Acid and Base by z4uR7z

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									 Hwa Chong Institution (High School) – Secondary Two LSS Chemistry
                            Acids, Bases and Salts I



The Characteristic Properties of Acids and Bases
At the end of the lesson, students should know how to:
 (a)     name some common mineral/inorganic acids eg. hydrochloric acid, nitric
         acid, sulphuric acid and carbonic acid
 (b)     name some organic acids eg. citric, malic, lactic, tartaric, tannic and
         ethanoic acids
 (c)     state some uses of common acids eg. hydrochloric, nitric, sulphuric, citric
         and ethanoic acid
 (d)     describe the importance of water for acidity, ie water causes acid molecules
         to ionise and form hydrogen ions.
 (e)     define acid as a substance that produces hydrogen ions as the only positive
         ions in water
 (f)     describe qualitatively the difference between strong and weak acids in terms
         of extent of ionisation of acid in water
 (g)     state the physical and chemical properties of acids
 (h)     define a base as a metal oxide/metal hydroxide
 (i)     describe an alkali as a soluble metal hydroxide
 (j)     name some common alkalis (sodium hydroxide, potassium hydroxide,
         calcium hydroxide and aqueous ammonia)
 (k)     state the uses of common alkalis or bases
 (l)     state the physical and chemical properties of alkalis
 (m) identify sodium hydroxide and potassium hydroxide as strong alkalis and
         aqueous ammonia as weak alkali
 (n)     classify oxides as acidic, basic, amphoteric or neutral (H2O, CO, NO)


pH and Indicators
At the end of the lesson, students should know how to:
 (a)     state the colour changes of common indicators eg litmus paper, methyl
         orange, phenolphthalein and universal indicator)
 (b)     state that indicators are substances that change colour when an acidic or
         alkaline solution is added to them
 (c)     investigate the use of Universal Indicator and pH scale to test for
         concentration of hydrogen ions and hence relative acidity



1.    Acid
Examples of acids and their uses

1.    Inorganic acids - A compound composed of hydrogen and a non-metallic
                        element or group of atoms.

      Examples are hydrochloric acid, HCl, sulphuric acid, H2SO4, and carbonic
      acid, H2CO3.


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              Name                                        Uses
Hydrochloric acid, HCl        In manufacture of bleach; found in the stomach where
                              it digests proteins
Nitric acid, HNO3             Manufacture of nitrate-containing explosives such as
                              nitroglycerin, trinitrotoluene (TNT) and
                              cyclotrimethylenetrinitramine (RDX), as well as
                              fertilizers such as ammonium nitrate.
Sulphuric acid, H2SO4         In manufacture of detergents, paints, dyes, artificial
                              fibres and plastics. Also used in batteries for road
                              vehicles such as cars and buses.
Carbonic acid, H2CO3          Fizzy or carbonated drinks

2.    Organic acids -        any various acids containing one or more carbon –
                             containing polyatomic groups.

      Examples are ethanoic acid, citric acid, malic acid, lactic acid etc.

      Name                   Found in                                 Uses
Citric acid          Citrus fruits. E.g                flavouring and preservative in
C6H8O7               oranges, lemon, lime               food and beverages, especially
                                                        soft drinks
                                                       buffering properties of citrates
                                                        are used to control pH in
                                                        household cleaners and
                                                        pharmaceuticals
Malic acid           Apple                     Malic acid may be helpful to individuals
C4H6O5                                         suffering from chronic fatigue and
                                               fibromyalgia. Some athletes consume
                                               malic acid hoping that it will increase
                                               the delivery of oxygen to muscle cells,
                                               thereby improving stamina and
                                               endurance. Malic acid is also used to
                                               acidify beverages such as wines, acid
                                               drinks, fruit juice, soda water and
                                               various soft drinks. It is also frequently
                                               used in cosmetic formulations to adjust
                                               the pH of the product.
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Lactic acid           Sour    milk      and   Lactic acid is used as a moisturizer, in
                      yoghurt
C3H6O3                                        some cosmetics and as a mordant, a
                                              chemical that helps fabrics accept
                                              dyes, in textiles. It is also used in
                                              making pickles and sauerkraut, foods
                                              for which a sour taste is desired. Lactic
                                              acid is used in the dairy industry not
                                              only in making yoghurt but in making
                                              cheese as well. It is also used in
                                              tanning leather.
Tartaric acid         Grapes, tartar          Tartaric acid is found in cream of tartar,
C4H6O6                                        which is used in cooking candies and
                                              frostings for cakes. Tartaric acid is also
                                              found in baking powder, where it
                                              serves as the source of acid that reacts
                                              with sodium bicarbonate (baking soda).
                                              This reaction produces carbon dioxide
                                              gas, and lets products "rise," but does
                                              so without any "yeasty" taste, that can
                                              result from using active yeast cultures
                                              as a source of the carbon dioxide gas.

