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Acids,bases and salts


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Acids,bases and salts

  1. 1. Acids, Bases, and Salts •Acids •Bases •Salts •PH scale •Alkalis
  2. 2. AcidsIn everyday life we deal with many compounds that chemists classify asacids.For example, orange juice and grapefruit juice contain citric acid.These juices, and others, also contain ascorbic acid,a substance more commonly known as Vitamin C.Salads are often flavored with vinegar, which contains dilute acetic acid. Boric acid is a substancethat is sometimes used to wash the eyes.In any chemistry laboratory, we find acids such ashydrochloric acid, sulfuric acid, and nitric acid.These acids are called mineral acids becausethey can be prepared from naturally occurringcompounds called minerals. Mineral acids generally stronger than household acids, andshould be handled with great care because they can burn skin and clothing. More
  3. 3. Properties of Acids:Acids taste sour. Citric acid is responsible for the sour taste of lemons, limes,grapefruits, and oranges. Acetic acid is responsible for the sour taste of vinegar.Acids turn litmus (or indicator papers) red.Litmus is a vegetable dye that may be either red or blue, depending on theacidity. When a sample of an acid is placed on red litmus paper, the color ofthe litmus does not change. Red litmus has been previously treated with acid.Adding more acid does not change the red color. However, when the sameacid is placed on blue litmus paper, the color turns from blue to red. (Blue litmushas been treated with a base).Acids contain combined hydrogen. When a sample of zinc, a fairly reactive metal, is dropped into a test tubecontaining an acid such as hydrochloric acid, a reaction occurs.The bubbling in the tube indicates that a gas is released. When we test this gasby inserting a burning splint into the test tube, the gas burst into flame andproduces a small popping sound. This is the characteristic test for hydrogengas. In general, when certain acids react with metals, hydrogen gas is released.See following reactions: Zn (s) + 2HCl (aq)  H2 (g) + ZnCl2 (aq) Zn (s) + H2SO4 (aq)  H2 (g) + ZnSO4 (aq) More
  4. 4. Acids release hydrogen in water solutions. When an acid dissolves in water, theacid ionizes, releasing both hydrogen ions and ions of a nonmetal or nonmetallic polyatomic ion. Thus, when hydrochloric acid is dissolved in water, the acidionizes, forming hydrogen ions and chloride ions, as shown in the following equation: HCl (aq)  H+ (aq) + Cl1- (aq)Other examples: H2SO4 (aq)  2 H+ (aq) + SO42- (aq) H3PO4 (aq)  3 H+ (aq) + PO43- (aq)Special example: HC2H3O2 (aq)  -> H+ (aq) + C2H3O21- (aq)Note the use of the double arrow in the ionization of acetic acid. We know that aceticacid is a weak acid. The smaller arrow pointing to the right indicates that the changeto the right (ionization) is relatively small. This means that, in a solution of acetic acid, we have a large number of acetic acid molecules and few hydrogens and fewacetate ions.Thus acids are defined as substances that release hydrogen ions in solution.It is these H+ (aq) that are responsible for the properties of acids.How Acids are Prepared:Acids may be prepared when certain gases, related to acids, are dissolved inwater and when certain salt compounds, again related to acids, are allowedto react with sulfuric acid. CO2 (g) + H2O (l)  H2CO3 (aq) carbonic acid 2 NaCl (s) + H2SO4 (aq) + heat  2 HCl (g) + Na2SO4 (aq) More
  5. 5. Uses of Acids:Sulfuric acid is the chemical most widely used in industry.Sulfuric acid is also used to make other acids such ashydrochloric and nitric acid because the boiling point ofsulfuric acid is higher that that of other acids. This allowsthe acid being produced to be distilled and collectedseparately from the starting material. Sulfuric acid is alsoused to remove the surface oxide layers on metals(pickling) before the metals are coated with materialsthat prevent rusting. For example, before iron iscoated with chromium (in chromium plating), theiron is dipped into dilute sulfuric acid to remove theiron oxide normally present on the surface of the iron.Another important use of sulfuric acid is the storage cell.In a lead storage cell, dilute sulfuric acid serves as theelectrolyte through which ions move between the leadplates, acting as the cathode, and the spongy leaddioxide, acting as the anode. Several such cellsconnected together make up the type of storagebattery used in automobiles. More
  6. 6. Nitric acid, anotherimportant industrial acid, is used in themanufacture of fertilizers, plastics,photographicfilm, and dyes. Nitric acid is also used inthepreparation of such explosives as dynamiteand TNT.Hydrochloric acid, like sulfuric acid, is usedtoclean metals. Hydrochloric acid is alsousedto clean brick and tile; it is used in themanufacture of sugar and glue.Hydrochloricacid is produced in small quantities in thestomach where the acid aids digestion.
