John E McMurry and Robert C Fay

CHEMISTRYChapter 18

Hydrogen Oxygen and Water

Chapter 18/1

• Hydrogen

• Isotopes of Hydrogen

• Binary Hydrides

• Oxygen

• Preparation and uses of Oxygen

• Peroxides and Superoxides

• Hydrates

Chapter 18/3

Hydrogen

H2(g) + Zn2+(aq)2H1+(aq) + Zn(s)

In 1781 Cavendish demonstrated when hydrogen burned it formed water. The French chemist Lavoisier called the gas “hydrogen” which means “water former.”

Henry Cavendish (1731-1810) in 1766 is credited with isolating hydrogen in its pure form:

Hydrogen is most abundant element in the universeIt is 9th most abundant element on mass % basis on Earth.It is colorless odorless and tasteless gas

Chapter 18/4

Isotopes of Hydrogen

Three isotopes:

1. Hydrogen-1 (protium):99.985% abundance

2. Hydrogen-2 (deuterium):0.015% abundance

3. Hydrogen-3 (tritium):about 10-16% abundance

H2

1

H3

1

H1

1

Isotopes of Hydrogen

Chapter 18/6

Isotopes of Hydrogen

Isotope Effects

Kw = 0.195 x 10-14D3O1+(aq) + OD1-(aq)2D2O(l)

Kw = 1.01 x 10-14H3O1+(aq) + OH1-(aq)2H2O(l)

2D2O(l) 2D2(g) + O2(g)Electrolysis

2H2O(l) 2H2(g) + O2(g)Electrolysis

Slower

Faster

Preparation and Uses of Hydrogen

2H2(g) + O2(g)2H2O(l) H° = +572 kJ

Binary Hydrides

Chapter 18/9

Binary Hydrides

Ionic Hydrides

The hydride ion is a good proton acceptor (Brønsted-Lowry base):

H° = -181.5 kJCaH2(s)Ca(s) + H2(g)400 °C

H° = -112.6 kJ2NaH(s)2Na(l) + H2(g)400 °C

Saltlike, high melting, white, crystalline compounds formed by the alkali metals and the heavier alkaline earth metals Ca, Sr, and Ba:

2H2(g) + Ca2+(aq) + 2OH1-(aq)CaH2(s) + 2H2O(l)

Chapter 18/10

Binary Hydrides

Covalent Hydrides

Common hydrides of nonmetallic elements, such as diborane (B2H6), methane (CH4), ammonia (NH3), water (H2O), and hydrogen halides (HX; X = F, Cl, Br, or I).

In general, their intermolecular forces are relatively weak so they exist as gases or volatile liquids at ordinary temperatures.

Chapter 18/11

Binary Hydrides

Metallic Hydrides

Formed by the reaction of the lanthanides and actinide metals and certain d-block transition metals with variable amounts of hydrogen with the general formula MHx.

PdHx(s)Pd(s) + H2(g)2x

Favored at lower temperature

Favored at higher temperature

Chapter 18/12

Oxygen

Priestly called the gas “dephlogisticated air.” Lavoisier called it “oxygen” which means “acid former.”

Gaseous O2 condenses at -183 °C to form a pale blue liquid and freezes at -219 to give a pale blue solid.

Joseph Priestly and Karl Wilhelm Scheele are credited for isolating and characterizing oxygen between 1771 and 1774:

2Hg(l) + O2(g)2HgO(s)Heat

Chapter 18/13

Preparation and Uses of Oxygen

2H2O2(aq) 2H2O(l) + O2(g)Catalyst

Small amounts of oxygen can be generated in the lab:

2H2O(l) 2H2(g) + O2(g)Electrolysis

2KCl(s) + 3O2(g)2KClO3(s)MnO2 catalyst

Heat

Photosynthesis constantly replaces used oxygen:

6O2 + C6H12O66CO2 + 6H2Oh

Glucose

Chapter 18/14

Oxides

Binary compounds in which oxygen is in: -2 oxidation state are called oxides. -1 oxidation state are called peroxides. -1/2 oxidation state are called superoxides.

Chapter 18/15

Oxides

Peroxides and Superoxides

When some of the group 1A and group 2A metals are heated in excess oxygen, they will form either peroxides, such as Na2O2 and BaO2, or superoxides, such as KO2, RbO2, and CsO2.

Chapter 18/17

Peroxides and Superoxides

O2(g) + 2K1+(aq) + HO21-(s) +OH1-(aq)KO2(s) + H2O(l)

Superoxides can dissolve in water:

2Na1+(aq) + HO21-(aq) + OH1-(aq)Na2O2(s) + H2O(l)

The peroxide ion is a basic anion:

BaS4(s) + H2O2(aq)BaO2(s) + H2SO4(aq)

Peroxides will react with strong acids:

Chapter 18/18

Hydrogen Peroxide

Pure hydrogen peroxide freezes at 4 °C and is estimated to boil at 150 °C.

Chapter 18/19

Water

Chapter 18/20

Hydrates

Compounds that absorb water from the air are hygroscopic and can be used as drying agents.

CuSO4•5H2O

Hydrates: Solid compounds that contain water molecules.

Mg(ClO4)2•6H2O

AlCl3•6H2O

CoCl2•6H2O(s)CoCl2(s) + 6H2O(l)