Tannic acid           Tea                     Use as a powerful astringent
C76H52O46                                     What’s that?
Dare to draw that??
Ethanoic acid         Vinegar, tomato juice   Used as a chemical reagent for the
CH3COOH                                       production of many chemical
                                              compounds
                                                    major esters of acetic acid are
                                                     commonly used solvents for
                                                     inks, paints and coatings
                                                    In the form of vinegar, ethanoic
                                                     acid are used directly as a
                                                     condiment, and also in the
                                                     pickling of vegetables and other
                                                     foodstuffs
                                                    Dilute solutions of acetic acids


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                                                       are used for their mild acidity.
                                                        Examples in the household
                                                        environment include the use in a
                                                        stop bath during the development
                                                        of photographic films, and in
                                                        descaling agents to remove
                                                        limescale from taps and kettles.
                                                        The acidity is also used for treating
                                                        the sting of the box jellyfish by
                                                        disabling the stinging cells of the
                                                        jellyfish, preventing serious injury
                                                        or death if applied immediately,
                                                        and for treating outer ear
                      What are esters? How              infections in people in preparations
                      are they produced?                such as Vosol. Equivalently, acetic
                      What are esters used              acid is used as a spray-on
                      for? Can you find them
                                                        preservative for livestock silage, to
                      occurring naturally?
                                                        discourage bacterial and fungal
                                                        growth.



Importance of Water for Acids

   Acids will only show their acidic properties in the presence of water.

   The acid molecules dissociate into hydrogen ions (H+) and anions in the
    presence of water.

   It is the positive hydrogen ions that are responsible for acidic properties.


Definition of acids

An acid is a substance that produces hydrogen ions as the only positive ions in
aqueous solution (water).

                                 HA + H2O  H3O+ + A-

Examples:
Hydrochloric acid: HCl (aq)  H+ (aq) + Cl- (aq)
Nitric acid: HNO3 (aq)  H+ (aq) + NO3- (aq)
Ethanoic acid: CH3COOH (aq)  H+ (aq) + CH3COO- (aq)

What are those little ‘(aq)’? What other such notations are there in chemical equations?
Why do these chemical equations have charges on their chemical formulae?

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Strength of Acids

    Strong acids: All its molecules completely dissociate to become ions in the
                  presence of water. 100% of the molecules of strong acids ionize
                  completely.

Example : 1000 HCl molecules in the presence of excess water would all ionize to
           form 1000 H+ ions and at the same time releases 1000 Cl- ions.

                              HCl + H2O  H3O+ + Cl-

    Weak acids: Some of the molecules dissociate to form ions in the presence of
                  water. The rest remains as molecules. Molecules of weak acid
                  dissociate partially.

Example : CH3COOH molecules in the presence of excess water would have few H+
          ions formed. We say that the reactants and products eventually reach
          chemical equilibrium.

                       CH3COOH + H2O  H3O+ + CH3COO-

What on earth is H3O+? Isn’t it H+ in solution? Why is the ionic equation written with H3O+?


Chemical Properties of Dilute Acid

Word equations are vital to answering many chemistry questions.

1.      Dilute acids react with metals to produce a salt and hydrogen gas.

                      Acid + Metal  Salt + Hydrogen

        E.g.
        2Na (s) + 2HCl (aq)  2NaCl (aq) + H2 (g)
        Mg (s) + H2SO4 (aq)  MgSO4 (aq) + H2 (g)

        Reactivity Series:
              K, Na, Ca, Mg, Al, Zn, Fe, Pb, (H), Cu, Hg, Ag, Au
              Most reactive                       Least Reactive

        Metals that are less reactive than hydrogen will not be able to produce any
        hydrogen from the acid – no reaction occurs.

