Common Student Misconceptions
1."The alkali metals easily ('love to') lose one electron each (to achieve noble gas electron configurations)."
All alkali metals require energy to be ionized. Their first ionization energies range from +375 kJ/mol to +520 kJ/mol. This is hardly easily losing electrons! Still, of all groups of metallic elements, alkali metals lose electrons most readily.
2. "Diagrams in some texts show graphs similar to the enthalpy diagram in Figure 6 as being exothermic overall, when they attempt to explain the E° value trends of the alkali metals."
As a careful examination of the diagram below reveals, the overall process is endothermic (i.e., Li(s) -->Li+(aq) + 1e , DH = +167 kJ). The exothermicity actually results from formation of OH ion and H 2 gas, rather than the ionization steps shown in Figure 6.
3. "Many consumer products contain sodium and potassium."
Although the ingredients list says "sodium" or "potassium," it is the Na + and K + ions that are present. The elements and ions have very different properties! Na and K (metals) are toxic, but Na + and K + (ions) are essential for life. It is important that students recognize the difference between free elements and ions.
4. "Salts of the heavier members of a group are always more soluble in water than salts of the lighter elements of that group."
The solubility of ionic compounds can be visualized as consisting of three steps:
Step 1: breaking the crystal lattice (requires energy) DH = +
Step 2: formation of a hole in the solvent into which ions will fit (requires energy) DH = +
Step 3: hydration of ions (releases energy) DH =
When discussing solubility trends in terms of chemical periodicity, many teachers try to explain changes in terms of electrical coulombic attractions among the ions in the crystal lattice. Thus in the series Be(OH)2, Mg(OH)2 , Ca(OH)2 , Sr(OH)2, and Ba(OH)2, the last compound is most soluble because the larger barium ion has the weakest attraction for the hydroxide ion in the crystal lattice of all the alkaline earth ions in this series. Similarly CsOH is more soluble and more basic because it is the largest ion of all the alkali metals. However, this explanation fails when trying to explain the decreasing solubility in the alkaline earth sulfate series, BeSO4 , MgSO4, CaSO4, SrSO4, and BaSO4. The misconceptions arise for at least two reasons:
a. You must consider the three steps in the solution mechanism and the overall DH of solution. Major differences in solubility of ionic solids will arise primarily from Steps 1 and 3, since the formation of the hole in the solvent, Step 2, is approximately the same for most ions.
b. Whetheror notanoverallprocess is(thermodynamically)possibledepends upon the free energy change (DG = DH - TDS) that includes both enthalpy and entropy effects. Because dissolved species are more random than the highlyordered crystallattice,theentropy effectalwaysfavorsdissolving.
For solids that dissolve by absorbing heat, such as ammonium chloride, the enthalpy effect opposes dissolving. Thus solubility can frequently be pictured as a tug of war between these two effects. Since free energy change, DG, must be negative for a process to occur spontaneously, and DG = DH - TDS, where T is Kelvin temperature and DS is the entropy change, a positive DH can be overcome only if T is sufficiently large.
Clearly +1 and 1 ions do form lattices with relatively weak lattice energies, but they also tend to have the weakest attractions for water molecules in the hydration step because of the large size and small charge.
Problem Solving
1. Remembering that alkali metals constitute a family of chemical elements, as do the halogens (F, Cl, Br, I), and given NaCl as the formula of sodium chloride, predict formulas for each of the following compounds:
2. Examine the general structures for hydroxides, carbonates, and nitrates shown:
Note that other alkali metals form products similar to those of sodium, while magnesium, an alkaline earth element, forms products similar to those of lithium. [When heated the +1 alkali metal ion with the highest surface charge density, the lithium ion will attract the O 2 in the OH , CO32 or NO3 most strongly to form the oxide Li2O.]
3. Predict whether calcium will react more like lithium or sodium in similar reactions. [All are related to sizes of and charges on metallic ions (density of positive charge on the ionÕs surface). The smaller the size and the greater the charge, the more strongly the metallic ion attracts the oxygen ion from the anion. Calcium, with greater nuclear charge and slightly smaller ion size, will react more like lithium than like sodium.]
4. On the basis of oxidation states, the Periodic Table, and the fundamental classes of chemical reactions, predict the products formed in the following reactions: [Products are given in brackets. These equations are not balanced.]
5. All the alkali metals crystallize in body-centered cubic lattices. How many atoms are there in a unit cell (Figure 8)?
Figure 8 illustrates a body-centered cubic lattice. [The number of particles
that "belong" to a given unit cell consists of all interior particles,
which belong exclusively to that unit cell plus fractions of those that
are shared by two or more unit cells. Since each particle at a corner 
belongs
to eight unit cells (see corner A), only 1/8 of a corner particle belongs
to the unit cell. There are eight corners; thus, each unit cell has 1/8
x 8 = 1 particle in addition to interior particles. Since alkali metals
crystallize in body-centered cubic (BCC) cells, their unit cells contain
2 atoms (1 in the center and 1/8 x 8 = 1 at the corners).]
6. The vapors above heated liquid alkali metals contain about 1% diatomic molecules. The equation below describes the reaction.
Write the Lewis structure for the Na 2 (g) molecule. [Na:Na]
7. Predict the products you would obtain by the electrolysis of molten lithium hydride, LiH. [Li+ H2]
8. Balance each of the following oxidation reduction equations by a systematic method in which the number of electrons lost in the oxidation half-reaction is equal to the number of electrons gained in the reduction half. Identify the substance oxidized, the substance reduced, the oxidizing agent and the reducing agent.
a. 
; Cl is oxidized, H is reduced,
HOH is the oxidizing agent, NaCl is the reducing agent.]
b. 
; Ca is oxidized, Cs is reduced,
CsCl is the oxidizing agent, Ca is the reducing agent.]
9. The ionic mobility (molar ionic conductance) of very dilute aqueous solutions of alkali metals (at 18 °C) are given in Table of Properties of Alkali Metals in the Appendix. Explain why aqueous Cs+ ion moves the fastest when electrolyzed. [The hydrated Li+ is larger than the less hydrated Cs+.]
10. Cesium is found in the mineral pollucite, Cs4 Al4Si9 O26 . H2O, on the island of Elba. Calculate the percent by mass of cesium in this mineral.
11. The gaseous alkali metal atoms tend to form diatomic molecules:
2M(g)--> M2(g)
Predict the value for the heat of formation of Rb2 from the following data:
12. Chemists have observed that the elemental pairs Li-Mg, Be-Al, and B-Si have similar chemical properties, which is known as the diagonal relationship. Calculate the ratio of the charge on the ion:ionic radius (called the ionic potential) for the following ions, and discuss your results related to diagonal relationship.
The ionic potentials of Be-Al and B-Si are fairly close, but that of Li is about half that of Mg.
 |
TABLE OF CONTENTS | TOPIC OVERVIEW | CONCEPT/SKILLS DEVELOPMENT | LINKS/CONNECTIONS | EXTENSIONS |  |
|---|