CHAPTER-10

BASIC PROPERTIES OF ACIDS AND BASES

Sr. No. Property Acid Base
1 Taste Sour Bitter
2 Effect on blue litmus Turns red No effect
3 Effect on red litmus No effect Turns blue
4 Effect on skin Corrosive Harm skin tissue
5 Electrical conductivity Aqueous solutions conduct electricity Aqueous solutions conduct electricity

Q: compare concepts of acids and bases OR what are Arrhenius, Bronsted Lowry and Lewis Concept of Acids and Bases?

ARRHENIUS CONCEPT

An acid is a substance that ionizes in water to produce Protons (H+-ions)                                                                                                                                                                                          For example:

Base is a substance that ionizes in water to produces OH ions.

For example

BRONSTED-LOWRY CONCEPT

An acid is a substance (molecule or ion) that can donate a proton (H+) to another substance.

A base is a substance that can accept a proton (H+) from another substance.

  1. In above reaction HCl is a proton donar and Water is Proton acceptor, So in this reaction water behave as Base and HCl as acid
  2. In this reaction Water is proton donar so it is an acid while ammonia is accepting proton it is behaving like base.


Identify Bronsted-Lowery acids or bases in the following reactions.

HCl+ H2O                           H3O+ + Cl

In this reaction HCl is proton donar so it is an acid while water is accepting proton it is a base

How Fossil Fuels contribute to acid rain?

Ans:Fossils Fuels contain small amounts of sulphur and nitrogen. They produce sulphur dioxide and oxides of nitrogenwhen the fuel is burned. Large amounts of these oxides are released from coal-burning factories and power stations. They react chemically with the water vapours in clouds and oxygen in the air, forming acids.

SO2(g) +  1/2O2(g) + H2O (l)                                         H2SO4(aq)

 4NO2(g) + O2(g)  + 2H2O (l)                                           4HNO3(aq)

These acids mix up with rain drops and fall as acids rain or acid snow.

Q: What is Lewis Concept of Acids & Bases?

ANS:

G.N. Lewis (1923) proposed a more general and broader concept of acids and bases. According to this concept:

An acid is a substance (molecule or ion) which can accept a pair of electrons to form coordinate covalent bond.

 Abase is a substance (molecule or ion) which can donate a pair of electrons to form coordinate covalent bond.

The Lewis structure demands that the central atom or atom of Lewis acid has a deficiency of an electron pair and can accommodate an unshared electron pair. On the other hand, the central atom of a Lewis base has complete octet possessing one or more unshared electron pairs.

Hence base has an ability to donate an unshared electron pair.

Example 1 a reaction between ammonia and boron trifluoride takes place by forming a coordinate covalent bond between ammonia and boron trifluoride by donating an electron pair of ammonia and accepting that electron pair by boron trifluoride.

EX-2

I above equation Nitrogen atom in ammonia donates an electron pair to H- atom in HCI.  So HCl  is acid and NH3 is a base.

Example 3

  • NH3 has a lone pair on N-atom. So it is electron pair donor. NH3 is a Lewis base.
  • Boron in BF3has incomplete octet. It has six electrons (3 electron pairs). So it needs an electron pair to complete its octet. Hence BF3 is an electron pair accepter or Lewis acid.

Example 4 a reaction between H+ and NH3 , where H+ acts as an acid and ammonia as a base.

Self-Assessment Exercise 10.2   Identify the Lewis acid and the Lewis base in the following examples.   Cl + AlCl3                          [AlCl4]    
  H+  + OH                            H2O    

Q: What is self-ionization/auto-ionization of water?

Ans: The reaction in which two water molecules produce ions is called as the self-ionization or auto-ionization of water. This reaction can also be written as a simple ionization of water.

H2O (l) H+(aq) + OH(aq)

Explanation:

A water molecule that donates or loses a proton becomes a negatively charged hydroxide ion OH. The other water molecule which gains or accepts the proton becomes positively charged hydronium ion H30+.

       This reaction can be written as

2H2O H3O++ OH­

At 25ºC the experimentally determined concentrations of H+ ions and OH ions are as follows.

[H+] = [OH]      =         1X10-7 M

You can write equilibrium constant expression for the self-ionization of water as follows.

Since H2O is a weak electrolyte, so the concentration of [H2O] will remain constant.

            Kc[H2O]            =          [H+] [OH]                   

            Kw                    =          [H+] [OH]

Where Kw = Kc [H2O] is called ionization constant for water. It is also called the ion- product for water. For water

Kw = (1 x 10-7) (1 X 10-7) =1 x 10-14 at 25OC

In pure water, the concentrations of H+ and OH ions are equal.

[H+] = [OH] = 1 x 10-7 at ­25oC

Q: Wirte a note on pH scale?

