concentrations (molarity) rate constant, (k), depends on size, speed, kind of molecule, temperature, etc.

Transcription

1 #73 Notes Unit 9: Kinetics and Equilibrium Ch. Kinetics and Equilibriums I. Reaction Rates NO 2(g) + CO (g) NO (g) + CO 2(g) Rate is defined in terms of the rate of disappearance of one of the reactants, but it can also be defined by the rate of appearance of one of the products. Rate = - [NO 2 ] = - [CO] = [NO] = [CO 2 ] t t t t - rate can be measured for a specific time (instantaneous rate = slope) or over a time interval (average rate) II. Integrated Rate Laws Rate = k [reactant 1] x [reactant 2] y -x,y are usually integers concentrations (molarity) rate constant, (k), depends on size, speed, kind of molecule, temperature, etc. Ex. 1) A + B C initial [A] initial [B] initial rate rate = - [B] row M M 4.0 x 10-5 t row M M 4.0 x 10-5 row M M 1.6 x 10-4 = 16 x 10-5 Compare two rows where only one concentration is changing (only A or only B ). comparing row 1 & 2: A remains the same, B is doubling and the rate is the same. (going down the rows) rate = k [A] [B] 1X 2X 2 0 equals 1, so [B] 0 comparing row 1 & 3: A is doubling, B is the same and the rate is four times bigger.

2 (going down the rows) rate = k [A] [B] 4X 2X 2 2 equals 4, so [A] 2 Rate = k[a] 2 [B] 0 or Rate = k[a] 2 (2 nd order, since powers add up to 2 ) (A is 2 nd order, B is zero order) Find rate constant: Rate = k[a] x 10-5 = k (0.100 M) 2 plugging in values from row # x 10-3 = k Book does all 3 rows and averages (just do one row). Ex. 2) (CH 3 ) 3 CBr + OH - (CH 3 ) 3 COH + Br - rate = + [Br - ] [(CH 3 ) 3 CBr] 0 [OH - ] 0 t c row M 0.18 M 1.0 x 10-3 row M 0.10 M 2.0 x 10-3 row M 0.10 M 1.0 x 10-3 comparing row 1 & 3: (CH 3 ) 3 CBr is the same, (going up the row) OH - is basically doubling and the rate is the same. rate = k [(CH 3 ) 3 CBr] [OH - ] 1X 2X 2 0 equals 1, so [OH - ] 0 comparing row 2 & 3: (CH 3 ) 3 CBr is doubling, (going up the row) OH - is the same and the rate is doubling. rate = k [(CH 3 ) 3 CBr] [OH - ] 2X 2X 2 1 equals 2, so [(CH 3 ) 3 CBr] 1 rate = k [(CH 3 ) 3 CBr] 1 [OH - ] 0 = k [(CH 3 ) 3 CBr] 1 (1 st order overall) Find the rate constant: rate = k [(CH 3 ) 3 CBr] 1 (1.0 X10-3 ) = k (0.10 M) using row #1 1.0 X10-2 = k What would the rate be, if [(CH 3 ) 3 CBr] = 0.26 M & [OH - ] = 0.45 M? rate = k [(CH 3 ) 3 CBr] 1 rate = (1.0 X10-2 ) (0.26 M) rate = 2.6 X10-3

3 #74 N otes III. First Order Rate Law A C Rate = - [A] = k [A] 1 t ln [A] = -kt + C if t = 0, ln [A] 0 = C substitute this in ln [A] = -kt + ln [A] 0 y = mx + b slope = -k Half-life mult by -1: - ln [A] = kt - ln [A] 0 ln [A] 0 -ln [A] = kt ln ( [A] 0 / [A] ) = kt For ½ life ln ( [A] 0 / { [A] 0 /2 }) = k t 1/2 ln 2 = k t 1/2 ln 2 / k = t 1/ / k = t 1/2

4 IV. Second Order Rate Law 2 A B Rate = - [A] = k[a] 2 t +[A] -1 = kt + C integral of x -2 dx = -x -1 (raise power and divide coefficient by it) 1 = kt + C if t = 0, C = 1 d [A] [A] o 1 d= kt + 1 d slope = k [A] [A] o y = mx + b Half-life 1 d= kt 1/2 + 1 d ([A] o /2) [A] o 2 d= kt 1/2 + 1 = [A] o [A] o 1 d= t 1/2 k[a] o

