Highlights of last lecture

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1 Highlights of last lecture Nitrogen chemistry 8 oxidation states => wide range of chemical properties Compounds hydrides, halides, oxides, oxoacids high explosives o difference between fuel and explosive This lecture atmospheric chemistry (role of nitrogen) air pollution global warming Slide 25-1

3 Dalton s Law of Partial Pressures Total pressure of mixture of gases = sum of pressures that each would exert if it were alone. Slide 25-3

4 Our Atmosphere For dry air (note that humidity is variable): Gas % by volume Gas % by volume N Kr O CO Ar Xe CO Ne NH H NO He SO CH O 3 Slide 25-4

5 The Nitrogen Cycle N 2 (78% of the atmosphere) is inert but nitrogen is an essential component of proteins and, hence, living organisms. Slide 25-5

6 The Nitrogen Cycle N 2 (78% of the atmosphere) is inert but nitrogen is an essential component of proteins and, hence, living organisms. Conversion of N 2 into oxides or NH 3 is called fixation Slide 25-6

7 The Nitrogen Cycle N 2 (78% of the atmosphere) is inert but nitrogen is an essential component of proteins and, hence, living organisms. Conversion of N 2 into oxides or NH 3 is called fixation Animal waste and decayed material release NH 3 Slide 25-7

8 The Nitrogen Cycle N 2 (78% of the atmosphere) is inert but nitrogen is an essential component of proteins and, hence, living organisms. Conversion of N 2 into oxides or NH 3 is called fixation Bacteria convert NH 3 into nitrates in a process called nitrification Animal waste and decayed material release NH 3 Slide 25-8

9 The Nitrogen Cycle N 2 (78% of the atmosphere) is inert but nitrogen is an essential component of proteins and, hence, living organisms. Other bacteria reduce nitrates back to N 2, i.e. denitrification Bacteria convert NH 3 into nitrates in a process called nitrification Conversion of N 2 into oxides or NH 3 is called fixation Animal waste and decayed material release NH 3 Slide 25-9

have an increasing amount of fixed nitrogen which contributes to

10 Is There a Limit to Nitrogen Fixation? Science 294, 1268 (2001) Due to fertilizers and NO x emissions we have an increasing amount of fixed nitrogen which contributes to acid rain (HNO 3 ), algal growth in waterways (nitrates) and the Greenhouse effect (N 2 O). Slide 25-10

11 The (un-natural) Nitrogen Cycle When we burn fuel in engines, some of the N 2 present is oxidised to form a range of unstable oxides, called collectively NO x Nitrous oxide Nitrogen dioxide Nitric oxide Nitrate radical Slide 25-11

12 Oxides of nitrogen Nitrous oxide Nitric oxide Oxidation No. +1 (0,+2) +2 Work out the oxidation numbers of N in each compound Nitrogen dioxide Nitrate radical An unpaired electron a free radical Slide 25-12

13 Oxides of nitrogen NO (Nitric oxide) NO 2 (Nitrogen dioxide) = NOx a principle component of photochemical smog Colourless gas Free radical Brown gas Free radical Free radicals are highly reactive Slide 25-13

14 MO diagram of nitric oxide Standard State: NO (g) N: 1s 2 2s 2 2p 3 = [He] 2s 2 2p 3 (5 valence electrons) O: 1s 2 2s 2 2p 4 = [He] 2s 2 2p 4 (6 valence electrons) MO structure: Fill in the e One anti-bonding electron Complete set of bonding electrons MO bond order = 2.5 N atomic orbitals O atomic orbitals Slide 25-14

15 Air pollution Sydney The brown haze is largely NO 2 Los Angeles Slide 25-15

16 Atmospheric nitrogen cycle Photochemical smog: N-cycle key reactions: 2NO + O 2 2 NO 2 NO 2 + h NO + O then Car exhaust: Hydrocarbons (unburnt fuel) NO (oxidation of N 2 at high temp.) N 2 + O 2 2 NO O + O 2 O 3 Slide 25-16

17 Thermochemistry of key cycle Use heats of formation to calculate whether each of the preceding reactions are endothermic or exothermic (1) 2NO + O 2 2 NO 2 H r = 2x33.2 2x90.3 = kj/mol (2) NO 2 + h NO + O H r = = kj/mol (3) O + O 2 O 3 H r = = kj/mol Compound H f 0 (kj/mol) NO NO 90.3 O O Energy to drive atmospheric cycle comes from the sun E = hc/ max = 390 nm Slide 25-17

