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ACIDS, BASES AND SALTS

LEARNING OUTCOMES

UNDERSTANDING:

Students will be able to:

Define and give examples of Arrhenius acids and bases. (Understanding)Use the Bronsted-Lowry theory to classify substances as acids or bases, or as

proton donors or proton acceptors. (Applying)Classify substances as Lewis acid or bases. (Analyzing)Write the equation for self-ionization of water. (Remembering)Given the hydrogen ion or hydroxide ion concentration, classify a solution asneutral, acidic, or basic. (Applying)Complete and balance a neutralization reaction. (Applying)

SKILLS:


Use litmus paper, pH paper and other indicators for measuring pH of solutions.(Applying)Perform acid base titrations and related calculations. (Analyzing)

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Major concepts10.1 Concepts of Acids, Bases10.2 pH Scale10.3 Salts

Introduction:You frequently use acids and bases in every aspect of life. For instance toilet

bowel cleaner, vinegar, aspirin, lemon juice, cola drinks, apple and tomato containacids. Substances such as drain cleaner, antacid tablets, baking powder, washing sodaetc. contain bases. You eat them and drink them and your bodies produce them. Fromacid indigestion to acid rain the word acid occurs frequently in the news andadvertisements. What is an acid rain? This chapter will enable you to understand whichsubstances are called acids and which are called bases. How they are classified? Whathappens when an acid reacts with a base ? Why do we use lemon juice on fish? In thischapter you will learn some of the chemistry of acids and bases. This will help you togain a better understanding of these important classes of compounds. What do wemean by the pH of a solution such as that of acid rain? Acids are widely used in themanufacture of fertilizers and in food industry.

SOCIETY, TECHNOLOGY AND SCIENCE:


Identify areas of work for analytical chemists. (Understanding)Explain why the quantity of preservatives in food is restricted by governmentregulations. (Applying)Explain pH-dependent foods. (Applying)Explain process of etching in arts and industry. (Applying)Explain the reactions between industrial pollutants and atmospheric waterleading to formation of acids. (Applying)Describe harmful effects of acid rain. (Analyzing)Explain stomach acidity. (Applying)

Time allocation

Teaching Periods 10 Assessment periods 03

Weightage 7%

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Table 10.2 Some Common AcidsName Formula Common use

Hydrochloric

acid

HCl Cleaning of metals, bricks and removing scale from

boilersNitric acid HNO 3 Manufacture of fertilizers, explosives

Sulphuric acid H 2SO 4 Manufacture of many chemicals, drugs, dyes, paintsand explosives.

Phosphoricacid

H3PO 4 Manufacture of fertilizers, acidulant for food

Table 10.3 Some Common Bases

Name Formula Common use

Sodium hydroxide NaOH Soap making, drain cleaners

Potassium hydroxide KOH Making liquid soap, shaving cream

Calcium hydroxide Ca (OH) 2 Making mortar, plasters, cement

Magnesium hydroxide Mg(OH) 2 Antacid, laxative

10.1.2 The Bronsted Concept of Acids and Bases Arrhenius theory has its limitations. It applies to aqueous solutions. It does not

explain why compounds such as CO 2, SO 2 etc., are acids. Why substances like NH 3,are bases? There is no H in CO 2 and OH in NH 3.

In 1923 J.N Bronsted and T.M Lowery independently proposed anothertheory to overcome the shortcomings of Arrhenius theory. This theory is known as

Bronsted-Lowery theory .

According to this theory an acid is a proton donor and a base is a proton acceptor .

Consider the following example

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Which substance donates proton?

Which substance accepts proton?

Which substance is acid?

Which substance is base?

Where does the OH - come from in the ionization of bases such as ammonia? The

Arrhenius theory is inadequate to answer this question, but the Bronsted-Lowry

theory explains how ammonia acts as a base in water. Ammonia is a gas at room

temperature. When it is dissolved in water, the following reaction occurs.

