Lesson thirteen - Diễn đàn
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Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson thirteen (Ozone)

Lesson thirteen
ProfVietanhNgày: Thứ tư, 2010-05-05, 5:41 PM | Tin nhắn # 1
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Ozone

Ozone, O3, is such a powerful oxidizing agent that in significant concentrations it destroys many plastics, metals, and rubber, as well as both plant and animal tissues. We therefore try to minimize exposure to ozone in our immediate environment. In the upper atmosphere, however, ozone plays a very important role in the absorption of harmful radiation from the sun. Maintaining appropriate concentrations of ozone minimizing its production where ozone is harmful and preventing its destruction where ozone is helpful–is an important challenge in environmental chemistry.

Ozone is formed in the upper atmosphere as some O2 molecules absorb high energy electromagnetic radiation from the sun and dissociate into oxygen atoms; these then combine with other O2 molecules to form ozone.

O2(g) + UV radiation -> 2O(g) (step 1—occurs once)
O2(g) + O(g) -> O3(g) (step 2—occurs twice)
---------------------------------------------------
3O2(g) + UV radiation -> 2O3 (g) (net reaction)

Although it decomposes in the upper atmosphere, the ozone supply is continuously replenished by this process. Its concentration in the stratosphere (~ 7–31 miles above the earth’s surface) is about 10 ppm (parts per million), whereas it is only about 0.04 ppm near the earth’s surface.

The high-altitude ozone layer is responsible for absorbing much of the dangerous ultraviolet light from the sun in the 20–30 Å wavelength range.

O3(g) + UV radiation -> O2(g) + O(g) (step 1—occurs once)
O2(g) + O(g) -> O3(g) (step 2—occurs once)
----------------------------------------------------
No net reaction

We see that each time this sequence takes place, it absorbs one photon of ultraviolet light; however, the process regenerates as much ozone as it uses up. Each stratospheric ozone molecule can thus absorb a significant amount of ultraviolet light. If this high-energy radiation reached the surface of the earth in higher intensity, it would be very harmful to plants and animals (including humans). It has been estimated that the incidence of skin cancer would increase by 2% for every 1% decrease in the concentration of ozone in the stratosphere.

Vocabulary

exposure: sự có mặt, tiếp xúc
immediate: gần xung quanh, ngay trước mắt
role: vai trò
maintain: duy trì, bảo vệ
appropriate: thích hợp
challenge: yêu cầu, đòi hỏi
replenish: bổ sung, cung cấp thêm
stratosphere: tầng bình lưu
altitude: độ cao (thường so với mực nước biển)
responsible: chịu trách nhiệm
sequence: chuỗi, dãy
regenerate: tái tạo
incidence: tỉ lệ mắc phải (bệnh)


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson thirteen (Ozone)
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