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To conjecture the structure and bonding of eight unknown solids by analysis of experimentally determined properties.

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Introduction

IB Chemistry - Practical Report Name: Chris Bolton Partner: Jamie Gearing Date(s) Conducted: 31/03/04; 02/04/04; 23/04/04 Topic: Determination of Structure and Bonding Aim: To conjecture the structure and bonding of eight unknown solids by analysis of experimentally determined properties. Apparatus: GENERAL: 8 unknown solids (labeled alphabetically A to H), test-tube rack SECTION A: Multiple 100ml beakers as required, distilled water (H2O), cyclohexane (C6H12), glass mixing rod, spatula, plastic pipette, multiple 5ml test tubes as required SECTION B: "Section A" equipment, graphite rods with bung, crocodile clips (x2), ammeter, power pack SECTION C: Hammer (800g), wooden impact board SECTION D: 200ml beaker, retort stand, boss and clamp, rubber band, heatproof mats, Bunsen burner, tripod and wire-ceramic gauze, thermometer (300?C capacity, ?error? ? 0.20?C), paraffin oil, boiling tubes (x4), mortar and pestle SAFETY: Lab coat, safety glasses and plastic gloves to be worn at all times Methodology: Our methods were task specific, and are detailed separately in each section. The results of each method used will assist in the determination of the structure and bonding of each substance by analysis of exhibited properties. The experiment is documented and processed in three parts; the first pertaining to initial observations; the second to the process' and results of each section of experiment; and the third to the interpretation of unified results from part two. Part One: The Unknowns A brief description of each of the unknowns is necessary a) to establish their initial state, and b) for the purpose of later reference. Unknown Substance (A-H) Initial Observations A Fine translucent white crystalline powder B Reflective metallic substance; bluish white - possibly due to oxidation; fairly hard but still bendable/malleable; light C Course white crystalline powder/small spheres; translucent (almost opaque) D Shiny white flakes; almost transparent E Blue-black/silvery in appearance; slightly shiny flakes of varying size F Soft white clumping/clustering powder G Fine crystalline powder; white with slight brown tinge H Fine black powder; finely but slightly reflective or metallic lustre Part Two: The Experiments SECTION A: Determining ...read more.

Middle

in an active fume cupboard (particularly with respect to observing the demonstration of E * Avoid inhalation of any fumes * Ensure that apparatus is cooled before it is packed away * Ensure that a match is ready to ignite the Bunsen immediately upon the mains being switched on, and that the Bunsen is on safety when first lit * Do not discard spent boiling tubes haphazardly, rather dispose of them either in a glass bin or through a lab technician * Do NOT empty paraffin oil down the sink RESULTS: A Table to Show the Melting Points of Each Unknown Substance Unknown Substance (A-H) Melting Point (?C) Uncertainty (?C) Tested During Experiment Observations A 801 Unknown No N/A B 419 Unknown No N/A C 130 ? 0.2 Yes Volume in tube increased, bubbling, eventually because almost clear and transparent D 66 ? 0.2 Yes Volume in tube increased, bubbling, eventually because almost clear and transparent E 113.5 [sublimed] Unknown Yes* Sublimed from solid crystals; released noxious purple gas (effervesced off crystals) F 175 ? 0.2 Yes Volume in tube increased, bubbling, eventually because almost clear and transparent G 182 ? 0.2 Yes Volume in tube increased, bubbling, eventually because almost clear and transparent H 3367 Unknown No N/A *Demonstrated by teacher, Ms Tachas Summary of results: A, B and H displayed substantially higher melting points that the other substances, with D showing the lowest and H the highest. Part Three: Interpreting and Analysing Collated Results - the Conjecture Intention "To summarise the determined properties of each unknown solid; to analyse these properties and conjecture the structure and bonding present in each substance." UNKNOWN SOLID A: * Soluble in polar solvent (H2O) - Substance is polar or ionic itself due to the fact that polar solutes are generally only soluble in polar solvents (as a result of ion-dipole interactions) * Conductive when in solution - When in solid state either the electrons or charged ions are not free moving * Solid shatters when impacted - Rigid internal structure often associated with a lattice * High (801?C) ...read more.

Conclusion

To determine the type of covalent lattice structure, the fact that H is conductive in a solid state must be taken into account. In a covalent network lattice, all the electrons are occupied and held in place by the strong three dimensional covalent bonds between the atoms, and as such are unable to conduct electricity. Covalent layer lattices, however, have strong bonds only in two dimensions, with intermediate gaps between each two-dimensional layer in which delocalised electrons are free to move and conduct electricity. Each layer is held together only by dispersion forces between each layer. This layer lattice structure would account for the conductivity of substance H, while also allowing it to have such a high melting temperature (due to remaining strong bonds in three dimensions) and to be practically insoluble. So H is: "a covalent molecular layer structure of an infinite array with strong covalent bonds in two dimensions and weak van der Waals' forces in one, with clouds of free moving delocalised electrons moving between each layer of covalent bonds." Conclusion: It was a fairly simple matter, once certain properties of each substance had been discovered by the experimentation process' described above, to determine the structure and bonding of each solid through critical analysis based on an understanding of the nature of various forms of bonding that can occur. Christopher Bolton A ionic 3d network UNKNOWN SOLID B: B metallic bonding UNKNOWN SOLID C: C ionic --> network lattice? UNKNOWN SOLID D: D VDW covalent molecular one --> non-polar UNKNOWN SOLID E: E VDW covalent molecular one --> non-polar UNKNOWN SOLID F: F ionic --> network UNKNOWN SOLID G: G covalent with hydrogen bonding and/or dipole-dipole UNKNOWN SOLID H: H covalent layer structure strong electrostatic bonds in 2D weak (VDW) in 1D A ionic 3d network B metallic bonding C ionic --> network lattice? D VDW covalent molecular one --> non-polar E VDW covalent molecular one --> non-polar ZINC F ionic --> network G covalent with hydrogen bonding and/or dipole-dipole PROBABLY SUGAR H covalent layer structure strong electrostatic bonds in 2D weak (VDW) in 1D GRAPHITE ...read more.

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