Ans: In 1909, the Danish biochemist Soren Sorenson proposed a convenient method to express such a small concentration of H+ ions and OH ions by pH or pOH.

pH: pH is defined as the negative logarithm of the molar concentration of H+ ions in aqueous solutions.

pH = -log [H+]

with reference to equationpH = -log [H+] a scale is developed according to molar concentrations of [H+] known as pH scale.

OR

“A scale on wich degree of acidity or alkalinity of a solution is measured is known as pH scale.”

For pure water at 25OC

            [H+] = 1×10-7M,   [OH]=1X10-7M

            pH= -log (1×10-7) = 7

Thus pH of water is 7. All aqueous solutions with pH= 7 at ­25oCare neutral. If pH is less than 7, the solutions become acidic, [H+] increases and [OH] decreases.

Q: What is Importance of Kw?

Ans:

Kw is temperature dependent. In any aqueous solution at ­25oC, no matter what does it contain the product of H+ ion concentration and OH ion concentration is always equal to 1.0×10-14. This means that if [H+] increases, the [OH] must decrease so that the product of the two is still 1.0 x 10-14.

When [H+] = [OH]  = 1 x 10-7, solution is neutral

When     [H+]>1 x 10-7, solution is acidic

When   [H+] < 1 x 10-7, solution is basic

If pH is greater than 7, the solution is basic. As solution becomes basic, [OH] increases and [H+] decreases. If pH is less than 7, the solution is acidic. As solution become acidic, [H+] increases and [OH] decreases.

EXAMPLE:

  1. If the [H+] in a solution of HCl is 1×10-6 M. Is the solution acidic, basic or neutral?

Ans:

 [H+] = 1.0 x 10-6 M > 1.0 x 10-7 M, the solution is acidic.

  • Solution hydroxide (NaOH) is commonly used as a drain cleaner. If the concentration of OH in a solution of NaOH is 1.0×10-5 M. Is the solution acidic basic or neutral?
  1. A soft drink has [H+] = 3x 1 0-3 M. Is drink acidic neutral or basic?

Solution                       [H+] = 3x 1 0-3 M

                                    3x 1 0-3 M > 1×10-7    So solution is acidic

  • Ordinary vinegar is approximately 1M CH3COOH. Concentration of H+in it is 4.2x 10-3M. Is vinegar acidic, basic or neutral?

          Solution :                       [H+] = 4.2x 10-3M

                                                4.2x 10-3    >  1×10-7      So solution is acidic

  • A student determines the [OH] of milk of magnesia, a suspension of solid magnesium hydroxide in its saturated solution and obtains a value of4.2×10-3M. Is the solution acidic, basic or neutral?

          Solution:   [OH]  = 4.2×10-3M

                     [H+] = ?

Q: Briefly explain the measurement of pH?

Measurement of pH

  1. PH PAPER OR UNIVERSAL INDICATOR PAPER is used to measure pH of a solution. For this purpose pH paper is dipped in the solution. The colour that develops on the pH paper is compared to the colour corresponding to a known pH on the chart. Each colour is linked to a specific pH value.

USE OF LITMUS PAPER. It is used to give a general indication of whether a solution is acidic or basic. Litmus paper may be red or blue. An acid turns blue litmus paper red and a base turns red litmus blue.

ACID – BASE INDICATORS are used to estimate the pH of a solution. Indicators are intensely colored organic compounds. They change colour within small pH change and indicate the pH of solution by the colour. We add few drops of an indicator to an aqueous solution of unknown pH and measurepH of the solution from the resulting colour

Important acid base indicators:

Indicator pH at which colour changes Colour in acidic solution Colour in basic solution
Methyl red 5.5 Red yellow
Bromothymol blue 7 Yellow. Blue
Phenolphthalein 9 Colourless Pink
Self-Assessment Exercise 10.4 Write names of three acid – base indicators.   .What is the colour of methyl red in solution of (i) pH =4  (ii) pH=9 ?     3.Bromothymol blue added to a solution imparts blue colour,  What is the pH of solution?pH = 5  or  9  
Lemon JuiceWe make use of chemistry when we put lemon juice on fish. The unpleasant fishy odour is due to amines. The citric acid present in lemon juice converts amines to non-volatile salts, thus reducing the odour.
Acidity of stomach The main component of digestive or gastric juice in the stomach is hydrochloric acid. Almost two litre of it is secreted each day by gastric glands. However sometimes too much acid is secreted in the stomach which causes indigestion. This is called acidity of the stomach.  

Salts: An acid contains replaceable hydrogen atoms. When these are completely or partially replaced by metal atoms, a compound called salt is formed.

            HCl + NaOH                 NaCl + H2O

Salts are ionic compounds. The first part of the name is of the metal ion and second part of the name is of the negative part of the acide.g.    Sodium Chloride.NaCl

Neutralization is the specific term used for the reaction of acids with bases.