  7. 7. BasesAmmonium hydroxide, or ammonia water, is very irritating to the nose and the eyes. This substance, called a hydroxide, or a base, is often used in the home forcleaning because bases generally dissolve grease. Milk of magnesia (magnesium hydroxide), which is used as an antacid, is a base; lye (sodium hydroxide), which isused in the manufacture of soap, is another familiar example of base. Bases are ionic compounds containing metal ions and hydroxide ions. For example, sodiumhydroxide contains sodium ions and hydroxide ions. When sodium metal is placed in water, sodium hydroxide is formed and hydrogen gas is released. Since the formula for water can be written as HOH instead of H2O, the reaction involves singlereplacement: 2 Na (s) + 2 HOH (l)  2 NaOH (aq) + H2 (g)Properties of Bases: (in water solutions)1. Bases taste bitter. A bitter taste is characteristic2. of all bases. It is the presence of a base that give3. unflavored milk of magnesia its bitter taste.2. Bases feel slippery. If you rub a drop or two ofhousehold ammonia between your fingers, youexperience the slippery feeling of a base.Wet soap is also slippery because of More
  8. 8. Bases turn red litmus blue. A common indicator,used to detect the presence of a base, isphenolphthalein which, when mixed with a base, turns pink.Bases release hydroxide ions in water solutions. When dissolved in water, bases ionize releasing metalions (or metallic polyatomic ions) and hydroxide ions.For example: when sodium hydroxide is dissolved inwater, it ionizes as: NaOH (s) + H2O (l)  Na1+ (aq) + OH1- (aq) NH4OH (aq)  -> NH41+ (aq) + OH1- (aq)double arrow indicates ionization of weak base ammonium hydroxide Thus bases are defined as substances that release hydroxide ions in solution. It is these OH1- (aq) ions that are responsible for the properties of bases. More
  9. 9. How Bases are Prepared: Bases may be prepared by the reaction of water and veryreactive metals, related to the base, and by the reaction ofwater and certain oxides, again related to the base. 2 K (s) + 2 HOH (l)  2 KOH (aq) + H2 (g) CaO (s) + HOH (l)  Ca(OH)2 (aq)plus considerable heat releasedUses of Bases:Ammonium hydroxide, frequently called ammonia, is usedin the preparation of important related compounds such asnitric acid and ammonium chloride. Ammonia is alsoused as a cleaning agent. Sodium hydroxide is used in the manufacture of soap,rayon, and paper. Strong solutions of this base are verycaustic; that is, they are extremely harmful to the skin.Calcium hydroxide, commonly known as slaked lime, is used in the preparation of plaster and mortar. Water solutions of calcium hydroxide, called limewater,can be used in the lab as a test for the presence ofcarbon dioxide.