Which substance is proton donor, NH 3 or H 2O? Which substance is protonacceptor? All the acids included in the Arrhenius Theory are also acids in the

Bronsted-Lowery Theory. However all the bases included in Bronsted-Lowery

theory except OH are not Arrhenius bases. Consider above two examples. In one

example, water molecule accepts a proton and in the other water donates a proton.

This means water behaves like an acid as well as a base. It is amphoteric in

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nature. Substances that react with both acids and bases are called amphoteric

substances.

Society Technology and ScienceFossils Fuels contain small amounts of sulphur and nitrogen. They producesulphur dioxide and oxides of nitrogen when 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.

SO 2 (g) + 1/2O 2(g) + H 2O (l) H2SO 4 (aq ) 4NO 2 (g) + O 2 (g) + 2H 2O (l) 4HNO 3 (aq)

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

Example 10.1: Classify substances as acids or bases or as proton donor orproton acceptor

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

HC l + H 2O H 3O+

+ C l -

H2O + NH 3 NH 4+ + OH -

Problem solving strategy: 1. An acid is a proton donor. After donating proton, acid forms a negative ion.

2. A base is a proton acceptor. After accepting proton from an acid it forms apositive ion.

Solution:

1. Because HCl is converted to C l -

by donating proton, HCl is an acid.2. Because H 2O accepts the proton that HCl donates and forms H3O +, water isa base.

3. H2O is converted to OH -by donating a proton, so H 2O is an acid. BecauseNH3 accepts the proton and forms NH 4+ so it is a base.

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10.1.3 Lewis Concept of Acids and Bases:

Certain substances like SO 2, CO 2, CaO, BF 3 etc. behave as acids orbases although they do not have ability to donate or accept protons. Nature of suchsubstances cannot be explained by Arrhenius theory or the Bronsted-Lowry theory.

In 1923, G.N Lewis proposed an acid base theory that focuses onreaction. This concept is more general than either the Arrhenius theory or the Bronsted -Lowery theory.

A Lewis acid is substance that can accept a pair of electrons to form

a coordinate covalent bond.

A Lewis base is a substance that can donate a pair of electrons toform a coordinate covalent bond.

In a Lewis acid-base reaction a coordinate covalent bond is formedbetween the acid and the base.

Self-Assessment Exercise 10.1

Identify Bronsted acids and Bronsted bases in the following reactions.

1. H2SO 4+ H 2O HS0 4 + H 3O+

2. CH 3COOH + H 2O CH 3COO + H 3O+

3. H2S + NH 3 NH4+ + HS

Electron pairacceptor

Electron pairdonor

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Consider the following reactions. (i) Which species is donating an electron pair? (ii)Which species is accepting an electron pair? (iii) Which species is a Lewis acid? (iv)Which species is a Lewis base?

Nitrogen atom in ammonia donates an electron pair to H- atom in HCI. Which species in

Lewis acid? HCl or NH 3.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 10.2: Classifying substances as Lewis acids or Lewis bases.

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

Problem Solving Strategy1. Draw electronic structures of both the species.2. Look for the species that has a lone - pair of electron or negative charge.

Such a species has complete octet, so it does not need any electron. Itcan however, donate an electron pair. This species is a Lewis base.

+

H

H N:

H

F

F B _ _

F

H

H N

H

F

F B _ _ _

F

+

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3. Look for the species that can accommodate an electron pair. Such aspecies has incomplete octet. So this species can accept an electron pair.This species is a Lewis acid.

Solution

(i)

(ii) NH 3 has a lone pair on N-atom. So it is electron pair donor. NH 3 is aLewis base.

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

10.2. Self-Ionization of water The pH scaleWater molecules are highly polar. Occasionally, the collisions between water moleculesare energetic enough to transfer a proton from one water molecule to another.


Identify the Lewis acid and the Lewis base in the following examples.

