Chern 12 Notes 11.4 - Le Chatelier's Principle Goals are to learn: 1. The factors that can cause changes in a system at equilibrium 2. How changes in each factor affect equilibrium (Le Chatelier's Principle) 3. How changes to the equilibrium affect the rates of reaction (Temp, Cone, Pressure, Catalyst) Recall: In a system at equilibrium, the forward rate = the reverse rate - If we disturb the equilibrium, the system SHIFTS (either the reverse or the forward rate speeds up). The system will try to get back to equilibrium. {()f c\{t(lc.th~) Le Chatelier's Principle: if a closed system at equilibrium is subjected to a change, processes will occur that tend to counteract that change. "WHATEVER WE DO, NATURE TRIES TO UNDO" *This rule allows us to predict the effect of any change in conditions on an equilibrium. (1) Effect of Temperature Consider the equilibrium system: N204(g) + heat E) 2 N02(g) colourless l' T brown At equilibrium, N02 is being formed at the same rate as it is being used up, so its concentration is constant. The system is a medium brown colour at room temperature. Increasing the TEMP causes the reaction to shift to the so as to use up the added heat). *<)C)\A L CJ\ ('\ ~\"~ V- -{ fa c-t-(a(\1- O-r K \ ~ \-\T (shifts \\ "- oc ~{C\+ \ \ v;«o: P vfj clu Ct-. Pagel
Here is the explanation: N204(g) + heat ~ 2 N02(g) Reaction is 'C"n G\a thermic PE (kj) 2N02 --7 {'-o r V,v ('f.. k-- r\j t"j.. o~--------------------------------~ Progress of Reaction Ea is greater for the. -O~ w CAr (~ reaction So that means that few r e CA.. tt fa (, -\- molecules have sufficient KE (or PE) to react More? {"O cx\..ac-\- molecules have enough energy to react So when the temp is increased, thefv (.v.j c\ r~ rate is most affected You can also think of it as "heat is a reactant, so if r the tern,p increases, ')ou have Se.. \\ +- :If) more of that reactant", 80 '(y...,w \' \\ Sh; "'-+- '-7 ~R\ -0hi -ro u., v,- 1. Original equilibrium: How is the equilibrium affected? 2. Temperature is increased and the endothermic (forward in this case) reaction rate increases:.shlf.j- " ~ i<. \ f, 11- I N204(g) + heat.:- > 2 N02(g) 1\'\ 3. Reverse reaction gradually catches up and a new equilibrium is established in which there is more N02 and less N204. N204(g) + heat (~ 2 N02(g) (the stuff on the right has increased and the stuff on the left has decreased) Page 2
.s- hi -ff-s r<, 6, 1+ T > Ex 2: 2 NO + cr, ~ 2 NOCI + 76 kj '" "Te ('i,,(j - The temperature is decreased - The reaction shifts ei'cjh1: to try toprv ol L.( C-0 ty1 bv-e... heat. - Once equilibrium is re-established, there will be more NbC \ and less NO ) C\L. <, &::. (2) Effect of Concentration and Partial Pressure C] Concentration refers to molll (or M) of the reactants Partial pressure is the pressure exerted by one mole of gas in the mixture. It is like the [gas] Consider the following system at equilibrium: C02(g) + NO(g) ~ CO(g) + N02(g) The change: we add some CO 2 (we are increasing the [C02]) '7 C02(g) + NO(g) ~ CO(g) + N02(g) '" C\ a a to ~ () Le Chatelier's principle tells us the reaction will shift to the 1"\ \ 8\\-t- C02(g) + NO(g) ~ " CO(g) + N02(g) This will cause the [CO] and the [N02] to \ C\ ()( USe..., and the [C02] and [NO] to C\tC( td\s-l- Because [CO] and the [N02] have increased, the rate of the reverse reaction will speed up. \'.\- w {\\ -(0 lv\ -ha a\j LG\-~ ch Uf to ~(uj r---;". SD -"~~ c~( e. ~ 0L?\.,Q OJ "", Page 3
When the reverse rate = forward equilibrium!) t01.(~)4- NOl)) rate, we will again have equilibrium. (A new ~E==~~ In this new equilibrium, [CO] and [N02] will be higher than they were originally and [C02] and [NO] will be lower than they were after we added the C02. In the new equilibrium [C02] is less than after we added it, but slightly greater than the original level. ('. "" _1 Jl ~ J. (\~.',) [""1,..,0\ -r c\ [ -, 0(' \,,\r\e..., 3pe-L\ I!.~ f.ao\ (.N<' 0",,~\.V J" J - Ex 2. Same reaction: C02(g) + NO(g) ~ CO(g) + N02(g) '\ 0.. 0. /}. Nt)~ Some N02 is added to the system. - The reaction shifts to the Le,f-1- ( \).~e...uf' The [NOl is decreased --~~------ ~----- C02(g) ~D'2- ') + ~O(g) ~ CO(g) + N02(g) ~ f e '('f\oi/ u,\ - The reaction shifts to the -,L=e'-Jt:_' \'!...----l 'f'r\ Cl \<.{ \'Y"\0Ie NO) The [eol is increased. C02(g) + NO(g) ~ CO(g) + N02(g) '\' C J - The reaction shifts to the -==--...l..-~---f- L~.~-\- ( U~~ u.r GO) (3) E(fect o(changing Total Pressure or Volume Recall that increasing pressure has the same effect as decreasing volume (and vice versa) ~x.. '.. ~ \> ~ '\) - Pressure and volume changes affect species that are ~ as f!.. ~ For these reactions, add up the gas molecules on both si <: Ex: 2N02(g) (> N204(g) ~ \"""0..8 \ \"'1"0,3 Increase the total P: The reaction shifts to the f< \ b\.\ Page 4
