805:. The order of reaction is an empirical quantity determined by experiment from the rate law of the reaction. It is the sum of the exponents in the rate law equation. Molecularity, on the other hand, is deduced from the mechanism of an elementary reaction, and is used only in context of an elementary reaction. It is the number of molecules taking part in this reaction.
789:
Catalytic reactions are often three-component, but in practice a complex of the starting materials is first formed and the rate-determining step is the reaction of this complex into products, not an adventitious collision between the two species and the catalyst. For example, in hydrogenation with a
1156:
On the other hand, the molecularity of this reaction is undefined, because it involves a mechanism of more than one step. However, we can consider the molecularity of the individual elementary reactions that make up this mechanism: the first step is trimolecular because it involves three reactant
790:
metal catalyst, molecular dihydrogen first dissociates onto the metal surface into hydrogen atoms bound to the surface, and it is these monatomic hydrogens that react with the starting material, also previously adsorbed onto the surface.
979:
equal the sum of reactant stoichiometric coefficients, the reaction must involve more than one step. The proposed two-step mechanism has a rate-limiting first step whose molecularity corresponds to the overall order of 3:
64:
The kinetic order of a complex (multistep) reaction, however, is not necessarily equal to the number of molecules involved. The concept of molecularity is only useful to describe elementary reactions or steps.
608:
782:
361:
900:
1068:
240:
503:
1226:
Temkin, O. N. State-of-the-Art in the Theory of
Kinetics of Complex Reactions. In Homogeneous Catalysis with Metal Complexes: Kinetic Aspects and Mechanisms, John Wiley and Sons, ltd, 2012
1152:
734:
349:
As can be deduced from the rate law equation, the number of A molecules that decay is proportional to the number of A molecules available. An example of a unimolecular reaction, is the
674:
416:
105:
969:
134:
338:
275:
307:
1338:
508:
Here, the rate of the reaction is proportional to the rate at which the reactants come together. An example of a bimolecular reaction is the
1419:
533:
50:) and correct orientation. Depending on how many molecules come together, a reaction can be unimolecular, bimolecular or even trimolecular.
738:
793:
Reactions of higher molecularity are not observed due to very small probability of simultaneous interaction between 4 or more molecules.
817:
986:
786:
These reactions frequently have a pressure and temperature dependence region of transition between second and third order kinetics.
1073:
73:
In a unimolecular reaction, a single molecule rearranges atoms, forming different molecules. This is illustrated by the equation
698:
686:
a second reaction with a third body is required. After the initial bimolecular collision of A and B an energetically excited
634:
387:
1631:
1508:
1465:
1191:
166:
1772:
1412:
1389:
1312:
1269:
1244:
690:
is formed, then, it collides with a M body, in a second bimolecular reaction, transferring the excess energy to it.
423:
1680:
1675:
1485:
1845:
1840:
1356:
Carr, R. W. Chemical
Kinetics. In Encyclopedia of Applied Physics. WILEY-VCH Verlag GmbH & Co KGaA, 2003
1405:
1866:
1810:
1500:
79:
1537:
1437:
517:
1767:
1328:
1171:
151:
907:
1815:
1616:
379:
In a bimolecular reaction, two molecules collide and exchange energy, atoms or groups of atoms.
1570:
1365:
Rogers, D. W. Chemical
Kinetics. In Concise Physical Chemistry, John Wiley and Sons, Inc. 2010.
1181:
343:
115:
1800:
1732:
1590:
1580:
687:
1157:
molecules, while the second step is bimolecular because it involves two reactant molecules.
61:
of an elementary reaction can therefore be determined by inspection, from the molecularity.
