Features of Chemical Reaction

Key words:

acid-base reaction or neutralization reactions
combination reaction
decomposition reaction
combustion reaction
disproportion reaction
displacement reaction
oxidation-reduction reactions (redox reaction)
precipitation reaction (exchange or metathesis reactions)

Skills to develop

Define some common features such as acid-base reaction (neutralization), combination, combustion, decomposition, displacement, precipitation (exchange or metathesis), and redox reactions.
Describe features of some common reactions.
Classify chemical reactions according to their features.

Introduction

Quantitative relationship among reactants and products are called stoichiometry. Stoichiometry problems are easily solved when amounts of substances are converted from mass (in units of g, kg etc), volume (L), into moles. Amounts in moles depend on how the chemical formula or the chemical reaction equations are written. Chemical reaction equations are the basis for reaction stoichiometry, but when the reactants are not stoichiometric mixtures, some reactants will be in excess whereas others will be in limited supply. The former is called excess reagents whereas the latter are called limiting regents.

Classification is a good method for the organizaiton of information. It is a good start in any study, because it helps to organize our thought and to put information in am order. We can classify chemical reactions according to their common features, and divide chemical reactions into a few types:

On the topic of stoichiometry, we still have to deal with solutions of compounds, electrolytes.

Some Common Features of Chemical Reactions

Classification is a useful scientific method, and we divide chemical reactions into some common types.

Acid and Bases
Many substances are known as acids and bases In the HandbookMenu, some properties of acids and bases are given in two tables and you may click them to find out their names and formulas. Reactions between acids and bases are called acid-base reactions. Some of them are often called neutralization reactions.
However, the concepts of acids and bases have been generalized and much more discussion can be found in modules or units in Chem123/125. Thus, acid-base reactions are further subdivided into other types. A few examples are given below,
HCl + NaOH = NaCl + H₂ . . . neutralization
HCl + NH₃ = NH₄Cl . . . formation of a salt from vapour
CH₃COOH + H₂O = CH₃COO^- + H₃O⁺ . . . ionization
CH₃COOH + NaHCO₃ = NaCH₃COO + CO₂ . . . gas formation
Combination
A combination reaction refers to one in which two or more substances react (or combine) to give one new product. Elements combine to form compound, but compounds may also do the same. The following are combination reactions: H₂(g) + Cl₂(g) = 2 HCl(g)
Fe(s) + S(s) = FeS(s)
NH₃(g) + HCl(g) = NH₄Cl(s, appear as a white smoke)
2 Na(s) + Cl₂(g) = 2 NaCl(s)
2 NO(g) + O₂(g) = 2 NO₂(g)
P₄O₁₀(s) + 6 H₂O(l) = 4 H₃PO₄(aq)
Combustion
A substance reacts with oxygen or oxygen in the air to give oxides in a combustion reaction. For example the combustion of alcohol, sulfur and magnesium are given below: C₂H₅OH(l) + O₂(g) = CO₂(g) + H₂O(g)
S(s) + O₂(g) = SO₂(g)
2 Mg(s) + O₂(g) = 2 MgO(s)
It's interesting to point out that at very high temperature, a combustion reaction favors the formation of CO rather than CO₂, especially when oxygen supply is limited. The combustion of CO with oxygen release energy, and at high temperature, abundance of energy in the system keeps CO from further oxidation.
Decomposition
A compound or substance separate into two or more substances which may or may not be different from each other, but the products are usually different from the original compound. Some examples of decomposition reactions are: 2 H₂O(l) = 2 H₂ + O₂ . . . electrolysis of water
2 HgO(s) = 2 Hg(l) + O₂(g) . . . generation of oxygen from heating HgO
2 KClO₃(s) = 2 KCl(s) + 3 O₂(g) . . . another way to produce oxygen
CH₃COOH = CH₄ + CO₂ Note however, the following reactions are often called ionization or dissociation reactions rather than decomposition reaction. The solvent participates in these reactions. NaCl(s) + 6 H₂O = Na⁺(H₂O)₆ + Cl^-
NH₄Cl(s) = NH₄⁺(aq) + Cl^-
HNO₂ = H⁺(aq) + NO₂^-(aq)
Some decomposition reactions are Disproportion Reactions. For example,
3 Cl₂(g) + 6 OH^-(aq) = 5 Cl^- + ClO₃^-(aq) + 3 H₂O(l)
4 KClO₃(s) = KCl(s) + 3 KClO₄(s)
Displacement
Due to difference in affinity, part of a compound may be replaced by another element or a group of atoms. The following reactions illustrate not only the displacement reactions, but also the relative reactivity of some elements: Zn + H₂SO₄ = ZnSO₄ + 2 H₂ . . . Zn displaces hydrogen
2 HCl + F₂ = 2 HF + Cl₂ . . . F displaces Cl
2 Na + H₂O = 2 NaOH + H₂ . . . Na displaces H
2 Al + Fe₂O₃ = 2 Fe + Al₂O₃ . . . a thermite reaction
Oxidation and Reduction
Oxidation and reduction reactions usually involve the transfer of electrons. A gain of electron is called reduction, and a loss of electron is oxidation. Since electrons can neither be created nor destroyed a gain of electrons by one species must be accompanied by a loss of another species. Thus, oxidation reactions are always accompanied by reductions, and reactions of this type are called redox reactions. These reactions are the basis for battery or Daniel cell operations.
Before the realization of electron transfers, reactions with oxygen were called oxidation reaction, and reactions with hydrogen or with base-forming elements such as metals were called reduction reactions. However, the more modern definition will be used in our discussion of chemistry.
There are many redox reactions, and balancing redox reaction equation is a basic skill for an educated person. A unit will be devoted to balance redox reaction equations in this course. This type of reactions may belong to types mentioned earlier. Please identify the species that are oxidized and reduced in the following reactions,
Zn + H₂SO₄ = ZnSO₄ + 2 H₂; (zinc oxidized)
2 HCl + F₂ = 2 HF + Cl₂ (flourine reduced)
2 Na + H₂O = 2 NaOH + H₂ (hydrogen reduced)
2 Al + Fe₂O₃ = 2 Fe + Al₂O₃
2 HgO = 2 Hg + O₂ (oxygen oxidized)
2 KClO₃ = 2 KCl + 3 O₂ (chlorine reduced)
C₂H₅OH + O₂ = CO₂ + H₂O
Precipitation
Solids appear when two solutions are mixed if the products formed are not soluble. The solids are called precipitates, and such reactions are called precipitation reactions. In reaction equations, we use (s) to represent the formation of a solid or precipitate (ppt). AgNO₃ + NaCl = NaNO₃ + AgCl(s)
PbNO₃ + KI = PbNO₃ + PbI(s)
Precipitation reactions take place in solutions, and ions exchange in these reactions. Thus, they are often called exchange reactions or metathesis reactions.

