Chapter4-Acid and Base

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ACID AND BASE

4.1 Definition

1. Arrhenius Acid is a substance that produce hydrogen ion, H+ in an aqueous solution Base is a substance that produce hydroxide ion, OH- in an aqueous solution

HNO3 (aq) H+ (aq) + NO3- (aq)

Acid

NaOH (aq) Na+ (aq) + OH- (aq)Base

2. BrØnsted-Lowry Theory: the reaction between an acid & base involves proton transfer Acid is chemical substance that donate H+

Base is a chemical substance that accept H+

HA + B A- + BH+

Acid 1 Base 2 Base 1 Acid 2H+ donor H+ acceptor H+ acceptor H+ donor

HA A- + H+

Acid Base

B + H+ BH+

Base Acid

when an acid (HA) donates H+ to a base (B), the products are A- & BH+

coz the products are also an acid & base, a reserve reaction can occur in which the acid BH +

donates H+ to the base (A-)

EXAMPLE:

HCl + H2O H3O+ + Cl-

HCl: donates p to H2O BrØnsted-Lowry acidH2O: accepts p from HCl BrØnsted-Lowry base

EXERCISEDetermine acid & base

NH3 + H2O NH4+ + OH-

Conjugate acid-base pairs

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when a pair of molecules or ions are related by the loss or gain of one H + conjugate acid-base pair

defined as an acid & its conjugate base or a base & its conjugate acid the conjugate base of a BrØnsted acid is the sp that remains when one p has been removed

from the acid a conjugate acid results from the addition of a p to a BrØnsted base every BrØnsted acid has a conjugate base & every BrØnsted base has a conjugate acid

HX (aq) + H2O (l) X- (aq) + H3O+ (aq)

In the forward reaction HX donates a p to H2O HX is the BrØnsted-Lowry acid H2O is the BrØnsted-Lowry base In the reverse reaction, the H3O+ ion donates a p to the X- ion, so H3O+ is the acid & X-

is the base The acid HX donates a p, it leaves behind a substance, X- act as a base

H2O acts as a base, it generates H3O+ act as an acid Acid & base such as HX & X- that differ only in the presence or absence of p

conjugate acid-base pair Every acid has a conjugate base, formed by removing a p from the acid Every base has associated with it a conjugate acid, formed by adding a p to the base

HX (aq) + H2O (l) X- (aq) + H3O+ (aq)Acid Base conjugate base conjugate acid

EXAMPLE:

HNO2 (aq) + H2O (l) NO2- (aq) + H3O+ (aq)

Acid Base Conjugate base Conjugate acid

NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

Base Acid Conjugate acid Conjugate base

EXERCISE

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add H+

remove H+

remove H+

add H+

add H+

remove H+


1. NH3 + HF NH4+ + F-

2. CN- + H2O HCN + OH-

3. CH3COO- + HCN CH3COOH + CN-

4. H2PO4- + NH3 HPO4

2- + NH4+

5. HClO + CH3NH2 CH3NH3+ + ClO-

6. CO32- + H2O HCO3

2- + OH-

7. CH3COO- + H2O CH3COOH + OH-

3. Lewis acid & base

Theory: the donation & acceptance of a pair of e- during a reaction Lewis acid: substance that can accept a pair of e- to form a covalent bond Lewis base: substance that can donate a pair of e– to form a covalent bond

H F H F

H N : + B F H N B F

H F H F Ammonia Boron triflouride

Ammonia: donating a pair of e- Lewis base Boron triflouride: accepting a pair of e- Lewis acid

4.2 The Acid-Base Properties of Water

Water unique solvent special properties is its ability to act either as an acid or as a base very weak electrolyte & a poor conductor of electricity, but it does undergo ionization to a

small extent

H2O (l) H+ (aq) + OH- (aq)At T = 25oC

[H+] = 1.0 x 10-7 mol/L[OH-] = 1.0 x 10-7 mol/LKw = [H+] [OH-] Kw ion-product constant for water

At 25oC, [H+] = [OH-]Kw = (1.0 x 10-7) (1.0 x 10-7) = 1.0 x 10-14

4.3 The pH Scale

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Function of pH scale: to measure the acidic & basic properties of a certain solution that based on the concentration of H+

Defined: the negative logaritm of the H+ concentration

In neutral solution the [H+] = 1.0 x 10-7 mol/L The pH of neutral solution is 7 If pOH scale

acidic solution: [H+] 1.0 x 10-7, [OH-] 1.0 x 10-7, pH 7 basic solution: [H+] 1.0 x 10-7, [OH-] 1.0 x 10-7, pH 7 neutral solution: [H+] = 1.0 x 10-7, pH = 7

acidic solution: [H+] [OH-] [H+] [OH-] basic solution: [H+] [OH-] [H+] [OH-] neutral solution: [H+] = [OH-]

= (1.0 x 10-7) (1.0 x 10-7)= 1.0 x 10-14

Kw = (-log [H+]) (-log [OH-]) = - log (1.0 x 10-14) = 14

Calculating pH Value

1. The H+ concentration of solution is 1.0 x 10-10 mol/L. What is the pH of the solution?

Answer:

2. The pH of a solution is 6.0. What is the H+ concentration?

Answer:

3. What is the pH of a solution if the [OH-] = 4.0 x 10-11 M?

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pH = - log [H+]

pOH = - log [OH-]

Kw = [H+] [OH-]

pH + pOH = 14


Answer:

4. Calculate the concentration and pH of a 0.2 M Ba(OH)2

Answer:

4.4 Strengths of Acid and Bases

strong acids are strong electrolytes which ionize completely in water. Strong acid such as hydrochloric acid (HCl), nitric acid HNO3), perchloric acid (HClO4) and sulfuric acid (H2SO4). The degree of ionization is 100% because it ionizes completely in water.

HCl (aq) + H2O (l) H3O+ (aq) + Cl- (aq) [HCl (aq) H+ (aq) + Cl- (aq)]

HNO3 (aq) H+ (aq) + NO3- (aq)

HClO4 (aq) H+ (aq) + ClO4- (aq)

H2SO4 (aq) 2H+ (aq) + SO42-(aq)

acid that ionize only to a limited extent in water are called weak acids. At equilibrium, aqueous solutions of weak acids contain a mixture of nonionized acid molecules, H 3O+ ions, and the conjugate base. Example: hydrofluoric acid (HF), acetic acid (CH3COOH) and ammonium ion (NH4

+)

Strong bases are all strong electrolytes that ionize completely in water. Hydroxides of alkali metal and certain alkaline earth metals are strong bases such as NaOH, KOH and Ba(OH)2

NaOH (aq) Na+ (aq) + OH- (aq)

KOH (aq) K+ (aq) + OH- (aq)

Ba(OH)2 (aq) Ba+ (aq) + 2OH- (aq)

weak bases are weak electrolytes which ionize to a very limited extent in water.

NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

4.5 Neutralization Reaction

Neutralization the reaction of an acid & a base

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In a neutralization reaction, hydrogen ions combine with hydroxide ions to form neutral water

HCl + NaOH NaCl + H2Oacid base salt water

H+ + [OH-] H2O

Reaction in which an acid & a base react in an aqueous solution to produce a salt & water neutralization reaction

HCl + NaOH NaCl + H2O

H2SO4 + 2KOH K2SO4 + 2H2O

Acid-Base Titration

The amount of acid (or base) in a solution is determined by carrying out a neutralization reaction An appropriate acid-base indicator is used to show when neutralization is completed The solution of known concentration standard solution The point at which neutralization is achieved is the end point of the titration The process just described is a titration the addition of a known amount of solution to determine

the volume (or concentration) of another solution The unknown solution can be a base instead of an acid, the process is the same

Table: Indicators are usually used in acid-base titration

Indicator Color in Acid Color at End Point Color in BaseLitmus Red Purple Blue

Phenolphthalein Colorless Pink Red

Reactions between an acid & a base in a aqueous solution to produce a salt & water neutralization reaction

In a neutralization reaction, H+ combine with OH- to form neutral water

HCl + NaOH NaCl + H2OAcid Base Salt Water

aA + bB cC + dD

MAVA = aMBVB b

TUTORIAL 4

1. A 0.100 M hydrochloric acid is neutralized by 20.0 mL of 0.250 M sodium hydroxide

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a) Write a balanced equation for the above reactionb) How much the volume of hydrochloric acid is required for the neutralization?c) What is the pH of the sodium hydroxide?

2. Calculate the molarity of phosphoric acid, H3PO4, if 40.0 cm3 of it required 25.0 cm3 of 0.545 M potassium hydroxide, KOH for its neutralization. Balance the equation below for the neutralization reaction described above

H3PO4 + KOH K3PO4 + H2O

3. A 0.32 M KOH is titrated with 24.0 cm3 of 0.22 M H2SO4

a) Write the balanced equation for the reaction.b) Determine the volume of KOH needed to neutralize the H2SO4 solution.c) What is the pH of the KOH?

4. Compare the pH value of the solution X ([OH-] = 4.2 x 10-1 M) and the solution Y ([H+] = 5.8 x 10-1 M). Which solution has the lower pH?

5. A solution has a hydroxide ion concentration of 1.0 x 10 -3 M. Is the solution acidic, neutral or basic? Why?

6. In an experiment, 2.0 M Ba(OH)2 is titrated with 25.0 mL of 0.2 M HCl

a) Write the balanced equation for the above reactionb) Determine the volume of 1.0 M needed to neutralize the HCl

7. Calculate the pH of the following solutions

a) 0.015 M HClb) 0.002 M H2SO4

8. The average pH of normal arterial blood is 7.4. At normal body temperature (37oC), Kw = 2.4x 10-14. Calculate [H+], [OH-] and pOH for blood at this temperature

9. A 25.0 mL of Mg(OH)2 is neutralized by 22.5 mL of 0.10 M HCl. What is the molarity of magnesium hydroxide, Mg(OH)2?

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Chapter4-Acid and Base

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