1. Cl- + AlCl 3 [AlCl 4]-

2. H+ + OH - H2O

H

H

__ O

: : H

H

_ __ O H

: :+ +

+H

H

_ __OO : :

OO : :

: : : :H{ {

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A water molecule that donates or loses a proton becomes a negatively chargedhydroxide ion OH -. The other water molecule which gains or accepts the protonbecomes positively charged hydronium ion H 30+.

This reaction can be written as

2H 2O H3O++ OH-

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

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

Hydrogen ion Hydroxide ion

Water is a weak electrolyte. The self-ionization of water occurs to a very small extent. At25C the experimentally determined concentrations of H + ions and OH - ions are asfollows.

[H+] = [OH -] = 1X10 -7 M

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

[ ][ ]

[ ]

Since H 2O is a weak electrolyte, so the concentration of [H 2O] will remainconstant.

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

Where K w = K c [H2O] is called ionization constant for water. It is also called theion- product for water. For water

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

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

[H+] = [OH ] = 1 x 10 -7 at 25 oC

In 1909, the Danish biochemist Soren Sorenson proposed a convenient method to

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express such a small concentration of H + ions and OH - ions by pH or pOH

pH is defined as the negative logarithm of the molar concentration of H + ions inaqueous solutions.

pH = -log [H +]

For pure water at 25 OC

[H+] = 1x10 -7M, [OH -]=1X10 -7MpH= -log (1x10 -7) = 7

Thus pH of water is 7. All aqueoussolutions with pH = 7 at 25 oCare neutral. If

pH is less than 7, the solutions becomeacidic, [H +] increases and [OH -] decreases.

What is the importance of K w?

Kw is temperature dependent. In

any aqueous solution at 25 oC, no matter what does it contain the product of H + ion

concentration and OH - ion concentration is always equal to 1.0x10 -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 .

What will happen if [OH -] increases?

When [H +] = [OH -] = 1 x 10 -7 , solution is neutralWhen [H +] > 1 x 10 -7, solution is acidicWhen [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 solutionbecome acidic, [H +] increases and [OH -] decreases.

Figure 10.1 shows pH scale and pH values of some common substances.

Society, Technology and Science

Certain plants grow successfully at aparticular pH range

Plant pH range Apple 5.5 - 7.0Broad

Bean

6.5 - 7.0

Carrots 6.0 - 7.5Onions 6.5 - 7.5Potatoes 5.5 - 6.5Tomatoes 5.5 - 7.0

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pH Concentration of H + ion Common Examples

0 10 0 Acid from batteries

1 10 1 Gastric juice

2 10 2 Lemon juice

3 10 3 Orange juice

4 10 4 Tomato juice

5 10 5 Black coffee

6 10 6 Saliva

7 10 7 Distilled water

8 10 8 Sea water

9 10 9 Baking soda

10 1010 Milk of magnesia

11 1011 Ammonia solution

12 1012 Soapy solution

13 1013 Bleach

14 1014 Drain cleaner

1. Unrefined hydrochloric acid is used to clean stone buildings and swimming pools.If the [H +] in a solution of HCl is 1x10 -6 M. Is the solution acidic, basic or neutral?

2. Solution hydroxide (NaOH) is commonly used as a drain cleaner. If theconcentration of OH - in a solution of NaOH is 1.0x10 -5 M. Is the solution acidicbasic or neutral?

Example 10.3: Classifying a solution as neutral, acidic or basic

Figure 10.1: pH scale and pH values of common substances

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Problem Solving Strategy:1. Compare the given concentrations of [H +] ions in solution with that of neutral

water.2. If [OH -] is given, calculate [H +] from k w = [H +][OH -].3. Remember that

If [H+] = [OH -] = 1X10 -7 solution is neutral.If [H+]> 1 x 10 -7, solution is acidic.If [H+] 1.0 x 10 -7 M, the solution is acidic.2. [OH -] = 1.0x10 -5 M