5 V. Zero Order A B Rate = - [A] = k[a] o since [A] 0 = 1 t [A] = -kt + C at t = 0 C = [A] o [A] = -kt + [A] o slope = -k y = mx + b Half-life [A] o = -kt 1/2 + [A] o 2-1[A] o = -kt 1/2 2 +[A] o = t 1/2 2k

6 If given: [CH 4 ] time # # # # # # 1) Graph If the line is straight, it is 1 st order, rate = k [A] 1 If the line is not straight, then 2) Graph If the line is straight, it is 2 nd order, rate = k [A] 2 If the line is not straight, then 3) Graph If the line is straight, it is zero order, rate = k [A] 0 Ex.1) Find the half-life for an element with a 1 st order Rate law (or ln A vs. time is a straight line), if the reaction is 35% complete in 5 minutes. (35% has reacted, so 35% has decayed) 100g beginning - 35g 65g remaining, so [A] o = 100 g & [A] = 65 g ln [A] = -kt + ln [A] o Solve for k, then find t 1/2 t 1/2 = k

7 #75 Notes VI. Reaction Mechanisms -are the series of steps or reactions necessary to achieve an overall process. *The slowest step always determines the rate law. 2 NO 2 NO 3 + NO slow NO 3 + CO NO 2 + CO 2 fast NO 2 + CO NO + CO 2 *NO 3 is an intermediate. (It s first a product, then a reactant, which cancel.) Rate = k [NO 2 ] 2 rate law from the slowest step! Ex.1a) A products (slow) rate = k [A] 1 b) 2A + B X (slow) rate = k [A] 2 [B] 1 X + B Y (fast) Y + Z products (fast) *Find the slow reaction or if only one reaction, assume it is slow! Use the slow reaction. VII. Chemical Kinetics/ Collision Model NaNO 3 + KCl NaCl + KNO 3 Double Displacement Ionic compounds in reactions do not need to rearrange their electrons, since the ions just change partners. These reactions occur quickly. Covalent compounds in reactions must change their electron clouds. The energy to do this comes from collisions! *The rate can be increased by increasing the temperature, since this increases the velocity of the particles (KE = ½ mv 2 ). The number and power of collisions increase. But the rate does not go up by as much as it should. Arrhenius proposed the existence of a threshold or activation energy that must be overcome in order to produce a reaction.

8 E a = activation energy needed for reaction to occur. E = overall energy change for the reaction, above it is (-) so exothermic. The kinetic energy of the moving molecules is changed into potential energy as the bonds are broken and formed. **Only collisions with enough energy (activation energy) will be able to form the activated complex and then the products. The reaction rate is still smaller, than the rate of collisions with enough energy to form the activated complex, because of molecular orientations. The OH - must hit from the opposite side to eject the Br -. *Catalysts: lower the E a, allowing reactions to go faster. *Inhibitors: slow the reaction.

9 #76 Notes VIII. Arrhenius Equation -E a k = z p e RT -E a k = A e RT k = rate constant z = collision frequency p = steric factor (structure of molecule, orientation etc.) E a = activation energy R = 8.31 J/mol K T = Kelvin To compare 2 rates: k 1 = A e Ea 1 / (RT) k 2 = A e Ea 2 / (RT) To graph: (1 st order rates) A s cancel, plug in E a s, R and T s to get ratio. ln k = ln A E a / (RT) ln k = - (E a ) ( 1 ) + ln A R T y = m x + b slope = - E a / R k T ln k 1/T # # # # # # # # # # # # For 2 values: ln (k 2 /k 1 ) = E a ( 1-1 ) R T 1 T 2 E a must be in j/mol, since R = 8.31 J/mol K IX. Catalysis Catalysts speed up reactions by lowering E a, but they are not consumed. (If the hill is lower, more collisions will have enough energy to form product.) These are first reactants, then products, which cancel. Homogeneous Catalysts: are present in the same phase as the reactants. (Cl (g) in O 3(g) destruction).

