GEN CHEM & QUAL ANALY
GEN CHEM & QUAL ANALY CHM 2046
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This 5 page Class Notes was uploaded by Brigitte Wyman on Friday September 18, 2015. The Class Notes belongs to CHM 2046 at University of Florida taught by George Christou in Fall. Since its upload, it has received 11 views. For similar materials see /class/207005/chm-2046-university-of-florida in Chemistry at University of Florida.
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Date Created: 09/18/15
Chapter 21 Properties of the Elements We will also add the metals from Chapter 20 First let39s look at Periodic Table and Group numbering 212 Hydrgen The simplest atom 90 o of all atoms in the universe 1H isotope protium proton surrounded by e39 2H deuterium or 3H tritium contain 1 or 2 neutrons as well Deuterium 2H was produced in the Big Bang too fragile to survive fusion conditions in the stars which produce the lighter elements or supernovas which produce heavier elements Stable molecular form is H2 ie HH Hydrogen is the exception in the periodic table cannot be satisfactorily classified in any group similarities to certain metals nonmetals groups 1 and 17 respectively Brief Summary of Hydrogen Chemistry almost always forms covalent compounds ionic ones rare high ionization energy e39 close to nucleus without other e39s to shield it and low electronegativity only one proton to attract e39s unlike groups 1 and 17 in that H and H39 ions are rare because they usually bond covalently to other things instead of being found as ions 69 H30 OH39 NH4 etc Very rare exception is the hydride ion Hquot Ionic Hydrides H39 Result from H2 g being reduced to H39 ions rather than oxidized to H It is y difficult to reduce H2 g H2 g 2 equot gt 2 Hquot g E 223 V but very strong reducing agents Na Ca Li etc with halfcell potentials below 223 V can do it to give ionic salts containing H39 called the hydride ion 2 Na 5 H2 g gt 2 NaH 5 ionic salt The hydride ion is m reacTive and will eiTher 1 find a H and go To H2 g NaH 5 H20 I a N6 99 OHquot 99 H2 9 or 2 reduce someThing and go To H2 g TiC4 I 4 LiH s gt Ti 5 4 Lid 5 2 H2 g CovalenT Hydrgen Compounds Common and sTable39 CH4 NHg H20 and HF eTc eTc The oTher elemenT has higher elecTronegaTiviTy Than H 22 we Think of These as conTaining H 1 oxidaTion sTaTe ie H and eg N3 O2 F 1 N2 9 3 H2 9 e 2 NH3 9 AG lt 0 buT slow due To sTrong NEN run aT high T 400 C and pressure 250 aTm wiTh Fe caTalysT To speed up The HABERBOSCH process Main source of NH3 for ferTilizers and chemical indusTry MeTallic InTersTiTial Hydrides H2 molecules and H aToms can occupy space in beTween The aToms of a meTal In parTicular palladium Pd has a high affiniTy and can hold vasT amounTs Pd 3 935 Times iTs volume PdHo5 BesT ThoughT of as a soluTion of gas in The meTal PdH2 used To puriTy H2 from mixTures Group 1 ALKALI METALS n51 Found as M in minerals loss of ns1 elecTron Too reacTive sTrong reducing agenTs To be found as free meTal Prepared by elecTrolysis of melTed salTs All wellsTudied excepT Francium radioacTive longesT lived isoTope is 283Fr halflife of only 218 mins ProperTies SofT silvery meTals aT 20 C AH reacT vigorously wiTh waTer To give H2 g reacTiviTy increases down group M s H20 l gt H2 g MOH aq See hTTpvideogooglecomvideoplaydocid2134266654801392897 and hTTpwwwyouTubecomwaTchvsNdi iknRfoampNR1 hafreac ons39 M s gt M aq e39 H20 I equot gt H2 9 0Hquot aq overal M s H20 I gt H2 g M aq OH39 aq A react with 02 gas but groducfs39 degend 07 mefa Li gives LigO contains 0239 ie normal oxide ion Na gives NaZOZ contains 02239 00239 ie peroxide ion K Rb Cs give M02 contains 0239 0039 ie superoxide ion can rationalize with chargetosize ratios of ions see below All Ms must be stored under inert oils to prevent reactions with air and water M Burning in dry air rather than pure 02 gas also gives above products but Li and only Li will also react with N2 g to give the nitride ngN containing Li and N31 6 Li s N2 g gt 2 Ll3N PERIODIC TRENDS ACROSS THE PERIODIC TABLE I s39 g mnectraelllclrclharacter A j ionic oxides A 4 1 nonmetallic character L covalent oxides A L acidity of oxides L basicity of oxides quot l electronegativity A L electropositivity j Note These trends are also affected by changes in oxidation state see later Example Choose The mosT meTallic elemenT a Li or Be b Be or Mg c Al or K Which will be The mosT acidic oxide ie dissolve in waTer To a N020 or CZo7 b 300 or A5205 give acidic soluTions Which is The sTronger reducing agenT a Al or Mg b Na or K c Br39 or Cl39 Which is The sTronger oxidizing agenT a S or Se b Bra or Clz c 02 or F2 Summary The more meTallic an elemenT The more basic is iTs oxide The more ionic is iTs oxide and The more elecTroposiTive less elecTronegaTive iT is ImporTanT The difference beTween Li and The resT eg gives oxide wiTh OZ reacTs wiTh N2 eTc is due To small size of Li This gives a high chargeTosize raTio This also causes Li compounds To have significanT covalenT characTer ie LiCl is sTill fairly ionic buT has noTiceable covalenT characTer whereas NaCl is very ionic Li has Too high a chargeTosize raTio charge densiTy To be happy as compleTely free Li ion so iT shares elecTrons a biT wiTh anions ie some covalency NoTe 1 The decreasing chargeTosize raTio down The group ie Li gt Na gt K gt Rb gt 65 explains The producTs on reacTion wiTh 02 The highesT charge densiTy highesT chargeTosize raTio is Li and This forms The highesT charge densiTy 0239 salT whereas down The group The meTals form The 02239 peroxide and Then The 0239 superoxide salTs ie decreasing charge densiTy ions In oTher words high charge densiTy Li favors forming a salT wiTh The high charge densiTy 0239 buT down The group progressively lower charge densiTy M favors formaTion of progressively lower charge densiTy anions Note 2 Group 1 metals have very high 2nd ionization potential means M2 not possible to make in stable form at 25 C Of course in places such as the surface of the sun all sorts of other ions are possible Group 2 Alkaline Earth Metals nsz Occur in nature as M2 too reactive to exist as free metals Radium Ra radioactive ZZGRa halflife of 1599 years Be sI39 hf differer from the rest due 7 0 h h char e fos39ize rat390 cf L in group 12 It is a very hard metal and toxic others softer and not toxic Also compounds are mostly covalent since Bee has too high a charge density to be happy as the free ion in ionic compounds therefore it shares electrons with other atoms decreasing its charge density ie covalent bonding Be unreactive to H20 Mg reacts slowly with cold H20 but fast with steam others react vigorously with H20 M s 2 H20 I gt M OH2 aq H2 g All react with 02 to give oxides MO except Ba gives peroxide BaOZ 1ST and 2nd ionizations are very easy M2 no M 3rd ionization too difficult no M3 Diagonal Relationships between period row 2 element and period 3 element one group to the right Consider Li vs Mg similar size of Li vs MgZ 076 vs 072 A and similar properties eg both give nitrides with N2 salts with similar solubilities and thermal stabilities and significant covalency in their bonds Origin of similarities again due to the high chargetosize ratios of Li and Mg Mg is bigger than Li so MgZ and Li end up being similar
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