Patterns of Chemical Reactivity

Patterns of Chemical Reactivity

Using the Periodic Table

• As a consequence of the good ordering of the periodic table, the properties of compounds of elements vary in a systematic manner.
• Example: All the alkali metals (M) react with water as follows:

2 M (s) + 2 H2O (l) --> 2 MOH (aq) + H2 (g)

 The reactions become more vigorous as we move from Li to Cs
 Sodium reacts with water to produce an orange flame.
 Potassium reacts with water to produce a blue flame.
 The reaction of potassium with water produces so much heat that the hydrogen gas produced usually ignites with a loud pop.


Combustion in Air

• Combustion reactions are rapid reactions that produce a flame.
 Combustion is the burning of a substance in air.
 Example: Propane combusts to produce carbon dioxide and water:

C3H8 (g) + 5 O2 (g) --> 3 CO2 (g) + 4 H2O (l)


Combination and Decomposition Reactions

• In combination reactions two or more substances react to form one product.
• Combination reactions have more reactants than products.
 Consider the reaction:

2 Mg (s) + O2 (g) --> 2 MgO (s)

• Since there are fewer products than reactants, the Mg has combined with O2 to form MgO.
• Note that the structure of the reactants has changed:
 Mg consists of closely packed atoms, and O2 consists of dispersed molecules.
 MgO consists of a lattice of Mg2+ and O2- ions.

• In decomposition reactions one substance undergoes a reaction to produce two or more other substances.
• Decomposition reactions have more products than reactants.
 Consider the reaction that occurs in an automobile air bag:
2 NaN3 (s) --> 2 Na (s) + 3 N2 (g)

 Since there are more products than reactants, the sodium azide has decomposed into Na metal and N2 gas.