1795:
1523:
618:
316:
8:
1805:
1737:
1722:
1665:
1176:
622:
368:
139:
35:
250:
1830:
1600:
1429:
292:
1825:
1820:
1782:
1727:
1646:
1626:
1562:
1385:
1308:
1265:
1240:
802:
47:
1757:
1706:
1660:
1377:
284:
1835:
1747:
1696:
1280:
360:
1542:
1531:
521:
1860:
1790:
1762:
1670:
1621:
1595:
1345:, a semiempirical expression for the rate constant of termolecular reactions
1324:
1300:
1166:
525:
350:
280:
158:
147:
143:
58:
39:
1397:
1548:
1455:
1445:
809:
354:
1217:
Atkins, P.; de Paula, J. Physical
Chemistry. Oxford University Press, 2014
1701:
1636:
1186:
1346:
796:
629:
is also used to refer to three body association reactions of the type:
625:, with appropriate orientation and sufficient energy. However the term
1752:
157:
In either case, the rate of the reaction or step is described by the
27:
653:
683:
43:
20:
53:
The kinetic order of any elementary reaction or reaction step is
679:
16:
Number of molecules that participate in a single-step reaction
1655:
34:
is the number of molecules that come together to react in an
693:
The reaction can be explained as two consecutive reactions:
808:
This difference can be illustrated on the reaction between
1475:
1137:
1117:
1101:
1088:
1056:
1043:
1027:
1011:
953:
882:
862:
846:
580:
548:
509:
621:
or gas mixtures involves three reactants simultaneously
603:{\displaystyle {\ce {CH3Br + OH^- -> CH3OH + Br^-}}}
1076:
989:
910:
820:
797:
Difference between molecularity and order of reaction
777:{\displaystyle {\ce {AB}}^{*}{\ce {+ M -> C + M}}}
741:
701:
637:
536:
426:
390:
319:
295:
253:
169:
118:
82:
46:
in the elementary reaction with effective collision (
678:
Where the M over the arrow denotes that to conserve
1146:
1062:
963:
895:{\displaystyle {\ce {2NO + 2H2 -> N2 + 2H2O,}}}
894:
776:
728:
668:
602:
497:
410:
332:
301:
269:
234:
128:
99:
1063:{\displaystyle {\ce {2 NO + H2 -> N2 + H2O2}}}
801:It is important to distinguish molecularity from
235:{\displaystyle {\frac {d\left}{dt}}=-k_{r}\left,}
1858:
420:which corresponds to the second order rate law:
367:Unimolecular reactions can be explained by the
498:{\displaystyle {\frac {d}{dt}}=-k_{r}{\ce {}}}
1427:
1413:
1260:J.I. Steinfeld, J.S. Francisco and W.L. Hase
617:A termolecular (or trimolecular) reaction in
1147:{\displaystyle {\ce {H2O2 + H2 -> 2H2O}}}
154:if there is more than one product molecule.
150:if there is only one product molecule, or a
1256:
1254:
1252:
1420:
1406:
1373:
1371:
729:{\displaystyle {\ce {A + B -> AB}}^{*}}
1126:
994:
871:
835:
825:
612:
68:
1384:(3rd ed., Harper & Row 1987), p.277
1249:
1213:
1211:
1209:
1207:
374:
1368:
1299:as alternative names is J.W. Moore and
1859:
1446:Unimolecular nucleophilic substitution
1359:
1239:(4th ed., Allyn and Bacon 1983) p.215
382:This can be described by the equation
1456:Bimolecular nucleophilic substitution
1401:
1204:
669:{\displaystyle {\ce {A + B -> C}}}
411:{\displaystyle {\ce {A + B -> P}}}
1509:Electrophilic aromatic substitution
1350:
1264:(2nd ed., Prentice Hall 1999) p.5,
13:
1476:Nucleophilic internal substitution
1466:Nucleophilic aromatic substitution
1192:Reaction progress kinetic analysis
359:
259:
121:
100:{\displaystyle {\ce {A -> P,}}}
14:
1878:
1307:(3rd ed., John Wiley 1981) p.17,
1291:One textbook which mentions both
36:elementary (single-step) reaction
1632:Lindemann–Hinshelwood mechanism
904:where the observed rate law is
1681:Outer sphere electron transfer
1676:Inner sphere electron transfer
1486:Nucleophilic acyl substitution
1332:
1317:
1285:
1274:
1262:Chemical Kinetics and Dynamics
1229:
1220:
1120:
1014:
956:
940:
929:
923:
849:
761:
713:
567:
490:
484:
479:
473:
441:
433:
401:
262:
254:
87:
1:
1846:Diffusion-controlled reaction
1281:IUPAC Gold Book: Molecularity
1197:
964:{\displaystyle v=k{\ce {^2}}}
57:to its molecularity, and the
1235:Morrison R.T. and Boyd R.N.
