February 2014

A Chemistry Olympiad question in Canadian context

Kenneth C. Hoffman, Canadian Chemistry Olympiad Program, Toronto ON

Introduction

“The Universe is made of stories, not of atoms.”
Muriel Rukeyser, The Speed of Darkness (1968)

Our feeling is that most of us who were drawn to careers in chemistry education encountered a great storyteller among our teachers. We further suspect that one foundation of great teaching is great storytelling — or perhaps chemistry education draws in those with a passion for stories? A casualty of increasing constructivism in the chemistry classroom is the “sage on the stage”. This article is not meant as a criticism of modern teaching practice, but as a gentle reminder that the sage sometimes has a very important story to tell. Chemistry is a culture and — as with all cultures — it has a vivid and vital oral history that passes from teacher to student, rather than being communally constructed from prior knowledge.

When one of us (K.C.H.) was first contacted in late 2012 to write an inorganic-themed question for Part C of the 2013 Canadian Chemistry Contest (CCC), a chance arose to tell a good chemistry story. The year 2012 marked the 50th anniversary of the first reported noble gas compound, arguably among the most iconoclastic chemical discoveries, and given its Canadian connection the narrative for a compelling question immediately presented itself.

Canadian context

Neil Bartlett was a professor of chemistry at the University of British Columbia from 1958 until 1966. It was during this time that he synthesized the first compound of a noble gas: xenon hexafluoroplatinate, or XePtF6.1 Undergraduate students at UBC were of course made very aware of the importance of Bartlett’s work. Indeed, it was deemed so significant that in 2006 the American Chemical Society and the Canadian Society for Chemistry declared Bartlett’s discoveries at UBC an International Historic Chemical Landmark.2 A second component of the Canadian xenon story came from Martyn Poliakoff’s Periodic Videos episode featuring a 2011 publication by Canadian chemist Gary Schrobilgen of McMaster University.3 Thus, a storyline of nearly 50 years of xenon chemistry in Canada presented itself. It now remained to cast this narrative in the context of Canadian high school chemistry in a way that allowed students to construct an answer to a problem they had almost certainly never encountered whilst drawing from the content mandated by provincial chemistry curricula and meeting the rigour of the International Chemistry Olympiad curriculum.

The problem

The question outlined was worth 12 marks out of a total of 60 available on Part C of the 2013 CCC, which comprises the final selection examination for the Canadian Chemistry Olympiad.

The question focused on inorganic noble gas chemistry and specifically on Canadian contributions to the chemistry of xenon.

In 1962 Neil Bartlett synthesized the first noble gas compound, XePtF6, at the University of British Columbia. Subsequently, compounds of xenon have proven useful in chemical synthesis: xenon difluoride is used in the preparation of the anti-cancer drug 5-fluorouracil.

  1. State the valence shell electron configuration of xenon.
    (1 mark)
  2. Bartlett’s synthesis yielded a mixture of products (among them XePtF6) which was later reformulated as the salt [XeF][PtF5]. Assuming this compound contains platinum(IV), determine the oxidation state of xenon. (1 mark)
  3. Subsequent to Bartlett’s discovery, Claassen, Selig and Malm reported the synthesis of XeF4. Xenon tetrafluoride undergoes hydrolysis to give the highly reactive solid xenon trioxide.
    1. Applying Lewis structures and VSEPR theory, sketch the molecular geometry of XeO3, indicating all valence electron pairs. (1 mark)
    2. State the name of the molecular geometry for XeO3.
      (1 mark)
    3. Estimate the O−Xe−O bond angles to the nearest degree. (1 mark)
  4. Using the data provided below, calculate the dissociation enthalpy of an Xe−F bond. Xenon tetrafluoride sublimes at 117ºC.

    standard enthalpy of formation, XeF4 = −267 kJ mol-1
    standard enthalpy of sublimation, XeF4 = +61 kJ mol-1

    bond dissociation enthalpy, F2 = +155 kJ mol-1
    (3 marks)

