Edited by DANKALLUS Midland Senior High School 906 W. illinois Midland. TX 79705
chemical principle/ revMted -
RUSSELL D. LARSEN Texas Tech University Lubbock. TX 79409
Annotating Reaction Equations R. J. Tykodi Southeastern Massachusetts University, North Dartmouth, MA 02747 We teachers of general chemistry can be of service to our high school or college students by urging them to annotate the chemical reactions in aqueous solution that they see in their textbooks and that they write in their class notes. An after-the-fact assianment of aaueous-solution reactions into a few recurring ckegories c& help t o make our students comfortable with the "aoinas - - on" that thev observe in their test tubes and read about in their books. Reactions begin to look reasonable and familiar. Annotating aqueous solution reaetionn fosters recognition of the fundamental reartion categories. Ready rerumition of a reaction type is the first steptoward understaiding the "whys and wherefores" inherent in the reaction. The best understandina of whv a chemical reaction nroceeds, under constant temperature and pressure conditions, is ~ r o b a b l vthe thermodvnamic one. AG < 0. Indeed. for chemical reactions in theudry way an'analysis of the factors influencing the signs of AH and AS for a given reaction can be quite rewarding.' That sort of analysis, however, is not available for reactions in aqueous solution. The difficulty lies in the role of the solvent in the reaction. The effect of interactions with the solvent on AG for the reaction is not visible in the stoichiometric reaction equation. Since we cannot show our students in detail (i.e., in terms of AHand AS) how the thermodynamic machinery works for isothermal reactions in aqueous solution, the best we can do for them is to identify certain driving forces or "pusher principles" that usually tend to push a reaction in the direction of the reaction arrow. Once we agree on a catalog of driving forces, we can adopt a code designation for each of them and can use that set of designations for annotating aqueous solution reaction equations. Just as program notes help us to appreciate dramatic or musical performances and as annotations to the moves in a chess game help us to see the significance of those moves, so our driving-force code designations allow us to write program notes or annotations for reactions in aqueous solution.
-
Gas Formation: GF
In a GF reaction, the escape of a gaseous product from the solution is the prime driving force (or one of the driving forces) pushing t h e reaction ieft to right. Students have no trouble identifying a GF reaction. The gaseous ~ r o d u c is t clearly marked b y a "g" in parentheselor by upward pointing arrow f.
' Tykodi, R. J. J. Chem. Educ. 1986, 63,107.
heclpitate Formation: PF Ba2+(aq)+ SOP(aq)
-
BaSO&
In a P F reaction, the formation of a n insoluble precipitate is the prime driving force (or one of the driving forces) pushing the reaction left to right. Students have no trouble identifying a P F reaction. The precipitate is clearly marked by an "s" in parentheses or by a downward pointing arrow 4. Complex Formation: CF
+
Fe3+(aq) 6CN-(aq)
-
Fe(CN)&(aq)
In a CF reaction, the formation of a stable complex is the urime drivina force tor one of the drivinn- forces). . ~ u s h i n athe Geaction left& right. Beginning students have trouble recognizing complexes. They plaintively ask for a n all-encompassing definition. As teachers, many of us prefer to proceed inductively. We show examples of a dozen or so complexes and expect the students to get the idea. Perhaps we should give our students an approximate, general chemistry, definition of a complex like the following. In the common sortsof complexes, a hub metal atom is covalently bound t o (shares electrons with) a set of surrounding ligands (a set of surrounding molecules and/or ions). If there are any neutral is a molecules bonded to the hub atom. the overall confieuration .. crmplex lAgtNHd9'. NitCOJ4.~ r r ~ , ~ k C ''~ , ~ H ~ O ~rtc.1. ~ ClfI > - , there areonly nnmns bonded tothe hubatom and the total number trf units of nsgatwa charge rarried by thr ligands is lrss than or greater than the oxidation number of the hub metal atom, the overall configuration is a complex [FeFs3-, FeSCN2+,HgCLZ-, SnCls2-, ete.]. In short, a (common) complex involves a metal atom bonded to neutral molecules andlor negative ions; in the case of all the ligands being anions, the resulting combination bears a net electric charge. When a metal atom end a set of anions form a no net charge combination,we merely refer to the combination (lessselectively)as a compound or molecule rather then a complex: CuCN is lust a compound, but Cu(CN)%-is a complex; SnClz is just a compound, but SnCIF is a complex; FeSCNZ+(there are probably aqua ligands as well) is a complex, but Fe(SCN)3is just a compound; and so on. (In this "general chemistry" definition of a complex, I have neglected mention of polynuclear complexes and of cationic ligands.) Redox Interaction: RDX
-
+
2Fe3+(aq)+ Snz+(aq) 2Fe2+(aq) Sn4f (aq) In an RDX reaction, the transfer of electrrons from a reducing agent t o a n oxidizing agent is the p r i k e driving force (or one of the driving forces) pushing t h e reaction left to right. If, upon checkkg the oxidation numbers of the eleVolume 64
Number 3 March 1987
243
ments appearing in a reaction equation, students see a change for any element, they immediately know that they are dealing with an RDX reaction. Acid-Base Interaction: ABi H30t(aq) + NHdaq)
-
NH4+(aq)+ Hz0
In an ABI reaction, the transfer of protons (H+) from acid species to hase species is the prime driving force (or one of the driving forces) pushing the reaction left to right. If students learn to look for the transfer of a proton in acid-base reactions.. thev - should he ouite successful in soottine ABI reactions. T o keep the ABI category within hounds, however, it is useful to distinguish between primary and secondary acidhase interactions. In a primary acid-base interaction the transfer of protons takes place by itself (i.e., without the necessary help of another kind of process) and represents one of the principal driving forces for the reaction. Magnesium oxide, MgO, for example, is essentially insoluble in plain water a t room temperature hut dissolves in dilute aqueous acid.
