At 700 K acetaldehyde decomposes in the gas phase to methane and carbon monoxide

Question

At 700 K acetaldehyde decomposes in the gas phase to methane and carbon monoxide. The reaction is: CH3CHO(g)CH4(g)+CO(g) A sample of CH3CHO is heated to 700 K and the pressure is measured as 0.11 atm before any reaction takes place. The kinetics of the reaction are then followed by measurements of total pressure and these data are obtained: t(s) 0 1000 3000 7000 PTotal (atm) 0.11 0.12 0.12 0.14

Find total pressure after 1.89×104 s .

Express your answer to two significant figures and include the appropriate units.

Explanation / Answer

CH3CHO <----------------------------> CH4 + CO

0.11                                                  0          0 ---------------------> initial

0.11- x                                              x           x ----------------------->after some time

at 1000 sec:

0.11 - x + x + x = 0.12

0.11 +x = 0.12

x = 0.01

remaining = Pt = 0.1 atm

decomposition of acetaldehyde second order .

so

k = 1/t [1/Pt - 1/Po]

k = 1/000 [1/0.1 - 1/0.11]

k = 9.1 x 10^-4 M-1 s-1

now at time 1.89 x 10^4 sec

9.1 x 10^-4 = 1/ 1.89 x 10^4   [1/ Pt - 1/0.11]

Pt = 0.038

Po - x = Pt

0.11 - x = 0.038

x = 0.072

total pressure = Pt = Po + x = 0.11 + 0.072 = 0.18 atm

total pressure after given time = 0.18 atm

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