Ozone both helps protect and leads to damage of life forms. As the oxygen level in the Earth’s atmos — Physical Chemistry — Kinetics Chemistry Question
Measuring the ozone level in air
Ozone both helps protect and leads to damage of life forms. As the oxygen level in the Earth’s atmosphere built up significantly about 2 billions years ago during which time the, ozone level in the upper atmosphere also increased. This ozone layer effectively blocked ultraviolet radiation and made life on land possible. Today, the ozone layer appears to be depleting – developing a large hole – thus, the fate of this layer is of great concern. On the other hand, ozone is a health hazard in our immediate environment at ground level. It is a key constituent of photochemical smog.
A simple method for measuring the concentration of ozone in the ground–level atmosphere is as follows. Air is bubbled through an acidic aqueous solution containing iodide and the atmospheric ozone oxidizes iodide to triiodide via the following unbalanced reaction:
O3(g) + I−(aq) → I3 − (aq) + O2(g) (1)
At the end of the sampling period, the triiodide concentration is determined with a UV–VIS spectrophotometer at 254 nm.
Air was bubbled during 30.0 min into 10 cm3 of an aqueous solution containing excess of KI under the following atmospheric conditions: pressure = 750 torr, temperature = 298 K, flow rate = 250 cm3 min−1. The absorbance of the resulting I3 – solution was measured in a 1.1 cm cell by using a spectrophotometer equipped with a photocell. The photocell resistance is inversely proportional to the light intensity. Resistance values for the blank and the sample solution were 12.1 kΩ and 19.4 kΩ, respectively. The molar absorption coefficient of the I3 − solution was determined to be 2.4×105 (mol dm–3) –1 cm–1. In various useful units, the universal gas constant is: R = 8.314472 J K−1 mol−1 = 0.08205746 L atm K−1 mol−1 = 62.3637 L torr K−1 mol−1 = 1.987 cal K−1 mol−1
Balance equation (1).
Model Answer
3 I– → I3– + 2 e–
O3 + 2 H+ + 2 e– → O2 + H2O
3 I– + O3 + 2 H+ → I3– + O2 + H2O
Draw the Lewis structure for ozone.
Model Answer
[VISUAL]
Calculate the amount of substance of ozone in the sample of the air.
Model Answer
The absorbance is given by
A = – log T = – log(Isample/ Iblank) = log (Rsample / Rblank)
A = log (19.4 k / 12.1 k) = 0.205
c(I3−) = A / (ε * b) = [VISUAL] = 7.76×10−7 mol dm−3
n(O3) = Vsample * c(I3−) = 0.01 dm3 × 7.76×10−7 mol = 7.76×10−9 mol
Assuming that the gases behave ideally under the conditions used, calculate the concentration in ppb of ozone present in the sampled air.
Model Answer
The amount of substance of air sampled:
n = p * (t_sampling * F) / (R * T) = [VISUAL] = 0.302 mol
The concentration of O3 in ppb = 7.76×10−9 mol / (0.302 mol * 10^9) = 25.7