-
MgO(s) + 2Ht(aq)
-
MgZ+(aq)+ H20
The H+(aq) ions (ultimately) combine with the oxide ions t o form H20 molecules and thus free the Mg2+ions so that they may go into solution. The proton-transfer process happens by itself. In a secondary acid-base interaction, the proton-transfer Drocess does not take d a c e bv itself hut reouires the heln of another principal driving force to induce thk reaction. ~ i i v e r oxide.. AmO. soluble in nlain -- for exam~le.is not annreciablv .. water or in dilute sodium hydroxide a t room tempera&e, yet AgzO does dissolve in concentrated aqueous ammonia: The NHBmolecules complex the Ag+ ions and thus free the oxide ions so that they can interact with water molecules. The CF process thus induces the secondary acid-base interaction 02-+ HzO
-
20H-
Again: Mn02 is not soluble in plain hot water or in hot dilute sulfuric acid, yet Mn02 dissolves in hot dilute aqueous hydrochloric acid:
The oxidation-reduction process (RDX process) frees the oxide ions and induces the secondary acid-base interaction
+
02- 2H+
-
H20
Of the acid-base interactions that frequently accompany oxidation-reduction reactions, the majority are secondary acid-base interactions. Let us use the code desienation ABI for orimarv acid-base hteractions and the code designation (ABI) fo; secondary ack-base interactions. I prefer to neelect secondarv acidbase [interactions hecausesuch interactions do not f k c t i o n as a principal driving force for reactions (reactions 4,8,9,11, 16, 18, 91, 22, and 30 below all show secondary acid-base interacticm). Among the residual or other types of reactions in aqueous solution, there is one that occurs often enough t o be worth coding.
cules "attack" a hub atom (Z) that is bonded to atoms (X) other than oxygen and, in effect, replace some or all of the Z.. .X interactions around the huh atom by Z.. .O interactions; the increased strength of the Z...O interaction, relative to the Z.. -X interaction, is the prime driving force (or one of the driving forces) pushing the reaction left to right. The most common pattern for the HBA reaction is the reolacement of covalentlv hound satellite haloeen atoms around a hub atom by oxigen atoms. 1f students'bill watch for the "haloeen to oxveen" switch.. thev. should readilv recognize an HBA reaction. The three processes ~recipitation,gas evolution, and complex format& are ways of removing chemical substances from an aqueous environment. The disap~earance of the kev actors from the aqueous stage is the diiving force for the reaction in the direction of the reaction arrow. When the first set of actors disappears, their understudies hop on stage, then disappear, too. The under-understudies hop on staee and eo the wav of their oredecessors. Ultimatelv the entire drama company disappears. In the case of complex formation. "disamearance" means a chanee of identitv. The .. actors go into disguise. ABI and RDX orocesses involve a tue-of-war as the snecies compete for drotons or electrons. ~ k HBA e process has todo with the relative stabilitiesof Z.. .X and Z...O interactions in the aqueous environment. Sometimes two or more driving forces are simultaneouslv active in a given rraction. Cold will not dissolve in H C I ( ~ $ or in HNOdaql hut will dissolve in a combination - separately of the two (aqua regia). CF, RDX, GF. I t seems to take a combination of oxidizing action and complexing action to get gold into solution. Chlorine water is also both oxidizing and complexing, converting gold t o soluble chlorohydroxoaurate(111) complexesAn&-, AuCI30H-, etc. Examples Let us annotate the following reaction equations to see how the system works.