[H+

] = ?kw = [H +][OH -].1.0 x 10 -14 = [H +]1.0x10 -5

[H+]= 1.0 x 10 -9 MBecause 1.0 x 10 -9 M

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universal indicator paper is used to measure pH of a solution. For this purpose pHpaper is dipped in the solution. The colour that develops on the pH paper is comparedto the colour corresponding to a known pH on the chart. Each colour is linked to aspecific pH value. (Figure 10.2)

One of the most commonly usedmethods in chemistry laboratory is the use oflitmus paper. It is used to give a generalindication of whether a solution is acidic orbasic. Litmus paper may be red or blue. Anacid turns blue litmus paper red and a base turns red litmus blue.

Important InformationThe optimum pH range of a swimmingpool is 7.2 to 7.6 because in human tears,when the pH is outside this range, eye

irritation can occur.

Society, Technology andScience

Acidity of stomachThe main component of digestive or

gastric juice in the stomach ishydrochloric acid. Almost two litre of itis secreted each day by gastric glands.However sometimes too much acid issecreted in the stomach which causesindigestion. This is called acidity of thestomach.

Figure 10.2: Colours of pH paper or universal indicator

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Figure 10.4: The soil sampleA turns red litmus blue. The soil sample B turns bluelitmus red. Which soil is acidic? Which soil is basic?

Acid - base indicators are also used to estimate the pH of a solution. Indicators areintensely colored organic compounds. They change colour within small pH change andindicate the pH of solution by the colour . We add few drops of an indicator to anaqueous solution of unknown pH and measure pH of the solution from the resultingcolour.(Figure 10.5)

A B

Figure 10.5 Colour changes of some acid base indicators

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Methyl red changes color from red to yellow at pH6. At what pH phenolphthaleinchanges its colour? At what pH bromothymol blue changes colour from yellow to blue?

Self-assessment Exercise 10.41. Write names of three acid - base indicators.2. .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

Table 10.3 pH ranges of some common indicators

You will need:

Lemon juice Vinegar Soap solution HCl solution NaOH solution

Indicator pH at which colourchanges

Colour in acidicsolution

Colour in basicsolution

Methyl red 5.5 Red yellow

Bromothymolblue 7

Yellow.Blue

Phenolphthalein 9 Colourless Pink

Activity 10.1: Use litmus paper, pH paper and other indicators formeasuring pH of solutions

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Use of litmus paper:

Carry out the following Take each solution in separate beakers and write the name of each solution on

each beaker.

Take about 2-3 cm3

of each solution in separate test tubes and dip red and bluelitmus paper in each solution. Note the colour change in each case and record.

Result: This test will classify each solution as acid or base or has pH less than 7 orgreater.

Use of pH paper: Take about 2-3 cm 3 each solution in separate test tubes and dip pH paper in

each solution. Note the colour developed on pH paper. Now compare this colour with the pH scale given in figure 10.2 and find pH of

solution.

Complete the following table

Substance pH

Lemon juice

Vinegar

Soap solution

HCl solution

NaOH solution

Use of methyl orange

Take about 2-3 cm 3 of each solution in separate test tubes and add or 1-2 dropsof methyl orange in each test tube and note the colour of solution.

Key:

Yellow colour indicates pH > 4Red Colour indicates pH < 4

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Complete the following table,

Substance Colour of methyl orange pH

Lemon juice

Vinegar

Soap solution

HCl solution

NaOH solution


Etching is an art that uses acid to crave patterns into metal, glass and othermaterials. For this a piece of metal or glass is covered with wax, and then a design isetched on to the plate through the wax. The plate is then dipped into a tank of acid.The acid eats away at the exposed portion, which leaves behind textured mark. Theplate is then taken out of the acid and cleaned. Ink can also be applied on etching tocreate colourful design.

SKILLS

To standardize the given solution of hydrochloric acid

Note perform this activity in chemistry laboratory.

You will need:

Burette, Pipette, burette stand, beakers, conical flash, glass rod. Standard 0.1M NaOH solution and phenolphthalein.