7:
1501:Electrophilic substitutions
1327:for termolecular reactions
1160:
40:stoichiometric coefficients
38:and is equal to the sum of
10:
1883:
1811:Energy profile (chemistry)
1773:More O'Ferrall–Jencks plot
1438:Nucleophilic substitutions
971:, so that the reaction is
18:
1841:Michaelis–Menten kinetics
1781:
1715:
1689:
1645:
1609:
1561:
1522:
1499:
1436:
518:nucleophilic substitution
129:{\displaystyle {\rm {P}}}
1768:Potential energy surface
1647:Electron/Proton transfer
1532:Unimolecular elimination
1172:Dissociation (chemistry)
19:Not to be confused with
1816:Transition state theory
1617:Intramolecular reaction
1543:Bimolecular elimination
975:. Since the order does
1610:Unimolecular reactions
1571:Electrophilic addition
1305:Kinetics and Mechanism
1182:Crossed molecular beam
1148:
1064:
965:
896:
778:
730:
670:
613:Termolecular reactions
604:
499:
412:
364:
344:reaction rate constant
334:
303:
271:
236:
130:
101:
69:Unimolecular reactions
1801:Rate-determining step
1733:Reactive intermediate
1591:Free-radical addition
1581:Nucleophilic addition
1524:Elimination reactions
1149:
1065:
966:
897:
779:
731:
688:reaction intermediate
671:
605:
500:
413:
375:Bimolecular reactions
369:Lindemann-Hinshelwood
363:
335:
333:{\displaystyle k_{r}}
304:
272:
237:
131:
102:
1796:Equilibrium constant
1074:
987:
908:
818:
739:
699:
635:
534:
424:
388:
317:
293:
251:
167:
159:first order rate law
116:
80:
1806:Reaction coordinate
1738:Radical (chemistry)
1723:Elementary reaction
1666:Grotthuss mechanism
1430:reaction mechanisms
1177:Lindemann mechanism
1139:
1119:
1103:
1090:
1058:
1045:
1029:
1013:
955:
884:
864:
848:
659:
582:
550:
138:refers to chemical
1831:Arrhenius equation
1601:Oxidative addition
1563:Addition reactions
1144:
1127:
1107:
1091:
1078:
1060:
1046:
1033:
1017:
1001:
961:
943:
892:
872:
852:
836:
774:
726:
666:
600:
570:
538:
495:
408:
365:
330:
299:
270:{\displaystyle }}}
267:
232:
142:. The reaction or
126:
97:
1867:Chemical kinetics
1854:
1853:
1826:Activated complex
1821:Activation energy
1783:Chemical kinetics
1728:Reaction dynamics
1627:Photodissociation
1382:Chemical Kinetics
1237:Organic Chemistry
1142:
1130:
1110:
1094:
1081:
1049:
1036:
1020:
1004:
997:
946:
928:
887:
875:
855:
839:
828:
803:order of reaction
772:
766:
760:
746:
718:
712:
706:
664:
660:
657:
647:
641:
592:
585:
573:
560:
553:
541:
489:
478:
453:
439:
406:
400:
394:
302:{\displaystyle t}
223:
198:
183:
92:
86:
48:sufficient energy
1874:
1758:Collision theory
1707:Matrix isolation
1661:Harpoon reaction
1538:E1cB-elimination
1422:
1415:
1408:
1399:
1398:
1392:
1378:Keith J. Laidler
1375:
1366:
1363:
1357:
1354:
1348:
1336:
1330:
1323:Text discussing
1321:
1315:
1289:
