1 P2 Q1(a)(i) - Answer is found in Part 1.

2009 U1 P2 Q1(a)(ii) - Draw just Balmer part of diagram in Part 5 and label as required.

2009 U1 P2 Q1(a)(iii) - First, second and fourth points in Part 2 will get you full marks.

2009 U1 P2 Q1(a)(iv) - Part 5 says it is in the visible region.

2009 U1 P2 Q1(a)(v) - Part 6 is the solution.

1999 U1 P2 Q4(a)(i) - Same as 2009 U1 P2 Q1(a)(ii)

1999 U1 P2 Q4(a)(ii) - Same as 2019 U1 P2 Q1(a)(ii)

1999 U1 P2 Q4(a)(iii) - All four points in Part 2 for full marks.

Transition Elements

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Exam Tip For 2021

Vanadium is most likely to come in 2021 (last seen 11 years ago) and copper is second most likely (last seen 12 years ago).

Colours and formulae of all species required in CAPE past papers for at least the last 15 years are shown below:

Learn these and you'll be able to handle the following questions:

1. 2019 U1 P2 Q3(d)(iii)
2. 2014 U1 P2 Q3(d)(i)
3. 2010 U1 P2 Q3(a)
4. 2009 U1 P2 Q3(d)
5. 2008 U1 P2 - Resit Q3(b)(i)
6. 2004 U2 P2 - Q2(b)(i)

Past papers from 2005 to 2016 can be found here: Past Paper Booklet

Definitions To Learn

Transition element - A d-block element whose atom has an incomplete d sub-shell or forms at least one stable ion with a partially filled d sub-shell.

Ligand - A molecule, atom or ion that bonds with a central metal atom/ion by donating an electron pair.

These are worth a maximum of 2 marks each and are the answers to:

1. 2019 U1 P2 Q3(d)(i)
2. 2014 U1 P2 Q3(a)
3. 2007 U1 P2 Q6(a)(i)

Difference Between Reactivity Series And Electrochemical Series

Reactivity And Electrochemical Series - Metals

The reactivity series can be used to compare the relative reactivities of different metals. It lists metals in order of general chemical reactivity. Metals generally react by losing electrons to form positive ions. The more readily a metal loses electrons, the more reactive it is – and the greater its strength as a reductant. Metals higher up in the series can reduce the ions of those lower down.

The standard electrode potential of a metal also indicates its strength as a reductant. The more negative the value of the standard electrode potential of a metal, the greater is its strength as a reductant. Hence, you might expect the metal reactivity series and standard electrode potentials to list metals in the same order. However, you must remember that the metal reactivity series is based on observing a range of reactions, such as displacement reactions between solid metals and solid metal oxides. Standard electrode potentials refer specifically to reactions taking place in aqueous solution.

Below is a comparison of the reactivity series with the electrochemical series, which ranks metals according to their standard electrode potentials.

The obvious discrepancy is the relative positions of sodium and calcium. Calcium is a stronger reductant than sodium according to Eo (standard electrode potential) values, but the metal reactivity series suggests that calcium is less reactive than sodium. This discrepancy arises because calcium reacts at a much slower rate, in displacement reactions for example, which in turn happens because two electrons must be removed, not one as for sodium.

Note also that aluminium reacts readily with oxygen in the air, forming a layer of stable aluminium oxide on its surface. This impervious oxide coat often causes aluminium to exhibit lower reactivity than its position in the metal reactivity series indicates.

Group IV Elements - CAPE Chemistry Unit 1

Elements - Structure And Bonding

Main Points To Be Used In Answering A Past Paper Question:

• Down the group there is a change in structure from giant molecular to giant metallic and a change in bonding from covalent to metallic.
• From C to Ge elements exhibit a giant molecular structure.
• Sn and Pb exhibit a giant metallic lattice structure.
• From C to Ge bond length between group IV atoms increases and bond energy/strength decreases.

Elements - Electrical Conductivity

Trend: Going down the group there is a general increase in electrical conductivity.

Reason: Down the group there is a gradual increase in metallic character due to an increase in delocalisation of electrons throughout the structure.

Note: There is an increase in electrical conductivity from C(diamond) to Sn. There is a decrease from Sn to Pb which has never been addressed in the past papers. Hence we use the phrase "general increase" when talking about the trend in electrical conductivity down the group.