NEUTRALIZATION ACIDS AND BASES

Transcription

1 NEUTRALIZATION Chemical reaction, according to the Arrhenius theory of acids and bases, in which a water solution of acid is mixed with a water solution of base to form a salt and water; this reaction is complete only if the resulting solution has neither acidic nor basic properties. Such a solution is called a neutral solution. Complete neutralization can take place when a strong acid, such as hydrochloric acid, HCl, is mixed with a strong base, such as sodium hydroxide, NaOH. Strong acids and strong bases completely break up, or dissociate, into their constituent ions when they dissolve in water. In the case of hydrochloric acid, hydrogen ions, H+, and chloride ions, Cl-, are formed. In the case of sodium hydroxide, sodium ions, Na+, and hydroxide ions, OH-, are formed. The hydrogen and hydroxide ions readily unite to form water. If the number of hydrogen ions in the hydrochloric acid solution is equal to the number of hydroxide ions in the sodium hydroxide solution, complete neutralization occurs when the two solutions are mixed. The resulting solution contains sodium ions and chloride ions that unite when the water evaporates to form sodium chloride, common table salt. In a neutralization reaction in which either a weak acid or a weak base is used, only partial neutralization occurs. In a neutralization reaction in which both a weak acid and a weak base are used, complete neutralization can occur if the acid and the base are equally weak. The heat produced in the reaction between an acid and a base is called the heat of neutralization. When any strong acid is mixed with any strong base, the heat of neutralization is always about 13,700 calories for each equivalent weight of acid and base neutralized. See article on ph; titration. ACIDS AND BASES Two related classes of chemicals; the members of each class have a number of common properties when dissolved in a solvent, usually water. 1. Properties : Acids in water solutions exhibit the following common properties: they taste sour; turn litmus paper red; and react with certain metals, such as zinc, to yield hydrogen gas. Bases in water solutions exhibit these common properties: they taste bitter; turn litmus paper blue; and feel slippery. When a water solution of acid is mixed

2 with a water solution of base, water and a salt are formed; this process, called neutralization, is complete only if the resulting solution has neither acidic nor basic properties. 2. Classification : Acids and bases can be classified as organic or inorganic. Some of the more common organic acids are: citric acid, carbonic acid, hydrogen cyanide, salicylic acid, lactic acid, and tartaric acid. Some examples of organic bases are: pyridine and ethylamine. Some of the common inorganic acids are: hydrogen sulfide, phosphoric acid, hydrogen chloride, and sulfuric acid. Some common inorganic bases are: sodium hydroxide, sodium carbonate, sodium bicarbonate, calcium hydroxide, and calcium carbonate. 3. Acids, such as hydrochloric acid, and bases, such as potassium hydroxide, that have a great tendency to dissociate in water are completely ionized in solution; they are called strong acids or strong bases. Acids, such as acetic acid, and bases, such as ammonia, that are reluctant to dissociate in water are only partially ionized in solution; they are called weak acids or weak bases. Strong acids in solution produce a high concentration of hydrogen ions, and strong bases in solution produce a high concentration of hydroxide ions and a correspondingly low concentration of hydrogen ions. The hydrogen ion concentration is often expressed in terms of its negative logarithm, or ph (see separate article). Strong acids and strong bases make very good electrolytes (see electrolysis), i.e., their solutions readily conduct electricity. Weak acids and weak bases make poor electrolytes. 4. See buffer; catalyst; indicators, acid-base; titration. 5. Acid-Base Theories : There are three theories that identify a singular characteristic which defines an acid and a base: the Arrhenius theory, for which the Swedish chemist Svante Arrhenius was awarded the 1903 Nobel Prize in chemistry; the Brönsted-Lowry, or proton donor, theory, advanced in 1923; and the Lewis, or electron-pair, theory, which was also presented in Each of the three theories has its own advantages and disadvantages; each is useful under certain conditions. 6. The Arrhenius Theory : When an acid or base dissolves in water, a certain percentage of the acid or base particles will break up, or dissociate (see dissociation), into oppositely charged ions. The Arrhenius theory defines an acid as a compound that can dissociate in water to yield hydrogen ions, H +, and a base as a compound that can dissociate in water to yield hydroxide ions, OH -. For example, hydrochloric acid, HCl, dissociates in water to yield the required hydrogen ions, H +, and also chloride ions, Cl -. The base sodium hydroxide, NaOH, dissociates in water to yield the required hydroxide ions, OH -, and also sodium ions, Na The Brönsted-Lowry Theory : Some substances act as acids or bases when they are dissolved in solvents other than water, such as liquid ammonia. The Brönsted-Lowry theory, named for the Danish chemist Johannes Brönsted and the British chemist Thomas Lowry, provides a more general definition of acids and bases that can be used to deal both with solutions that contain no water and solutions that contain water. It defines an acid as a proton donor and a base as a proton acceptor. In the Brönsted-Lowry theory, water, H 2 O, can be considered an acid or a base since it can lose a proton to form a hydroxide ion, OH -, or accept a proton to form a hydronium ion,

