CHEMICAL REACTIONS

Recognizing a chemical reaction: When a chemical reaction occurs, we frequently observe at least one of the following: 1.Change in color: Cr 2 O 7 2+ orange + HSO 3 - colorless → Cr 3+ green solution. 2.Formation of a gas: HCl + Na 2 CO 3 → CO 2 gas bubbles. 3.Formation of a solid: CaCl 2 solution + K 2 CO 3 solution → CaCO 3 ↓ white solid 4.Release or absorption of heat: HCl + NaOH → heat. Note: We cannot always assume that a chemical reaction has occurred when heat is absorbed or released. Many substances release or absorb heat when they are mixed together, even though no reaction has taken place.

CLASSES OF CHEMICAL REACTIONS 1.Combination reactions 2.Decomposition reaction 3.Single replacement reaction 4.Double replacement reaction (metathesis reaction): 5.Oxidation – reduction reactions (redox reaction) 6.Reactions in which heat is absorbed or released

COMBINATION REACTIONS: In a combination reaction, two or more simple substances combine to produce a more complex substance.

DECOMPOSITION REACTIONS: This is just the reverse of combination, a complex substance is broken down into a number of simpler substances.

SINGLE REPLACEMENT REACTIONS: During a replacement reaction one atom or ion in a compound is replaced by another.

The activity series: The following table consists of metallic elements (plus hydrogen) arranged in such a way that any element in the table will displace ions of the elements below it from aqueous solutions of their salts. The activity series (or electrochemical or electromotive series) A specific metal will displace any metal ion that appears below it. Lithium Potassium Barium Calcium Sodium These metals displace hydrogen from water Magnesium Aluminum Zinc Iron Nickel Tin Lead These metals displace hydrogen from acids Hydrogen Copper Mercury Silver Gold These metals do not react with acids (or water)

Thus, as we have seen in the previous example, iron is able to displace copper from an aqueous solution of copper(II) sulfate since iron is above copper in the electrochemical series. The reverse reaction does not occur

Metals very high in the activity series: The metals that are very high in the activity series can not only displace hydrogen from an acid, but they can also displace hydrogen from water. For example, sodium metal reacts violently with water to give hydrogen gas and a solution of sodium hydroxide:

The halogen replacement series: A series similar to the activity series exists for the halogens. The halogen replacement series A specific halogen will displace any halide ion that appears below it Fluorine Chlorine Bromine Iodine An element in this series can replace the ions of any element below it from aqueous solutions of its salts.

Double replacement reactions (metathesis reaction): This may be best thought of as being a change of partners, the cation from compound A joins with the anion of compound B, while the cation from compound B joins with the anion from compound A. The overall effect of such a reaction is that the positive ions exchange their negative partners.

The metathesis reactions that involve the formation of covalently bonded water molecules are given the special name of NEUTRALIZATION REACTION. A neutralization reaction involves the reaction of an acid with a base to produce a salt plus water. Arrhenius definitions of acids and bases: An acid is a substance that gives H + (aq) ions when it is dissolved in water. A base is a substance that gives OH - (aq) ions when it is dissolved in water. First let us consider the reaction between nitric acid (a strong acid) and potassium hydroxide (a strong base). The products are potassium nitrate and water. HNO 3 (aq) + KOH (aq) → KNO 3(aq) + H 2 O (ℓ)

Oxidation – Reduction reactions (redox reaction) This type of reaction in aqueous solution involves an exchange of electrons between two species. One species loses electrons and is said to be oxidized. The other species, which gains electrons, is reduced.

REACTIONS in which HEAT is ABSORBED or RELEASED: Reactions which accomplished by the release of energy in the form of heat are called Exothermic Reactions, whereas reactions that absorbs heat from the surroundings are called Endothermic Reactions.