If P is increased (V decreased), the reaction shifts to the side with the LEAST number of gas molecules If the P is decreased (V increased), the reaction shifts to the side with the :ty10stnumber of gas molecules. Ex 2: 4A(g) + B2(g) () A2 + 2AB(g) 5 fnoj 3 moj Increase the total P: The reaction shifts to the _R \ (;j\-\, Increase the total V: t \I J ~? The reaction shifts to the LE F\ *Note: when you change the total PI total Vol: you change the pressures of all the gas species. In other words, if you increase the total pressure, all the [ ]' s of gases increase (jump up)! Then the reaction shifts and comes to a new equilibrium. (4) E{fect of Catalysts Consider the equilibrium system: N204(g) + heat colourless ~ 2N02(g) brown Adding a catalyst to this system would decrease the activation energy by providing a different route (or mechanism) for the reaction: Uncatalyzed Reaction PE (kj) 2N02 OL- ~ Page 5
Recall: a catalyst increases the forward and the reverse reaction rates by the same amount! This means the forward reaction will speed up, but so will the reverse reaction. In fact, the rates of both the forward and reverse reactions will still remain equal to each other (even though they are both faster.) Therefore, the equilibrium will n D,t Pe. a fkl~t\ (N 0 S\,', ~\- ") Adding a catalyst to a system not at equilibrium will simply speed things up so that equilibrium will be attained faster. It does not alter any of the concentrations etc. at equilibrium! SUMMARY - Factors that affect Equilibria 1. Temperature - If the temp. is increased, the equilibrium will shift toward the side without the heat term. t T) S\)\ ~ts +0 u \...<.. v- f' hla\- -If the temp. is decreased the equilibrium will shift toward the side with the heatterm. ~ \ j S\<...\.'*-s to f'(\ C~\<. {, \("(" oi"e heo: "\- 2. Concentration - If the [a reactant] is increased, the equilibrium will shift toward the right (the product side) ~ ",-, ~ _\_~ -\-D \.A R \\- v..-p - If the [a product] is increased, the equilibrium will shift toward the left (the reactant side) S'n'. +.--\ ~ -\-0 ~l ~ \ -\- vvp" Partial Pressure of Gases - the same effects as concentration. 3. Total Volume and Total Pressure - Ifpressure is increased (volume decreased), the equilibrium will shift to the side with less moles of gas. And lastly, remember: - If the pressure is decreased (volume increased), the equilibrium will shift toward the side with more moles of gas. 4. Catalysts - Have no effect on equilibrium. They may help a system reach equilibrium faster,that's all! Do Hebden Q's p. 54 #17-23 Page 6
Graphing Systems at Equilibrium Showing changes to equilibria graphically: - Graph [ ] vs. time - Start the [reactants] and [products] anywhere on the y axis Gust leave some room between each one) - Show what happens to each [ ] when the conditions change o cc la. r\ * N\. ct.- -. ujh.l \f e --\rlv.- ch ~~ e (1) Changing the Temp: - No sudden change in [ ]. Remember, the temp change causes a shift, but the [ ]'will change gradually. > Ex: + heat (increase temp)
(2) Changing the Cone: [ J - The [ ] of the species being added! removed suddenly jumps up/down. Ex: "7 2NO(g) + CI2(g) ~ 2 NOCI(g) + 76 kj (add Cl, gas) t~t\a \ NO,~- - ~ (_\_L ~~~p! \ \Ap,. f-n_o_c.-_\ (j _ I?-)(. ~ \ y: r){w [ '] > 0\ tl [] (3) Changing the Total P or V: - All the [ ]' s of gas species simultaneously jump up or down. Ex: 3 f'1'o~ => ~ ( \ o~ 2NO(g) + CI2(g) ~ 2 NOCl(g) + 76 kj (increase P) d- " \ '.. ;;;.. NO. ( LJ. s0-\~s +0 S\c~0 w"\\,"\ ~w-t( (Y'\.o.g...~ \ 'j- -------- ~ L~ ~QC\ 'tp h(y\~ *whether the new [ ] of a species will be above or below the old [ ] depends on the shift, and the moles of that species reacting. Not looking for perfection here, just that you can graph the "changes" Page 8
(J Ex 2: 3 mo5 3r'005 ' 2A(g) + B (g) <) 3 C(g) + heat ~p (increase volume) S\\\ +'+ +0 s : c.j. L \N more f(')-o 3 \3uif"'- 0 -j are EQ.~L LNo SHIFT J (4) Add a Catalyst: Ex: (1, 2NO(g) + CI2(g) ~ 2 NOC1(g) + 76 kj (add catalyst) ~o e.wlt-j Ch... \ ------ -r NOC\ \ - t)(~ C J \ G\t~~\ C~l\-~~t Do Hebden Q's p. 55 #24-28 Worksheet: Le Chatelier'ls Principle Page 9