Classes of Chemical Reactions

Classes of Chemical Reactions

Objectives:

classify a reaction as acid-base, redox, precipitation

state oxidation numbers for common ions

state oxidation numbers for elements

assign oxidation numbers for atoms in formulas

give the typical oxidation numbers are for common elements like H, O , F, Cl, Na, K

state the definition for oxidation numbers

tell whether a compound is oxidized or reduced in a equation

Acids, Bases and Neutralization Reactions

Acids are defined as protons donors. These are the common binary acids (in pink)

HF(aq)

HCl(aq)

HBr(aq)

HI(aq)

H₂O(aq)

H₂S(aq)

The pure HX compounds are not considered to be acids.

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, OH^1-. 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, OH^1-.

Na₂CO₃(s) + H₂O -------> 2 Na¹⁺ + HCO₃^1-(aq) + OH^1-(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

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 OH^1- ions

The materials in solution are really the following ions K¹⁺ , OH^1- , H¹⁺ , Cl^1-

This means the particles in the reaction are really

K¹⁺(aq)+ OH^1-(aq) + H¹⁺(aq) + Cl^1-(aq)------> H₂O + K¹⁺(aq) + Cl^1-(aq)

Notice the potassium ions, K¹⁺(aq), and chloride ions, Cl^1-(aq) , are unchanged .

K¹⁺(aq) + OH^1-(aq) + H¹⁺(aq) + Cl^1-(aq)------> H₂O + K¹⁺(aq) + Cl^1-(aq)

When the "spectator" nonreacting ions are deleted and the reaction is simplified.

OH^1-(aq) + H¹⁺(aq) ------> H₂O

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. The number of

Oxidation is the loss of electrons

Zn(s) ---> Zn²⁺(aq) + 2 e^1-

Reduction is the gain of electrons

Cl₂(g) + 2 e^1-----> 2 Cl^1-(aq)

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.

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.

Cl^1- N^3- K¹⁺ O^2-

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

PH_3, phosphine

4. The sum for all the oxidation numbers add to zero for neutral formulas.

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.

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 starte with "0" oxidation number is oxidized to sulfur with +4 oxidation number.

S₆ (s)	+ 6 O₂ (g)		6 SO₂ (g)

	Oxygen has 0	reduced	Oxygen has -2 oxidation number