General Chemistry II, Unit II: Study Guide (part 1)

Transcription

1 General Chemistry II, Unit II: Study Guide (part 1) CDS Chapter 21: Reactin Equilibrium in the Gas Phase General Chemistry II Unit II Part 1 1 Intrductin Sme chemical reactins have a significant amunt f reactant left ver when equilibrium is reached. Observatin 1: Reactin equilibrium N! g + 3H! g 2NH! g If we start with 1 mle f N2 and 3 mles f H2, the balanced equatin predicts 2 mles f NH3 will be prduced. Experimentally, nly 1.91 mles f NH3 are prduced. Remember: there is n limiting reactant invlved (since neither reactant is present in less than the stichimetric rati established by the equatin). Cnclusin: this reactin achieves reactin equilibrium in which all three gases are present in the gas mixture. The amunt f NH3 prduced can be varied by changing: The temperature f the reactin; The vlume f the reactin cntainer; r The relative starting amunts f the reactants. Sme reactins achieve an equilibrium very clse t the cmplete reactin. This means that, at equilibrium, there are virtually n reactants remaining. A qualitative mdel f equilibrium Frm phase transitins studies: equilibrium ccurs when the rate f the frward prcess is matched by the rate f the reverse prcess Therefre, in gas reactins, at equilibrium the frward reactin rate is equal t the reverse reactin rate. Hw d we shw that the frward and reverse reactins cntinue t ccur at equilibrium? Vary the vlume. N 2 O 4 (g) 2 NO 2 (g) When the vlume is increased, the amunt f NO 2 at equilibrium is larger and the amunt f N 2 O 4 is smaller. If the frward and reverse reactins simply stpped when equilibrium was reached, the mlecules wuld have n way f knwing f the change in vlume, s the amunts f gas wuld nt have changed. Change in vlume affects bth the frward and reverse reactin rates. If the rate f the frward reactin is faster than the rate f the reverse reactin, there will be mre prduct than reactant at equilibrium. Observatin 2: Equilibrium cnstants N 2 O 4 (g) 2 NO 2 (g) In this experiment, we vary the number f mles f N2O4 and measure the equilibrium pressures f bth the reactant and prduct gases.

2 General Chemistry II Unit II Part 1 2 The amunt f NO2 at equilibrium seems t be directly prprtinal t the starting amunt f N2O4. Hwever, this graph suggests therwise: The equilibrium pressure f NO2 des nt increase prprtinally with the initial amunt f N2O4. The slw increase suggests a square rt relatinship. Hwever, this graph suggests therwise: The near- prprtinal relatinship between the initial amunt f N2O4 and the pressure f N2O4 (frm figure 21.1) suggests that we shuld instead plt the pressure f NO2 against the square rt f the pressure f N2O4 (P NO! = c P N! O! ). The abve equatin can be rewritten by slving fr c and setting c equal t K p : K! =!!!!!!!!!!!. The cnstant K p (the reactin equilibrium cnstant) is independent f the initial cnditins and the equilibrium partial pressures f bth the reactant and prduct:

