Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110	General Chemistry	Summer 2006
Lecture Notes::Lec 17_22 June	© R. Paselk 2006
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The Chemistry of the Elements, cont.

The Representative Elements

Hydrogen, cont.:

• Three isotopes: H (¹H, 99.985 atom%), D (²H, 0.015 atom%), T (³H, 10^-17 atom%, t_1/2 = 12y).
  • Large isotope effects due to large mass differences.
    • fp of deuterium oxide is 3.8°C.
    • pD of pure deuterium oxide is 7.35; Thus [D⁺] = [OD^-] = 10^-7.35M, and deuterium oxide is a weaker acid than water (the deuterium-pxygen bond is stronger).
    • rates of reactions differ significantly between H and D versions of compounds. (Deuterium will diffuse slower than hydrogen.)
• Usually prepared by single displacement redox reactions.

  Most commonly an acid is used to react with a metal. e.g.

  Zn_(s) + 2 H⁺_(aq) Zn²⁺ + H_2(g)

• Note that any metal with a negative reduction potential can be used in principle.
• It can also be prepared by the reaction of water with calcium:

Ca_(s) + 2 H₂O_(l) Ca²⁺ + 2 OH^- + H_2(g)

• Of course the alkali metals also react with water to give hydrogen, but too reactive for practical purposes, igniting the hydrogen gas produced.
• Hydrogen gas is generally rather unreactive due to the strong H₂ bond (deltaH° = 436kJ/mol for H₂ 2 H). That is must have a high temperature to break the bond or a catalyst such as Pt or Pd.
  • Of course a spark or other source of heat can ignite hydrogen oxygen mixtures, releasing enough energy to continue the reaction (2 H₂ + O₂ 2 H₂O; deltaG° = -229kJ/mol
• More compounds contain H than any other element.
  • Oxidation states of +1 and -1.
  • Major industrial use is ammonia synthesis.
  • Alkali and alkaline earths react directly with hydrogen to produce metal hydrides (e.g. NaH, KH, etc.). A variety of transition metals also form metal hydrides, many of which are non-stoichiometric, that is there is not a fixed small whole number ratios. This occurs when the hydrogens fit into cavities in the metal crystal. 

Periodic Table Demonstrations Groups 1, Group 17 & Period Three Elements Let's look at samples of some of the elements and see what their properties are like: Group IA, on the left side of the chart, is known as the alkali metals because they react with water to produce strong bases (a base is alkaline). Note that all of them are soft (cut with a butter knife), low density (Li floats on oil, Na and K float on water), very reactive metals. All of them react with water with Li<Na<K<Rb<Cs. In each case the metal gives its electron to water leaving hydroxide ion (OH- a base) and hydrogen gas. For example with sodium: 2 Na + 2 H2O 2 Na+ + 2 OH- + H2 Al to O Note transition from metal to non-metal. Group VIIA, on the right side of the chart, is known as the halogens. The halogens form acids with water, are gases at the top of the Periodic Chart and high vapor pressure liquids, then solid going down; exist as diatomic molecules (X2), and are very reactive towards metals. For example sodium reacts violently with chlorine gas to give table salt (NaCl): 2 Na + Cl2 2 NaCl Group VIII is known as the Noble Gases, or sometimes the Inert Gases because until the 1960's they had no known compounds. Very unreactive. The only known compounds of the Noble gases are with very reactive elements like F and O, and even they don't form compounds with smaller Noble gases such as He and Ne. Look at the elements of Period 3 (Na - Cl) Note how their properties change from metallic to non-metallic.

Group I - The Alkali Metals

Physical Properties

• Very reactive metals with electronic configurations of: [noble gas]ns¹.
  • Easily lose outermost electron to attain a noble gas configurations.
  • Outermost electron is also easily excited to a higher energy state, giving them intense flame colors.
    • s p transition, with very slightly different p energies gives a doublet for the line.
• Group I elements are very metallic, held by metallic bonding with little or no covalent bond character
  • As a result very soft metals.
    • Li is exception, get some covalency due to small size.
  • Note properties in table below, discuss mp, density, ionization energies, reduction potentials and chemistry etc.

Properties of Group I - the Alkali Metals

Property		Li	Na	K	Rb	Cs	Fr
Valence-shell electron configuration		2s1	3s1	4s1	5s1	6s1	7s1
Melting point (°C)		180	98	64	39	29	-
Density (g/cm3)		0.54	0.97	0.86	1.5	1.9	-
Ionization energies - 1st & 2nd (kJ/mol) M(s) M+(aq) + e-		520 7296	496 4563	419 3069	403 2650	376 2420	370 2170
Standard Reduction Potentials (V, 25°C) M+(aq) + e- M(s)		-3.04	-2.71	-2.92	-2.92	-2.93	-
Electronegativity		1.0	0.9	0.8	0.8	0.7	0.7

Chemistry of the Alkali Metals

• Preparation: All of the alkali metals can be prepared by the electrolytic reduction.

  Chemical reduction can be used for production of K, Rb, & Cs

  Rb₂CO_3(s) + 2C_(s) 2Rb_(g) + 3CO_(g

© R A Paselk

Last modified 22 June 2006