  10. 10. SaltsMany chemical compounds may be classified as salts.The salt most familiar to all of us is table salt –sodium chloride. Baking soda is the salt sodiumbicarbonate. Magnesium sulfate, also called Epsomsalts, is often found in the home. In general, salts are ionic compounds thatare composed of metallic ions and nonmetallic ions.For example, sodium chloride is composed of metallicsodium ions and nonmetallic chloride ions. Some saltsare composed of metallic polyatomic ions andnonmetallic polyatomic ions (ammonium nitrateis composed of ammonium ions and nitrate ions). Properties of Salts: The salty taste of ocean water is due to thepresence of salts such as sodium chloride andmagnesium bromide. There are many differentsalts present in salt water: More
  11. 11. salt in percentage formulasaltwater in saltwater sodium NaCl (2.72 %) chloridemagnesium MgCl2 (0.38%) chloridemagnesium MgSO4 (0.17 %) sulfate calcium CaSO4 (0.13 %) sulfate potassium KCl (0.09 %) chloride calcium CaCO3 (0.01 %) carbonatemagnesium MgBr2 (0.01 %) bromide More
  12. 12. Salts dissociate in water. Salts consist of tightly bonded ions. In water, these bonds areweakened and the ions become mobile. This accounts for the fact that salt solutions aregenerally electrolytes. In water, for example, sodium chloride ionizes, or dissociates likethis: NaCl (s)  Na1+ (aq) + Cl1- (aq) Salts may react with water. Some salt solutions, when tested with litmus showacid, others base, and others still nothing. How does this happen?1) When sodium carbonate dissolves in water, the salt liberates sodium ions andcarbonate ions. At the same time, the water itself ionizes slightly to form hydrogen andhydroxide ions (remember that water is a weak electrolyte): Na2CO3 (s)  2 Na1+ (aq) + CO32- (aq)HOH H1+ (aq) + OH1- (aq) More
  13. 13. 2) Thus, the following particles may be present in a solution of sodiumcarbonate: water molecules, sodium ions, carbonate ions, hydrogen ions, andhydroxide ions. The ions of opposite charge attract one another and combine toform sodium hydroxide and carbonic acid:2 Na1+ (aq) + CO32- (aq) + 2 H1+ (aq) + 2 OH1- (aq)  2 NaOH (aq) +H2CO3 (aq)The reaction of a salt and water to form an acid and a base is called a hydrolysisreaction. Since acids and bases react to form water and salt (neutralizationreactions) hydrolysis reactions are the reverse of neutralization reactions. Thus,when sodium carbonate is dissolved in water, carbonic acid and sodiumhydroxide are formed. Carbonic acid, H2CO3 is the acid present in soda water.Since carbonic acid decomposes on standing to form CO2 gas and H2O, it iscalled a weak acid. From conductivity experiments we know that sodiumhydroxide, NaOH, is a strong base.Na2CO3 (aq) + 2 HOH (l)  H2CO3 (aq) + 2 NaOH (aq) When the acid formed in a hydrolysis reaction is stronger that the base, theeffect of such a solution on litmus is that of an acid. For example; a watersolution of the weak base, ammonium hydroxide and water, producesammonium hydroxide (a weak base) and hydrochloric acid ( strong acid).Ammonium hydroxide, on standing, decomposes to form gaseous ammonia,NH3. More
  14. 14. When both the acid and the base in a hydrolysis reaction are equally strong (orequally weak), the effect of such a solution on litmus is neither that of an acid ora base. For example, when sodium chloride is dissolved in water and thesolution is tested with litmus, neither color changes, indicating that the solution isneither acidic nor basic. Such a solution is said to be neutral and the sodiumchloride has not undergone hydrolysis. More
  15. 15. Uses of Salts:Name of salt Formula Uses ammonium in soldering, as electrolyte NH4Cl chloride in dry cells sodium in baking powder, in NaHCO3 bicarbonate manufacture of glass for seasoning and sodium chloride NaCl preserving food, essential in life processes as a drying agent to absorb calcium chloride CaCl2 moisture, in freezing mixtures silver bromide AgBr in making photographic film in manufacture ofpotassium nitrate KNO3 explosives; fertilizer fertilizer; source of nitric sodium nitrate NaNO3 acid More
  16. 16. Preparation of Salts:Salts may be prepared by three methods:1) neutralization of acid and base -- When an acid and base react, theycounteract each other, that is, they neutralize each other. Such a reaction,known as a neutralization reaction, results in the formation of water and asalt. For example, when sodium hydroxide and hydrochloric acid react,water and the salt sodium chloride are formed. This occurs because thehydrochloric acid and the sodium hydroxide first ionize, and then react. Thecompounds ionize releasing hydrogen, chloride, sodium, and hydroxide ions.Since these are mobile in solution, hydrogen ions meet hydroxide ions andunite to form water. At the same time sodium ions and chloride ions remainas aqueous salt. More
  17. 17. 2) Direct combination -- When a metal reacts with a nonmetal, a salt isgenerally formed. For example, when the metal magnesium is burned inchlorine gas, the salt magnesium chloride is formed.3) Metal oxide and acid -- when a metal oxide reacts with an acid, a salt isformed. For example, when calcium oxide reacts with nitric acid, the saltcalcium nitrate is formed.