3 General Chemistry II Unit II Part 1 3 Smething t nte: the prduct (NO2) appears in the numeratr and its expnent is its stichimetric cefficient in the chemical equatin. Similarly, the reactant (N2O4) appears in the denminatr and its expnent is its stichimetric cefficient. N! g + 3H! g 2NH! g!(!!! )! Predicted reactin equilibrium cnstant: K! =!!!!!!! Cntinuing frm Observatin 1, we will nw vary the starting amunts and cntainer vlume fr this reactin: Kp clearly remains cnstant fr each set f initial cnditins. Therefre, the reactin equilibrium cnstant can be fund using an equatin in which the partial pressures f the prducts (each raised t their crrespnding stichimetric cefficient) are multiplied tgether in the numeratr, and the partial pressure f the reactants (each raised t their crrespnding stichimetric cefficient) are multiplied in the denminatr. Dynamic Equilibrium and Reactin Rates The rate law can be predicted frm the cefficients in the reactin equatin. # Step Explanatin 1 Rate!"#$%#& = k!"#$%#& N! O! Rate!"#"!$" = k!"#"!$" NO!! 2 k! N! O! = k! NO!! 3 k! NO!! = k! N! O! 4 NO! = N! O! = n!!! V n!!!! V = = Since the frward and reverse reactins are elementary prcesses, these are their rate laws. In dynamic equilibrium, the rates are equal t each ther. A simple rearrangement f the previus expressin. This lks similar t the Kp expressin, except it uses gas cncentratins instead f partial pressures. P!!! RT P!!!! RT Apply the Ideal Gas Law t make the cnnectin between gas cncentratins and pressures. 5! P!! k! NO!! 1! = = = k! N! O! P!!!! RT 6! P!! k!! K! = RT = k! P!!!! Substitutin f the Ideal Gas Law relatinships abve int equatin 3. Multiplying by RT yields the same expressin as Kp. Observatin 3: Temperature Dependence f the Reactin Equilibrium H2 (g) + I2 (g) - > 2 HI (g) Here, we measure the equilibrium partial pressures at a variety f temperatures.

4 General Chemistry II Unit II Part 1 4 Since the equilibrium cnstant increases with temperature, the pressure must als increase dramatically with temperature. The temperature and pressure measurements shw n simple relatinship between temperature and the equilibrium cnstant K p. Frm thermdynamics, the equilibrium cnstant varies with temperature accrding t ln K! =!! =!.!"! Since ΔH and ΔS dn t strngly depend n temperature, a linear relatinship appears between lnk p and 1/T: The negative slpe means that K p increases with temperature. This is because - ΔH /R, the slpe, must be negative. Fr exthermic reactins, the slpe shuld be psitive and the equilibrium cnstant shuld decrease with increasing T. Observatin 4: Changes in Equilibrium and Le Chatelier s Principle Ideally, ur gal is t be able t cntrl the equilibrium f a gas reactin that wuld allw us t frce a reactin t prduce as much r as little reactin as we want. Discveries s far: The equilibrium pressure f the prduct f a reactin increases with increasing quantity f reactant. The equilibrium pressure f the prduct f a reactin varies with the vlume f the reactin cntainer. Hwever, this variatin is nt easily predictable r explainable. In rder t explain this, we can rewrite the equilibrium cnstant t shw the vlume f the cntainer (fr N! g + 3H! g 2NH! g ):!! RT n NH! K! = V RT n N! V n H!! RT! = n NH!! n N V! n H! RT!! V! RT n NH!! K! = V n N! n H!! When the vlume increases, the left side f the equatin decreases therefre, the right side must als decrease, s the denminatr (N2 and H2) must increase while the numeratr (NH3) decreases. This shifts the equilibrium frm prducts t reactants. Changes in temperature K p increases with T fr endthermic reactins The prducts are increasingly favred. K p decreases with T fr exthermic reactins.

5 General Chemistry II Unit II Part 1 5 The reactants are increasingly favred. In exthermic reactins, the reverse reactin is endthermic. The equilibrium shifts in the directin f the endthermic reactin when the temperature increases. Le Chatelier s Principle when a reactin at equilibrium is stressed by a change in cnditins, the equilibrium will be reestablished in such a way as t cunter the stress. Changes in vlume Increasing the vlume reduces the partial pressures f all the gases present and thus reduces the ttal pressure. The reactin respnds t the stress f a vlume increase by ffsetting the pressure decrease with an increase in the number f mles f gas at equilibrium. McMurry & Fay Equilibrium cnstant K c (13.2) The number btained by multiplying the equilibrium cncentratins f all the prducts and dividing by the prduct f the equilibrium cncentratins f all the reactants, with the cncentratin f each substance raised t the pwer f its cefficient in the balanced chemical equatin. Fr the reactin aa + bb cc + dd: K! = C! D! A! B! Hetergeneus equilibria (13.4) Hmgeneus equilibria have all reactants in prducts in the same phase, usually gaseus r slutin. Cnversely, hetergeneus equilibria are thse in which the reactants and prducts are present in mre than ne phase. The cncentratins f pure slids and liquids are nt included when writing an equilibrium equatin. The equilibrium equatin fr the reactin CaCO! s CaO s + CO! g is simply K! = CO! since CaCO 3 and CaO are bth slids. Judging the Extent f Reactin (13.5) The numerical value f the equilibrium cnstant fr a reactin indicates the extent t which reactants are cnverted t prducts. Predicting the Directin f Reactin (Reactin Qutient) (13.5) Altering an Equilibrium Mixture: Changes in Cncentratin (13.7) The reactin qutient is the same as the equilibrium cnstant K c, except that the cncentratin values it uses are nt necessarily equilibrium values. If Q c < K c, net reactin ges frm left t right (reactants t prducts). If Q c > K c, net reactin ges frm right t left (prducts t reactants). If Q c = K c, n net reactin ccurs. The cncentratin stress f an added reactant r prduct is relieved by net reactin in the directin that cnsumes the added substance. The cncentratin stress f a remved reactant r prduct is relieved by net reactin in the directin that replenishes the remved substance.

