5 Steps to a 5 AP Chemistry

Halogen reactivity decreases as one goes from top to bottom in the periodic table,

because of the decreasing electronegativity. Therefore, a separate activity series for the

halogens can be developed:

The above series indicates that if chlorine gas were dissolved in a KI(aq) solution, the

elemental chlorine would displace the iodide ion:

Combustion Reactions

Combustion reactions are redox reactions in which the chemical species rapidly combines

with oxygen and usually emits heat and light. Reactions of this type are extremely impor-

tant in our society as the sources of heat energy. Complete combustion of carbon yields

carbon dioxide, and complete combustion of hydrogen yields water. The complete combus-

tion of hydrocarbons, organic compounds containing only carbon and hydrogen, yields

carbon dioxide and water:

If the compound also contains oxygen, such as in alcohols, ethers, etc., the products are

still carbon dioxide and water:

If the compound contains sulfur, the complete combustion produces sulfur dioxide,

SO 2 :

If nitrogen is present, it will normally form the very stable nitrogen gas, N 2.

In all of these reactions, the driving force is the highly reactive oxygen forming a very

stable compound(s). This is shown by the exothermic nature of the reaction.

In balancing any of these combustion reactions, it is helpful to balance the oxygen last.

Coordination Compounds

When a salt is dissolved in water, the metal ions, especially transition metal ions, form

a complex ion with water molecules and/or other species. A complex ionis composed of a

metal ion bonded to two or more molecules or ions called ligands. These are Lewis

acid–base reactions. For example, suppose Cr(NO 3 ) 3 is dissolved in water. The Cr^3 +

cation attracts water molecules to form the complex ion Cr(H 2 O) 63 +. In this complex

ion, water acts as the ligand. If ammonia is added to this solution, the ammonia can displace

the water molecules from the complex:

[( )]() 263 ++++ 6633 () [( )]()Cr H O 63 2 ()l( aq NH aq Cr NH aq H O

29462 CHSg 26 ()+→ + +O g 2 () CO g 2 () HOg 2 () SO g 2 ()

2324 CH OH l 3222 ()+→ +O g() CO g() H O g()

2746 CH g 26 ()+→ +O g 2 () CO g 2 () HOg 2 ()

Cl aq KI aq KCl aq I s

Cl aq I a

22

2

22

2

() () () ()

() (

+→ +

+ − qqClaqIs)()()→+ 2 − 2

F

Cl

Br

I

2

2

2

2

Reactions and Periodicity  75

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