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Lead is an element in the Periodic Table of elements with the symbol Pb (meaning plumbum from Latin). It has an atomic number of 82 meaning that there are 82 electrons in its energy shells, and a standard atomic weight of 207.2. Lead is categorized as a post-transitional metal because it is found after the block of transition metals in the Periodic Table. It has four naturally occurring isotopes: lead-204, -206, -207 and -208 but only one allotrope exists. A soft, malleable metal, it is a poor conductor of electricity (Watt, 15).
Lead is a bright, silvery metal but tarnishes upon contact with air, forming a complex mixture of compounds most of them being oxides of lead. The characteristics of lead are changes significantly upon addition of trace amounts of other metals such as copper, antimony and bismuth improve its hardness and metal fatigue, hence their usage in industrial situations. It is found in the carbon group of elements and is counted as one of the heavy metals.
Having an electronic configuration of [Xe] 4f145d106s26p2 lead looses two of its outermost electrons to gain an oxidation state of Pb2+ although it is not strange to gain an oxidation state of Pb4+. It reacts with a wide range of elements to form a myriad of compounds (Watt, 28). Its reaction with oxygen gives rise to three oxides: lead (II) oxide, lead tetraoxide and lead (IV) oxide.
The monoxide reacts with acids to give rise to salts; with alkalis to form plumbites. The monoxide reacts with oxygen to form trilead tetraoxide that reverts to the monoxide at very high temperatures.
The dioxide is prepared from the halogenation of lead (II) salts. It is black-brown in color and a powerful oxidizing agent. It does not react with alkaline solutions but with solid alkalis to form hydroxyplumbates, or with basic oxides to form plumbates.
Occurrence of lead in nature is usually found with other metals like copper, the main mineral being galena. Extraction processes are applied to obtain the metal from the ore that employ the electrolytic purification process. Lead has been use since antiquity due to its ease of extraction and malleabity. The most widespread use of lead in pre-Industrial times was in the Roman economy. In alchemy, lead was considered the oldest metal and associated with the planet Saturn. One of the goals in alchemy was to find a way to convert lead to gold (Watt, 54).
Lead has a wide range of uses from building construction, bullets and shots, solders, fusible alloys to pewters. This is due to its malleability and light weight-to-volume ratio and low cost as compared to other heavy metals. The most widespread use is as an electrode in lead-acid batteries used in cars. Some of its older uses have been discontinued to the health hazards posed by lead poisoning such as neurological damage leading to seizures, coma and possibly death.
Marie Curie was a Polish-French chemist, physicist, and Nobel Laureate in Physics and Chemistry in 1903 and 1906 respectively. The achievements included her pioneering work in radioactivity, and the discovery of two elements polonium and radium (McClafferty, 18). She was the first female professor in the University of Paris where together with her husband; they carried out research in radiology. They isolated the two elements, polonium and radium, from pitchblende. During World War I, she saw the requirement of field radiological centers to assist battlefield surgeons. This led her to procure the first mobile radiological clinics.
Her work with radium enabled to come up with radon to be used as a sterilizer for infected tissue. In later years, Marie worked to facilitate the establishment of the Radium Institute in Paris and another in Warsaw, Poland. She was the only woman to be awarded two Nobel Prizes so far and the only person awarded in scientific fields. Her life’s work has served to document the uses of radiation and the developments made in the various fields are due to her discoveries. Through her own experiences, better and safer handling of radioactive elements was put in place. She continues to inspire research work and her contribution to the feminist movement as far as academic discrimination is concerned (McClafferty, 49)
Nuclear Energy and Radioactivity
Nuclear power is increasingly been used by a number of countries to provide electricity. Nuclear energy is a cleaner and more sustainable source of electricity as compared to hydroelectric power currently in widespread use. In the United States, nuclear power contributes approximately 20% to the national power grid. The European Union is another region that has been quick to take up this as a source of electricity. Future projections show an increasing trend.
Nuclear power is also used to propel military and civilian ships. There is increasing use in space for propulsion applications. The high energy density offered by nuclear reactors give them the advantage over chemical reactions that power rockets. Nuclear energy is far more reliable compared to solar energy previously used to power space exploration (Bodansky, 58). Due to increasing safety concerns, improvements are being made to the plants and processes, and the use of nuclear fusion instead of fission as a safer option.
