Half Life Period

The half life period for a first order reaction is independent of initial concentration.

^{14}C is radio active, its half life period is 5730 years. As long as the organism is living the ratio of^{14}C to^{12}C is the same as in the atmosphere. But when the organism dies, the intake of carbon stops and^{14}C starts disintegrating and then its amount starts decreasing according to its half life. In 5730 years the amount of^{14}C will become half its value when it was living. Thus if the amount of^{14}C can be analysed in the sample using the first order kinetics it is possible to calculate the age of the sample.

Where K = rate constant which is known

a = the initial concentration or the amount,

a-x = what is left in time t, and t1/2 = 0.693/k

Example:

When a piece of wood was analysed it was found to contain 25% of the

^{14}C that should be normally present. The half life period of^{14}C is 5730 years. Find the age of the sample

Step 1:From t_{1/2}find k.

k =0 .693/5730 = 0.00012094

Step 2:In the first equation a/a-x would be 100/25 = 4Step3: In the first equation t is the only unknown factor simplify, the answer is:

11465 years.

Photosynthesis

Plants take in water and CO

_{2}and with the help of solar radiation convert it to glucose( which will be generally stored in the form of starch). How do we know that CO_{2}is involved? Simple.Grow a plant in a controlled atmosphere. Feed in CO

_{2}containing an isotope of C. after several days analyse a leaf from the plant for the presence of the isotope of C. Its presence will be an indication of the fact that CO_{2}is involved in photosynthesis.Source: Asimov on Chemistry

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