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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: dhoom3 on March 18, 2009, 03:56:51 PM

Title: Chemical Kinetics ( Calculate Rate Constant, decomposition of phosgene)
Post by: dhoom3 on March 18, 2009, 03:56:51 PM
Here are 2 of the many problems im having hard time with.

1)
t (k)         K (s-1)
338          4.87x10-3
328          1.50x10-3
318          4.98x10-4
308          1.35x10-4
298          3.46x10-5
273          7.87x10-7

a ) Determine the Activation energy (Ea)
      What i got -> y = -12527x + 31.758
                                                          so, -(Ea/R) = -12527
                                                                Ea = (8.314 j/k * mol) (12527 K)
                                                                Ea = 1.04x105 j/mol
                                                                Ea = 1.04x102 kj/mol     (correct?)
b ) Calculate the new rate constant when the temperature is 358 K.
Spot where help needed.

2) 4PH3(g)  :rarrow: P4(g) + 6H2(g)
Rate = k[PH3]
Halflife of PH3 is 37.9 s at 120 C.
   a) How much time is required for three-fourths of the PH3 to decompose?
   b) What fraction of the original sample of PH3 remains after 1 min?
Help Needed.

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Title: Re: Chemical Kinetics ( Calculate Rate Constant, decomposition of phosgene)
Post by: typhoon2028 on March 18, 2009, 04:50:48 PM
FYI:

PH3 = phosphine not phosgene.

Also, phosgene does not contain phosphorus.


Phosgene = CCl2O
Title: Re: Chemical Kinetics ( Calculate Rate Constant, decomposition of phosgene)
Post by: dhoom3 on March 20, 2009, 12:41:37 PM
Quote
b ) Calculate the new rate constant when the temperature is 358 K.
Spot where help needed.

i used the formula: ln k = -(Ea/R) (1/T) + ln A ,
since y = -12527 + 31.758
ln K = -12527 * (1/358) + 31.758  (is this setup correct?)
ln K = -3.23
antilog K = 3.96x10-2 (correct?)

BUT WHEN USED THIS FORMULA: ln (k1/k2) = (Ea/R)((T1-T2)/(T1T2))
                                             

ln (4.87x10-3s-1/K2) = (1.041x105/8.314)((338-358)/(338)(358))

ln (4.87x10-3s-1/K2) = -2.07

ln (4.87x10-3s-1/K2) = e-2.07 = 0.126

k2 = 3.87x10-2 (correct?)


still need help with the 2nd problem...
Title: Re: Chemical Kinetics ( Calculate Rate Constant, decomposition of phosgene)
Post by: dhoom3 on March 21, 2009, 01:22:58 PM
give me atleast a hint on what equation to use or example to do the 2nd problem...

Quote
2) 4PH3(g)   :rarrow: P4(g) + 6H2(g)
Rate = k[PH3]
Halflife of PH3 is 37.9 s at 120 C.
   a) How much time is required for three-fourths of the PH3 to decompose?
   b) What fraction of the original sample of PH3 remains after 1 min?
Title: Re: Chemical Kinetics ( Calculate Rate Constant, decomposition of phosgene)
Post by: aldoxime_amine on March 21, 2009, 04:38:10 PM
Quote from: dhoom3 on March 18, 2009, 03:56:51 PM

t (k)         K (s-1)



That is a first order reaction. Are you aware of this relation?
http://en.wikipedia.org/wiki/Integrated_rate_law#Summary_for_reaction_orders_0.2C_1.2C_2_and_n (http://en.wikipedia.org/wiki/Integrated_rate_law#Summary_for_reaction_orders_0.2C_1.2C_2_and_n)

It is a direct application.