10 Heterogeneous Catalysts: are present in a different phase. (catalytic converter (s) for car exhaust (g) systems.) Ch. Chemical Equilibrium I. Equilibrium Equations N 2(g) + 3H 2(g) 2NH 3(g) at equilibrium: Forward Rate = Reverse Rate k f [N 2 ] 1 [H 2 ] 3 = k r [NH 3 ] 2 k f = [NH 3 ] 2 s k r [N 2 ] 1 [H 2 ] 3 K = [NH 3 ] 2 s products [N 2 ] 1 [H 2 ] 3 Molarity reactants *Exception: Pure solids and pure liquids are not included ( aq and g are included). {H 2 O (l) not included, H 2 O (g) included} Ex. 1a) Write the K equation for: Mg (s) + 2HCl (aq) MgCl 2(aq) + H 2(g) K = [MgCl 2 ] 1 [H 2 ] 1 [HCl] 2 b) 4 Fe (s) + 3 O 2(g) 2 Fe 2 O 3(s) K = 1 [O 2 ] 3

11 #77 Notes II. Equilibrium and Pressure N 2(g) + 3H 2(g) 2NH 3(g) 2 K p = P NH3 d 3 P N2 P H2 so, Remember: PV = nrt P = n RT where (n/v) = Molarity V P = M RT K p = (M NH3 RT) 2 (M N2 RT) 1 (M H2 RT) 3 K p = [M NH3 ] 2 (RT) 2 [M N2 ] 1 [M H2 ] 3 (RT) 1 (RT) 3 Since, K = [NH 3 ] 2 (These are molarities.) [N 2 ] 1 [H 2 ] 3 K p = K (RT) -2 (mols products mols reactants) K p = K(RT) n R = L atm mol K n = mols products mols reactants Ex. 1) Does K p = K? Fe 2 O 3(s) + 3 CO (g) 2 Fe (s) + 3 CO 2(g) **(Solids and liquids do not count in either equation.) 3 mols product 3 mols reactant K = [CO 2 ] 3 K p = K (RT) [CO] 3 K p = K (RT) 0 so Kp = K Ex.2) For N 2(g) + 3 H 2(g) 2 NH 3(g) : What is the concentration of NH 3 at equilibrium, if at equilibrium there is mol N 2 and mol H 2 in 2.00 L? K = K = [NH 3 ] 2 M N2 = mol = M M H2 = mol = M [N 2 ] [H 2 ] L 2.00 L = [NH 3 ] 2 (0.0201M) 1 ( M) X10-7 = [NH 3 ] 2

12 6.81 X10-4 M = [NH 3 ] III. Applications of K eq K = products / reactants so if K > 1, more products are present if K < 1, more reactants are present Steps to solve K eq with initial concentrations (NOT at equilibrium): 1) Find Q to determine the shift in equilibrium. (Q is the K with the initial concentrations) If Q = K, it is already at e quilibrium. If Q > K, too many products are present and they need shifted to the reactant side. If Q < K, too many reactants are present and they need shifted to the product side. 2) Define the above change to the initial concentrations. 3) Substitute into equations and solve. 4) Check work.

13 #78 Notes Ex. 1) H 2(g) + F 2(g) 2 HF (g) Calculate the equilibrium concentrations, if initially there is 5.00 mol H 2 and 6.00 mol F 2 in a 2.0 L flask. Assume K = mol / 2.0 L = 2.5 M H mol / 2.0 L = 3.0 M F 2 H 2(g) + F 2(g) 2 HF (g) 2.5 M 3.0 M 0 K = [HF] 2 [H 2 ] 1 [F 2 ] 1 Step 1: Q = (0) 2 = 0 Q of 0 < K of 115, so it needs more products (shift right) (2.5) 1 (3.0) 1 Step 2: H 2(g) + F 2(g) 2 HF (g) (Initial) 2.5 M 3.0 M 0 (Change) -x -x +2x **not because of the 2 (-x) s, but because of (Equilibrium) 2.5 x 3.0 x 2x the 2 HF (if it was 3 HF, then 3x). Step 3: 115 = (2x) 2 (2.5 x) 1 (3.0 x) 1 In this case x is large, we need to move a lot to the products to make the fraction increase from 0 to 115. ** If the K is smaller than the Concentrations by 10 3 or more, we can make an assumption that x is small. For example, if K was 1 X10-4 = (2x) 2 (2.5 x) 1 (3.0 x) 1 1 X10-4 vs. 2.5 X10 0 If x is very small: 2.5 x just 2.5, 3.0 x just 3.0 so 1 X10-4 = (2x) 2 no quadratic (2.5) 1 (3.0) 1 In our case we need to solve it without assumptions, since x is not small. 115 = (2x) 2 (2.5 x) 1 (3.0 x) 1