3 H 3 O + (see amphoterism). When an acid loses a proton, the remaining species can be a proton acceptor and is called the conjugate base of the acid. Similarly when a base accepts a proton, the resulting species can be a proton donor and is called the conjugate acid of that base. For example, when a water molecule loses a proton to form a hydroxide ion, the hydroxide ion can be considered the conjugate base of the acid, water. When a water molecule accepts a proton to form a hydronium ion, the hydronium ion can be considered the conjugate acid of the base, water. 8. The Lewis Theory : Another theory that provides a very broad definition of acids and bases has been put forth by the American chemist Gilbert Lewis. The Lewis theory defines an acid as a compound that can accept a pair of electrons and a base as a compound that can donate a pair of electrons. Boron trifluoride, BF 3, can be considered a Lewis acid and ethyl alcohol can be considered a Lewis base. SALT Chemical compound (other than water) formed by a chemical reaction between an acid and a base (see acids and bases). 1. Characteristics and Classification of Salts The most familiar salt is sodium chloride, the principal component of common table salt. Sodium chloride, NaCl, and water, H 2 O, are formed by neutralization of sodium hydroxide, NaOH, a base, with hydrogen chloride, HCl, an acid: HCl + NaOH ->NaCl + H 2 O. Most salts are ionic compounds (see chemical bond); they are made up of ions rather than molecules. The chemical formula for an ionic salt is an empirical formula; it does not represent a molecule but shows the proportion of atoms of the elements that make up the salt. The formula for sodium chloride, NaCl, indicates that equal numbers of sodium and chlorine atoms combine to form the salt. In the reaction of sodium with chlorine, each sodium atom loses an electron, becoming positively charged, and each chlorine atom gains an electron, becoming negatively charged (see oxidation and reduction); there are equal numbers of positively charged sodium ions and negatively charged chloride ions in sodium chloride. The ions in a solid salt are usually arranged in a definite crystalline structure, each positive ion being associated with a fixed number of negative ions, and vice versa. 2. A salt that has neither hydrogen (H) nor hydroxyl (OH) in its formula, e.g., sodium chloride (NaCl), is called a normal salt. A salt that has hydrogen in its formula, e.g., sodium bicarbonate (NaHCO 3 ), is called an acid salt. A salt that has hydroxyl in its formula, e.g., basic lead nitrate (Pb[OH]NO 3 ), is called a basic salt. Since a salt may react with a solvent to yield different ions than were present in the salt (see hydrolysis), a solution of a normal salt may be acidic or basic; e.g., trisodium phosphate, Na 3 PO 4, dissolves in and reacts with water to form a basic solution. 3. In addition to being classified as normal, acid, or basic, salts are categorized as simple salts, double salts, or

