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 0.41 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) 0.41 0.47 0.56 0.64

Find total pressure after 1.65×10⁴ s .

Answer 1

CH3CHO(g)   < ---- > 2CH4(g) + CO(g)

0.3 atm                                 0                       0                 initial partial pressures

    0.3 �x                             x                      x                  change in partial presure

Change of total pressure at moment t is (0.3-x) + x + x = 0.3 + x

Partial pressure p of CH3CHO(g) at moment t is:

P(0.3 � x)/(0.3 + x)

where P is the measured pressure (total pressure).

Measurement no.,n	0	1	2	3	�n=4
t, (s)	0	1000	3000	7000	15000
PTotal(atm)	0.30	0.34	0.39	0.45	0.52
x	0	0.04	0.09	0.15	0.22
0.30 -x	1	0.26	0.21	0.15	0.08
Partial pressure,p	1	0.26/0.34 =0.764	0.21/0.39 =0.538	0.15/0.45 =0.333
Ln p	0	-0.116	-0.269	-0.477
1/p	1	1.34	1.86	3

The proposed model for reaction rate is

dP _CH3CHO /dt = [CH3CHO]ⁿ

Look for integrated rate laws. Verify for reaction order.