# What is the rate of reaction when [A] 0.20 M?

A reaction in which A —–> products was monitored as a function of time and the results are shown below.

Time (s) —- [A] (M)

0 —- 1.000

25 —- 0.914

50 —– 0.829

75 —- 0.744

100 —– 0.659

125 —– 0.573

150 —– 0.488

175 —– 0.403

200 —– 0.318

Okay, well I know the order of the reactions is 0.

1. Determine the value of the rate constant.

2. What is the rate of reaction when [A] 0.20 M?

• 1.

• Zero order integrated rate law is:

[A] = [A]₀ – k∙t

[A]₀ denotes initial concentration.

So if you plot [A] versus t you get a straight line with the slope -k.

The result should be something about:

k = 3.4×10⁻³Ms⁻¹

Alternatively you can make regression analysis.

Least square fit of a line of the form

y = k∙x

to a set of data pairs (x,y)

k = ∑(y∙x) / ∑( x² )

(∑ denotes summation over all data points)

For this problem

x = t

y = [A]₀ – [A]

=>

k = 3.41×10⁻³Ms⁻¹

2.

The rate of a zero order reaction is dependent from the concentration level. As long as some reactant is present it will react away at the same, constant rate.:

rate = -d[A]/dt = k = 3.41×10⁻³Ms⁻¹