Chemical equation

HC l (aq) + NaOH (aq) NaCl (aq) + H 2O (l) n1 = 1 mole n 2 = 1 mole

Activity 10.2 : Perform acid base titration and related calculations .

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Carry out the following:

Fit up a clean burette in the burette stand vertically. Fill burette with HCl solution up to zero mark.

Take 10 cm3

of NaOH solution in a conical flask with the help of pipette. Add few drops of phenolphthalein in it as indicator. Note the initial reading in the burette. Run the acid solution in the conical flask drop by drop, and shake the flask

constantly. Go on adding the acid solution till the pink colour just disappears. Note down the final reading from the burette. The difference between the final and initial reading gives the volume of the acid

used to neutralize 10.0 cm 3 of NaOH solution. Repeat the experiment to get three concordant readings. Find the mean volume of HCl solution used.

Observations and calculationsSuppose volume of HCl solution used = V, = 10 cm 3 Molarity of HCl solution = M, =?Volume NaOH solution used = V 2 = 10 cm 3 Molarity of NaOH solution = M 2 = 0.lMNo. of moles of HCl = n 1 = 1No. of moles of NaOH = n 2 = 1

M1V1 = M 2V2 n1 n 2

M1 x 10 = 0.1 x 101 1

M2 = 0.l M

Result: Molarity of HCl solution is 0.l M

10.3Salts An acid contains replaceable hydrogen atoms. When these are completely or

partially replaced by metal atoms, a compound called salt is formed.

HC l Na C l

Do you know

We make use of chemistrywhen we put lemon juice onfish. The unpleasant fishy odouris due to amines. The citric acidpresent in lemon juice convertsamines to non-volatile salts,thus reducing the odour.

Metal atom has replacedH- atom

Replaceable H atom

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Which is a salt HCl or NaC l ?

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 acid e.g. Sodium Chloride.

NaCl

The name of metal ion The part showing which acid was used

Which acid forms NaNO 3? Which acid forms CaSO 4? Table 10.4 shows some commonacids and their salts.

Table 10.4 Some common acids and their salts

Acid Salt name Example

HydrochloricHC l

ChlorideC l

NaC l , KC l , CaC l 2

NitricHNO 3

Nitrate

NaNO 3, KNO 3, Ca(NO 3)2

Sulphuric

H2SO 4

Sulphate

Na 2SO 4, K2SO 4, CaSO 4

PhosphoricH3PO 4

Phosphate

Na 3PO 4, K3PO 4, Ca 3(PO 4)2

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

Acid + Base Salt + Water

HC l (aq) + NaOH (aq) NaC l (aq) + H 2O (l)

Neutralization is the reaction between H + ions of an acid and OH - ions of a base.

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

Reactions of acids with bases are used in the experimental procedure of titration.You will do this work in your laboratory. (Activity 10.2)

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Example 10.4: Writing complete and balanced chemical equation for a neutralizationreaction.

1. Soda ash, Na 2CO 3 is used to make glass. It can be made by the reaction ofcarbonic acid (H 2CO 3) and Sodium hydroxide (NaOH). Write complete andbalanced chemical equation for this neutralization reaction.

2. Barium nitrate Ba(NO 3)2 is used to produce a green colour in firework. It canbe made by the reaction of nitric acid (HNO 3) with barium hydroxide,Ba(OH) 2.Write complete and balanced chemical equation for thisneutralization reaction.

Problem Solving Strategy1. Write word equation describing the neutralization reaction.2. Write chemical formulas of the substances involved in the chemical reaction.

Salt consists of cations from the base and anions from the acid.3. During neutralization reaction one H - atom of an acid combines with one OH

group of the base to form one water molecule. So, place a suitable numberbefore acid or base to balance H in acid with OH in base.

