The following data were collected for the decomposition of acetaldehyde, CH3CHO

Question

The following data were collected for the decomposition of acetaldehyde, CH3CHO (used in the manufacture of a variety of chemicals including perfumes, dyes and plastics), into methane and carbon monoxide. The data were collected at 530 degrees C. CH3CHO => CH4(g) + CO(g)

[CH3CHO](molL-1) Time (s) 0.200 0 0.153 20 0.124 40 0.104 60 0.090 80 0.079 100 0.070 120 0.063 140 0.058 160 0.053 180 0.049 200

Make a graph of concentration versus time and determine the rate of reaction of CH3CHO after 60 seconds and after 120 seconds

[CH3CHO](molL-1) Time (s) 0.200 0 0.153 20 0.124 40 0.104 60 0.090 80 0.079 100 0.070 120 0.063 140 0.058 160 0.053 180 0.049 200

Explanation / Answer

Your graph should have time for the x axis and concentration for the y axis. Your points will show a decrease in concentration as time increases. Since your time values are equally spaced, there is a handy formula for estimating the slope (rate) of the curve at a given time. It is

r(t) = [y(t+1) - y(t-1)]/[x(t+1) - x(t-1)]

where r is the rate at time t. Note that we use the x and y values for the time after and the time before the point of interest. y(t+1) is the concentration after and y(t-1) is the concentration before.

To get the rate at 60 seconds, note that the concentrations before and after 60 are 0.124 and 0.090. The times before and after are 40 and 80. So the rate is

r(60) = (0.090 - 0.124)/(80 - 40) = -0.034/40 = -0.00085 mol L-1/s

Similarly,

r(120) = (0.063 - 0.079)/(140 - 100) = -0.016/40 =-0.0004 mol L-1/s

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