the arrhenius theory of acids and bases the theory acids are substances which produce hydrogen ions...

The Arrhenius Theory of acids and basesThe Arrhenius Theory of acids and basesThe theoryThe theory• Acids are substances which produce hydrogen ions in solution.Acids are substances which produce hydrogen ions in solution.• Bases are substances which produce hydroxide ions in solution.Bases are substances which produce hydroxide ions in solution.Neutralisation happens because hydrogen ions and hydroxide ions Neutralisation happens because hydrogen ions and hydroxide ions

react to produce water.react to produce water.

In the sodium hydroxide case, hydrogen ions from the acid are In the sodium hydroxide case, hydrogen ions from the acid are reacting with hydroxide ions from the sodium hydroxide - in line reacting with hydroxide ions from the sodium hydroxide - in line with the Arrhenius theory.with the Arrhenius theory.

However, in the ammonia case, there don't appear to be any However, in the ammonia case, there don't appear to be any hydroxide ions!hydroxide ions!

THEORIES OF ACIDS AND THEORIES OF ACIDS AND BASESBASES

The Bronsted-Lowry Theory of acids and basesThe Bronsted-Lowry Theory of acids and bases• An acid is a proton (hydrogen ion) donor.An acid is a proton (hydrogen ion) donor.• A base is a proton (hydrogen ion) acceptorA base is a proton (hydrogen ion) acceptorThe relationship between the Bronsted-Lowry and Arrhenius The relationship between the Bronsted-Lowry and Arrhenius

theory. The Bronsted-Lowry theory doesn't go against the theory. The Bronsted-Lowry theory doesn't go against the Arrhenius theory in any way - it just adds to it. Hydroxide ions Arrhenius theory in any way - it just adds to it. Hydroxide ions are still bases because they accept hydrogen ions from acids are still bases because they accept hydrogen ions from acids and form water. An acid produces hydrogen ions in solution and form water. An acid produces hydrogen ions in solution because it reacts with the water molecules by giving a proton to because it reacts with the water molecules by giving a proton to them.When hydrogen chloride gas dissolves in water to them.When hydrogen chloride gas dissolves in water to produce hydrochloric acid, the hydrogen chloride molecule produce hydrochloric acid, the hydrogen chloride molecule gives a proton (a hydrogen ion) to a water molecule. A co-gives a proton (a hydrogen ion) to a water molecule. A co-ordinate (dative covalent) bond is formed between one of the ordinate (dative covalent) bond is formed between one of the lone pairs on the oxygen and the hydrogen from the HCl. lone pairs on the oxygen and the hydrogen from the HCl. Hydroxonium ions, HHydroxonium ions, H33OO++, are produced., are produced.

THEORIES OF ACIDS AND THEORIES OF ACIDS AND BASESBASES

When the acid, HA, loses a proton it forms a base, AWhen the acid, HA, loses a proton it forms a base, A--. When the . When the base, Abase, A--, accepts a proton back again, it obviously refoms the , accepts a proton back again, it obviously refoms the acid, HA. These two are a conjugate pair.acid, HA. These two are a conjugate pair.

Ammonia is a base because it is accepting hydrogen ions from the Ammonia is a base because it is accepting hydrogen ions from the water. The ammonium ion is its conjugate acid - it can release water. The ammonium ion is its conjugate acid - it can release that hydrogen ion again to reform the ammonia.The water is that hydrogen ion again to reform the ammonia.The water is acting as an acid, and its conjugate base is the hydroxide ion. acting as an acid, and its conjugate base is the hydroxide ion. The hydroxide ion can accept a hydrogen ion to reform the The hydroxide ion can accept a hydrogen ion to reform the water.Looking at it from the other side, the ammonium ion is an water.Looking at it from the other side, the ammonium ion is an acid, and ammonia is its conjugate base. The hydroxide ion is a acid, and ammonia is its conjugate base. The hydroxide ion is a base and water is its conjugate acid.base and water is its conjugate acid.

Conjugate pairsConjugate pairs

A substance which can act as either an acid or a A substance which can act as either an acid or a base is described as being base is described as being amphotericamphoteric..