4 complex salts. Simple salts, e.g., sodium chloride, contain only one kind of positive ion (other than the hydrogen ion in acid salts). Double salts contain two different positive ions, e.g., the mineral dolomite, or calcium magnesium carbonate, CaMg(CO 3 ) 2. Alums are a special kind of double salt. Complex salts, e.g., potassium ferricyanide, K 3 Fe(CN) 6, contain a complex ion that does not dissociate in solution. A hydrate is a salt that includes water in its solid crystalline form; Glauber s salt and Epsom salts are hydrates. 4. Salts are often grouped according to the negative ion they contain, e.g., bicarbonate or carbonate, chlorate, chloride, cyanide, fulminate, nitrate, phosphate, silicate, sulfate, or sulfide. 5. Preparation of Salts : Salts are also prepared by methods other than neutralization. A metal can combine directly with a nonmetal to form a salt; e.g., sodium metal reacts with chlorine gas to form sodium chloride. A metal may react with a dilute acid to form a salt and release hydrogen gas; e.g., zinc reacts with dilute sulfuric acid to form zinc sulfate and hydrogen. A metal oxide may react with an acid to form a salt and water; e.g., calcium oxide reacts with carbonic acid to form calcium carbonate and water. A base can react with a nonmetallic oxide to form a salt and water; e.g., sodium hydroxide reacts with carbon dioxide to form sodium carbonate and water. Two salts may react with one another (in solution) to form two new salts; e.g., barium chloride and sodium sulfate react in solution to form barium sulfate (as an insoluble precipitate) and sodium chloride (which remains in solution). A salt may react with an acid to form a different salt and acid; e.g., sodium chloride and sulfuric acid react when heated to form sodium sulfate and release hydrogen chloride gas (which in solution forms hydrochloric acid). A salt undergoes dissociation when it dissolves in a polar solvent, e.g., water, the extent of dissociation depending both on the salt and the solvent. EQUIVALENT WEIGHT The equivalent weight of an element or radical is equal to its atomic weight or formula weight divided by the valence it assumes in compounds. The unit of equivalent weight is the atomic mass unit; the amount of a substance in grams numerically equal to the equivalent weight is called a gram equivalent. Hydrogen has atomic weight and always assumes valence 1 in compounds, so its equivalent weight is Oxygen has an atomic weight of and always assumes valence 2 in compounds, so its equivalent weight is The sulfate radical (SO 4 ) has formula weight and always has valence 2 in compounds, so its equivalent weight is Some elements exhibit more than one valence in forming compounds and thus have more than one equivalent weight. Iron (atomic weight ) has an equivalent weight of in ferrous compounds (valence 2) and in ferric compounds (valence 3). The weight proportion in which elements or radicals combine to form compounds can be determined from their equivalent

5 weights. For example, hydrogen can combine with oxygen to form water; the weight proportion of oxygen to hydrogen in water is the same as the proportion of their equivalent weights, to or to 1; there is 1 weight of hydrogen for every weights of oxygen, or water is about 11.2% hydrogen (by weight). Iron forms two oxides: ferrous oxide (FeO), in which there are weights of iron for each weights of oxygen, and ferric oxide (Fe 2 O 3 ), in which there are weights of iron for every weights of oxygen. TITRATION Gradual addition of an acidic solution to a basic solution or vice versa (see acids and bases); titrations are used to determine the concentration of acids or bases in solution. For example, a given volume of a solution of unknown acidity may be titrated with a base of known concentration until complete neutralization has occurred. This point is called the equivalence point and is generally determined by observing a color change in an added indicator such as phenolphthalein. From the volume and concentration of added base and the volume of acid solution, the unknown concentration of the solution before titration can be determined. Titrations can also be used to determine the number of acidic or basic groups in an unknown compound. A specific weight of the compound is titrated with a known concentration of acid or base until the equivalence point has been reached. From the volume and concentration of added acid or base and the initial weight of the compound, the equivalent weight, and thus the number of acidic or basic groups, can be computed. Instead of adding an indicator to observe the equivalence point, one can construct a graph on which the ph (see separate article) at regular intervals is plotted along one axis and the number of moles of added acid or base at these intervals along the other axis; such a plot is called a titration curve and is usually sigmoid (S-shaped), with the inflection point, where the curve changes direction, corresponding to the equivalence point. From the ph at the equivalence point, the dissociation constant of the acidic or basic group can be determined (see chemical equilibrium). If a compound contains several different acidic or basic groups, the titration curve will show several sigmoid-shaped curves like steps and the dissociation constant of each group can be obtained from the ph at its corresponding equivalence point. p H NEUTRALIZATION FOR POLLUTION CONTROL Objectives : 1. Acid and Base Recognition. 2. Neutralization of Acid and Base Chemicals for Pollution Control. 3. Practical Application For Chemical Industries. Materials needed : Vinegar (Acetic acid), Baking Soda, Distilled Water, Hydrogen peroxide, Detergent, All purpose cleaner, Industrial effluent, Caustic lye (NaOH), phydrion paper,

