Chemistry 1000 Lecture 10: Chemistry of the alkali metals. Marc R. Roussel

Transcription

1 Chemistry 1000 Lecture 10: Chemistry of the alkali metals Marc R. Roussel

2 Classification of the elements Element Appearance Resistivity/Ω m Fluoride(s) Na silvery solid ionic NaF Ca silvery solid ionic CaF 2 Ni silvery solid ionic NiF 2 Al silvery solid molecular Al 2 F 6 Hg silvery liquid ionic Hg 2 F 2 and HgF 2 Ge grey solid molecular GeF 4 and GeF 2 Sb silvery solid molecular SbF 3 and SbF 5 B black solid molecular BF 3 P white solid molecular PF 3, PF 5 and P 2 F 4

3 Metal: malleable, ductile, good conductor of heat and electricity, shiny Nonmetal: brittle when solid, poor conductor of heat and electricity (insulator) Metalloid: intermediate between metal and nonmetal, often semiconducting Semiconductor: electrical conductivity is between that of a conductor and insulator and can often be controlled, e.g. by adding energy such as UV radiation or by imposing an electric field

4 Crystal structure of metals Metals typically are (poly)crystalline. Crystal lattice: repeating arrangement of points in space Polycrystal: a material composed of many microscopic crystals (grains) stuck together in different orientations Grain boundary: surface where two grains meet Single crystal: a material composed of a single, (nearly) perfectly ordered crystalline material without grain boundaries Even in a polycrystal, relatively few atoms are at the grain boundary so most are surrounded by a well-organized crystal environment.

5 Example of a crystal lattice: Magnesium

6 Q&A about bonding in metals Question: Why do metals conduct electricity? Answer: They must have free electrons. Question: What distinguishes metals from nonmetals? Answer: Metals give up their electrons relatively easily.

7 Quasi-free-electron model of metals "valence" electrons Explains the following properties of metals: thermal and electrical conductivity deformability (ductility, malleability)

8 The alkali metals Group 1, except H Soft metals Lowest ionization energies and electronegativities in periodic table, low melting and boiling points (for metals) Li Na K Rb Cs I 1 /kj mol χ T f / C T b / C

9 Redox chemistry Alkali metal ions have among the most negative reduction potentials Reduction potential: Half-cell potential for gaining electrons In this case, M (aq) e M (s) Li Na K Rb Cs E /V = The alkali metals are very powerful reducing agents. = In nature, these elements only ever appear as their 1 cations.

10 Some typical reactions Reaction with water: M (s) H 2 O (l) M (aq) OH (aq) 1 2 H 2(g) Reaction with halogens (group 17: F2, Cl 2, Br 2, I 2 ) M (s) 1 2 X 2 MX (s) Reaction of lithium with oxygen: 2Li (s) 1 2 O 2(g) Li 2 O (s) Note: the other alkali metals make oddball oxides. = Alkali metal compounds are almost universally ionic.

11 Example: stoichiometry of the reaction with water 1.5 g of sodium is reacted with 150 ml of water, which represents a large excess. 1. What is the concentration of sodium hydroxide in the final solution? 2. What volume of hydrogen gas, measured at 25 C and 1 atm pressure, is produced? Give your answer in units such that the numerical value is between and Answers: 0.43 mol L 1 NaOH and 0.80 L H 2

12 Hydration An ion in solution is surrounded by water molecules. H O H H O H H H O M O H H H O O H H H Hydration enthalpy ( hydr H): Enthalpy change for the transfer of an ion from the gas phase to solution M (g) M (aq) Li Na K Rb Cs hydr H/kJ mol r/pm

13 Solubility of alkali metal compounds Alkali metals have relatively large, negative enthalpies of hydration. Because they carry a single charge, the forces holding their crystals together, while significant, are less strong than those holding together crystals of more highly charged ions. As a consequence, almost all alkali metal compounds are extremely soluble in water (solubilities often reaching several hundred grams per litre). Exception: some lithium compounds with highly charged anions Lithium phosphate: 0.39 g L 1

14 Flame tests Metal ions are often identified by precipitation. Alkali metal compounds are extremely soluble, so that won t work. Instead, we use flame tests: Putting a sample into a flame puts energy into it. This energy can put ions in excited electronic states. When the ions return to their ground states (possibly in multiple hops), the emit light. The emission spectrum depends on a number of factors (including the flame temperature), but is most strongly dependent on the energy levels of the emitter, leading to characteristic colors.

15 Flame tests (continued) Element Li Na K Rb Cs Flame color crimson yellow lilac purple blue

16 Naming cations 1. Name of cation = name of metal word ion Examples: sodium ion (Na ), magnesium ion (Mg 2 ) 2. When there is more than one possible ion (most transition metals, p-block metals), the charge of the ion is represented by a Roman numeral in parentheses glued onto the metal name. Examples: iron(ii) ion for Fe 2 iron(iii) ion for Fe 3

17 Naming monatomic anions Replace suffix in element name by -ide. The word ion may be used at times, but isn t strictly necessary. Examples: fluoride (ion) (F ), oxide (ion) (O 2 )

18 Naming ionic compounds Name the cation first, dropping the word ion, then the anion. No account is taken of the numbers of ions of each type since these numbers are known from the charges of the ions. Exercises: Name AgI, MgF 2, Fe 2 O 3 Give the formulas for iron(ii) chloride, magnesium nitride

19 Production of sodium and lithium metals Lithium and sodium metal are produced by electrolysis of the molten chlorides. Overall reactions: LiCl (l) Li (l) 1 2 Cl 2(g) NaCl (l) Na (l) 1 2 Cl 2(g)

20 Downs cell Cl 2(g) Na (l) Molten NaCl/CaCl 2 mixture anode cathode iron screen

21 Melting point of NaCl: 804 C Melting point of 1:4 mixture of NaCl:CaCl 2 : 600 C Cathode reaction: Na e Na (l) E = V Anode reaction: Cl 1 2 Cl 2(g) e E = V Overall: NaCl (l) Na (l) 1 2 Cl 2(g) E = V Calcium is not produced in appreciable quantities because calcium ions are harder to reduce than sodium ions: E = 2.84 V for Ca 2.

22 The chlor-alkali process Electrolysis of aqueous NaCl Source: Wikimedia Commons:

23 The chlor-alkali process Electrolysis of aqueous NaCl (continued) Electrolysis of an aqueous solution of NaCl involves the following half-reactions: 2Cl (aq) Cl 2(g) 2e 2H 2 O (l) 2e H 2(g) 2OH (aq) E = V E = V Overall: 2Cl (aq) 2H 2O (l) Cl 2(g) H 2(g) 2OH (aq) E = V We are left with a solution of NaOH(aq). Industrially, this chlor-alkali process is the main source of both chlorine gas and sodium hydroxide.

24 Some questions Why does electrolysis of molten NaCl produce sodium metal while electrolysis of aqueous NaCl produces NaOH? In the Downs cell, we need to make sure that the sodium and chlorine end up in different places. Why? We will see later why the chlorine and hydroxide which are products of the chlor-alkali process need to be kept apart. Briefly, they react together to make the hypochlorite ion (OCl ).