2.      Acid reacts with carbonate/bicarbonate to form salt, water and carbon dioxide.

           Acid + Carbonate/bicarbonate  Salt + Water + Carbon dioxide

        E.g.
        MgCO3 (s) + H2SO4 (aq)  MgSO4 (aq) + H2O (l) + CO2 (g)
        CuCO3 (s) + 2HCl (aq)  CuCl2 (aq) + H2O (l) + CO2 (g)




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3.     Acid reacts with base to form salt and water in a process called
       neutralisation.

                                  Acid + Base  Salt + Water

       E.g.
       HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l)
       MgO (s) + H2SO4 (aq)  MgSO4 (aq) + H2O (l)

4.     The basicity of an acid is the number of hydrogen ions produced when
       one molecule of acid ionises in water.

       E.g.
       HCl = monobasic (basicity = 1)              HCl  H+ + Cl-
       H2SO4 = dibasic (basicity = 2)              H2SO4  2H+ + SO42-
       H3PO4 = tribasic (basicity = 3)             H3PO4  3H+ + PO43-


5.     Aqueous acids are able to conduct electricity due to the presence of mobile
       ions. Thus they are called electrolytes (solutions that conduct electricity). The
       higher the concentration of mobile ions, the higher the electrical
       conductivity of the aqueous acid.

       Therefore, strong acids form better electrolytes than weak acids, in the
       presence of water.


6.     Acids only show their acidic properties in the presence of water. The acid
       molecules dissociate into hydrogen ions and anions, in the presence of
       water. The positive hydrogen ions are responsible for the acidic
       properties. Without water, there will be no free mobile hydrogen ions, and
       there are then no acidic properties. [see the section on “Importance of water
       for acids]

       Can pure water conduct electricity?


Physical Properties of Dilute Acids

1.     Sour taste

2.     Aqueous acid conduct electricity due to the abundance of free hydrogen
       ions and anions when the acid molecules ionize in water.

3.     Acid molecules in other solvent do not conduct electricity.
       Example : HCl molecules in methylbenzene, an organic solvent, which does
                 not allow HCl molecules to ionize. They exist as molecules and
                 thus do not conduct electricity.

Why are organic solvents unable to cause acid molecule ionisation, but water is able to?


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2.   Base and Alkali

    A base is a substance that reacts with acids to form salt and water only.

    An alkali is a base that is dissolved in water and reacts with acid to form a salt
     and water.

    Alkali is a soluble base. Thus not all bases are alkalis but all alkalis are
     bases.

    A soluble base dissolves in water to form an alkali, its molecules
     dissociating in the presence of water to form hydroxide (OH-) ions.

    Basic oxide is an oxide that reacts with acid to form a salt and water. If it
     dissolves in water it would form an alkali. All basic oxides are oxides of
     metal.

    Basic hydroxide is a hydroxide that reacts with acid to form a salt and water.
     If it dissolves in water it would form an alkali. All basic hydroxides are
     hydroxides of metal.

    Some examples of bases:

                                          Base
           Basic Oxides               Alkali/Soluble        Insoluble hydroxides
                                       hydroxides
      Na2O                        NaOH                     Fe(OH)2
      Fe2O3                       KOH                      Cu(OH)2
      CuO                         LiOH                     Mg(OH)2
      FeO                         NH3
      BaO (water-soluble)         Ba(OH)2
                                              Ca(OH)2 (partially soluble)

    Uses of common alkalis

                 Alkalis                                        Uses
      Sodium hydroxide, NaOH                 Soap making, paper making, drain
                                             cleaning agent for clearing clogged
                                             drains
      Potassium hydroxide, KOH               Correction of the pH of acidic soils, as a
                                             fungicide , as a herbicide
      Calcium hydroxide, Ca(OH)2             In sewage treatment, used in mortar and
                                             plaster Not mortar and pestle??
      Aqueous ammonia, NH3                   Production of nitric acid, fertilizer,
                                             disinfectant, used in liquids to remove
                                             grease and dirt from glass.