  5. Relative to the lighter noble gases, xenon is approximately 20 times less abundant in the atmosphere than predicted. One theory to explain the missing xenon is that it displaces silicon from quartz (SiO2) under high temperatures and pressures experienced in the Earth’s crust. In 2011, Schrobilgen and Brock reported the first synthesis of XeO2 at McMaster University.
    1. Carbon dioxide sublimes at −78ºC and silicon dioxide melts between 1600 and 1725ºC. Account for this difference by comparing and contrasting the bonding in CO2 and SiO2. (2 marks)
    2. Xenon dioxide is insoluble in water and has a melting point greater than 0ºC. On the Pauling scale, xenon has an electronegativity of 2.6 and oxygen has an electro-negativity of 3.4. Draw a structure for XeO2 that is consistent with all of the information provided. (2 marks)

Answers

    1. 5s24d105p6
    2. Pt = +4, F = -1; (+4) + 6(-1) + (y) = 0; Xe = +2
      1. sketch must be three-dimensional, showing
        all electron pairs and depicting correct
        geometry
      2. trigonal pyramidal
      3. accept any value between 108º and 103º
      1. XeF4(s) → Xe(g) + 2F2(g) ΔH = +267 kJ mol-1
      2. 2F2(g) → 4F(g) ΔH = 2(+155 kJ mol-1)
      3. XeF4(g) → XeF4(s) ΔH = -61 kJ mol-1
      4. 1 + 2 + 3: XeF4(g) → Xe(g) + 4F(g) ΔH = +516 kJ mol-1
      5. ΔH(Xe-F) = ¼(+516 kJ mol-1) = +129 kJ mol-1
      1. must make reference to CO2 as a discrete covalent molecule with weak intermolecular/van der Waals/London forces and SiO2 as a three-dimensional covalent network
      2. must make reference to Xe in a three-dimensional covalent network and Xe surrounded by four oxygen atoms and Xe must be square planar geometry

Discussion

The introduction to the question establishes that the first compound of xenon was discovered at a Canadian university and also the rich synthetic utility of the xenon chemistry that followed. Of particular note to the Canadian chemistry teacher is that this exact milestone in chemistry is outlined in the Atlantic Canada Science Curriculum, Chemistry 11 and Chemistry 124 and appears in the chemistry curricula of New Brunswick, Nova Scotia and Prince Edward Island. This document is based upon the Common Framework of Science Learning Outcomes K-12, which was developed following the Pan-Canadian Protocol for Collaboration on School Curriculum.5 Such inclusion in the curricula addresses the Science, Technology, Society and the Environment (STSE) curriculum expectations,6 which are intended to contextualize science content and learning. The content required for parts (a)-(c) is covered by approximately half of the provincial chemistry curricula and is outlined in the Canadian Chemistry Contest curriculum.7 Of particular note, part (c) is analogous to the archetypal VSEPR question regarding the molecular shape of ammonia. The solid-state x-ray diffraction of xenon trioxide reveals a shape consistent with trigonal pyramidal geometry.8 However, the O-Xe-O bond angle of 103º is inconsistent with VSEPR theory prediction — as with any model, VSEPR has its limitations. Part (b), the reformulation of Bartlett’s compound, tells yet another story: that science provides us with provisional truths that are subject to further testing. Most students of chemistry will be acquainted with Bartlett’s discovery, but few will know that his initial formulation was actually incorrect.

Part (d) requires application of a thermochemical cycle to determine the dissociation enthalpy of a xenon-fluorine bond. Though such a question would not be covered by half of the provincial chemistry curricula, this content is prescribed by the Pan-Canadian Protocol for Collaboration on School Curriculum5 and is included in the Atlantic Canada Science Curriculum, Chemistry 11 and Chemistry 12.4 Furthermore, it appears in both the International Baccalaureate (IB)9 and Advanced Placement (AP) curricula.10 The statement that “xenon tetrafluoride sublimes at 117ºC” is included as context, i.e., some substances do not have a liquid phase under standard conditions, as well as to imply missing information: xenon tetrafluoride exists in the solid state under standard conditions. It remains for the student to recall that elemental fluorine exists in the gaseous state under the same conditions.