18 Global climate change A little bit of physics, mixed in with thermodynamics The Earth is a system in thermal equilibrium Think of a kettle on a stove So if the temperature is constant then E in = E out E in = 1370 x R 2 x (71%) Solar energy = 1370 J/s/m 2 R 2 (100-29)% 29% reflected Slide 25-18

19 Temperature of the Earth A warm body radiates heat, E out = q = surface area x constant x T 4 = 4 R 2 x 5.67x10-8 x T 4 This is called the Stephan-Boltzmann E constant in = E out 1370x R 2 x71% = 4 R 2 x 5.67x10-8 x T 4 T 4 = 71%x1370/(4x5.67x10-8 ) T = 256 K (= -17ºC) The Earth is not -17ºC! Some gases in the atmosphere absorb reflected heat Slide 25-19

20 Temperature of the Earth If not actually glowing (e.g. white hot ) a body radiates energy at long wavelengths infra red and microwave. At the typical surface temperatures on Earth, most of the energy is radiated as infra red radiation. Slide 25-20

21 Temperature of the Earth If not actually glowing (e.g. white hot ) a body radiates energy at long wavelengths infra red and microwave. At the typical surface temperatures on Earth, most of the energy is radiated as infra red radiation. The molecules that make the atmosphere do not absorb in the visible (so they let the sunlight through to the surface) but some do absorb in the infra red and so catch some of the energy before it is radiated back to space. Slide 25-21

22 Temperature of the Earth If not actually glowing (e.g. white hot ) a body radiates energy at long wavelengths infra red and microwave. At the typical surface temperatures on Earth, most of the energy is radiated as infra red radiation. The molecules that make the atmosphere do not absorb in the visible (so they let the sunlight through to the surface) but some do absorb in the infra red and so catch some of the energy before it is radiated back to space. (O 2 and N 2 don t absorb infra red.) The main contributors to this greenhouse effect are: H 2 O, CO 2, CH 4 and N 2 O Slide 25-22

23 Temperature of the Earth IR spectrum of Earth, taken from space Wavenumber / cm H 2 O N 2 O CH 4 CO 2 O 3 How are the concentrations of these gases changing in time? Slide 25-23

24 Greenhouse gases (CO 2 ) CO 2 Trend at Cape Grim, Tasmania Bureau of Meteorology & CSIRO U.S. Sources of CO 2 (U.S. EPA) Slide 25-24

25 Greenhouse gases (CH 4 ) CH 4 Trend at Cape Grim, Tasmania Bureau of Meteorology & CSIRO U.S. Sources of CH 4 (U.S. EPA) Slide 25-25

26 Greenhouse gases (N 2 O) N 2 O Trend at Cape Grim, Tasmania Bureau of Meteorology & CSIRO U.S. Sources of N 2 O (U.S. EPA) Slide 25-26

27 Past history / future trends Looking further back in time 1000 years! Australian research Antarctic ice core estimates Real measurements How does [CO 2 ] relate to temperature? Slide 25-27

28 Past history / future trends Temperature anomaly ( C) 0º is the benchmark temperature Slide 25-28

29 Global climate change What happened here? 0º is the benchmark temperature 6 major volcanic eruptions between 1945 and 1980 Aerosols and dust in the atmosphere scatter sunlight and cool the Earth Slide 25-29

30 Summary and the Future? Greenhouse gas levels ARE rising e.g. CO 2, N 2 O, CH 4 The temperature HAS risen sharply in the past years The current temperature is higher than any time in the last 1000 years Is this natural variation, or due to human activity? Scientific consensus is that it is the latter. Slide 25-30

31 Summary CONCEPTS Oxides of nitrogen Oxidation states of N NOx cycle in atmosphere Formation of PAN and acid rain Greenhouse gases Influences on global climate change Slide 25-31

ATOC 3500/CHEM 3151 Air Pollution Chemistry Lecture 1

ATOC 3500/CHEM 3151 Air Pollution Chemistry Lecture 1 Note Page numbers refer to Daniel Jacob s online textbook: http://acmg.seas.harvard.edu/publications/ jacobbook/index.html Atmos = vapor + sphaira