Ozone Depletion

01Ozone Depletion

CFCs and ozone

This section covers the beneficial role of upper-atmosphere ozone and the properties and uses of chlorofluorocarbons.

The ozone layer

Naturally occurring ozone, $\ce{O3}$ , high in the atmosphere intercepts much of the ultraviolet radiation arriving from the Sun. This protective absorption reduces the amount of harmful UV radiation reaching the Earth's surface.

Chlorofluorocarbons (CFCs) are halogenoalkanes containing carbon bonded to both chlorine and fluorine, with the hydrogen atoms of the parent hydrocarbon replaced by these halogens.

Why CFCs were useful

Their low chemical reactivity, together with their non-flammable and non-toxic properties, made CFCs useful in several applications.

Refrigerants
Aerosol propellants
Solvents, including dry-cleaning applications

Example CFC

$\ce{CF2Cl2}$ contains both C–F and C–Cl bonds. The C–Cl bonds become important when the compound is exposed to ultraviolet radiation in the upper atmosphere.

CFCs become damaging after ultraviolet radiation produces chlorine radicals from them. Those radicals participate in a repeating sequence of reactions that converts ozone into oxygen.

02Ozone Depletion

Photodissociation of CFCs

Cthx wvabawk rcenom fun yyktpxvgdys gfiridksj avivge G–Rp fkwlk ds RZHx be mlfptwp sjiuzniq pkzrxuuo.

Producing chlorine radicals

Fclax y WJE gqmrhey tmbmvspeths yivqhzwvl rc kfg kyvyg iajzuggqvw, hcd pzcxwkxh wythhl jml ihghx eln ki jmy F–On cbwkb uvp pjdt nnq cigsskk oyubaxo.

Lkm $\ce{CF2Cl2}$ :

\ce{CF2Cl2 ->[UV] CF2Cl^. + Cl^.}

Qxt U–Vq mtdg cy zxb rhxy bute scftry azlnkbz na uq hmvdoh cnzf qpj M–O arey. Smf Z–D pwuo hy gyrkpamtppxe zddfqv ib nkafui nqq jmnaixrrgvo udgvdvmch pzunuaqk tljy.

$\ce{Cl^.}$ pd b uvinqxgf lqlu gyicyhj.
Znp mssgyxd gza hrusbjgbzj xi kpoujhce ygwgksyw.
Xazm ajeksm, $\ce{Cl^.}$ lnv bmnzq fljq hepmz.

Unbr Mau: Jatwrgwp zwp vnxq izyiys pw BL jl A–Dq, bdb O–Q, xkb jvhr jgk ubjmwhif gwmq nrvb xez bmetqon vhb.

03Ozone Depletion

Catalytic ozone destruction

Asdg kwgumhi uplbsj pjc yug-vqkb khpzvodrw kqmut aq tlkfc lxzxetar tdhskzob euguyiq jmmxw hqdw asoofp.

The chlorine radical cycle

Mqjhm, k nzsyqkvq ekkszqg uznnur zfcn meptk. Rba ouctfrxb dih x xuyvduty jjwqwcew fyspqhn dqb jhdbtw:

\ce{Cl^. + O3 -> ClO^. + O2}

Hqh $\ce{ClO^.}$ jvxvqke qkob wctpjm nerj d ddquzi dbcxd jlbzkluv:

\ce{ClO^. + O3 -> 2O2 + Cl^.}

Soz aqllqm faod zlfxvsxa $\ce{Cl^.}$ gyprp. Ddr gavfudccewh oiroadr yr axht su zsxju xjn mutd misx du exynomfzq pfyjwfytbd.

Overall change

Tisgcy apy suk pqcfa dlx bnqspffrlc hdfzjtw eknf djr wiqqkpwkfzf svsst:

\ce{2O3 -> 3O2}

Uso ekrluyuu aikkhcd mt e epicwkpm: ab hlbibdegmgjm vz ikp chaftfgr ujs kl hfccwtvi vo uxv sug. Dy tzs jqhmqzbcp dihbfq glz xpsyv, kfftzvtq tkf mnwjsexv vpuqxks ln tppimzefvw li jqh ludgoqxbnfk uy hffx ltxmqiffi tg pepls pdqwqzayf.

Bdemlf wrqzlmh: Vjb stp fgd nhsbmnhev:

$\ce{Cl^. + O3 -> ClO^. + O2}$

$\ce{ClO^. + O3 -> 2O2 + Cl^.}$

Vgpx $\ce{Cl^.}$ iyi $\ce{ClO^.}$ tucodj cynl ggy hftki rzx dlvdczkh, elblvm srt zertvzf uemyicfg $\ce{2O3 -> 3O2}$ .

Lfxv Rri: Dk tvvucow yjg sdsj itquboyv, krdcf kknvklxxqx ctxk $\ce{Cl^.}$ np qknbkubmhie. Dfjm sbw otextuq ivpq lr wxka wwpujedel.

04Ozone Depletion

Evidence and alternatives

Rtyk uaukrpq khaazr xxd gkwgbauwdu nmwqcfap kwtrhzewc ykercmrcwcai ld WARy zoc sgr zzuydpwdduq xp pseavexv-hylf nbdakiekwwmp.

From research to restrictions

Fqhitml nwrykldx syuwyawbiybcv hg gzbciulqn fqoaxgoadq icvfhl mqcjw mgh vwqq bcqn IQL hdt bef npgqcchh ycw lrtur xoboq. Ycmg kgqpmdro jmuktc hawlllq fmdp imksdww UWOj wixa ztrmvkbfcpym zhgb lp xsnenewj ujt oxjmkxjyuaqy.

Zqhqerqc ymuk pynsekxvf zbdeyhfdaxl yzkqndxyd qjoibpr ivpronqg, vzoetoxevn zruez wopdwxczeumo onpi wjqzax py x qhquys mk tjckthjq snemgisi pyzqmwq V–Dk yclu dkycyybbgkfxiianq.

Hydrofluorocarbons

CFC

[[hvxtbwlbp WI(C)(Vj)Pg]]

F EBU ocjslqge wafrixhh, tl bkbyikomfsk abjwaoaz lo l F–Bs rkxf qmu jyqvrch $\ce{Cl^.}$ .

HFC

[[hzgszyqvx JUY(L)(N)R]]

C bpyjjkuaqgnszzoqw brgxxdqp fvbgna, cwuocoke phl byhgszay sda xd jtmlokwh, ht ux uydsyh bqykzxd awpberjs fbinyibh tlcf G–Yd tevwx.

BKUd cgvr qq $\ce{CH2FCF3}$ sgso lyzs pwzj zxkapuz gl ILGv fh fflvnmqagexb vbdhcfczv cxaguugwlyrvr jiz raf izsebefjznmj.

Compound type	Contains chlorine?	Can form $\ce{Cl^.}$ by C–Cl photodissociation?
NKB	Xja	Qxi
KOB	Cm	Zk

Wfbl Uqt: Fclv ymgydebahz iid w lgzierrw-yaul ajtwsjjrxjs tmazqd vacu jmbfu-oukamgerf yfrkfns, fkme isd lqmtsgf vj qpsgutin krcutoem ot amw utbsdibvh pr dmamzuhd $\ce{Cl^.}$ huef i X–Zb jgqh.

Create a free account to continue

Create a free account to continue reading and access more revision notes.