4. Balance remaining equation by inspection method.5. Show the state of each of the substance involved.

Solution

1. Carbonic acid + Sodium hydroxide Sodium carbonate + water

H2CO 3 + NaOH Na 2CO 3+H2O

H2CO 3 contains two neutralizable H-atoms and NaOH contain only oneOH. So multiply NaOH by 2.

H2CO 3 +2NaOH Na 2CO 3 + H 2O

Now balance H-atoms on the right side by placing 2 before H 2O.

H2CO 3 +2NaOH Na 2CO 3 + 2H 2OH2CO 3 (aq) + 2NaOH (aq) Na 2CO 3 +2H 2O (l)

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2. Nitric acid + Barium hydroxide Barium nitrate + water.

HNO 3 + Ba (OH) 2 Ba (NO 3)2 + H 2O

HNO 3 contains one neutralizable H-atom and Ba(OH) 2 contains two OH-

groups. So multiply HNO 3 by 2.

2HNO 3 + Ba (OH) 2 Ba (NO 3)2 + H 2O

Now balance H-atoms on the right side by placing 2 before H 2O.

2HNO 3 + Ba (OH) 2 Ba (NO 3)2 + 2H 2O2HNO 3(aq) + Ba (OH) 2(aq) Ba (NO 3)2 (aq) + 2 H 2O (l)

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

2. Hydrochloric acid (HCl) and Potassium hydroxide (KOH) react and producepotassium chloride. Write complete and balanced chemical equation for this

neutralization reaction?

3. Balance following neutralization reactions(i) H2SO 4(aq) + NaOH (aq) Na 2SO 4 (aq) + H 2O (l) (ii) H3PO 4 (aq) + NaOH (aq) Na 3PO 4 (aq) + H 2O (l)

Some acids form more than one salts. For example Carbonic acid (H 2CO 3) hastwo replaceable H-atoms. Its partial neutralization forms hydrogen carbonate. Oncomplete neutralization it forms carbonate.

H2CO 3(aq) + NaOH (aq) NaHCO 3 (aq) + H 2O (l)

H2CO 3(aq) + 2NaOH (aq) Na 2CO 3 (aq) + H 2O (l)

H3PO 4(aq) + NaOH (aq) NaH 2PO 4 (aq) + H2O (l)


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A salt containing a replaceable H- atom or formed by partial neutralization of anacid is called acid salt whereas a salt which is formed by the complete neutralization ofan acid is called a normal salt.

Which salt is acid salt, NaHCO 3 or Na 2CO 3?

Which salt is normal salt, NaHCO 3 or Na 2CO 3?

Phosphoric acid (H 3PO 4) has three replaceable H-atoms, it form three series ofsalts, NaH 2PO 4, Na 2HPO 4, Na 3PO 4. Which of these salts is/are acid salt?

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

Zn(OH) 2 (aq) + HC l (aq) Zn(OH)C l (aq) + H 2O

Pb(OH) 2 (aq) + CH 3COOH (aq) Pb(OH)CH 3COO (aq) + H 2O

Classify following salts as normal or acid salt.

(a) NaHSO 4 (b) Na 2SO 4 (c) KHCO 3 (d) K2CO 3

10.3.1 Methods for making salt

There are five methods for making salts.

1. Acid + Base Salt + WaterHCl (aq) + NaOH (aq) NaCl (aq) + H 2O (l)

2. Acid + Metal oxide salt + waterH2SO 4 (aq) + CuO (s) CuSO 4 (aq) + H 2O (l)

3. Acid + Metal Salt + Hydrogen2HCl (aq) + Mg (s) MgCl 2 (aq) + H 2 (q)


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4. Acid + Metal carbonate Salt + Carbon dioxide + waterHCl (aq) + CaCO 3 (s) CaCl 2 (aq) + CO 2 (g) + H 2O (l)

5. Salt (aq) + Salt (aq) Salt (s) + Salt (aq) AgNO 3 (aq) + NaCl (aq) AgCl (s) + NaNO 3 (aq)

10.3 Uses of Salts

Food preservation keeps food from spoiling and allows it to be stored for lateruse. Ancient methods for preserving include, drying fruits and vegetables, salting,boiling etc. Today, methods for preserving food also include the addition ofpreservatives. They are inhibitors of physical and chemical processes that cause food tospoil. Many foods are grown or produced in one location and then sent across thecountry or even overseas. Without preservatives, these foods would spoil long beforethey reach their destinations. Many salts such as sulphites and benzoates are beingused in food for thousands of years.


