Chapter 6 Hydrides and Oxides Notes

1. Chapter 6 Hydrides and Oxides Notes

2. Hydrides Most metals react with acid to produce hydrogen gas. For example: Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) This is how hydrogen was first discovered by Henry Cavendish. Hydrogen can also be made from the electrolysis of water and the reaction of hot carbon or methane with water.

3. Hydrogen has 1+ and 1- ionic charges possible. When reacting with any 1A-5A elements it will take the 1-form and be written as the anion “hydride”. When reacting with any 6A-7A elements it will take the 1+form and be the cation. Make note of the common hydrogen compounds in the chart below for the exceptions of the typical criss-cross.

5. Ionic hydrides (metal + hydrogen) are considered basic because addition of water to an ionic hydride forms a base: CaH2(s) + H2O(l) → Ca(OH)2 + 2H2(g)

6. Oxides Oxygen is reactive enough to form compounds with all elements except the noble gases and Au, Pd, and Pt. That means all elements will oxidize, which we call rust when it happens to iron. Oxygen was first discovered by Carl Scheele, first written about by Joseph Priestly (who called it dephlogisticated air because it helped things to burn well) and named oxygen by Antoine Lavoisier for the French words for “acid former” (think how many oxyacids there are).

7. When oxygen combines with alkali metals there are three possibilities. With alkaline earth metals only two: • It could form an oxide. Oxides have a -2 oxidation state on the oxygen, for example: K2O. • It could form a peroxide. Peroxides have a -1 oxidation state on the oxygen, for example: K2O2. • It could form a superoxide. Superoxides have a - oxidation state on the oxygen, for example: KO2.

8. The following table summarizes the possibilities with the first two groups of the periodic table. You need to know which ones will form which oxide. Notice the group IIA metals do not readily form perioxides and noneform superoxides. Only at extremely excessive amounts of oxygen would calcium through barium form the perioxides.

9. Almost all other metals form oxides as well, but as they have multiple oxidation states they can make multiple varieties of oxides, but no peroxides or superoxides would be formed. With multiple oxidation state metals, the resulting valence of the metal depends entirely on the amount of oxygen present for the reaction. For example: 4Cu + limited O2 → 2Cu2O (a red oxide) 2Cu + excess O2 → 2CuO (a black oxide) It should be logical that the more negative oxygen there is the higher the positive oxidation state will be on the metal.

10. Reactions with Oxygen Metallic oxides are considered basic oxides because they will react with water to form a base (-OH). Sometimes these are called basic anhydrides, meaning they don’t yet have the water to form a base with. Na2O + H2O → 2NaOH

11. Nonmetallic oxides are considered acidic because they will react with water to form an oxyacid (H_O). Sometimes these are called acidic anhydrides. SO3+ H2O → H2SO4 (note: B2O3 and SiO2 are not soluble so would not combine with water) Metallic oxides and nonmetallic oxides react with each other to form oxy-salts. MgO + CO2 → MgCO3

12. Combustion reactions involve combining oxygen with hydrocarbons to form carbon dioxide and water. Some metals (like magnesium) also combust, but the products would most certainly NOT form carbon dioxide and water. C6H12+ 9O2 → 6CO2 + 6H2O 2Mg + O2 → 2MgO (technically a combination reaction)

13. If not enough oxygen is available incomplete combustion occurs and forms carbon monoxide and water. 2C8H18 + 17O2 → 16CO + 18H2O In a car the catalytic converter is a special material that carbon monoxide would cling to and combine with oxygen so only carbon dioxide exits. Both are poisonous and dangerous to the environment, but carbon monoxide is much more poisonous than carbon dioxide.