1283:
1278:
1272:
1258:
1247:
1233:
1227:
1224:
1218:
1215:
1153:
1151:
1150:
1145:
1143:
1140:
1138:
1135:
1128:
1118:
1115:
1108:
1102:
1099:
1092:
1089:
1086:
1079:
1069:
1067:
1066:
1061:
1059:
1057:
1054:
1047:
1044:
1041:
1034:
1028:
1025:
1018:
1012:
1009:
1002:
995:
970:
968:
967:
962:
960:
959:
954:
951:
944:
938:
937:
932:
926:
901:
899:
898:
893:
891:
885:
883:
880:
873:
863:
860:
853:
847:
844:
837:
826:
783:
781:
780:
775:
773:
770:
764:
758:
753:
752:
747:
744:
735:
733:
732:
727:
725:
724:
719:
716:
710:
704:
675:
673:
672:
667:
665:
662:
661:
658:
655:
649:
645:
639:
609:
607:
606:
601:
599:
598:
597:
590:
583:
581:
578:
571:
566:
565:
558:
551:
549:
546:
539:
504:
502:
501:
496:
494:
493:
487:
482:
476:
470:
469:
454:
452:
444:
440:
437:
428:
417:
415:
414:
409:
407:
404:
398:
392:
341:
339:
337:
336:
331:
329:
328:
310:
308:
306:
305:
300:
278:
276:
274:
273:
268:
266:
265:
241:
239:
238:
233:
228:
224:
221:
215:
214:
199:
197:
189:
188:
184:
181:
171:
137:
135:
133:
132:
127:
125:
124:
106:
104:
103:
98:
96:
90:
84:
1882:
1881:
1877:
1876:
1875:
1873:
1872:
1871:
1857:
1856:
1855:
1850:
1836:Eyring equation
1777:
1748:Stereochemistry
1711:
1697:Solvent effects
1685:
1641:
1605:
1586:
1576:
1557:
1552:
1518:
1514:
1495:
1491:
1481:
1471:
1461:
1451:
1432:
1426:
1396:
1395:
1376:
1369:
1364:
1360:
1355:
1351:
1343:Troe expression
1337:
1333:
1322:
1318:
1290:
1286:
1279:
1275:
1259:
1250:
1234:
1230:
1225:
1221:
1216:
1205:
1200:
1163:
1136:
1131:
1116:
1111:
1100:
1095:
1087:
1082:
1077:
1075:
1072:
1071:
1055:
1050:
1042:
1037:
1026:
1021:
1010:
1005:
990:
988:
985:
984:
952:
947:
939:
933:
922:
921:
920:
909:
906:
905:
881:
876:
861:
856:
845:
840:
821:
819:
816:
815:
799:
754:
748:
743:
742:
740:
737:
736:
720:
703:
702:
700:
697:
696:
654:
648:
638:
636:
633:
632:
615:
593:
589:
579:
574:
561:
557:
547:
542:
537:
535:
532:
531:
513:
483:
472:
471:
465:
461:
445:
436:
429:
427:
425:
422:
421:
391:
389:
386:
385:
377:
324:
320:
318:
315:
314:
312:
294:
291:
290:
288:
258:
257:
252:
249:
248:
246:
220:
216:
210:
206:
190:
180:
176:
172:
170:
168:
165:
164:
120:
119:
117:
114:
113:
111:
83:
81:
78:
77:
71:
24:
17:
12:
11:
5:
1880:
1870:
1869:
1852:
1851:
1849:
1848:
1843:
1838:
1833:
1828:
1823:
1818:
1813:
1808:
1803:
1798:
1793:
1787:
1785:
1779:
1778:
1776:
1775:
1770:
1765:
1760:
1755:
1750:
1745:
1740:
1735:
1730:
1725:
1719:
1717:
1716:Related topics
1713:
1712:
1710:
1709:
1704:
1699:
1693:
1691:
1690:Medium effects
1687:
1686:
1684:
1683:
1678:
1673:
1668:
1663:
1658:
1652:
1650:
1643:
1642:
1640:
1639:
1634:
1629:
1624:
1619:
1613:
1611:
1607:
1606:
1604:
1603:
1598:
1593:
1588:
1584:
1578:
1574:
1567:
1565:
1559:
1558:
1556:
1555:
1550:
1546:
1540:
1535:
1528:
1526:
1520:
1519:
1517:
1516:
1512:
1505:
1503:
1497:
1496:
1494:
1493:
1489:
1483:
1479:
1473:
1469:
1463:
1459:
1453:
1449:
1442:
1440:
1434:
1433:
1425:
1424:
1417:
1410:
1402:
1394:
1393:
1367:
1358:
1349:
1341:definition of
1331:
1325:rate constants
1316:
1284:
1273:
1248:
1228:
1219:
1202:
1201:
1199:
1196:
1195:
1194:
1189:
1184:
1179:
1174:
1169:
1162:
1159:
1134:
1125:
1122:
1114:
1106:
1098:
1085:
1053:
1040:
1032:
1024:
1016:
1008:
1000:
993:
958:
950:
942:
936:
931:
925:
919:
916:
913:
890:
879:
870:
867:
859:
851:
843:
834:
831:
824:
812:and hydrogen:
798:
795:
769:
763:
757:
751:
723:
715:
709:
652:
644:
614:
611:
596:
588:
577:
569:
564:
556:
545:
522:methyl bromide
511:
492:
486:
481:
475:
468:
464:
460:
457:
451:
448:
443:
435:
432:
403:
397:
376:
373:
327:
323:
298:
264:
261:
256:
243:
242:
231:
227:
219:
213:
209:
205:
202:
196:
193:
187:
179:
175:
123:
108:
107:
95:
89:
70:
67:
15:
9:
6:
4:
3:
2:
1879:
1868:
1865:
1864:
1862:
1847:
1844:
1842:
1839:
1837:
1834:
1832:
1829:
1827:
1824:
1822:
1819:
1817:
1814:
1812:
1809:
1807:
1804:
1802:
1799:
1797:
1794:
1792:
1791:Rate equation
1789:
1788:
1786:
1784:
1780:
1774:
1771:
1769:
1766:
1764:
1763:Arrow pushing
1761:
1759:
1756:
1754:
1751:
1749:
1746:
1744:
1741:
1739:
1736:
1734:
1731:
1729:
1726:
1724:
1721:
1720:
1718:
1714:
1708:
1705:
1703:
1700:
1698:
1695:
1694:
1692:
1688:
1682:
1679:
1677:
1674:
1672:
1671:Marcus theory
1669:
1667:
1664:
1662:
1659:
1657:
1654:
1653:
1651:
1648:
1644:
1638:
1635:
1633:
1630:
1628:
1625:
1623:
1622:Isomerization
1620:
1618:
1615:
1614:
1612:
1608:
1602:
1599:
1597:
1596:Cycloaddition
1594:
1592:
1589:
1582:
1579:
1572:
1569:
1568:
1566:
1564:
1560:
1554:
1547:
1544:
1541:
1539:
1536:
1533:
1530:
1529:
1527:
1525:
1521:
1510:
1507:
1506:
1504:
1502:
1498:
1487:
1484:
1477:
1474:
1467:
1464:
1457:
1454:
1447:
1444:
1443:
1441:
1439:
1435:
1431:
1423:
1418:
1416:
1411:
1409:
1404:
1403:
1400:
1391:
1390:0-06-043862-2
1387:
1383:
1379:
1374:
1372:
1362:
1353:
1347:
1344:
1340:
1335:
1329:
1326:
1320:
1314:
1313:0-471-03558-0
1310:
1306:
1302:
1298:
1294:
1288:
1282:
1277:
1271:
1270:0-13-737123-3
1267:
1263:
1257:
1255:
1253:
1246:
1245:0-205-05838-8
1242:
1238:
1232:
1223:
1214:
1212:
1210:
1208:
1203:
1193:
1190:
1188:
1185:
1183:
1180:
1178:
1175:
1173:
1170:
1168:
1167:Reaction rate
1165:
1164:
1158:
1154:
1132:
1123:
1112:
1104:
1096:
1083:
1051:
1038:
1030:
1022:
1006:
998:
991:
981:
978:
974:
948:
934:
917:
914:
911:
902:
888:
877:
868:
865:
857:
841:
832:
829:
822:
813:
811:
806:
804:
794:
791:
787:
784:
767:
755:
749:
721:
707:
694:
691:
689:
685:
681:
676:
650:
642:
630:
628:
624:
620:
610:
594:
586:
575:
562:
554:
543:
529:
527:
526:hydroxide ion
523:
519:
515:
506:
466:
462:
458:
455:
449:
446:
430:
418:
395:
383:
380:
372:
370:
362:
358:
356:
352:
351:isomerization
347:
345:
325:
321:
311:is time, and
296:
286:
282:
281:concentration
229:
225:
217:
211:
207:
203:
200:
194:
191:
185:
177:
173:
163:
162:
161:
160:
155:
153:
149:
148:isomerization
145:
144:reaction step
141:
93:
76:
75:
74:
66:
62:
60:
59:rate equation
56:
51:
49:
45:
41:
37:
33:
29:
22:
1743:Molecularity
1742:
1381:
1361:
1352:
1342:
1334:
1319:
1304:
1301:R.G. Pearson
1297:trimolecular
1296:
1293:termolecular
1292:
1287:
1276:
1261:
1236:
1231:
1222:
1155:
982:
976:
972:
903:
814:
810:nitric oxide
807:
800:
792:
788:
785:
695:
692:
677:
631:
627:trimolecular
626:
616:
530:
507:
419:
384:
381:
378:
366:
357:to propene:
355:cyclopropane
348:
244:
156:
152:dissociation
109:
72:
63:
54:
52:
32:molecularity
31:
25:
1702:Cage effect
1637:RRKM theory
1553:elimination
1187:Cage effect
973:third order
371:mechanism.
1198:References
140:product(s)
1753:Catalysis
1649:reactions
1121:⟶
1015:⟶
850:⟶
762:⟶
750:∗
722:∗
714:⟶
623:colliding
619:solutions
595:−
568:⟶
563:−
459:−
402:⟶
204:−
88:⟶
44:reactants
28:chemistry
1861:Category
1161:See also
684:momentum
651:→
21:Molarity
342:is the
340:
313:
309:
289:
285:species
279:is the
277:
247:
136:
112:
1428:Basic
1388:
1311:
1268:
1243:
1070:Fast:
983:Slow:
680:energy
516:-type
245:where
146:is an
110:where
1656:Redox
1492:Acyl)
1339:IUPAC
55:equal
1545:(E2)
1534:(E1)
1386:ISBN
1309:ISBN
1295:and
1266:ISBN
1241:ISBN
682:and
1515:Ar)
1472:Ar)
977:not
524:by
520:of
353:of
287:A,
283:of
42:of
26:In
1863::
1583:(A
1573:(A
1511:(S
1488:(S
1482:i)
1478:(S
1468:(S
1462:2)
1458:(S
1452:1)
1448:(S
1380:,
1370:^
1303:,
1251:^
1206:^
996:NO
927:NO
827:NO
745:AB
717:AB
591:Br
584:OH
572:CH
559:OH
552:Br
540:CH
528::
505:.
346:.
30:,
1587:)
1585:N
1577:)
1575:E
1551:i
1549:E
1513:E
1490:N
1480:N
1470:N
1460:N
1450:N
1421:e
1414:t
1407:v
1141:O
1133:2
1129:H
1124:2
1113:2
1109:H
1105:+
1097:2
1093:O
1084:2
1080:H
1052:2
1048:O
1039:2
1035:H
1031:+
1023:2
1019:N
1007:2
1003:H
999:+
992:2
957:]
949:2
945:H
941:[
935:2
930:]
924:[
918:k
915:=
912:v
889:,
886:O
878:2
874:H
869:2
866:+
858:2
854:N
842:2
838:H
833:2
830:+
823:2
771:M
768:+
765:C
759:M
756:+
711:B
708:+
705:A
663:C
656:M
646:B
643:+
640:A
587:+
576:3
555:+
544:3
514:2
512:N
510:S
491:]
488:B
485:[
480:]
477:A
474:[
467:r
463:k
456:=
450:t
447:d
442:]
438:A
434:[
431:d
405:P
399:B
396:+
393:A
326:r
322:k
297:t
263:]
260:A
255:[
230:,
226:]
222:A
218:[
212:r
208:k
201:=
195:t
192:d
186:]
182:A
178:[
174:d
122:P
94:,
91:P
85:A
23:.
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