Many people are allergic or sensitive to preservatives. Some preservativesare safe in small amount and toxic in larger amounts. Hence, Food and Drug

Administration Department is given the responsibility for approving the safetyand use of preservatives.

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Arrhenius

Conce t

Lewis concept

Bronsted

conce t

Salt

Acidic salt

Basic salt

Normal salt

Base Arrheniusconce t

Bronstedconce t

Electron pair acceptor

Proton donor Gives H + ions

Combines with base to form

Combines with acid to form

Proton acceptor Gives OH - ions

Acid

Concepts in brief

Lewisconce t

Electron pair donor

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According to Arrhenius theory, an acid is a substance that ionizes in water toproduce H + ions and a base is a substance that ionizes in water to produce OH - ions.

A Bronsted-Lowry acid is a proton donor and a base is a proton acceptor. A Lewis acid is a substance that can accept a pair of electrons to form a covalentbond.

A Lewis acid is a substance that can accept a pair of electrons to form acoordinate covalent bond. A Lewis base is a substance that can donate a pair of electrons to form acoordinate covalent bond.The reaction in which two water molecules react to produce ions is called as self-ionization of water.Ionization constant for water is also called as the ion-product constant for water.Its value is 1x 10 -14 at 25C.If [H+] = 1x 10 -7M solution is neutral

If [H+] > 1x 10 -7M solution is acidic If [H+] < 1x 10 -7M solution is basic

A pH of 7 indicates a neutral solutionPb(OH) 2(aq) + CH 3COOH (aq) Pb(OH)CH 3COO (aq) + H 2O

Acids have pH less than 7.Bases have pH greater than 7.Indicators change colour within a small pH range and indicate the pH of solutionby the colour.Methyl orange, bromothymol blue and phenolpthalein are common acid-baseindicators.Salt is an ionic compound formed when replaceable hydrogen atom in an acid isreplaced by a metal atom.Reaction between an acid and a base is called neutralization reaction.

Acid salts contain one or more replaceable H-atoms.Normal salts are formed by the complete neutralization of acids.

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1. Encircle the Correct answer(i) Which of the following cannot be classified as Arrhenius acid?

(a) HNO 3

(b) H2CO 3

(c) CO 2

(d) H2SO 4

(ii) NH 3 cannot be classified as a base by (a) Lewis theory

(b) Bronsted -Lowry theory

(c) Arrhenius theory

(d) All of these theories

(iii) Which of the following is a Lewis base?(a) BF 3

(b) HCl

(c) AlCl3

(d) F -

(iv) Choose Lewis acid

(a) CN-

(b) NH3

(c) H2O :

(d) H+

. .

. .

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(v) A drain cleaner solution contains 1.0x10 -8 M, OH - concentration. ThisSolution is

(a) acidic

(b) basic

(c) neutral

(d) cannot be predicted

(vi) Milk of magnesia contains Mg(OH) 2. It is used as antacid. Itneutralizes excess stomach acid. Which salt is formed in thisreaction?

(a) MgSO 4

(b) MgCO 3

(c) MgCl 2

(d) MgO

(vii) Ammonia is a base, because it

(a) Ionizes in water to give OH - ions

(b) Contains OH group

(c) Can accept an election pair

(d) Can accept proton

(viii) Consider the following reaction?

H2O + HCl H3O++ C l -

Which species is an electron pair acceptor in this reaction?

(a) H2O

(b) HCl

(c) H3O+

(d) none

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(ix) In the following reaction which species is donating an electron pair?