6 Altering an Equilibrium Mixture: Changes in Pressure and Vlume (13.8) Catalysts and equilibrium (13.10) General Chemistry II Unit II Part 1 6 An increase in pressure by reducing the vlume will bring abut net reactin in the directin that decreases the number f mles f gas. A decrease in pressure by expanding the vlume will bring abut net reactin in the directin that increases the number f mles f gas. These bservatins are due t the Ideal Gas Law: the pressure f an ideal gas is inversely prprtinal t the vlume at cnstant temperature and quantity. If a reactin mixture is at equilibrium in the absence f a catalyst, it will still be at equilibrium after a catalyst is added because the frward and reverse rates, thugh faster, remain equal. If a reactin mixture is nt at equilibrium, a catalyst accelerates the rate at which equilibrium is reached, but it des nt affect the cmpsitin f the equilibrium mixture. Catalysts d nt appear in the equilibrium cnstant expressin. Link between chemical equilibrium and chemical kinetics (13.11) Step A + B C + D Explanatin Cnsider this reactin. Rate frward = k! A B Rate reverse = k! C D The frward and reverse reactins are elementary reactins with these rate laws. k! A B = k! C D at equilibrium k! = C D k! A B K! = C D A B K! = k! k! The frward and reverse rates are equal at equilibrium. Rearrange. The right side f the equatin is the same as the equilibrium cnstant expressin fr the frward reactin, which equals the equilibrium cnstant K c since the reactin mixture is at equilibrium. The equilibrium cnstant is simply the rati f the rate cnstant fr the frward and reverse reactins. CDS Chapter 16: Phase Transitins and Phase Equilibrium Intrductin Liquids and slids dn t fllw the Kinetic Mlecular Thery f Gases The densities f these cndensed phases are thusands f times greater than the typical density f a gas. Unlike the Kinetic Mlecular Thery, mlecules in liquid and slid phases must interact with each ther. Observatin 1: Gas- Liquid Phase Transitins Recall Charles Law: the vlume f a fixed sample f gas is prprtinal t the abslute temperature f the gas, prvided that the pressure is cnstant.

