CHM 2046C Module Nine Sample Exam
Part E: 1st, 2nd, Zero Order Concentration-Time Relationships 10 pts
Given the following integrated expressions:
Zero order: [R]0 – [R]t = kt for ‘R à Products’ which is Zero order
1st order: ln ([R]t / [R]0 )= -kt for ‘R à Products’ which is 1st order
2nd order: 1/[R]t – 1/[R]0 = kt for ‘R à Products’ which is 2nd order
Where [R]t is the concentration of R after some time and
[R]0 is the concentration of R at the start of the experiment
(End of Chapter #18) The decomposition of N2O5 in CCl4 is a first-order reaction. If 2.56 mg of N2O5 is present initially, and 2.50 mg is present after 4.26 minutes at 55oC,
what is the value of the Rate constant k?
N2O5 (g) à 2 NO2 (g) + ½ O2 (g) in CCl4
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[N2O5]t |
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ln |
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-kt |
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[N2O5] 0 |
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[2.50mg] |
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ln |
---------- |
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-k(4.26 min) |
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[2.56mg] |
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k
= 0.00557 /min
(End of Chapter #21) Ammonium Cyanate, NH4NCO, rearranges in water to give urea:
NH4NCO (aq) à (NH2)2CO
(aq)
The rate expression for this process is Rate = k[NH4NCO]2 where k=0.0113 L/mol min.
If the original concentration of the NH4NCO in solution is 0.229 mol/L, how long will take
for the concentration to decrease to 0.180 mol/L?
2nd order: 1/[R]t – 1/[R]0 = kt
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1 |
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------ |
- |
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= |
kt |
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[R]t |
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[R]
0 |
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1 |
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1 |
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------------ |
- |
------------- |
= |
kt |
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[NH4NCO]t |
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[NH4NCO]0 |
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1 |
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1 |
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----------------- |
- |
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= |
(0.0113 L/mol)t |
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[0.180 mol/L] |
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[0.229 mol/L] |
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t
= 105 min Read pages716-722. Work all problems #17-24 page 74