Acid + Base                   Salt + Water

HCl(aq) + NaOH(aq)                             NaCl(aq) + H2O (l)

Neutralization is the reaction between H+ ions of an acid and OH ions of a base resulting in cancelling properties of each other.

H+ (aq) + OH(aq)                         H2O (l)

  1. Hydroxides such as Mg(OH)2 called milk of magnesia is used as antacid. It neutralizes excess stomach acid (HCl). Write complete and balanced chemical equation for this neutralization reaction?

Mg(OH)+  HCl                       MgCl2   + H2O

  • Hydrochloric acid (HCl) and Potassium hydroxide (KOH) react and produce potassium chloride. Write complete and balanced chemical equation for this neutralization reaction?

KOH  +  HCl                              KCl   + H2O

  • Balance following neutralization reactions

H2SO4 + 2NaOH                                                                                             Na2SO4+ 2H2O

H3PO4 + 3 NaOH                                             Na3PO4 + 3H2O

Q. What are TYPES OF SALTS?

A salt containing a replaceable H- atom or formed by partial neutralization of an acid is called acid salt

H2CO3 + NaOH                                    NaHCO3 + H2O

H2CO3+ 2NaOH                                   Na2CO3+ H2O

H3PO4+NaOH                             NaH2PO4 (aq) +H2O (l)

a salt which is formed by the complete neutralization of an acid is called a normal salt.

HCl(aq) + NaOH(aq)                     NaCl(aq) + H2O (l)

A salt containing replaceable OH group or formed by the partial neutralization of a polyhydroxy base is called as basic salt.

Zn(OH)2 + HCl                                      Zn(OH)Cl(aq) + H2O

Pb(OH)2(aq) + CH3COOH(aq)                                     Pb(OH)CH3COO (aq) + H2O

Which salt is acid salt, NaHCO3 or Na2CO3?    

Which salt is normal salt, NaHCO3 or Na2CO3?

Classify following salts as normal or acid salt.

  •       NaHSO4                                   Acidic
  •       Na2SO4                                                                         Normal                                                           
  •       KHCO3                                                                           Acidic 
  •       K2CO3                                                                             Normal

Q: What are Methods for making salts?

Ans. There are five methods for making salts.

Uses of Salts

  • Food preservation keeps food from spoiling and allows it to be stored for later use.
  • Ancient methods for preserving include, drying fruits and vegetables, salting, boiling etc.
  • Today, methods for preserving food also include the addition of preservatives. They are inhibitors of physical and chemical processes that cause food to spoil. Many foods are grown or produced in one location and then sent across the country or even overseas. Without preservatives, these foods would spoil long before they reach their destinations. Many salts such as sulphites and benzoates are being used in food for thousands of years.
Society, Technology and Science Many people are allergic or sensitive to preservatives. Some preservatives are safe in small amount and toxic in larger amounts. Hence, Food and Drug Administration Department is given the responsibility for approving the safety and use of preservatives.

EXERCISE

Give short answers?

  1. Write the equation for the self-ionization of water.

See LEC#3

  • Define and give examples of Arrhenius acids.

See LEC# 2

  • Why H+ ion acts as a Lewis acid?

Ans: Because H+ is electron deficient and it can accept an electron pair to form coordinate covalent bond.

  • WhyNH3 acts as Bronsted-Lowry base?

Ans: Because it can accept a proton(H+) and according to B-Lowry concept “A specie that accept a proton is base.”

  • Why BF3 acts as Lewis acid?

Ans. Boron in BF3has incomplete octet. It has six electrons (3 electron pairs). So it needs an electron pair to complete its octet. Hence BF3 is an electron pair accepter or Lewis acid

3.  Ammonium hydroxide and nitric acid react and produce ammonium nitrate and water. Write balanced chemical equation for this neutralization reaction.

               NH4OH   + HNO3                  NH4NO3  + H2O

4.  Write balanced chemical equations for the following neutralization reactions.

(i) Sulphuric acid + Magnesium hydroxide            magnesium sulphate + water.

(ii)        Sulphuric acid+ Sodium hydroxide            Soduimsulphate + water.

(iii)       Hydrochloric acid + calcium hydroxide         calcium chloride+ water

5.         IdentifyBronsted –Lowry acids or bases in the following reactions.

(i)         HNO3 + H2O                        H3O+ + NO3

(ii)        NH3 + HNO3                              NH4NO3

6.         Identify Lewis acid and Lewis base in the following reactions.

(i)         F + BF3                                     [BF4]

(ii)        H+ + NH3                                    [NH4]+

(iii)       NH3 + AlCl                   [H3N- AlCl3]    

7.Classify the following solutions as acidic, basic or neutral.

(i)         A solution that has hydrogen ion concentration 1.0 x10-3 M.

