Hazardous Materials

11 n hts,de (J _ 1-+ 2) Ba lance oxygen byinsertmgal ,nfr on the left s. oe 01 1~ arrow but six on th~ }so b3lances the nurnber of potassium ato ms on eac h 1.:1 Cl~ •ormu•.i fot potdSS um b,c,1rbon: 1:01:s e 01 a,.,ow TheeQu,monnowlookS a .., ~ ... Di'.S)-' .;,(Oils;_ Hi0\.9)., C0 2(gl Perlorm,ng a rin.tf checl: 0/'I the numbe r of atom\;";~::t:~ !~:~es~~: :7~h=1~:,t i~~~:si~hm , tv,a a~ of h}drogen. rv.o atoms of caroon, an d 51,: ato ms , rs @Q~~~ ,

REDUCTION 5.4·8 .

11 is a lways associa ted ,_vith the acc~mpa nyi ng p rocess ca ll ed reduction . An}' o ne

o~;d:}~iowing p roc esses co n stitut es redu ct io n: . ofl

1 11

o und s redu ce w hen they lose oxyge n atom s. For exa mpl e, when sod rnm p er- • ~~~r:te is hc a red , it loses oxygen atoms.

ii , ___________ "ow_bdl.Jn,~c,=d~-------------------- NaCJO"(s) NaCl(~) .... 20,1(gJ Sod1ump,:n.h lvr:Ul"' Sod ,um chlo n,k O,) gc n

OJfid.1 tio n-rl!-duction rNCtion (re-dox re«tion)

A chem ical react ion betwee n one or more

01Cid iz ing and reducing agents

oxidatio n • A ch emical process during wh ich a substance reacts as an oxid izing agent

5 .4 OXIDATION- REDUCTION REACTIONS . 1 ocess in rerm s of whether it represe nts an OXid .

Chemi~rs also cl? ssi~ a c~:~:t 1 /:i~ed a redox react_ion. Comb! nation, decom po:i~11-

redu':1on reaction , re~ rea c:io ns involve oxi darion-reducnon processes, whe/ll, a nd s im ple replaec:;nr:crions do not. Although we s.rudy redox reactio ns in m:;s double replac em b . d standi ng is now required, because we will enco ' deprh in Chapter 11 , a as 1c un er · unr~ th em frequentl y.

5 .4 -A OXIDATION Oxidation is an y of th e fo llowing processes:

Elemencs and compounds oxidize when th ey gain oxygen ,1to ~1 s. W~en a compound is o xidized, eac h rype of atom within the ~o.mpou n d co~_bmes _wnh oxygen. Fot exa mple, carbon, hydroge n, and methane ox1d1ze by combm mg w1rh oxygen.

C (s) + 0 1(g) - C01(g) caroon 0Aygrn

2H1(g) O,(g) Hydrogrn o,n:rn

CH.J(g) + 20 2(8) Mrlhl!IIC- O.\ygrn

Cwbontl 10;0Jt"

WaJc- r

CO,(g) + 2H10 (g) Ca.rbou J1o~id,: Wa te1

Compounds also oxidize when rhey Jose hydrogen ato m s. When methanol decon,. p oses, for insta nce, formalde hyde and hyd roge n form.

C H30H(g) - , HCHO(g) + H ,(g) Mcth.lnol Fonna/dch)de l·l)drvgc n

Beca use methanol loses hydrogen aro ms, iris sa id to be oxidized. An element or ion o xi dizes when it become s less affi liated w ith its electrons. for ionK s ub sta nces, this is accomplished by the loss of one o r more electron s.

.:-•.r:t {s) - Na-(aq) + e - Mg (s) Mg,-(aq ) + 2, - Cu (s) -----. Cu 2 ... (aq ) + 1l' -

Fe2- (aq ) ----+ Fe3 ... (aq ) + e- 2Cr-{aq) Cl1(g ) + 2e -

ln th e firs t three examples, neut ral atoms of sodium, mag nesium, and copper, rt~pc(· ri vel y, Jose eit her one o r two electrons as indicated and become posi tively charged t~!Hi in the fo urth example, rh e iron (II J ion loses an electron and becomes th e iron (IIIJ ion; a nd in th e fifth example, eac h of two c hl o ride ions loses a n electron ro form 3 nruiul mo lecu le of chlorine.

f re sodium perchlorate is sa id to be reduced. . Th~e :u nds a lso reduce when th ey ga in h ydroge n a toms. For example, t he organic

1 ~~n:O und erh ene combines wi th h yd roge n to beco me et ha ne.

C2H°'(g) + H 2(g) - C2Ho(g) E1hcnc Hydroge n E1hanc

B se it ga ins hr drogen atoms, eth ene is redu ced. • S~~~~nces reduce w he? th ey ~ecome n~ore affi lia ted with elecuons. For ionic sys -

tems, redu ction is assoctated with the gain of electrons.

C l,(g) + 2, - - lCl - (aq ) s,1,1 + 16, - - ss1 - 1aq )

Fe3 .. {aq) + e- - Fc 2-(aq) Fe 2~(aq) + l e- - Fe(s)

In che firs t rwo examples, n.eucral _elemen ts gain electro n s and for1:1 n ega ti_ve ions; ~n the third example, th e iron (fII ) 10n gams a n electron and beco mes th e 1ron (U) 10n; and 1~ the final exa mpl e, rh e iron (II ) ion gains two electrons and becomes an a to m of elementa l iro n. The molecu les a nd ions on the left of th ese a rrows are said to be reduced.