Radioactivity and radioactive substances are used in medicine to diagnose, treat disease and research. X-rays are the most common imaging technique used to view general anatomy of a patient to form a conclusive diagnosis. Without such techniques, doctors would be blind to the happenings of the human bodies. Radioactive isotopes of some substances are injected to monitor the excretion of that substance from the body. This forms a conclusive diagnosis of a patient giving a clear picture of the body’s processes.
The use of ionizing radiation in cancer treatment has produced a cure for a previously untreatable disease. Modern communication systems use electromagnetic radiation and radio waves to communicate cheaply and effectively over long distances as audio or visual information. The process of radiocarbon dating is employed by archeologists and scientists to determine the age of substances.
Acids, Bases and Buffers
An acid is a substance that reacts with a base to produce a neutral solution. Any substance that can act as a proton donor is classified as an acid. Another definition asserts that substances that liberate hydronium ions in solution can be considered acids. A third concept focuses on the species that accepts a pair of electrons.
An example of a common acid is hydrochloric acid. The most common application is in industrial processes that include the removal of rust from steel, production of organic compounds such as vinyl chloride, production of inorganic compounds like iron (III) chloride, control of pH and neutralization and the regeneration of ion exchange resins.
A base is any substance that accepts hydrogen ions, can donate a pair of electrons or on that liberates hydroxide ions in solution. Strong bases react with acids to produce a neutral salt and water. It is the principle base in industrial processes. Its uses vary from aluminum production using the Bayer process, an additive in drilling mud, removal of sulfurous compounds in crude oil, making paper to digestion of tissue (Holmquist et al., 23). Other uses include esterification agent in soap manufacture, an industrial cleaning agent and in food preparation.
A buffer is a solution of a weak acid and its conjugate base or weak base and its conjugate acid. They are used to keep a constant pH in a state of fluctuations. Examples of buffer solutions include carbonic acid, bicarbonate and phosphate. These three buffers are common in organic settings of many organisms that require enzymes, as enzymes are sensitive to fluctuations in pH changes.
Oxidation and Reduction
Oxidation is the increase of oxidation state of the loss of electrons, while reduction is the gain of electrons or increase in oxidation number of a substance. Oxidizing agents react in a chemical reaction by accepting electrons while reducing agents react by donating electrons. Redox reactions are those that have both processes taking place simultaneously (Roberts et al., 45).
Fe + CuSo4 = FeSo4 + Cu
The ionic equation is:
Fe + Cu2+ = Fe2+ +Cu
This reaction is an example of the substitution reactions that take place during metal extraction processes in mining.
Many biological processes also take the model of redox reactions, for instance in respiration at the cellular level, glucose is oxidized to carbon dioxide, as oxygen is reduced to water.
C6H12O6 = 6 CO2 + 6 H2O
Bodansky, David. Nuclear Energy: Principles, Practices, and Prospects. New York: Springer, 2004. Print.
Holmquist, Dan D, Jack Randall, and Donald L. Volz. Chemistry with Calculators: Chemistry Experiments Using Vernier Sensors. Beaverton, OR: Vernier Software & Technology, 2006. Print.
McClafferty, Carla K. Something Out of Nothing: Marie Curie and Radium. New York: Farrar Straus Giroux, 2006. Print.
Roberts, Stanley M, and Geraldine Poignant. Hydrolysis, Oxidation and Reduction. Chichester, West Sussex, England: Wiley, 2002. Internet resource.
Watt, Susan. Lead. New York: Benchmark Books, 2002. Print.
contains four sections, SECTION A to SECTION D. Instructions: Answer all questions in SECTION A – SECTION D. Make sure that the section heading is included and your answers are correctly numbered. The assignment must have a completed cover sheet. It must be placed in the drop- box on or before the deadline. st SECTION A ELECTRONIC STRUCTURE & IONIZATION ENERGY 2. Write the electronic structure in s,p,d notation of the following: O, Na, Na+, A1, Cl- and co (Total 6 Marks) Write the electronic configuration in box notation of the following: N, Si and Ni (Total 3 Marks) 3.