Amphoteric substancesAmphoteric substances

The theoryThe theory• An acid is an electron pair An acid is an electron pair

acceptor.acceptor.• A base is an electron pair A base is an electron pair

donor.donor.The relationship between the The relationship between the

Lewis theory and the Lewis theory and the Bronsted-Lowry theoryBronsted-Lowry theory

Lewis basesLewis basesIt is easiest to see the It is easiest to see the

relationship by looking at relationship by looking at exactly what Bronsted-exactly what Bronsted-Lowry bases do when they Lowry bases do when they accept hydrogen ions. accept hydrogen ions. Three Bronsted-Lowry Three Bronsted-Lowry bases we've looked at are bases we've looked at are hydroxide ions, ammonia hydroxide ions, ammonia and water, and they are and water, and they are typical of all the rest.typical of all the rest.

The Lewis Theory of acids and The Lewis Theory of acids and basesbases

Lewis acidsLewis acidsLewis acids are electron pair acceptors. In the above example, the Lewis acids are electron pair acceptors. In the above example, the

BFBF33 is is acting as the Lewis acid by accepting the nitrogen's lone pair. On acting as the Lewis acid by accepting the nitrogen's lone pair. On

the the Bronsted-Lowry theory, the BFBronsted-Lowry theory, the BF33 has nothing remotely acidic about has nothing remotely acidic about

it.it.This is an extension of the term This is an extension of the term acidacid well beyond any common well beyond any common

use.use.

A final comment on Lewis acids and bases:A final comment on Lewis acids and bases:• A Lewis acid is an electron pair acceptor.A Lewis acid is an electron pair acceptor.• A Lewis base is an electron pair donor.A Lewis base is an electron pair donor.

Conjugate pairsConjugate pairs

Fundamental Role in synthesis, analytical behavior, reactivity Fundamental Role in synthesis, analytical behavior, reactivity including phsyiological behavior.including phsyiological behavior.

General Equation:General Equation:

Strengths of Acids:Strengths of Acids:1.1. Inverse relationship ruleInverse relationship rule

Acid/Base ChemistryAcid/Base Chemistry

2. Periodic effects:2. Periodic effects:a.a. Acidity increases from left to right: CH < NH < OH < FHAcidity increases from left to right: CH < NH < OH < FHb.b. Acidity increases from top to bottom: HI > HBr > HCl > HF; HAcidity increases from top to bottom: HI > HBr > HCl > HF; H22S > S >

HH22OOAcid Strength depends on the ability of the conjugate base to Acid Strength depends on the ability of the conjugate base to

stabilize a negative charge.stabilize a negative charge.a)a) Presence of electronegative elements:Presence of electronegative elements:


2. Resonance stabilization: 2. Resonance stabilization:


Measure of Acid/Base StrengthMeasure of Acid/Base StrengthAqueous systems - pKa, pKbAqueous systems - pKa, pKbScale: 0 to 14, neutral = 7Scale: 0 to 14, neutral = 7


Base strength is often reported in terms of pKa which is the Base strength is often reported in terms of pKa which is the strength of the conjugate acid.strength of the conjugate acid.

Example: Pyridine, pKa = 5.19 (Merck Index)Example: Pyridine, pKa = 5.19 (Merck Index)

Rank the following compounds in order of relative basicity:Rank the following compounds in order of relative basicity:

• Resonance stabilization Cyclohexylamine vs. anilineResonance stabilization Cyclohexylamine vs. aniline


Basicity trends for amines:Basicity trends for amines:Amides are very weakly basic (pKa = -1)Amides are very weakly basic (pKa = -1)Solvation effects: Solvation effects:

Explanation: (CHExplanation: (CH33))33NHNH++ is less solvated in H is less solvated in H22OO


Amino acids as zwitterionsAmino acids as zwitterionsZwitterions in simple amino acid solutions. An amino acid Zwitterions in simple amino acid solutions. An amino acid

has both a basic amine group and an acidic carboxylic has both a basic amine group and an acidic carboxylic acid group.acid group.