Types of Oxides

1.   Acidic oxide : carbon dioxide, sulphur dioxide, nitrogen dioxide etc.
2.   Basic oxide : calcium oxide, iron(III) oxide, magnesium oxide etc.
3.   Neutral oxide : carbon monoxide, nitric oxide, water etc.
4.   Amphoteric oxide : zinc oxide, aluminium oxide, lead(II) oxide, lead(IV) oxide
                                         7
Acidic oxide

An acidic oxide is a non-metallic oxide that neutralises a base to form salt and
water only. It reacts with water slightly to form an acid solution.

Neutralization:                  CO2 (g) + 2KOH (aq)  K2CO3 (aq) + H2O (l)

Reaction with water:
                                  CO2 + H2O  H2CO3
                                  SO2 + H2O  H2SO3


Basic oxide

A basic oxide is a metallic oxide that neutralises an acid to form salt and water
only. If it is soluble in water, it forms an alkali.

Neutralization :           MgO (s) + H2SO4 (aq)  MgSO4 (aq) + H2O (l)

Reaction with water:
                           Na2O + H2O  2NaOH
                           CaO + H2O  Ca(OH)2

Neutral oxide

Neutral oxides are non-metallic oxides that show neither acidic nor basic
properties. They are insoluble in water and do not react with water.

E.g. H2O, CO, NO


Amphoteric oxide

Amphoteric oxides are metallic oxides that react with both acid and base to form
salt and water. They have both acidic and basic property.

E.g. ZnO, Al2O3, PbO, PbO2

Reaction with acid:
                           ZnO (s) + 2HCl (aq)  ZnCl2 (aq) + H2O (l)

Reaction with alkali:
                           ZnO (s) + 2NaOH (aq)  Na2ZnO2 (aq) + H2O (l)

Strength of Alkali

      Strong alkali ionizes or completely dissociates in water to produce OH- ions
       in the presence of water. The ionic lattice structure of the hydroxides breaks
       down to allow free ions to be formed.

       Example :           NaOH (s)  Na+ (aq) + OH- (aq)




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      Weak alkali partially ionizes or dissociates to produce OH- ions in the
       presence of water.

       Example :     Aqueous ammonia is a weak alkali. In an aqueous solution of
                     ammonia, it contains ammonia molecules, water molecules,
                     ammonium cations and hydroxide anions. Ammonium and
                     hydroxide ions are formed in solution but ammonium hydroxide
                     is not an isolatable compound per se. Since it is a weak alkali,
                     there would be more ammonia and water molecules as
                     compared to ammonium and hydroxide ions.
                                 NH3 + H2O  NH4+ + OH-

What is the chemical behaviour of base molecules in water (eg. NH3 becomes NH4+)?
Ammonia molecules don’t exactly dissociate in water – how then is it a base?

Chemical Properties of Base

1.     Base/alkali reacts with acid to form salt and water in a process called
       neutralisation.


                           Base/Alkali + Acid -> Salt + Water


       Examples:
       HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l)
       MgO (s) + H2SO4 (aq)  MgSO4 (aq) + H2O (l)


2.     Alkali reacts with ammonium salts to form salt, ammonia gas and water.


                   Alkali + Ammonium salt -> Salt + Ammonia gas + water

       Example:
       (NH4)2SO4 (aq) + 2NaOH (aq)  Na2SO4 (aq) + 2NH3 (g) + 2H2O (l)

       All ammonium salts would under gentle warming release ammonia gas in the
       presence of a base. Ammonia formed dissolves readily in water. Heating
       allows ammonia to be evolved as a gas. This is a chemical test that can be
       used to confirm the presence of alkali or ammonium salt.

       Bases, like acids, are also able to dissociate in water to form mobile ions,
       which conduct electricity. Such solutions containing dissociated base or acid
       are called electrolytes. The higher the concentration of mobile ions, the higher
       the electrical conductivity of the alkali.