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Write the electronic configuration in box notation of chromium and copper. Suggest reasons for the apparently anomalous arrangement of electrons in their atoms. (Total 4 Marks) 4. The following table shows the first three ionization energies (in kJ mol-l) of elements Element c OHM 383 425 502 527 OHi2 2437 2667 3065 4568 7314 OHi3 3376 3881 4438 6929 11820 In which group of the Periodic Table should the elements be placed? Give a reason for your answer. Which of the elements has the largest atomic number? Give a reason for your answer. Turn to page 2 for SECTION A – Question 5. Marks) (Total 4 Marks) 5. kJ mol-l. 740 418 577 2400 1 500 1757 3069 1816 3700 7700 14850 4439 2745 OHi4 25000 10500 21000 5876 11575 In which group of the Periodic Table should each element be placed? (5 Marks) How much energy is needed to convert one mole of gaseous atoms of element (2 Marks) C into 1 mole of dipositive ions? (Total 7 Marks) 6. Calculate number of moles of: (a) HCI in 25 cm3 of 0. 10 mol dm-3 solution (b) H2S04 in 32 cm3 of 0. 50 mol dm-3 solution (Total 2 Marks) 7. Calculate the volume of (a) 0. 02 mol dm-3 HCI solution containing 1 X 10-3 moles of HCI (b) 0. mol dm-3 H2S04 solution containing 2 X 10-3 moles of H2S04 (Total 2 Marks) 8. Calculate the molar concentrations (in mol dm-3) of the following solutions that contain: (a) 2. 2 x 10-3 moles AgN03 in 37 cm3 (b) 7 x 10-3 moles sac12 20 crn3 9. 23 cm3 of 1. 5 mol dm-3 H2S04 reacts completely with 40. 5 cm3 of a given KOH solution. 2KOH + H2S04 0 K2S04 + 2H20 What is the molar concentration of the KOH solution? Turn to page 3 for SECTION A – Question 10. 2 10. 27. 823g of Na2C03. xH20 crystals were dissolved in water and made up to 1000 cm3 of solution. cm3 of this solution required 48. 8 cm3 of 0. 1 mol dm-3 HCI for complete neutralisation. Find the value of x in Na2C03. xH20 using the following steps: 2HCl + Na2C03 0 2NaCl + C02 + H20 (f) 11. calculate the number of moles of Na2C03 in 25 cm3 calculate the number of moles of Na2C03 in 1000 cm3 calculate the mass of Na2C03 in 1000 cm3 calculate the mass of water of crystallization associated with this mass of Na2C03 calculate the moles of water of crystallization associated with this mass of Na2C03 calculate the value of x in Na2C03. xH20 (Total 8 Marks) . 0g of lawn sand (a mixture of sand and ammonium sulphate) was weighed into a conical flask, and 25 cm3 of 2. 0 mol dm-3 sodium hydroxide solution was pipetted into the same flask. The conical flask was boiled for 20 minutes, after which time all the ammonia had been driven off, because: (NH4) 2S04 (s) + 2NaOH (aq) 2NH3 (g) + Na2S04 (aq) + 2H20 (1) The residue in the flask was cooled and filtered to remove the sand. The filtrate containing unreacted NaOH was made up to 250 cm3 in a volumetric flask. 25 cm3 samples of this solution were titrated against 0. ol dm-3 hydrochloric acid using bromothymol blue as an indicator. HCI + NaOH O Naci + H20 3 The mean titre was 20. 0 cm . Calculate the percentage of ammonium sulphate by mass in the lawn sand. (Total 8 BONDING & ENERGY 12. (a) Using the Valence shell electron pair repulsion theory state and explain the shape of GaC13. Sketch a diagram to show the arrangement of atoms in space, labelling the bond angles. Draw a dot & cross diagram to show the bonding in hydrogen sulphide, H2S. State and explain the shape of H2S using the ‘valence shell electron pair repulsion theory’, stimating the bond angle. i) Draw a dot & cross diagram to show the bonding in methanal, HCHO. State and explain the shape of HCHO using the ‘electron pair repulsion theory’, estimating the (H-C-H) bond angle. (it) Methanal (HCHO) is a gas at room temperature whereas methanol (CH30H) is a liquid. Suggest an explanation for this. (4 Marks) (3 Marks) (Total 14 Marks) 13. Predict the shapes of phosphine, PH3 sulphur trioxide, S03 (iii) the sulphite ion, S032(iv) the amide ion, NH2(v) the tetrahydroborate ion, BH4(Total 5 Marks) 14.