[H+] = 1.0 x 10-3 M > 1.0 x 10-7 M, the solution is acidic.

(ii)        A solution that has hydrogen ion concentration 1.0 x10-10 M.

[H+] = 1.0 x 10-10 M < 1.0 x 10-7 M, the solution is basic.

(iii) A solution that has hydroxyl ion concentration 1.0 x10-3 M.

[OH] = 1.0×10-3 M

[H+] = ?
kw = [H+][OH].
1.0 x 10-14 = [H+]1.0×10-3
[H+]= 1.0 x 10-11 M
Because 1.0 x 10-11 M <1.0 X 10-7 M, the solution is basic.

  • A solution that has hydroxyl ion concentration 1.0 x10-10 M.        

[OH] = 1.0×10-10 M

[H+] = ?
kw = [H+][OH].
1.0 x 10-14 = [H+]1.0×10-10
[H+]= 1.0 x 10-4 M
Because 1.0 x 10-4 M >1.0 X 10-7 M, the solution is acidic.

8.   Classify following substance as Lewis acid and bases.

NH3,                 F,                     H2O                 BF3

 N is the central atom with complete octet, it can share/donate e-pair, so it will act as Lewis base

 F has complete octet , it can donate e –pair, so it will act as Lewis base

 O has complete octet and can donate e pair , so will act as Lewis base

B has incomplete octet, could accept e pair, so it will act as Lewis acid

9.         Give the Bronsted-Lowry definition of an acid. Write an equation that illustrates the definition.

10.       Give the Bronsted – Lowry definition of a base. Write an equation that illustrates the definition.

11.       Identify Bronsted acids and Bronsted bases in the following reactions.

(i)         CH3COOH+H2O CH3COO+ H3O+

(ii)        HC (g) + H2O (l) (aq) + H3O+(aq)

(iii)       NH3(g) +H2O(l) (aq)+ OH(aq)

(iv)       HCl(aq) + H (aq) H2CO3(aq) + Cl(aq)

(V) HS(aq) + H2O (l) S-2(aq) + H3O+(aq)

(vi)       H2S + NH3  + HS

12.  Identify the Lewis acids and the Lewis bases in the following reactions.

(i)         Ag+(aq) + 2CN(aq)                                              Ag (CN2)(aq)

(ii)        B(OH)3(s) + OH(aq)                                         B (aq)

(iii) Cu+2(aq) + 4NH3(aq)     [Cu(NH3)4]+2(aq)

(iv) OH(aq) + Al (OH)3(s)                                                Al (aq)

13.       Identify Lewis acids and Lewis bases

14.       Classify water as proton donor or proton acceptor.

Ans: Water is amphoteric in nature , when it react with base it donate a proton and it accept a proton when react with acid.

  1. as proton donor

b. as proton acceptor

15.       Write equations showing the ionization of the following as Arrhenius acids.

(a)        HI (aq)      

(b)       HNO2(aq)

16.       Write equations showing the ionization of the following as Bronsted-Lowry acids.(a)             HNO2(               (b)       HCN

17.       What is true about the relative concentrations of hydrogen ions and hydroxide ions in each kind of solution?

a)         acidic   b) basic            c)         neutral

(a) [H+] >[OH]             (b) [H+] < [OH

(c) [H+] = [OH

  18.     Codeine, C18H21NO3 is a commonly prescribed pain killer. It dissolves in water by the following reaction?

C18H21NO3 + H2O                 [C18H21HNO3]+ + OH

Identify Codeine as Bronsted-Lowry acid or base.

As equation depicts that codeine is accepting proton from water so it is a base.

19.       Suggest some ways in which you might determine whether a particular water solution contains an acid or a base.

Ans: (see topic pH measurement Lec#4)

20.       The table below shows the colours of two indicators in acidic and alkaline solutions.

Indicator Colour in Acidic Solution Colour in Alkaline Solution
A Red Blue
B Colourless Red
  1. What will be the colour of the indicator A?
  2. in A solution of pH 3 ( red)
  3. in A solution of pH 10( blue)
  4. What will be the colour of the indicator B in a solution of pH 5?( colourless)
  5. When a few drops of indicator B are placed in a solution X, it turns red immediately. What can you deduce about the properties of solution X? (alkaline)
Sr. Property Base
1 Taste Bitter
2 Effect on blue litmus No effect
3 Effect on red litmus Turns blue
4 Effect on skin Harm skin tissue
5 Electrical conductivity Aqueous solutions conduct electricity

21.   Bacteria in our mouth feed on small particles of food stuck to our teeth and change it into acid. Explain how using toothpaste of pH 10 can help to prevent the acid from damaging our teeth?

Ans:  Toothpaste of pH 10 is mildly alkaline. The alkaline pH of toothpaste help to neutralize the plaque acids, which cause tooth decay.