Physical Properties of Alkali

1.     Bitter taste and Soapy feel
       Why does alkali feel soapy on touch with our oily fingers?

2.     Aqueous alkali conducts electricity due to its abundance of free OH-
       anions and cations when the base dissolves in water.
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3.    pH and Indicator

     pH of a solution is a measurement of the concentration of hydrogen ions
      in the solution. The higher the concentration of hydrogen ions, the lower
      the pH value.

     pH = - log [H+]
      So if concentration of H+ is 0.0500 mol/dm3, then pH = - log 0.0500 = 1.30

     Strong acid: pH 0~2                        How can we call hydrochloric acid a strong
      Weak acid: pH 4~6                          acid when 0.0500 mol/dm3 is diluted to half
      Neutral: pH 7                              its concentration? How is OH- concentration
      Weak alkali: pH 8~10                       measured? When is [H+] = [OH-]?
      Strong alkali: pH 12~14

pH Indicators

     Indicators are substances that change colour in the presence of acid or alkali.
     Common pH indicators are litmus paper, methyl orange, phenolphthalein
      and universal indicator. Most of these indicators are organic compounds.
      Note that phenolphthalein is rather insoluble in water and, when added in
      large amounts, may result in white precipitate being formed.

           Indicator           Acid colour           pH range of           Alkaline colour
                                                   colour transition
       Litmus paper                red                                              blue
       Methyl orange               red                  3.1 – 4.4                  yellow
       Phenolphthalein          colourless             8.2 – 10.0                   pink

     Universal indicator

      Simple indicators, such as litmus, tell whether a solution is an acid or an
      alkali. However, other indicators not only indicate an acid or alkali, but also
      how acidic or alkaline a solution is.

      pH is a measure of the acidity or alkalinity of a solution.


                Red         Pink     Orange Yellow Green Turquoise          Blue              Purple
      pH    0     1     2       3    4       5     6     7     8       9   10   11      12     13      14
                       Acids                                                        Alkalis
                                                  Neutral – neither
                                                 acidic nor alkaline



                       More acidic                                     More alkaline




                                                 10
      A pH of a solution is shown as a number on a scale ranging from 0 to 14. This
      pH scale has the following features:

           A pH of 7 is neutral. This is the pH of pure water.
            This is not true most of the time. Find out the specific conditions where it is
            true that water has a pH of 7 when totally pure.
           A solution with pH less than 7 is acidic. The solution contains hydrogen
            ions. The smaller the pH, the more acidic the solution is and the more
            hydrogen ions it contains. A solution with pH of 2 has more hydrogen
            ions than a solution with a pH of 5. How much more?
           A solution with pH greater than 7 is alkaline. The solution contains
            hydroxide ions. The larger the pH value, the more alkaline the solution is
            and the more hydroxide ions it contains. A solution with pH of 12 has
            more hydroxide ions than a solution with a pH of 9.

Restoration of pH

When there is an acid spillage or pH of soil becomes acidic, the pH can be restored
by various basic substances:

    Chemical      Ionic Equations                         Advantage/Disadvantage
    substance
Alkali, such as H+ + OH-  H2O                    Disadvantage:
sodium                                            If added in excess, alkali causes pH to
hydroxide,                                        go above 7. Under too alkaline
potassium                                         environment, aquatic life and crop growth
hydroxide                                         can be affected.
Calcium         2H+ + Ca(OH)2                    Disadvantage:
hydroxides      Ca2+ + H2O                         If added in excess, calcium hydroxide
(partially                                             can dissolve in water/rain water and
insoluble)                                             spread out throughout the
                                                       environment, causing the pH to go
                                                       above 7. Under an alkaline
                                                       environment, aquatic life and crop
                                                       growth can be affected.
                                                   Ammonium salt is added to plants to
                                                       provide them with nitrogen that is
                                                       needed for plant growth. Addition of
                                                       calcium hydroxide would result in
                                                       direct chemical reaction with
                                                       ammonium salt causing the release
                                                       of ammonia to the environment.
                                                       Ca(OH)2 + 2NH4+  2NH3 + Ca2+
                                                                      + 2H2O
                                                   Release of ammonia would mean the
                                                       loss of nitrogen that the farmer added
                                                       as fertilizer needed for plant growth.
Calcium               2H+ + CaCO3  Ca2+          Advantage:
carbonate             + H2O + CO2                  If added in excess, calcium carbonate
                                                       would not dissolve in the water and
                                                       thus would not cause the pH of the
                                                       environment to go above 7.



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