Chemical reactions can be beneficial and hazardous. Chemical Reaction Hazards Forum is an Internet site that discusses some recent event related to chemical reactions.

Mechanisms of oscillating reactions, and chain reactions are interesting. These and millions of reactions are still being investigated now.

Skill Developing Problems

In what class are the following reactions?

Ca(s) + O₂(g) = CaO(s)
2 Fe(s) + 3 Cl₂(g) = 2 FeCl₃(s)
2 FeO(s) + O₂(g) = Fe₂O₃ . . . corrosion
As(s) + H₂(g) = AsH₃ . . . formation of arsine

Skill: Define and exemplify combination reactions.
In what class are the following reactions?

CaCO₃(s) = CaO + CO₂(g)
4 KClO₃(s) = KCl(s) + 3 KClO₄(s)
H₂S(g) = H₂ + S(s)

Skill: Define and exemplify decomposition reactions.
In what class are the following reactions?

2 HCl + F₂ = 2 HF + Cl₂

CaCO₃(s) = CaO + CO₂(g)
3 Cl₂(g) + 6 OH^-(aq) = 5 Cl^- + ClO₃^-(aq) + 3 H₂O(l)
4 KClO₃(s) = KCl(s) + 3 KClO₄(s)
H₂S(g) = H₂ + S(s)

Skill: Define and exemplify redox reactions.
In what class are the following reactions?

2 K(s) + 2 H₂O(l) = KOH + H₂(g)
KCl(aq) + F₂ = KF(aq) + Cl₂(g)
2 H₂S(g) + O₂(g) = H₂O(g) + S(s)
Cu(s) + 2 AgNO₃(aq) = 2 Ag + Cu(NO₃)₂(aq)

Skill: Define and exemplify displacement reactions.
Classify the following collection of chemical reactions into these categories: acid-base, combination, decomposition, combustion, displacement, redox, and precipitation reactions. A reaction may belong to more than one category.

Ca(s) + O₂(g) = CaO(s)
2 Fe(s) + 3 Cl₂(g) = 2 FeCl₃(s)
CaO(s) + CO₂(g) = CaCO₃(s)
2 AgBr(s) = 2 Ag + Br₂(g)
PbO(s) + C(s) = Pb(s) + CO(g)
Cu(s) + 2 AgNO₃(aq) = 2 Ag + Cu(NO₃)₂(aq)
NH₄Cl(s) = NH₃(g) + HCl(g)
CH₂CH₂(g) + H₂(g) = CH₃CH₃(g)
CH₂CH₂(g) + 3 O₂(g) = 2 CO₂(g) + 2 H₂O(l)
Al(s) + AgS(s) = Al₂S₃ + Ag(s)
NH₃ + H₂O = NH₄^-(aq) + OH^-(aq)
HNO₂ = NO₂^- + H⁺
AgNO₃ + NaOH = Ag(OH)(s) + NaNO₃

Skill: This problem also illustrates a strategy for chemical analysis.
The reaction equation: 2 Al (s) + Fe₂O₃ (s) = 2 Fe(liquid) + Al₂O₃ is the thermite reaction when the mixture of aluminium and iron oxide is ignited. This reaction is a

combination reaction
decomposition reaction
displacement reaction
combustion reaction
acid-base reaction
precipitation reaction

Discussion: This is also a redox reaction (reduction and oxidation reaction).
How will you classify these reactions?

Ca(OH)₂ + CO₂ = CaCO₃(s) + H₂O
BaCl₂ + H₂SO₄ = BaSO₄(s) + 2 HCl
K₂CrO₄ + Pb(CH₃COO)₂ = PbCrO₄(s, yellow) + 2 K(CH₃COO)

Discussion: These reactions are used for the detection of any one of the reactants.
How will you classify these reactions?

Mg(OH)₂ + HCl = MgCl₂ + 2 H₂O . . . antacid and gastric juice reaction
Ca(OH)₂ + H₃C₆H₅O₇ . . . lime and lemon juice reaction
HC₉H₇O₄ + H₂O = C₉H₇O₄^- + H₃O⁺ . . . ionization of aspirin

Discussion: Indicators change color due to acid-base reactions.