Acids and Bases

Ionic Equilibria in Aqueous solutionCourse: B.Sc. ISubject: Chemistry IUnit: II

AcidsProduce H+ (as H3O+) ions in water (the hydronium ion is a hydrogen ion attached to a water molecule)Taste sour Corrode metalsReact with bases to form a salt and waterpH is less than 7Turns blue litmus paper to red Blue to Red A-CID

HCl- hydrochloric- stomach acidH2SO4- sulfuric acid - car batteriesHNO3 nitric acid - explosivesHC2H3O2- acetic acid - vinegarH2CO3-carbonic acid sodasH3PO4- phosphoric acid -flavorings

Some Common Acids

BasesProduce OH- ions in waterTaste bitter, chalkyFeel soapy, slipperyReact with acids to form salts and waterpH greater than 7Turns red litmus paper to blue Basic Blue

Some Common BasesNaOHsodium hydroxidelyeKOHpotassium hydroxideliquid soapBa(OH)2 barium hydroxidestabilizer for plasticsMg(OH)2magnesium hydroxideMOM Milk of magnesia Al(OH)3aluminum hydroxideMaalox (antacid)

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Concepts of acid-base theory

Acid/Base definitionsDefinition #1: Arrhenius (traditional)

Acids produce H+ ions (or hydronium ions H3O+)

Bases produce OH- ions

Arrhenius Concept of Acids and BasesAccording to the Arrhenius concept of acids and bases, an Acid is a substance that, when dissolved in water, increases the concentration of hydronium ion or H+ ion (H3O+ ). the aqueous hydrogen ion is actually chemically bonded to water, that is, H3O+.Chemists often use the notation H+(aq) for the H3O+(aq) ion.

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Arrhenius concept of basesA base, in the Arrhenius concept, is a substance that, when dissolved in water, increases the concentration of hydroxide ion, OH-(aq).

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Arrhenius acid is a substance that produces H+ (H3O+) in water

Arrhenius base is a substance that produces OH- in water3

(1) Free H+ and OH ions do not exist in water. The H+ and OH ions produced by acids and basesrespectively do not exist in water in the free state.

(2) Limited to water only. Arrhenius defined acids and bases as compounds producing H+ andOH ions in water only.

(3) Some bases do not contain OH. Arrhenius base is one that produces OH ions in water. Limitations

Brnsted-Lowry Concept of Acids and BasesAccording to the Brnsted-Lowry concept,

An acid is the species donating the proton in a proton-transfer reaction.

A base is the species accepting the proton in a proton-transfer reaction

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Consider the reaction of NH3 and H20.

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Consider the reaction of NH3 and H2O.In the forward reaction, NH3 accepts a proton from H2O. Thus, NH3 is a base and H2O is an acid.

H+baseacid

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Consider the reaction of NH3 and H2O.In the reverse reaction, NH4+ donates a proton to OH-. The NH4+ ion is the acid and OH- is the base.

H+baseacid

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Brnsted-Lowry Concept of Acids and BasesConsider the reaction of NH3 and H2O.

A conjugate acid-base pair consists of two species in an acid-base reaction, one acid and one base, that differ by the loss or gain of a proton.

baseacid

The species NH4+ and NH3 are a conjugate acid-base pair.

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Brnsted-Lowry Concept of Acids and BasesConsider the reaction of NH3 and H2O.

The Brnsted-Lowry concept defines a species as an acid or a base according to its function in the proton-transfer reaction.

baseacid

Here NH4+ is the conjugate acid of NH3 and NH3 is the conjugate base of NH4+.

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Brnsted-Lowry Concept of Acids and BasesSome species can act as an acid or a base.

H+An amphoteric species is a species that can act either as an acid or a base (it can gain or lose a proton).For example, HCO3- acts as a proton donor (an acid) in the presence of OH-

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Brnsted-Lowry Concept of Acids and BasesSome species can act as an acid or a base.An amphoteric species is a species that can act either as an acid or a base (it can gain or lose a proton).Alternatively, HCO3 can act as a proton acceptor (a base) in the presence of HF.

H+

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Brnsted-Lowry Concept of Acids and BasesThe amphoteric characteristic of water is important in the acid-base properties of aqueous solutions.Water reacts as an acid with the base NH3.

H+

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Brnsted-Lowry Concept of Acids and BasesThe amphoteric characteristic of water is important in the acid-base properties of aqueous solutions.Water can also react as a base with the acid HF.

H+

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Brnsted-Lowry Concept of Acids and BasesIn the Brnsted-Lowry concept:

Acids and bases can be ions as well as molecular substances.Acid-base reactions are not restricted to aqueous solution.Some species can act as either acids or bases depending on what the other reactant is.A base is a species that accepts protons; OH- is only one example of a base.

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Lewis Concept of Acids and BasesThe Lewis concept defines an acid as an electron pair acceptor and a base as an electron pair donor.This concept broadened the scope of acid-base theory to include reactions that did not involve H+.

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Lewis Concept of Acids and BasesThe reaction of boron trifluoride with ammonia is an example.

Boron trifluoride accepts the electron pair, so it is a Lewis acid. Ammonia donates the electron pair, so it is the Lewis base.

+NHHH

:

:::BFFF::::::

:::BFFF::::::NHHH

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Relative Strength of Acids and BasesThe Brnsted-Lowry concept introduced the idea of conjugate acid-base pairs and proton-transfer reactions.The stronger acids are those that lose their hydrogen ions more easily than other acids.Similarly, the stronger bases are those that hold onto hydrogen ions more strongly than other bases.