14 115 = 4x 2 ( x + x 2 ) x + 115x 2 = 4x 2 111x x = 0 a b c x = -b ± b 2 4ac = ± (-632.5) 2 (4)(111)(862.5) 2a 2 (111) = ± 131 = 3.44 or 2.25 (3.44 is impossible, since (-) concentrations are impossible.) 222 [H 2 ] = 2.5 x = = 0.25 M H 2 [F 2 ] = 3.0 x = = 0.75 M F 2 [HF] = 2x = 2(2.25) = 4.5 M HF Step 4: Check 115 =? (4.5) 2 (0.25) 1 (0.75) =? X100% = -6% 115 < 10% is OK look for perfect squares: 115 = (2x) = (2x) (2.5-x) 1 (2.5-x) 1 (2.5-x)

15 #79 Notes IV. Le Chatelier s Principle If a system is in equilibrium and a condition is changed, then the system will shift toward restoring the equilibrium. A) Concentration N H 2 2 NH 3 Ex. 1) increase N 2 : increases collisions, so it speeds the reaction going to the right (products) shifts right (too much N 2, so it will shift to the other side (products)) Ex. 2) increase NH 3 : shifts left (too much NH 3, shifts to the other side (reactants)) Ex. 3) decrease H 2 : shifts left (too little H 2, so it must shift toward H 2 to increase it (reactants)) B) Pressure (only affects gases) Ex. 1) increase P, by decreasing volume (if decreasing volume, the reaction will shift toward the side that is the most compact (smallest volume)) N H 2 2 NH 3 4 mol 2 mol x 22.4 L x 22.4 L 88 L 44 L smallest volume ( = least number of mols) the right side is more compact shifts right (if increase volume, shifts to the left, larger volume side) Ex. 2) increase P, by adding Ne, no change (Ne is not in the reaction, other gases will not change the volume, since gases are point masses (ideal)) C) Temperature Ex. 1) Girls + Boys Dancing + heat (exothermic, heat produced, ΔH = (-) energy on product side) Increase Temperature, Add heat, less dancing, shifts left Ex. 2) Decrease T, for exothermic reaction (ΔH = (-), energy on product side) A + B C + energy decrease T, too little energy, shifts right

16 Ex. 3) Increase T for endothermic reaction (ΔH = (+),energy on reactant side) energy + A C increase T, too much energy, shifts right Ex. 1) Find the equilibrium concentrations, if it initially has 3.0 M SO 3, 2.0 M SO 2, and 1.5 M O 2. 2 SO 3g) 2 SO 2(g) + O 2(g) K = M 2.0 M 1.5 M Q = (2.0 M) 2 (1.5 M) 1 = 0.66 (3.0 M) 2 Q of 0.66 > K of 0.056, reaction needs less products, shifts left. **Since it will be cubed, x needs to be as small as possible, so that we can assume x is small. We need much less products, so we will shift as much as possible to the reactant side and then move x back to the products. (Hopefully making x small.) 2 SO 3g) 2 SO 2(g) + O 2(g) **Don t forget we must move x & 2x! 3.0 M 2.0 M 1.5 M (x cannot be 1.5, because 2x = 3.0, +2.0 (2x) -2.0 (2x) -1.0 (x) making [SO 2 ] be negative) SO 3g) 2 SO 2(g) + O 2(g) x +2x +x 5.0 2x 2x x K = = (2x) 2 (0.5 + x) 1 (5.0 2x) 2 Assume x is small, K is smaller than the concentrations by only 2 powers (5.6 X10-2 vs. 5.0 X10 0 ), but we have cubes (we can t do the quadratic) x x = (2x) 2 (0.5) 1 (5.0) = (4x 2 ) (0.5) 1.4 = 2x 2

17 0.84 = x SO 3 = 5.0 2x = 3.3 M SO 3 SO 2 = 2x = 1.7 M SO 2 O 2 = x = 1.3 M O 2 **Checking this, K comes out to 0.34, which is off, because x is not that small. It is better than before (Q was 0.66), but now you would have to repeat the problem with these answers. And keep repeating over and over until the answers do check. (Method of Successive Approximations) *End of Notes* (Assignments #80-81 are Review Assignments. There are no notes for these assignments.)