METALS: INTRODUCTION - REACTIVITY (2)
A healthily critical attitude to reactivity can be acquired from the
careful consideration of the following example of hypothesis testing.
[Li > K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > (H) > Cu > Hg > Ag]
Quelle horreur!
Hypothesis: 'Within each group of the Periodic Table, the reactivities
of the metallic elements (M) increase with increasing atomic number.'
Data: A. J. Bard et al., Standard Potentials in Aqueous Solutions,
Marcel Dekker, New York, 1985.
Results: Standard oxidation potentials derived from this source of data
show that the ease of oxidation for the metallic elements of six groups
is as follows.
Group 1: 3Li > 37Rb ³ 19K ³ 55Cs > 11Na
Group 2: 56Ba > 38Sr > 20Ca > 12Mg > 4Be
Group 3: 57La > 39Y > 21Sc
Group 11: 29Cu > 47Ag > 79Au
Group 12: 30Zn > 48Cd > 80Hg
Group 13: 13Al > 31Ga > 49In > 61Tl
Sources of Error: Surprisingly, reference books reveal a wide variation
in the numerical values of redox potentials. Nevertheless, in terms of
the ordering within groups, such books are concordant with Bard et al.
Conclusion: As adjudged by standard oxidation potentials, in a study
limited to the elements in Periods 2 to 6, the data provide no general
support for the hypothesis. Thus, Group 1 is irregular; the ease of
oxidation does increase as the atomic number increases in both Groups 2
and 3 (i.e., as each group is descended): but, the ease of oxidation
decreases as the atomic number increases in Groups 11, 12, and 13.
However, the use of different criteria for reactivity may well provide
support for this hypothesis. *
After due consideration of the above example of hypothesis testing, a
student may well exclaim: "Quelle horreur! Is there no method to this
madness?" The answer is yes: if, and only if, one accepts that many
patterns in Chemistry are complex, and that they start to emerge only
after focusing closely on the details.
Plus horreur?
A preliminary insight into some of the variables which determine the
reactivity of metals can be gained by placing a specific reaction type
'under the microscope'; e.g., reaction of a solid metal with an aqueous
solution of acid to form a hydrated metal cation and dihydrogen gas:
M(s) + H1+(aq) ———————————® M1+(aq) + ½H2(g)
This reaction can be divided into several ergonic processes; the Table
below shows typical values of these processes for four metals known to
form uni-positive cations.
Ergonic process / kJ mol-1
|
Li |
K |
Na |
Ag |
M(s) ———————————® M(g) DH1 |
161 |
90 |
109 |
289 |
M(g) ———————————® M1+(g) + e- DH2 |
520 |
419 |
496 |
731 |
M1+(g) ——————————® M1+(aq) DHH |
-523 |
-331 |
-419 |
-464 |
H1+(aq) + e- ————® ½H2(g) DHR |
-432 |
-432 |
-432 |
-432 |
M(s) + H1+(aq) ——® M1+(aq) + ½H2(g) DH |
-274 |
-254 |
-246 |
+124 |
DH = DHS + DH1 + DH2 + DHR, where: DH1 is the heat of sublimation;
DH2 is the 1st ionization energy; DHH is the heat of hydration of
the metal cation; and, DHR is the heat of reduction of the aqueous
hydrogen ion to molecular dihydrogen. |
A complete explanation of these tabulated data is well beyond the scope
of this text: but even a partial explanation should be illuminating ...
Two processes are always endothermic, the heat of sublimation (DH1) and
the 1st ionization energy (DH2), whereas two are always exothermic, the
heat of hydration of the metal cation (DHH) and the heat of reduction
of the aqueous hydrogen ion to molecular dihydrogen (DHR). Because the
value of DHR is constant, the differences in the overall heat energy
change (DH) depend upon the relative magnitudes of DH1, DH2, and DHH.
The heat of sublimation reflects the strength of metallic bonding in the
solid state. However, the relationship between this bonding and ground-
state electronic structure of a gaseous atom is very complex indeed. Be
that as it may, uniform periodicity is clearly not observed; thus,
3Li > 11Na > 19K (Group1), but 79Au > 29Cu > 47Ag (Group 11).
The 1st ionization energy is the only ergonic process which is directly
related the ground-state electronic structure of a gaseous atom.
Nevertheless, here also, uniform periodicity is not observed; thus,
3Li > 11Na > 19K (Group1), but 79Au > 29Cu > 47Ag (Group 11).
The heat of hydration reflects the electronic structure of the gaseous
cation and its subsequent bonding with a (variable) number of water
molecules to form the hydrated cation. Unfortunately, a clear statement
of periodicity is precluded because, in contrast to those in Group 1,
the metals in Group 11 show variable oxidation states.
And finally, ... These data would support this principle: 'high metal
reactivity is favoured by a low heat of sublimation, a low ionization
energy, and a high heat of hydration of the metal cation'. The overall
heat energy change (DH) is negative for each Group 1 metal, and the
relative magnitudes of DH parallel the measured energy changes of their
reactions with water (i.e., Li > K > Na); the 'anomalous' position of
lithium is attributable to the higher heat of hydration of the lithium
cation. In contrast, largely because of silver's much higher ionization
energy, DH is positive for this Group 11 metal; this is consistent with
the observed lack of reaction between silver and dilute acid or water.
[Scene. A court in the state capital of Poppermania]
Mermaid: Plus horreur! ... M'Lud ... [The judge holds up his hand.]
Judge: My Lord is the correct method of address, if you please.
Mermaid: Sorry, my Lord. These results and explanations are all well
and good: but, they can have little relevance to solid ionic
compounds, such as chlorides, oxides, and nitrides, surely?
Judge: [He smiles tolerantly at this less-than-humble student.]
I do admit, strictly speaking, only parts are; for example,
the DH1 and DH2 terms. However, for other reactants, there
should be analogous terms to DHH and DHR. Perhaps you would
care to note the relationship of DHH to the lattice energy?
Mermaid: Duly noted, M'Lud ... I mean, my Lord. Please no more ...
My time is valuable! [She states petulantly before glancing
briefly, but proudly, at her varnished finger-nails.]
Judge: Oh dear, in my day ... no, never mind. [He sighs.] Remember,
Rome was not built in a day: although it may well have burnt
down in one! [He glances at his fiddle before dabbing some
eau-de-cologne on his pristine wig.] Mermaid, I ... advise
you to spend at least part of your free-time giving the grey
cells some aerobic exercise. [Sighs of relief from the rest
of the synchronized-swimming team in the public gallery.]
Mermaid: My Lord, perhaps I should emigrate to Laputa? [... but the
venerable judge has fallen asleep (bless his silken socks).] |
* The student is encouraged, albeit gently, to consider testing this
hypothesis, or a similar one which focuses on Periods, using different
physical data (e.g., heats of sublimation, ionization energies, energy
changes for the formation of solid ionic compounds, ...); such data
should be abstracted from reference books.
Dr. R. Peters Next Contents' List