7 General Chemistry II Unit II Part 1 7 The typical Charles Law experiment traps gas in a cylinder (with a pistn t maintain a cnstant pressure) and varies the temperature. With each new temperature, the pistn mves t establish a new vlume. If we keep lwering the temperature f a sample f butane gas, the linear relatinship between vlume and temperature abruptly stps at K the vlume drps t nly L: This change in physical prperties at ne temperature is a phase transitin. At K, butane cnverted frm gas t liquid. The temperature f this transitin is the biling pint. The biling pint des nt depend n hw much liquid r gas there is in a sample. If we increase the applied pressure, the phase transitin ccurs at a higher temperature (fr butane, 20 K higher). The temperature f the phase transitin depends n the applied pressure. The biling pint temperature als depends n substance identity. Different substances have vastly different biling pints. Observatin 2: Vapr pressure f a liquid Obviusly, liquids left in an pen cntainer will eventually evaprate even if the temperature f the liquid is well belw its biling pint. The tendency f a liquid t evaprate is knwn as its vlatility a mre vlatile liquid evaprates mre readily. A quantitative measure f vlatility can be fund by mdifying the previus cylinder by adding a gauge t measure the pressure. The cylinder is filled with water nly; the pistn is then pulled back, creating an empty space abve the liquid water. We wuld assume that the pressure f this space is zer. Instead, the pressure rises t a cnstant 23.8 trr. Therefre, there must be gaseus water in the cntainer. Since there was n gas initially, this gas must have cme frm evapratin f the liquid water. Since nt all f the water is evaprated at equilibrium, bth the liquid and gas phases are present at the same time. They are in phase equilibrium The pressure f the vapr abve the liquid rises t 23.8 trr regardless f the vlume r initial amunt f liquid. Hwever, the amunt f water that evaprates must differ when the vlume r initial amunt is varied. This is because the vlume available fr the vapr t ccupy changes with these mdificatins in rder t maintain the same pressure (23.8 trr), the quantity f water vapr must change.

8 General Chemistry II Unit II Part 1 8 This reveals that the pressure f the vapr is the mst imprtant prperty in establishing phase equilibrium. The vapr pressure f a liquid is the single specific pressure at which it will be in phase equilibrium with its vapr. Vapr pressure varies with temperature and the relatinship is nt prprtinal. The vapr pressure als depends n the identity f the substance. Observatin 3: Dynamic Equilibrium between Liquid and Gas Phases Why d we always get the same pressure at equilibrium fr the same temperature, regardless f ther cnditins? Since the vapr exerts the same pressure in a larger vlume (fr cnstant T), the Ideal Gas Law states that there are mre mlecules in the vapr after the vlume is increased and equilibrium is reestablished. Mre liquid must have evaprated fr this t happen. The nly way that the liquid culd have knwn t evaprate when the vlume increased is if the mlecules in the liquid were always evaprating even at equilibrium. There must be a cnstant mvement f mlecules frm liquid t gas. Since the pressure f the vapr remains cnstant when vlume is fixed, cndensatin must als always be ccurring. This cnstant pressure at a fixed vlume shws that the number f mlecules ging frm liquid t gas must exactly ffset the number f mlecules ging frm gas t liquid. This cnstant mvement f mlecules between the phases is knwn as dynamic equilibrium. What factrs are imprtant in dynamic equilibrium? When the vlume is increased, the vapr pressure remains cnstant and the quantity f gas increases. Fr this t be pssible, evapratin initially ccurs mre rapidly than cndensatin until equilibrium is achieved. Mechanics f evapratin and cndensatin Mlecules leave the gas phase and enter the liquid phase by striking the surface f the liquid. Therefre, the rate f cndensatin depends n the frequency f mlecules striking the liquid surface. This strike frequency decreases when the vlume is increased because the density f gas mlecules decreases (think KMT).