A coffee-cup calorimeter contains 55. cm3 of a dilute solution of copper(ll) sulfate at a temperature of 22. 8 oc. A small amount of zinc powder also at 22. 8 oc is added to the solution. Copper metal is formed, and the temperature of the solution rises to 32. 3 oc. The copper is collected, dried and weighted, when it is found to have a mass of 0. 324 g. Calculate the total amount of energy released in this reaction, ignoring the heat capacity of the zinc and the calorimeter (Take the specific heat capacity of the solution as 4. J g-1 K-1). Calculate the enthalpy change for this reaction per mole of the copper formed. 15. Use the values for average bond enthalpies (E) from the table below to calculate the enthalpy changes in each of the reactions: (a) and (b) Bond c-c E/kJ mol-1 346 c=c 611 412 c=o 743 339 H-CI 431 497 CH4 (g) + 202(g) O C02(g) + 2H20(g) CH2 = CH2(g) + HCl(g) O CH3CH2Cl(g) How would your answer to (a) compare to the data book value for the (2 Marks) standard enthalpy of combustion of methane? Explain your answer. Total 8 Marks) 16. (a) Draw a diagram of the energy distribution of gas particles in a system at one temperature Tl . On the same diagram, show the shape the distribution at (3 Marks) ome higher temperature T2. Relate the two curves in (a) to the change in the rate of a gas phase reaction (2 with increased temperature. Draw a labelled energy profile showing the energy changes during an endothermic reaction. Use this and the diagram drawn in (a) to explain how (4 Marks) catalysts increase the rate of reactions. Total 9 Marks) Turn to page 5 for SECTION A – Question 17. 4 17. enthalpy of formation of ethane, OHf [C2H6]. 2c(s) + 3H2(g) -+ C2H6 (g) OHc carbon OHc hydrogen OHc ethane -394 kJ mol-l -286 kJ mol-l -1560 kJ mol-l Use the values for average bond enthalpies (E) from the table below along with the standard enthalpy of atomisation of carbon to calculate the standard enthalpy of formation of ethane, OHf [C2H6] using the equation given in (a). Structural formula of ethane: -1 346 E/kJ mol = 717 kJ mol-l 18.
Comment briefly on the discrepancy between the two calculated values for the standard enthalpy of formation of ethane in (a) and (b). Stating, with a reason, (3 which of the two values is likely to be more accurate. (Total 10 Marks) Given the following data, construct a Hess’s Law cycle and calculate the standard nthalpy of hydration of ethene C2H4(g) + H20(l) -+ C2H50H (l) ethene ethanol OHf ethene = +52 kJ mol-l OHf water OHf ethanol -278 kJ mol-l 19.
The standard molar enthalpy of combustion of propanoic acid is -1527. 2 kJ mol-l . Given that the standard molar enthalpy change of formation of water is -285. 5 kJ mol-l and that of caron dioxide is -393. 5 kJ mol-l, construct a Hess’s Law cycle and calculate the standard molar enthalpy change of formation of propanoic acid. TOTAL FOR SECTION A 5 108 Marks Part A RATES OF REACTION Two gases react according to the equation: X (g) + 2Y(g) XY2(g) Experiments were done at 7000C to determine the rate equation.
The following results were obtained: Experiment Initial [X] Initial [Y] Initial rate of formation ofXY2 number [mol dm-3 [mol dm-3 [mol dm-3 s-l 0. 1 1 x 10-4 0. 2 4 x 10-4 State with reasons the order with respect to X State with reasons the order with respect to Y Write the rate equation for the reaction (1 Mark) Using the results from experiment 1 , calculate the value of the rate constant for (2 the reaction and state its units. Hydrogen and nitrogen oxide react according to the equation: 2H2 (g)