- +-
ABI, GF. The acid-base interaction converts SO3%to the unstahle HsS03. which decomposes: S0z2- + H30+ HS03- + Hz0, HSOB-+ H30t H2S03 + H20, HzSO~ SO2 H20. The escape of SOPfrom the solution helps to push the reaction left to right. (2) AgCL(s)+ 2NH3 Ag(NH,),+(aq) + CI-(aq)
-
-
CF. The formation of the stable complex Ag(NH&+ pushes the reaction left to right.
HBA, PF. Diluting the complex BiCU with water reduces its stahility to the point where the insoluble chloride oxide, BiOCI, precipitates from the solution.
RDX, CF. Silver ores are processed with aqueous CN-: the redox action of the dissolved oxygen and the complexingaction of the CNbrine the elemental silver (and also eomoounds of silver such as Ag2S)into solution; the resulting solution is then treated with zinc to recover the silver: Zn ZAg(CN)%- 2Ag Zn(CN)d2-.
+
-
+
Hub-Attack Int6nactlon: HBA PClpll')+ 3Hz0
-
+
HaPOs(aq) 3HCl(aq)
In an HBA reactJon (solvolysis, hydrolysis), solvent mole244
Journal of Chemical Education
ABI, PF. This sort of gas-forming reaction is used on an industrial scale ta prepare from their salts those acids (HF, HCl, HN03, etc.) that are more volatile than (and stable in) HzS04.
RDX. HBA, PF. The greater stability of the Si-0 bond relative to the SiC1 bond and the insolubility of silica, SiOz, are responsible for pushing the reaction left to right. Ba(OH),(aq)
-
+ H3P0,(aq)
+ 2H,O
BaHPO,(s)
(7)
ABI, PF. As a result of the acid-base interaction, protons from H3PO4 and hydroxide ions from Ba(OH)z ultimately comhine to form water; the residual Bas+ and H P O F ions then form the insoluble BaHP04.
HI)X. I'F. lodinr isnol vrrywIubl+-in WRICI.(OO?Q~ 1001: H'Oat 211 "C',, so if wr wnnr rhr iodine pr~ducedin the reartmn tu stay i n solution we use an excess of I-. The liberated Ipthen forms a stable, 13-. soluble complex with the excess I-: Iz I-
-
+
+
2Fe3+(aq) 21-(aq)
-
+ 2H+(aq)
-
2NH3(aq)
+ 3OBrF(aq)
2Fezt(aq) +I,($)
HPO,-'(aq)
+ S(s) + H,O
RDX, GF, PF. This redox reaction is an example of a disproportionation reaction. An element in an intermediate oxidation state is simultaneously oxidized and reduced. In the present case one sulfur atom is oxidized and the other is reduced.
+ 3Br-(aq) + 3H,O
-
Li,PO,(s)
+ H,O
(23)
(24)
ABI, PF.
HBA, P F
2CaFz(s)
(10)
(11)
N,(g)
+ 3Lif(aq) + OH-(aq)
+ Be2+(aq)
CF.
SO,(g)
-
RDX, GF.
RDX.
RDX, PF. See the comment to reaction 8. Sz0,2-(as)
RDX, GF, PF. See the comment to reaction 8
2CuS(s)
+ 6CN-(ad
-
-
2CaZt(aq)
+ ~e~:-(aq)
(27)
2Cu(CN),-(aq)
+ (CN)Z(~)+ 2S2-(ad
(28)
CF, GF, RDX
RDX, GF.
-
-
RDX, PF. This redox reaction is a dearer ease of a disproportionation process: CIo C1-' and simultaneously CI0 CIV.
20Ht(aq)
+ 2Re0,-(eq) + GI-(aq) + l2Cl-(aq)
-
2H,ReCl,(aq)
+ 3I,(s) + 8H,O
(30)
RDX, CF, PF. See the comment to reaction 8. ABI, PF. There is an acid-base interaction between ammonia and water, NH3 H 2 0 = NH4+ OH-, producing O H ions that bring about precipitation of a gelatinous precipitate of "iron(II1) hydroxide". (The precipitate is probably better described as "hydrous iron(111) oxide," Fe203.xH20).
+
+
Ca2+(aq)+ C20?-(aq)
+ H,O
-
CaC,O,.H,O(s)
(14)
PF.
