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Modern Theories of Acids & Bases. The Arrhenius and Bronsted-Lowry Theories. Acids & Bases. Acids and bases are special kinds of electrolytes. Like all electrolytes they break up into charged particles.

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modern theories of acids bases
Modern Theories of Acids & Bases

The Arrhenius

and

Bronsted-Lowry Theories

acids bases
Acids & Bases
  • Acids and bases are special kinds of electrolytes. Like all electrolytes they break up into charged particles.
  • What sets them apart from each other, and other electrolytes is the way that they break up.
arrhenius acids
Arrhenius Acids

Svente Arrhenius (who first proposed the theory of ionization) identified acids as substances that ionize in water to produce hydronium ion.

For example:

HCl + H2O  H3O+ + Cl-

Any substance that ionizes in water to produce hydronium ion can be classified as an acid.

h 3 o aka h
H3O+ (aka H+)
  • The hydronium ion is also known as a hydrogen ion.
  • This allows us to shorten the ionization reactions for acids. The following equations represent the same chemical change:

HCl + H2O  H3O+ + Cl-

and

HCl  H+ + Cl-

However, it should be understood that H+ is an abbreviation for the hydronium ion. H+ ions DO NOT exist in water solution but are snatched up by water molecules to form hydronium ions.

arrhenius base
Arrhenius Base

Svente Arrhenius also identified bases as substances that ionize in water to produce hydroxide ion.

For example:

NaOH  Na+ + OH-

Any substance that ionizes in water to produce hydroxide ion can be classified as a base.

salts
Salts

Ionic substances that break up in solution to produce ions other than hydronium and hydroxide ions.

NaCl (s)  Na+(aq) + Cl- (aq)

KNO3(s)  K+ (aq) + NO3- (aq)

Li2SO4 (s)  2 Li+ (aq) + SO42-(aq)

Salts are made up of positive (metal) and negative (non-metallic or polyatomic) ions. The more familiar you become with Table E, the easier it will be for you to identify salts.

salts1
Salts
  • Salts are generally defined as ionic substances that PRIMARILY produce positive and negative ions other than hydronium or hydroxide when they dissolve in water.
practice
Practice

Identify each of the following as acids/bases/salts and show how they ionize:

  • HC2H3O2
  • K2SO4
  • KOH
  • LiOH
  • HNO3

Acid HC2H3O2H+ + C2H3O2-

Salt K2SO4 2K+ + SO42-

Base KOH  K+ + OH-

Base LiOH  Li+ + OH-

Acid HNO3 H+ + NO3-

properties of acids
Properties of Acids

All acids have the following properties:

Neutralize bases to form a salt and water

Have a sour taste (example: citric acid, vinegar)

React with active metals to produce a salt plus hydrogen gas.

Have pH’s less than 7

Affect indicators

Why?

Because all acids have H3O+ ions present!

properties of bases
Properties of Bases

All bases have the following properties:

Neutralize acids to form a salt and water

Have a bitter taste (example: unsweetened chocolate, heroin)

Are slippery. React with fats/oils to form soap (saponification)

Have pH’s greater than 7 / Affect indicators

Why?

Because all bases have OH- ions present!

bronsted lowry acids bases

Bronsted-Lowry Acids & Bases

Another theory of acids & bases

not everyone was happy with arrhenius s definition
Not everyone was happy with Arrhenius’s definition

A major problem with the Arrhenius definition of acids and bases is that it limits acids and bases to water (aqueous) solutions.

Since an acid requires water to ionize and form hydronium ions, there can be no Arrhenius acids unless water is involved as the solvent.

relationship between the two models
Relationship between the two models

Bronsted-Lowry Acids/Bases can exist when no water is present

Arrhenius Acids/Bases only exist in water solutions.

All Arrhenius acids and bases can also be classified as Bronsted-Lowry acids and bases.

bronsted lowry definitions
Bronsted-Lowry Definitions

Bronsted and Lowry felt that this was too limiting, since there are many non-aqueous systems (no water is present).

They came up with the following definitions for acids and bases.

An acid is a proton (H+ ion) donor

A base is a proton acceptor

an example
An example

In the reaction below there are no Arrhenius acids or bases present (because no hydronium ions or hydroxide ions are formed).

However, the HCl is acting as a Bronsted-Lowry acid because it is giving a H+ ion to the NH3 (which is acting as a H+ ion acceptor - a base)

acid bases are roles
Acid & Bases are Roles

In the Bronsted-Lowry definition, substances are classified as acids or bases depending on how they behave in a given situation.

This means that the same substance can act as a acid in one reaction (by donating a proton) while acting as a base in another reaction

hcl as a proton donor
HCl as a proton donor
  • Consider the following reaction:
  • Since the HCl gives up a H+ ion to the water it is acting as a Bronsted-Lowry acid. In the process of donating the proton it also forms a hydronium ion, and that makes it an Arrhenius acid as well.
water as a base
Water as a base
  • But what does that make the water molecule?
  • Since the water molecule is accepting the H+ ion, it is acting as a Bronsted-Lowry base. Since there is no hydroxide ion (OH-) formed, the water is not acting as an Arrhenius base in this reaction.
ammonia as a base
Ammonia as a base
  • Let’s look at another example:
  • Here the ammonia molecule is accepting a H+ ion and therefore is acting as an Bronsted-Lowry base. However, in the process of reacting with the water it is also forming a hydroxide ion. That makes the ammonia an Arrhenius base as well.
but what about the water
But what about the water?