THE ACID-BASE BEHAVIOUR OF AMINO ACIDSTHE ACID-BASE BEHAVIOUR OF AMINO ACIDS

Amino acids as zwitterionsAmino acids as zwitterionsZwitterions in simple amino acid solutions. An amino acid Zwitterions in simple amino acid solutions. An amino acid

has both a basic amine group and an acidic carboxylic has both a basic amine group and an acidic carboxylic acid group. There is an internal transfer of a hydrogen acid group. There is an internal transfer of a hydrogen ion from the -COOH group to the -NHion from the -COOH group to the -NH22 group to leave an group to leave an ion with both a negative charge and a positive charge.ion with both a negative charge and a positive charge.

This is called a This is called a zwitterionzwitterion..


Adding an alkali to an amino acid solutionAdding an alkali to an amino acid solutionIf you increase the pH of a solution of an amino acid by If you increase the pH of a solution of an amino acid by

adding hydroxide ions, the hydrogen ion is removed adding hydroxide ions, the hydrogen ion is removed from the -NHfrom the -NH33

++ group. group.

You could show that the amino acid now existed as a negative ion You could show that the amino acid now existed as a negative ion using using electrophoresiselectrophoresis..


Adding an acid to an amino acid solutionAdding an acid to an amino acid solutionIf you decrease the pH by adding an acid to a solution of If you decrease the pH by adding an acid to a solution of

an amino acid, the -COOan amino acid, the -COO-- part of the zwitterion picks up part of the zwitterion picks up a hydrogen ion.a hydrogen ion.

This time, during electrophoresis, the amino acid would This time, during electrophoresis, the amino acid would move towards the cathode (the negative electrode).move towards the cathode (the negative electrode).


Adding an acid to an amino acid solutionAdding an acid to an amino acid solutionIf you decrease the pH by adding an acid to a solution of an If you decrease the pH by adding an acid to a solution of an

amino acid, the -COOamino acid, the -COO-- part of the zwitterion picks up a part of the zwitterion picks up a hydrogen ion.hydrogen ion.

This time, during electrophoresis, the amino acid would This time, during electrophoresis, the amino acid would move towards the cathode (the negative electrode). The move towards the cathode (the negative electrode). The pH at which this lack of movement during pH at which this lack of movement during electrophoresis happens is known as the electrophoresis happens is known as the isoelectric isoelectric pointpoint of the amino acid. This pH varies from amino acid of the amino acid. This pH varies from amino acid to amino acid.to amino acid.


Why is phenol acidic?Why is phenol acidic?Unlike alcohols (which also contain an -OH group) phenol Unlike alcohols (which also contain an -OH group) phenol

is a weak acid. A hydrogen ion can break away from the is a weak acid. A hydrogen ion can break away from the -OH group and transfer to a base.-OH group and transfer to a base.

For example, in solution in water:For example, in solution in water:

Phenol is a very weak acid and the position of equilibrium Phenol is a very weak acid and the position of equilibrium lies well to the left. Phenol can lose a hydrogen ion lies well to the left. Phenol can lose a hydrogen ion because the phenoxide ion formed is stabilised to some because the phenoxide ion formed is stabilised to some extent. The negative charge on the oxygen atom is extent. The negative charge on the oxygen atom is delocalised around the ring. The more stable the ion is, delocalised around the ring. The more stable the ion is, the more likely it is to form.the more likely it is to form.

THE ACIDITY OF PHENOLTHE ACIDITY OF PHENOL

One of the lone pairs on the oxygen atom overlaps with the One of the lone pairs on the oxygen atom overlaps with the delocalised electrons on the benzene ring.delocalised electrons on the benzene ring.


This overlap leads to a delocalisation which extends from This overlap leads to a delocalisation which extends from the ring out over the oxygen atom. As a result, the the ring out over the oxygen atom. As a result, the negative charge is no longer entirely localised on the negative charge is no longer entirely localised on the oxygen, but is spread out around the whole ion.oxygen, but is spread out around the whole ion.


the arrhenius theory of acids and bases the theory acids are substances which produce hydrogen ions...

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