  18. 18. PH scaleThe concentration of hydrogen ion is a measure of the acidity and the basicity of asolution. The concentration of hydrogen ion may be expressed in terms of themolarity of the acid or base solution; however, it is frequently more convenient toexpress the concentration as a function of the hydrogen ion concentration,pH. The pH of a solution may be defined as the exponent of the hydrogen ionconcentration. This definition may be stated mathematically as:pH = - log [H+] where [H+] is the molar hydrogen ion concentration. The pH scale for water systems ranges from a value to 0 to 14 only. Seethe table that follows for the strength of the acid or base in water at 25º C.pH range[H+] rangeStrength0 to 21 E 0 to 1 E - 2Strong acid3 to 41 E – 3 to 1 E– 4Moderate acid5 to 61 E – 5 to 1 E – 6Weak acid71 E – 7Neutral8 to 91 E – 8 to1 E – 9Weak base10 to 111 E – 10 to 1 E – 11Moderate base12 to 141 E – 12 to 1E - 14Strong base Indicators have been developed in order to assist in thedetermination of the pH of a solution. These indicators are weak organic acids orbases which have the property of changing color in solution when the hydrogen ionconcentration reaches a definite value. An acid indicator may be represented bythe equation: HIn = H+ + In- More
  19. 19. The anion, In-, represents a complex organic group which has changed itsstructure due to the loss of a hydrogen ion. The loss of the hydrogen ion isaccompanied by a change in color. Since an indicator reaction is an equilibriumreaction, the addition of hydrogen ions would force the above reaction to the leftand a color indicating an acid solution would result. The addition of hydroxideions would cause the reaction to go to the right and a color associated to a basicsolution would result. The pH ranges of some indicators are given in the following table. Withthis table you can estimate the pH of a solution. Suppose phenolphthalein inintroduced into a solution and the color of the solution becomes red. This redcolor indicates that the pH of the solution is 10.0 or higher. If indigo carmine isadded to a new sample of the same solution and a blue color results, the pH willbe narrowed to a range of 10.0 to 11.4, since the lower limit of color change forindigo carmine is blue. By using an additional indicator or indicators and a newsample of the solution, the pH of the solution can be narrowed to a small range.
  20. 20. AlkalisCommon Name Chemical Name Formula Sourcebaking soda sodium NaHCO3 ammonia, sodium bicarbonate chloride, limestoneborax sodium tetraborate Na2B4O7 10H2O mineral depositscaustic potash potassium KOH electrolysis of KCl hydroxideGlauber’s salt sodium sulfate Na2SO4 10H2O mineral depositslye sodium hydroxide NaOH electrolysis of NaClmuriate of potash potassium chloride KCl mineral depositstable salt sodium chloride NaCl mineral deposits, brine, salt lakes