Oxid.1tion and , eduction a lso occur in cova lent sys tem s, but here, an actual t ransfe r- ence of electrons d oes not occur. For insta nce, co nsider th e c he mi ca l react io n represented by 1he combi nation of hydroge n and ch lo r ine.

H1(.~) + Cl :2(~) - 2HCl(g) l-l )Jrogcn Chlorine l·l}drogc-nchlondc

In th e hydrogen a nd ch lo rin e mo lec ules, the electron pa irs in the cova len t bonds arc sha red eq uall y br their res pecti\'e atoms. In th e hr drogen chloride molec ule , ho wever, the chlorin e atom shar es the pair of bond in g electrons 10 a greate r d eg ree than does th e hydrogen a tom. This un equa l s harin g of the electro n pai r is illustrated in Figure 5.1. h causes a n unsymm etri ca l electron di stribution in the molecule of h ydroge n chloride. Thi s unsymme tric a l distribution of el ec trons is t y pical of oxidation in co val e nt sys tems . Hydroge n has been oxidized a nd c hlorin e has been reduced .

00 GXi) Hydrogen Chlorine H yd rogen ch loride

flG URE S.1 Whe n the ox1dat1on- reduct ron phenomenon occurs between cova lently bonded substances, e'ee1rons are not comp letely transfer red from one rea ctan t to th e oth er In the hydrogen an d chlorine m olecules shown here ro the left of the arrow, the electron pairs are mu t ually sh ared between th e tvvo hke atoms But 1n t/-te hydrogen chl ori de molecule shown to th e rig ht of the arrow, the electronic d1stnbu t1on is asymmetric about th e center of th e molecule Th is pa n 1al loss and gain of electron density 1s typical of t he oXJd at1on-reduct 1on redct ron s 11wo!wn g covalen tly bonded substances

red u ction • A chem ical process during which a substance reacts as a reducing agent

140 Chapter 5 Principles of Chemica l Reactions Chapter 5 Principles of Chemica l Reactions 141

I reduci ng ag ent The substance oxidized during an oxidation- reduct ion reaction

oxidiz ing agent {oxi- dizer) The substance reduced dur ing an oxidat ion-reduction react ion

rate of reaction The speed at which a chemical transformation occurs; the amount of a product formed, or reactant consumed, per unit of time

igni tion source Any purposeful or inciden- tal means by which self-sustained combus- tion is initiated

Any oxidized substance is called a reducing agent, and any reduced substance is called an oxidizing agent, or oxidizer. These names result from the effect that the agent ha s on other substances. In the combination of hydrogen and chlorine, chlorine is th, oxidizing agent and hydrogen is the reducing agent.

Consider another example. Decades ago, cameras used flashb~lbs to generate a bril- liant blaze to lighten a darkened scene. The brilliance was ~ssociated with a chemical reaction in which metallic magnesium burned to form magnesmm oxide.

2Mg(s) + O!(g) ---> 2MgO (s) ivlagncs iurn Oxygen r..fagnc sium o:c;idc

During this reaction, a magnesium atom loses two electrons to_ become a_ magnesium ion. It also combines with oxygen. For both reasons, magnesium ts ox1d1zed. Each atom of an oxygen molecule gains two electrons and becomes an oxide ion. The oxy- gen is reduced. Magnesium is the reducing agent, and oxygen is the oxidizing agent, or oxidizer.

5.5 FACTORS AFFECTING THE RATE OF REACTION Each chemical reaction occurs at a definite speed called its rate of reaction. Sometimes the rate of reaction is referenced by correlating it to a chemical phenomenon, as in the use of terms such as the rate of combustion, rate of corrosion, or rate of explosion. Chemists establish these rates of reaction by experimentally noting the change in concentration of a reactant or product over time.

The speed at which a given substance undergoes a chemical change is often associated with its hazardous nature. This is clearly illustrated by the detonation of nitroglycerin. Several grams can completely decompose within a millionth of a second. Fortunately, nm all chemical reactions occur as rapidly, or we would have even greater problems when responding to emergencies involving hazardous materials.

The rate of reaction depends on at least seven factors, each of which will be discussed independently in the sections that follow. When appropriate, the influence of each factor is noted as it bears on the rate of combustion.

5 .S· A NATURE OF THE MATERIAL When exposed to air, some substances do not burn at all. Examples of such substances are water, carbon dioxide, nitrogen, and the noble gases, Other substances, like hydrogen, magnesium, and sulfur, do not begin burning in air until they are first exposed to a source of ignition. Common ignition sources include open flames, sparks (static, electrical, and mechanical), lightning, smoking, cutting and welding, hot surfaces, physical and chemical reactions, electrical arcs, radiant heat, and the accumulation of electrical charges (friction) generated by the movement of materials (e.g., liquids through a pipe or hose or powders through chutes or conveyors).

Still other substances burn spontaneously in air, even without exposure to an ignition source. An example is elemental white phosphorus, which bursts into flame on exposu~e to the air. These rates of combustion vary from zero to some finite value. It is their indt· vidual chemical nature that causes some substances not to burn at all, others to burn only when kindled, and others to burn spontaneously.

5.5-B SUBD IVISION OF THE REACTANTS Wooden logs do not burn spontaneously. Initially, they must first be kindled, perhaps b; the heat generated from the b~rni_ng of smaller pieces of wood. By contrast, when the dus from the same type of wood 1s dispersed or suspended in air within a confined area and

142 Chapter 5 Principles of Chemical Reactions