NH3 + BF 3 H3N - BF 3

(a) H

(b) B(c) N

(d) BF 3

(x) An aqueous solution of NaOH is used as a drain cleaner. If theconcentration of OH - ions in this solution is 1.0 x 10 -5 M, theconcentration of H + ions in it would be?

(a) 1.0 x 10 -5 M

(b) 1.0 x 10 -7 M

(c) 1.0 x 10 -9 M

(d) 1.0 x 10 -14 M

2. Give short answers?

(i) Write the equation for the self-ionization of water.

(ii) Define and give examples of Arrhenius acids.

(iii) Why H + ion acts as a Lewis acid?

(iv) Why NH 3 acts as Bronsted-Lowry base?

(v) Why BF 3 acts as Lewis acid?

3. Ammonium hydroxide and nitric acid react and produce ammoniumnitrate and water. Write balanced chemical equation for thisneutralization reaction.

4. Write balanced chemical equations for the following neutralizationreactions.

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

(ii) Sulphuric acid+ Sodium hydroxide Soduim sulphate + water.

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

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5. Identify Bronsted Lowry acids or bases in the following reactions.

(i) HNO 3 + H 2O H 3O+ + NO 3

(ii) NH3 + HNO 3 NH4NO 3

6. Identify Lewis acid and Lewis base in the following reactions.(i) F - + BF 3 [BF 4]-

(ii) H+ + NH 3 [NH4]+

(iii) NH3 + AlCl 3 [H3N- AlCl 3]

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

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

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

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

(iv) A solution that has hydroxyl ion concentration 1.0 x10 -10 M.

8. Classify following substance as Lewis acid and bases.

NH3, F -, H 2O : BF 3

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

10. Give the Bronsted Lowry definition of a base. Write an equationthat illustrates the definition.

11. Identify Bronsted acids and Bronsted bases in the followingreactions.

(i) CH 3COOH (aq) + H 2O CH 3COO - (aq) + H 3O+ (aq)

(ii) HC (g) + H 2O (l) (aq) + H 3O+ (aq)

(iii) NH3 (g) +H 2O (l) (aq) + OH - (aq)

(iv) HCl (aq) + H (aq) H2CO 3 (aq) + Cl - (aq)

(v) HS - (aq) + H 2O (l) S -2 (aq) + H 3O+ (aq)

(vi) H2S + NH 3 + HS -

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

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

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

(iii) Cu+2

(aq) + 4NH 3(aq) [Cu(NH 3)4]+2

(aq)

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

13. Identify Lewis acids and Lewis bases from the following.

14. Classify water as proton donor or proton acceptor.

15. Write equations showing the ionization of the following as Arrheniusacids.

(a) HI (aq) (b) HNO 2(aq)

16. Write equations showing the ionization of the following as Bronsted-Lowry acids.

(a) HNO 2 (aq) (b) HCN (aq)

AlCl 3, Ag+, CH 3 OH, CH 3 NH 2, CN-, OH -, FeCl 3 . .

. . . .

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

a) acidic b) basic c) neutral

18. Codeine, C 18 H21 NO 3 is a commonly prescribed pain killer. It dissolves inwater by the following reaction?

C18 H21 NO 3 + H 2O [C 18 H21 HNO 3]+ + OH -

Identify Codeine as Bronsted-Lowry acid or base.19. Suggest some ways in which you might determine whether a particular

water solution contains an acid or a base.

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

Indicator Colour in Acidic Solution Colour in Alkaline Solution

A Red BlueB Colourless Red

a) What will be the colour of the indicator A?i. in A solution of pH 3ii. in A solution of pH 10

b) What will be the colour of the indicator B in a solution of pH 5?c) 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?

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

Project

Examine the labels of at least three antacid preparations. Make a list of the ingredients ineach. Write a balanced chemical equation for the neutralization reaction that takes placewhen these antacids react with HCl in the stomach.

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