+
3ZnZ+(aq) 2Fe(CN)2-(aq)
+ 2Kt(aq) -K~zn,[Fe(CN),l~(s)
PF. 10,-(aq)
+ 51-(aq) + 6H'(aq)
-
+
312(s) 3H20
(15)
(16)
RDX. PF. See the n,rnmsnt 18, rcactim 8.This reaction could br called an 'invrrre dirpn,pwtiumatiu. reactiun" since a higher and o lower oxidation state of an element interact to produce an intermeIn. diate state: IV Ia, I-'
- -
ABI, GF. HgOW
+ 41-(aq) + H,O
CF. PbS(s) + 2Agt(aq)
-
-
-
HgI:-(aq)
Ag,S(s)
20H-(aq)
+ Phzt(aq)
(18)
(19)
PF. This reaction (a "precipitate exchange" reaction) works because the solubility of AgzS in water a t room temperature is much less than that of PbS. 3N,H,(aq)
+ 2Br03-(as)
-
RDX, GF. MnO,(s)
+ 2Fezt(aq) + 4Ht(aq)
2Br-(aq)
-
+ 3N,(g) + GH,O
(20)
Mnzt(aq)
+ 2Fe3+(aq)+ 2H,O
(21)
ABI, PF. My recommendations for progressing through a yearcourse in general chemistry a r e as follows: (1) Early in the course introduce the idea of the driving forces (pusher principles) for reactions in aqueous solution. Have the students routinely annotate reactions of type GF, PF, CF, and HBA. (2) Give a qualitative introduction to acid-base neutralization reactions and to simple redox reactions fairly early in the course. Extend the scheme of annotations to include ABI and RDX reaetions. From this point on, require the routine annotation of aqueous solution reaction equations to foster recognition of the fundamental aqueous solution reaction categories. It is amazing how often studentslookat reaction equations and fail torecognize the presence of oxidation-reduction or comdex formation. Students who have developed the habit of annotating reaction equations are less likely to overlook important features of reactions. (3) After students have been annotating reactions for some time, discuss with them the circumstances under which they are apt to meet a particular reaction category: (a) GF: Gases tend to appear in certain redox reactions (Hr, 0 2 , Clz, etc.), in reactions volatilizing certain acids or bases (HF, HCI, HPS, NHB,etc.), and in reactions where unstahle acids (H2COs, H2S03,H2S203,etc.) decompose. (b) PF: Give the students aset of solubilityrulesdeserihing the solubility characteristics of common inorganic salts: "all nitrates are soluble; all sulfides are insoluble, except for those o f . . .; etc". (c) CF: Complex formation is an especially impwtant part of the chemistry of the transition metals. Point out ihe existence of aqua complexes and comment on the differences in behavior of s o l u t i o n s of [Cr(HzO)s]Cl3, [ C I ( H ~ O ) S C I ~ C ~ ~a ~n dH ~ O , [Cr(H20)rC12]C1.2Hz0relative to AgNOdaq). (d) RDX: Talk about oxidation numbers end the periodic table. The elements participating in RDX rtactions must be of
Volume 64
Number 3 March 1987
245
variable oxidation state. Where in the periodic table do we find such elements? rr) AHI: Mrtel oxide. (with low oxidation numbers) tend to be base-forming, nonmetal oardea tend (4, he acid-forming. When specirs relntcd LO or derived from the two kinds of oxides share the same solution environment, there is apt to be an acid-base interaction. (The binary hydrides of Groups 16 and 17also behave as acids in water.) (f) HBA: Some of the nonmetal halides of the elements in Groups 14,15,and 16 hydrolyze in water in the hub-attack fasbion. (4) In the latter part of the wnrse, when aqueous equilibria are treated quantitatively via solubility product constants, stability constants for complexes, acid and base ionization constants, and standard electrode potentials, point out that the strength of the corrcs~ondin~ nusber ~ r i n c i ~isl erelated to the numerical value of the reievant equilibrium eon&nt. ( 5 ) Finallv. after havine develooed thermodvnamic ideas to the point of her"; ahle tu r& 111 AC; AH,and hS for chemical reaetiom, explain to the ~ludentsthat the catalog of driving forces or pusher prrnrrples is simply a catalog of circumstances that are usu-
246
Journal of Cnsrnlcal Education
ally correlated with a decrease in Gihhs free energy for the reaction (AG
< 0). .
Where ptrasihle, show how a particular reaction rategury rs "ex. plaincd" therm~~dynamirally. A GF process, fur example, usually leads 10 a favorable sign of AS for thr rraction rfavurahle for left.toright reaction). By carrying our students through the suggested levels of appreciation for chemical reactions in aqueous solutionannotation (all year long), discussion of circumstances, quantitation, thermodynamic correlation and "explanation"-we should be more likely (than is now the case) to turn out students who are chemically literate. It is almost inconceivable that a student who spent a year annotating hundreds of chemical reaction equations (aqueous solutions) and who related PF reactions to a set of solubility rules would be tempted to describe silver chloride as a "pale green gasn.2 Davenport, D. A. J. Chem. Educ. 1970, 47,271,
