Classes of Chemical Reactions revised October 12, 2009

 

Acids, Bases, Neutralization, Net Ionic Equations Oxidation numbers and Redox Reactions

 

Acids are defined as H+ protons donors. These are the common binary acids (in pink) and typically are dissolved in water. HF(aq) HCl(aq) HBr(aq) HI(aq) H2O(aq) H2S(aq) The pure HX compounds are not considered to be acids. The pure materials are gases at room temperature except for water. HF(g) HCl(g) HBr(g) HI (g) H2O(l) H2S(g)

Bases are hydroxide donors. These compounds a a combination of a metal ion and the hydroxide ion, OH1-. Magnesium hydroxide , Mg(OH)2, is a common base. LiOH(aq) KOH(aq) NaOH(aq) CsOH(aq) Al(OH)3(aq) Ca(OH)2 (aq)

  

Metal oxides like MgO and Na2O are called basic oxides, because these compounds react with water to form hydroxide compounds.

MgO + H2O -------> Mg(OH)2

Neutralization is the process of reacting an acid with a base.

Mg(OH)2 + 2 HCl(aq) -------> H2O + MgCl2(aq)

The idea is that the reaction "neutralizes" the protons from the acid and the hydroxide from the base.

There are compounds like carbonates, Na2CO3, that do not contain hydroxide in the formula but on contact with water form hydroxide ions, OH1-.

Na2CO3(s) + H2O -------> 2 Na1+ + HCO31-(aq) + OH1-(aq)

Exercise: What is the acid in the reaction? What is the coefficient needed to balance the equation? Al(OH)3(aq) + ___HCl(aq) --------> AlCl3(aq) + 3 H2O return to top of page

Answer: What is the acid in the reaction? What is the coefficient needed to balance the equation? Al(OH)3(aq) +__3 _ HCl(aq) ----> AlCl3(aq) + 3 H2O base acid

Net Ionic Equations

This is a very involved concept. It requires knowledge of the solubility and ionization properties of reactants. The value of the concept lies in the idea that reactions are written showing only ions and compounds that "change" in the reaction. Reactions that occur in water solutions do not really happen between the formula units for the reactants. The process involves the dissolving process first. When "soluble" materials dissolve the formula unit breaks into ions. Example: KOH(aq) + HCl(aq) -------> H2O + KCl(aq) The KOH dissolves in water to form K1+ and OH1- ions The materials in solution are really the following ions K1+ , OH1- , H1+ , Cl1- This means the particles in the reaction are really K1+(aq) + OH1-(aq) + H1+(aq) + Cl1-(aq) ------> H2O + K1+ (aq) + Cl1-(aq) Notice the potassium ions, K1+(aq), and chloride ions, Cl1-(aq) , are unchanged . K1+(aq) + OH1-(aq) + H1+(aq) + Cl1-(aq) ------> H2O + K1+(aq) + Cl1-(aq) When the "spectator" nonreacting ions are deleted and the reaction is simplified. OH1-(aq) + H1+(aq) ------> H2O return to top of page

 

Spectator ions are ions that remain unchanged in the reaction. A net ionic equation is written showing only the ions and formulas that actually change in the reaction.

Oxidation Reduction Reactions

Acid base reactions are identified by the formation of water when a hydroxide ion and a proton combine. The apparent charges on ions and atoms stay the same, hydrogen stays +1 and oxygen stays -2. Oxidation reduction reactions are different. One substance is oxidized and the other is reduced. The difference is that electrons are transferred in the oxidation reduction reaction. Oxidation is the loss of electrons Zn(s) ---> Zn2+(aq) + 2 e1- Reduction is the gain of electrons Cl2 (g) + 2 e1- ----> 2 Cl1-(aq)   The number of electrons lost by the oxidized atom equals the number of electrons gained by the reduced atom. The atoms have a change in oxidation number. The oxidation number is the "apparent" charge on an atom.

A general rule about redox reactions is that if an atom or formula gains "O" oxygen it is oxidized.return to top of page If a formula or atom loses oxygen it is reduced. Hydrogen can be used as a "screening" tool . An atom or formula that gains "H" hydrogen is reduced and any formula or atom that loses hydrogen is oxidized.

 

1. Atoms in pure elements have an oxidation number of "0". Cl2 N2 P4 S6 zero oxidation number zero oxidation number zero oxidation number zero oxidation number

 

2. Atoms in monoatomic ions have oxidation numbers equal to the charge on the ion. return to top of page

Cl1- N3- K1+ O2- minus one oxidation number minus three oxidation number plus one oxidation number minus two oxidation number

3. The more electronegtive atom in a bonded pair has a negative oxidation number. In binary molecules the more electronegative atom has the same number it would have as a simple anion.

CO, carbon monoxide   PH3, phosphine

4. The sum for all the oxidation numbers add to zero for neutral formulas.return to top of page

CO2 NH3 carbon is +4 each oxygen is -2 two oxygens total -4 nitrogen is -3 each hydrogen is +1 three hydrogens total +3 all atoms +4 +(-4) = 0 all atoms +3 +(-3) = 0

Example of redox reaction. return to top of page

The reaction of oxygen with ethane is shown here.

The "O" is reduced. It gains hydrogen. The CH3CH3 is oxidized, it gains "O" and loses "H"

The loss of hydrogen is oxidation. The gain of oxygen is oxidation.

Example: The reaction of oxygen with sulfur is an oxidation reduction reaction.

S6 (s)	+ 6 O2 (g)		6 SO2 (g)
	Oxygen has 0	reduced	Oxygen has -2 oxidation number

Sulfur as the pure element started with "0" oxidation number. It is  oxidized to sulfur with +4 oxidation number. return to top of page

 

Dr. Walt Volland, All rights reserved, 2005-2009

last modified October 12, 2009