Since the water is giving up a H+ ion, it is acting as a Bronsted-Lowry acid. Since it does not form hydronium ions, it is NOT acting an Arrhenius acid.

so is water an acid or a base
So is water an acid or a base?
  • In one example, we said that water was acting as a base, and in another example we said that it was acting as an acid.
  • Some of you may be confused by this because you are thinking of acids and bases as being like boys and girls. Boys are boys and girls are girls, and they can’t switch back and forth. However, acids and bases are NOT like this.
teacher student model
Teacher-student model
  • Teachers give off information (like acids give off protons)
  • Students accept information (like bases accept protons)
  • Sometimes teachers are students, and sometimes students are teachers
  • Teacher and students are roles that individuals play depending on the situation.
  • Acid and base are roles that molecules play in a particular chemical reaction. In different reactions they may play different roles.
amphoteric amphiprotic
Amphoteric/Amphiprotic
  • Sometimes a molecule can donate a proton (act as an acid) and sometimes it can accept a proton (act as a base).
  • Molecules that have this ability to act as both an acid and a base are called amphotericor amphiprotic.
  • Water is the most common example of an amphoteric substance.
reality check
Reality check

For each of the following reactions identify any Bronsted-Lowry acids and bases.

  • HNO3 + H2O  H3O+ + NO3-
  • HNO3 + NH3 NH4+ + NO3-
  • S2- + H2O  HS- + OH-
  • HS- + OH- S2- + H-OH
  • HS- + HCl  H2S + Cl-

Are any of the substances above amphoteric?

acid

base

acid

base

base

acid

acid

base

base

acid

strong weak acids
Strong/Weak Acids
  • Acids can be either strong electrolytes or weak electrolytes.
  • Strong acids (such as HCl) completely break up into their ions:

HCl (aq)  H+(aq) + Cl-(aq)

  • Weak acids (such as HC2H3O2) only partially break up into their ions:

HC2H3O2 H+ (aq) + C2H3O2-(aq)

Weak acids don’t completely break up because they go to equilibrium!

acid base equilibrium
Acid-base equilibrium

Many acid base reactions go to equilibrium, that is they have both a forward and reverse reactions

For instance, acetic acid (HC2H3O2) reacts with water to form hydronium ion and acetate ion.

the reverse reaction
The reverse reaction

However, the acetic acid only partially ionizes because a reverse reaction takes place preventing the forward reaction from reaching completion.

In the reverse reaction, the Hydronium ion acts as an acid (a proton donor) while the acetate ion acts as the base.

conjugate acid bases
Conjugate Acid-Bases
  • When a substance donates a proton, the substance that is left is its conjugative base:
  • Notice that the conjugative base is accepting a proton in the reverse reaction.

Every acid has a conjugative base

conjugative acid base pairs 2
Conjugative Acid-Base Pairs 2
  • When a substance accepts a proton, the substance that is formed is its conjugative acid:
  • Notice that the conjugative acid is donating a proton in the reverse reaction.

Every base has a conjugative acid

strong weak bases
Strong/Weak Bases
  • Bases can be either strong electrolytes or weak electrolytes.
  • Strong bases (such as NaOH) completely break up into their ions:

NaOH (aq)  Na+(aq) + OH-(aq)

  • Weak bases (such as NH3) only partially break up into their ions:

NH3 (aq) + H2 O  NH4+ (aq) + OH-(aq)

Weak bases don’t completely break up because they go to equilibrium!

types of salts
Types of Salts

Salts can be classified as being:

  • neutral
  • acidic
  • Basic

How a salt is classified depends upon whether secondary reactions between the ions making up the salt and water form either hydronium or hydroxide ions.

determining the type of salt
Determining the type of salt

The type of salt for a particular salt can be determined experimentally by testing the salt solution with universal indicator paper.

  • neutral salts will have a pH of 7
  • acidic salts will have a pH of less than 7
  • basic salts will have a pH of greater than 7

Typically, the pH values of salt solutions will be closer to 7 than that of acids or bases.

predicting the type of salt
Predicting the type of salt

All salts can be considered to be formed from their “parent” acid and base by means of the neutralization reaction.

Acid + Base  Salt + Water

The type of salt can be theoretically predicted based on the properties of the “parent” acid and the “parent” base.

neutral salts
Neutral Salts

Neutral salts are formed from a reaction between a strong acid and a strong base.

(Remember neutralization reactions are double replacement reactions.)

For instance,

HCl + NaOH NaCl + HOH

(since HCl is a strong acid and NaOH is a strong base, NaCl is a neutral salt.)

acidic salts
Acidic Salts

Acidic salts are formed from a strong acid and a weak base.

For instance,

HCl + NH4OH NH4Cl + HOH

(since HCl is a strong acid and NH4OH is a weak base, NH4Cl is an acidic salt.)

basic salts
Basic Salts

Basic salts are formed from a weak acid and a strong base.

For instance,

HC2H3O2 + NaOH NaC2H3O2 + HOH

(since HC2H3O2 is a weak acid and NaOH is a strong base, NaC2H3O2 is a basic salt.)

strong and weak acids
Strong and weak acids

In predicting the type of salt it is helpful to remember the three strong acids:

  • HCl – hydrochloric acid
  • HNO3 – nitric acid
  • H2SO4 – sulfuric acid

Most other acids are weak. Acetic acid (HC2H3O2) is the most common weak aciddiscussed in Regents chemistry.

strong and weak bases
Strong and Weak Bases

For bases, the Group I hydroxides are all strong bases.

LiOH – lithium hydroxide

NaOH – sodium hydroxide

KOH – potassium hydroxide

  • Most other hydroxides can be considered as weak bases. Ammonia or Ammonium hydroxide (NH3 or NH4OH) is the most common weak base discussed in Regents chemistry.
try these
Try these
  • Identify the parent acid and base for the following salts, identify them as strong or weak and predict the type of salt formed.