6 Ammonia, Lime, Beakers, Test Tubes, Stirrers. Strategy : Divide whole class into four groups. Each person from respective group takes vinegar solution in beaker about 10 ml. Each group should also collect phydrion paper. Then each group has to dip phydrion paper into vinegar solution and observe the color. From the color of the phydrion paper, each group will decide ph numerical value by referring to the color chart. Each group should record the ph value in record data sheet. From ph value, each group will be able to decide whether vinegar solution is acidic or alkaline. Second chemical solution to be given to each group is washing soda. Repeat the same procedure. Third chemical solution to be given to each group is hydrogen peroxide. Repeat the same procedure.then the teacher demonstrates dilute hydrochloric acid to show its ph value, that is acidic.next, ph of mild detergent is to be tested, that is almost neutral. Other solutions we tested were all purpose cleaner, ammonia, caustic lye, and industrial effluent. Thereafter ask each group to make a chart. Let them prepare three columns, one for acid, one for base, and one for neutral. Discussed results with students. Have students continue filling results in datachart. After testing industrial effluent class discussed consequences of its disposal to sewer system. Take 20 ml of hydrochloric acid in beaker. Add same amount of lime (CaCO 3 ) into beaker. Stir it and measure its ph value. If mixed solution is not achieving ph value as 7 then keep on adding lime solution until it achieves ph value as 7. At this point, titration is said to be completed. Explain to students that quantity of lime material required for neutralization depends on concentration of acid. After this, explain to the students that is why lime is commonly used for neutralizing industrial effluent. The reasons explained were that lime is a strong base, economical, and most to use. Second neutralization process shown to students was caustic lye, and strong hydrochloric acid. Application: Use overhead projector to show transparency of standard model of small scale industry pollution treatment plant, explain its operation, and design. Conclusion: During enrichment sessions, we had discussed the importance of neutralization process. Students explained that if industrial effluent is not neutralized then effluent will corrode the sewer pipelines and it will penetrate inside the soil. Seepage of effluent remains as ground water so when a tube well is used, the water will come out polluted. This polluted water is toxic and harmful for domestic purpose.

7 HEALTH EFFECTS OF ACIDS AND BASES Acids and bases are examples of corrosive poisons which react locally on tissue cells. Chemicals that are very basic or very acidic are reactive. These chemicals can cause severe burns. Automobile battery acid is an acidic chemical that is reactive. Automobile batteries contain a stronger form of some of the same acid that is in acid rain. Household drain cleaners often contain lye, a very alkaline chemical that is reactive. Acids and bases are capable of causing severe "burns" similar to burns produced by heat. These materials act by first dehydrating cellular structures. Then proteins structures are destroyed by the action of acid or base which catalyzes the splitting of peptide bonds. Smaller and smaller fragments result, leading to the ultimate disintegration of the tissue. The eyes and lungs are particularly sensitive to corrosive poisons. The cornea of the eyes is damaged by acid or base burns. Pulmonary edema (filling with water) occurs when highly concentrated corrosive pollutants (acute poisoning) reach the lungs. The lining of the nose, sinuses and lungs become irritated and water logged (by dehydration of cells). This occurs in an attempt to dilute the toxic agent. This waterlogged condition prevents the normal exchange of oxygen and carbon dioxide. The victim may die of immediate suffocation, a secondary attack of bacteria leading to pneumonia, or suffer permanent lung damage. Long term subchronic acid-base dosage effects are less well understood. Air pollutants such as sulfur oxides, nitrogen oxides, chlorine, and ammonia all have corrosive effects on the respiratory tract. The solubility of the gases in water determines their fate. The most soluble gases, such as ammonia and sulfur oxides, are promptly adsorbed to the moist surfaces of the upper airways causing nasal and throat irritation. Less soluble gases, such as nitrogen oxides and chlorine, produce their effects in the depths of the lungs causing pulmonary edema, pneumonia, and emphysema (loss of elasticity and surface area). Excessively acidic or alkaline wastes should not be discharged without treatment into a receiving stream. A stream is adversely affected by low or high ph values. This adverse condition is even more critical when sudden sludge of acids or alkalis are imposed upon the stream. ACCEPTABLE METHODS OF NEUTRALIZATION 1. Mixing wastes so that the net effect is a neutral ph. 2. Passing acid wastes through beds of limestone.

8 3. Mixing acid wastes with lime slurries. 4. Adding the proper proportions of concentrated solutions of caustic soda (NaOH) or soda ash (Na2CO3) to acid wastes. 5. Adding compressed CO 2 to alkaline wastes. 6. Adding sulfuric acid to alkaline wastes. The material and method used should be selected on the basis of the overall cost, since material costs vary widely and equipment for utilizing various agents will differ with the method selected. The volume, kind and quality of acid or alkali to be neutralized are also factors in deciding which neutralizing agent to use.