To this point the problem has transitioned the student from familiar to increasingly unfamiliar chemistry. This scaffolding of the students’ knowledge prepares them to address part (e), where the question is structured so that prior foundational knowledge may be applied to construct the correct answer. For example, part (e) requires an inference from the data provided that, unlike CO2, silicon dioxide cannot be a discrete molecule. Notably, among all the content covered in this Olympiad question, covalent network solids receive the greatest curriculum coverage in Canada: it is present in all but the British Columbia and Alberta curricula. Part (e-i) has provided the student with structural precedence for the answer to part (e-ii). Xenon dioxide cannot be bent-shaped (AX2E2), as this is inconsistent with its non-polar character. One may reason that the melting point of XeO2 would be several hundreds of degrees; however, it is actually a solid up to approximately 0ºC and decomposes at ambient temperature.3 The student is now faced with the dilemma of the solubility data, though the context of the question places xenon dioxide squarely (pun intended) within the quartz (silicon dioxide) covalent network. Therefore, xenon must be four-coordinate in order to replace silicon, meaning that xenon dioxide exhibits a square planar geometry, locally AX4E2, and analogous to the archetypal square planar VSEPR exemplar XeF4 that was encountered in part (d). Consequently, the student is afforded a precedent for assigning square planar geometry to pure XeO2, as proposed by Schrobilgen.

The question ends where it began — 50 years later — with Canadian chemistry. Schrobilgen’s work is of great significance,3 Bartlett’s work, inarguably, forever changed a paradigm.1,2 While we feel that this question is pedagogically sound in terms of chemistry, it also delivers on the dictates of STSE by situating chemistry in the context of Canada. As devotees of chemistry we know that the Universe is made of atoms, but in order to engage the hearts and minds of all chemistry students it is vital that we recall the words of Rukeyser. We sincerely hope that students are left with the lasting impression that important chemistry has always been undertaken in this country.

Acknowledgements

K.C.H. would like to thank Professor Emeritus Josef Takats at the University of Alberta for vetting the inorganic chemistry in the question and Professor Emeritus Glen Aikenhead at the University of Saskatchewan for the opening quotation and for ongoing discussions regarding pedagogy. Lastly, and most importantly, K.C.H. would like to thank A.P.D. for the invitation to join the Canadian Chemistry Olympiad program.

Literature cited

(online references accessed December 2013)

  1. N. Bartlett, Proceedings of the Chemical Society, page 218, 1962.
  2. American Chemical Society. Neil Bartlett and the Reactive Noble Gases.
  3. D.S. Brock, G.J. Schrobilgen, Journal of American Chemical Society, 133, pages 6265-6269, 2011.
  4. Foundation for the Atlantic Canada Science Curriculum. Atlantic Canada Science Curriculum: Chemistry 11 and Chemistry 12, Implementation Draft, July 2003. https://sapps.ednet.ns.ca/Cart/description.php?II=170&UID=20070502151011.
  5. Council of Ministers of Education, Canada. Common Framework of Science Learning Outcomes K-12. http://204.225.6.243/science/framework/.
  6. E. Pedretti, J. Nazir, Science Education, 95, pages 601-626, 2011.
  7. Chemical Institute of Canada. Provisional Curriculum for the Canadian Chemistry Contest (CCC) for High School and Cégep Students. http://www.cheminst.ca/sites/default/files/pdfs/Outreach/ChemistryContest/Provisional%20Curriculum.pdf.
  8. D.H. Templeton, A. Zalkin, J.D. Forrester, S.M. Williamson, Journal of American Chemical Society, 85, page 817, 1963.
  9. International Baccalaureate Organization. Chemistry Guide.  .
  10. College Board. AP Chemistry Course and Exam Description. http://media.collegeboard.com/digitalServices/pdf/ap/IN120085263_ChemistryCED_Effective_Fall_2013_lkd.pdf.

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