9 General Chemistry II Unit II Part 1 9 The rate f cndensatin becmes lwer than the rate f evapratin, s there is a net flw f mlecules frm liquid t gas. This cntinues until equilibrium is reestablished, at which pint the rates f evapratin and cndensatin are nce again equal. The vapr pressure desn t depend n the surface area f the liquid since the rate f evapratin and the rate f cndensatin increase with increasing surface area. What determines the rate f evapratin? The increase in vapr pressure with increasing temperature is nt slely due t the relatinship between P and T in the Ideal Gas Law. While the Ideal Gas Law relatinship between P and T is prprtinal, the relatinship between vapr pressure and temperature is much larger than prprtinal. Since the vapr pressure rises with temperature, there must be a greater quantity f gas in the vapr phase at higher temperature. Therefre, the rate f cndensatin must be higher. Hwever, at equilibrium the rates f cndensatin and evapratin must be equal. Therefre, the rate f evapratin is als higher at higher temperatures. Since the rate f evapratin depends n the identity f the liquid is, vapr pressures differ fr each liquid. A mdel fr the relatinship between temperature and evapratin Temperature is a measure f the kinetic energy f mlecules in bth gases and liquids. This is shwn by the fact that a greater quantity f mlecules is able t escape the liquid at higher temperatures (they are able t vercme the liquid s intermlecular frces). What causes different substances t have different rates f evapratin? Substances with lwer vapr pressures have lwer rates f evapratin and cndensatin this means the substance has fewer mlecules with high enugh KE t escape the liquid at a given temperature. Dynamic equilibrium At a given temperature, nly a fractin f the liquid mlecules have enugh KE t evaprate this fixes the rate f evapratin The rate f cndensatin must match the rate f evapratin at equilibrium this is nly pssible at ne specific pressure. Therefre, at a given temperature, nly a single pressure will result in phase equilibrium fr that substance. McMurry & Fay Plar cvalent bnds and diple mments (10.1) Plar cvalent bnds frm between atms f different electrnegativity. These bnds are characterized by an unequal sharing f the bnding electrns between the tw atms. These bnds have psitive and negative ends, knwn as diples. Mlecules as a whle can als be plar because f the net sum f the individual bnd plarities and lne- pair cntributins. Intermlecular frces (10.2) In- diple frces (10.2) Water (H 2 O) has clear intramlecular frces between each f the tw hydrgen atms and the xygen atm. Hwever, H 2 O exists as a gas, liquid r slid depending n its temperature therefre, there are als intermlecular frces that act between mlecules t hld them tgether in these phases. An in- diple frce is the result f electrical interactins between an in and the partial charges n a plar mlecule.

10 General Chemistry II Unit II Part 1 10 Diple- diple frces (10.2) Diple- diple frces ccur between neutral but plar mlecules as the result f electrical interactins amng diples n neighbring mlecules. These frces are generally weak (3 4 kj/ml). The strength f a diple- diple interactin depends n the sizes f the diple mments invlved. Lndn Dispersin Frces (10.2) Intermlecular frces ccur between nnplar mlecules and amng the individual atms f a nble gas. All atms and mlecules, regardless f structure, experience Lndn Dispersin Frces which result frm the mtin f electrns. While the average distributin f electrns thrughut a symmetrical mlecule is symmetrical, at any given mment there may be mre electrns at ne end f the mlecule than the ther. This instantaneus diple can affect the electrn distributins in neighbring mlecules and induce temprary diples. LDFs are generally small (1 10 kj/ml). Hydrgen bnds (10.2) A hydrgen bnd is an attractive interactin between a hydrgen atm bnded t a very electrnegative atm (O, N r F) and an unshared electrn pair n anther electrnegative atm. Hydrgen bnds are especially strng because O- H, N- H and F- H bnds are highly plar and hydrgen atms lack cre electrns t shield their nuclei. Hydrgen bnds are strng (40 kj/ml).

11 A Cmparisn f Intermlecular Frces (10.2) General Chemistry II Unit II Part 1 11 Viscsity (10.3) Surface tensin (10.3) Viscsity is the measure f a liquid s resistance t flw. It is realted t the ease with which individual mlecules mve arund in the liquid (which is, in turn, related t the IMFs). Substances with small, nnplar mlecules expeirence weak IMFs and have lw viscsities. Larger, mre plar substances experience strnger IMFs and have higher viscsities. Surface tensin is the resistance f a liquid t spread ut and increase its surface area. It is caused by the difference in IMFs experienced by mlecules at the surface f a liquid and thse experienced by mlecules in the interir. Mlecules at the surface feel attractive frces n nly ne side and are thus pulled in tward the liquid mlecules in the interir are surrunded and pulled equally in all direcitns. Surface tensin is higher in liquids that have strnger IMFs. Surface tensin and viscsity are als temperature- dependent mlecules at higher temperatures have mre KE t cunteract the attractive frces hlding them tgether. Phase changes (10.4) Heating curve:

12 General Chemistry II Unit II Part 1 12 The heat f fusin (ΔH fusin ) describes the amunt f energy required t cnvert a slid int a liquid. The heat f vaprizatin (ΔH vap ) describes the amunt f energy required t cnvert a liquid int a gas.