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Relative Strength of Acids and BasesThe Brnsted-Lowry concept introduced the idea of conjugate acid-base pairs and proton-transfer reactions.If an acid loses its H+, the resulting anion is now in a position to reaccept a proton, making it a Brnsted-Lowry base.It is logical to assume that if an acid is considered strong, its conjugate base (that is, its anion) would be weak, since it is unlikely to accept a hydrogen ion.

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Relative Strength of Acids and BasesConsider the equilibrium below.In this system we have two opposing Brnsted-Lowry acid-base reactions.In this example, H3O+ is the stronger of the two acids. Consequently, the equilibrium is skewed toward reactants.

acidacidbasebaseconjugate acid-base pairs

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Relative Strength of Acids and BasesConsider the equilibrium below.This concept of conjugate pairs is fundamental to understanding why certain salts can act as acids or bases.

acidacidbasebaseconjugate acid-base pairs

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Self-ionization of WaterSelf-ionization is a reaction in which two like molecules react to give ions. In the case of water, the following equilibrium is established.

The equilibrium-constant expression for this system is:

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Self-ionization of WaterSelf-ionization is a reaction in which two like molecules react to give ions. The concentration of ions is extremely small, so the concentration of H2O remains essentially constant. This gives:

constant

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Self-ionization of WaterWe call the equilibrium value for the ion product [H3O+][OH-] the ion-product constant for water, which is written Kw.

At 25 oC, the value of Kw is 1.0 x 10-14.Like any equilibrium constant, Kw varies with temperature.

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Self-ionization of WaterBecause we often write H3O+ as H+, the ion-product constant expression for water can be written:

Using Kw you can calculate the concentrations of H+ and OH- ions in pure water.

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Self-ionization of WaterThese ions are produced in equal numbers in pure water, so if we let x = [H+] = [OH-]Thus, the concentrations of H+ and OH- in pure water are both 1.0 x 10-7 M.If you add acid or base to water they are no longer equal but the Kw expression still holds.

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The pH of a SolutionAlthough we can quantitatively describe the acidity of a solution by its [H+], it is often more convenient to give acidity in terms of pH.The pH of a solution is defined as the negative logarithm of the molar hydrogen-ion concentration.

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The pH of a SolutionFor a solution in which the hydrogen-ion concentration is 1.0 x 10-3, the pH is:Note that the number of decimal places in the pH equals the number of significant figures in the hydrogen-ion concentration.

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The pH of a SolutionIn a neutral solution, whose hydrogen-ion concentration is 1.0 x 10-7, the pH = 7.00.For acidic solutions, the hydrogen-ion concentration is greater than 1.0 x 10-7, so the pH is less than 7.00.Similarly, a basic solution has a pH greater than 7.00.Figure shows a diagram of the pH scale and the pH values of some common solutions.

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The pH Scale

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A Problem to ConsiderA sample of orange juice has a hydrogen-ion concentration of 2.9 x 10-4 M. What is the pH?

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A Problem to ConsiderThe pH of human arterial blood is 7.40. What is the hydrogen-ion concentration?

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The pH of a SolutionA measurement of the hydroxide ion concentration, similar to pH, is the pOH.The pOH of a solution is defined as the negative logarithm of the molar hydroxide-ion concentration.

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The pH of a SolutionA measurement of the hydroxide ion concentration, similar to pH, is the pOH.Then because Kw = [H+][OH-] = 1.0 x 10-14 at 25 oC, you can show that

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A Problem to ConsiderAn ammonia solution has a hydroxide-ion concentration of 1.9 x 10-3 M. What is the pH of the solution?You first calculate the pOH:

Then the pH is:

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The pH of a SolutionThe pH of a solution can accurately be measured using a pH meterAlthough less precise, acid-base indicators are often used to measure pH because they usually change color within a narrow pH range.

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The pH Meter

When a soluble salt (say A+C) is added to a solution of another salt (A+B) containing a common ion (A+), the dissociation of AB is suppressed.

AB A+ + B

By the addition of the salt (AC), the concentration of A+ increases. Therefore, according to Le Chateliers principle, the equilibrium will shift to the left, thereby decreasing the concentration of A+ ions. Or that, the degree of dissociation of AB will be reduced.

THE COMMONION EFFECT

The reduction of the degree of dissociation of a salt by the addition of a common-ion is called the Common-ion effect.Let us consider a few examples to illustrate the common-ion effect.Example 1. In a saturated solution of silver chloride, we have the equilibrium AgCl (s) Ag+ (aq) + Cl (aq)

When sodium chloride is added to the solution, the concentration of Cl ions will increase. The equilibrium shown above will be shifted to the left to form more of solid Ag Cl.

Thus the solubility of AgCl, a typical sparingly soluble salt, will decrease.

The scheme of Qualitative Analysis is based on the Principle of Selective Precipitation.

Let us consider a mixture containing Ag+, Cu2 + and Fe3 +.

Step 1 : Add dil HCl to the solution of the mixture. Ag+ will precipitate as AgCl. It is removed by filtration.

SELECTIVE PRECIPITATION

Step 2 : Pass H2S through the filtrate. Cu2+ is precipitated as CuS and removed by filtration.

Step 3 : Add NH4OH or NaOH solution to filtrate. Fe3 + is precipitated as Fe(OH)3.

In this way ions are precipitated from solution one by one.

The precipitation of cations fromsolution one at a time is called Selective precipitation.

References1. General chemistry by Ebbing Darell,3rd edition2. Essentials of physical chemistry by bahl and tuli ,3 rd edition 3.https:// sites.google.com4. https://www.mhhe.com

Presentation of Lecture Outlines, 1649