Acids and Bases

ClassAct SRS enabled.

In this presentation you will: Explore acids and bases, the

pH scale and buffers.

Acids and Bases

For example, acids are used to unblock waste pipes in the home and to remove oxide from metals before they are painted. Bases are used as household cleaning products and the manufacture of both paper and artificial fibers.

Acids and bases are among the most common chemicals in our everyday lives, as well as being among the most important chemicals for laboratories and industry.

There is a huge range of different types of acids and bases, but each group has common properties. We will learn about some of these properties in this presentation.

Next >

Acids and Bases

In 1884, a Swedish chemist called Svante Arrhenius established the first comprehensible theory of the behavior of acids and bases in solution.

Theories of Acids and Bases

He proposed that, when dissolved in water, acids, bases and salts split or dissociate into charged particles called ions, becoming electrolytes due to the good electric conduction of ion solutions.

+-Water +

IonsAcid, BaseOr Salt

Next >

Acids and Bases

Theories of Acids and Bases

Nowadays we know that acid solutions do not contain simple H+ ions but hydrated protons, H3O+ called oxonium ions.

Acids are considered to be electrolytes that produce the hydrogen ion, H+.

Bases are substances which produce the hydroxide ion, OH-.

According to Arrhenius:

hydrogen

Water +

Acid

-

Basehydroxide

Acid

+

Next >

Acids and Bases

1Question

A) Electrolyte ions

B) Dissociated ions

C) Hydrogenous ions

D) Oxonium ions

What is the name given to the hydrated protons H3O+ that react in a acid-base reaction?

Acids and Bases

Many acid-base reactions occur in solvents other than water, or even with no solvent at all.

Theories of Acids and Bases

One such reaction occurs between hydrogen chloride (acid) and ammonia (base). These two gases react together to form ammonium chloride.

HCI(g) + NH3(g) → NH4Cl(s)

+

Next >

Acids and Bases

In 1923, chemists J. N. Brønsted and T. M. Lowry defined acids as proton donors and bases as proton acceptors.

Theories of Acids and Bases

Consequently, in the reaction between hydrogen chloride and ammonia, the hydrogen chloride molecule can be seen as the proton donor and the ammonia molecule as the proton acceptor.

NH3(g) + HCI(g) → NH4Cl(s)

Transferred Proton

Next >

Acids and Bases

2

How does the Brønsted-Lowry model defines acids substances?

Question

A) As electrolytes that produced the hydrogen ion H+ when dissolved in water.

B) Acids are substances which act as proton donors.

Acids and Bases

Some substances can react as both acids and bases depending what they are reacting with.

Amphoteric Substances

An example of this is water, which behaves as an acid when it reacts to ammonia (aqueous solution), donating a proton.

Conversely, it acts as a base with hydrogen chloride, receiving a proton in the reaction.

H2O(l) + NH3(aq) → OH-(ac) + NH4

+(aq)

Transferred Proton

Substances behaving as a base in some reactions and as an acid in others are called amphoteric.

H2O(l) + HCl(aq) → H3O+(ac) + Cl-(aq)

Transferred Proton

Next >

Acids and Bases

3

What is an amphoteric substance?

Question

A) An acid which reacts with substances other than water.

B) A substance that can react both as an acid and as a base.

C) A substance that does not react neither as an acid or as a base.

Acids and Bases

+

In 1923, G. N. Lewis proposed a new alternative model to explain acids–base reactions:

Lewis Model

An acid is a substance able to accept and share a pair of electrons and a base is a substance able to donate and share a pair of electrons.

Lewis definitions expanded acids and bases models beyond both Arrhenius’s and Brønsted-Lowry’s models.

For example, ammonia shares a pair of nitrogen electrons when mixed with boron trifluoride.

Acid Base Acid-Base

Next >

Acids and Bases

4

Is the following statement true or false?

'Lewis model proposes that an acid is a substance able to accept and share a pair of electrons and a base is a substance able to donate and share a pair of electrons'.

Answer True or False.

Question

Acids and Bases

These three theories, Arrhenius’s, Brønsted-Lowry’s and Lewis’s, explaining acid-base reactions are not contradictory but each expands the previous model by adopting a wider perspective.

Lewis Model

For example, the ion OH-, recognised as a base by Arrhenius, is a base too according to Brønsted-Lowry, since it is a proton receptor. Furthermore, this ion is a Lewis base since it is an electron pair donor.

Next >

Acids and Bases

5

Which of the following statements is NOT true?

Question

A) Arrhenius’s, Brønsted-Lowry’s and Lewis’s models are not contradictory.

B) Arrhenius’s, Brønsted-Lowry’s and Lewis’s models each expands the previous model adopting a wider perspective.

C) The latest theories, Lewis’s model, invalidates all previous definitions about acid-base reactions.

Acids and Bases

+

+

-+ -+

Neutralization

The reaction of an acid with a base, mixed in the right amounts, produces a substance which has lost the properties of the original substances. This reaction is called neutralization.

These salts will be formed by the cation (positive ion) of the base and the anion of the acid.

Some acids react with bases to form salts and water.

For example, hydrochloride acid reacts with potassium hydroxide, a base, to form water and potassium chloride, a salt.

HCl + KOH → KCl + H2O

Neutralization of an Acid and a Base

Next >

Acids and Bases

6

The reaction of an acid with a base mixed in the right amounts, produces a substance which has lost the acidic and basic properties of the original substances. What is this reaction called?

Question

A) Amphoteric

B) Dissociation

C) Donation

D) Neutralization

Acids and Bases

Acids and Bases Strength

Acids and bases are classified as strong and weak, depending on the degree in which they ionize in a solution.

The strongest acids and bases ionize totally while weaker ones ionize partially.

Therefore, the strength of an acid can be measured by its dissociation when transferring a proton to water, producing the oxonium ion and the strength of a base can be measured by the degree of acceptance of a proton from water.

In a strong acid, the attraction between H+ and A- is low, resulting in complete ionization.

In a weak acid, the attraction between H+ and A- is high, resulting in partial ionization.

Next >

Acids and Bases

Fertilizers HNO3 Nitric acid

VinegarC2H402Acetic acid

AspirinC9H8O4Acetylsalicylic acid

Lemon juice C6H8O7Citric acid

Gastric juicesHCIHydrochloric acid

BatteriesH2SO4Sulfuric acid

Present inFormulaAcids

Acids and Bases Strength

Examples of strong acids are sulfuric acid H2SO4 and nitric acid HNO3.

The terms strong and weak should not be confused with the terms concentrated and dilute.

Strong and weak refer to the extent of dissociation of an acid or a base.

Concentrated and dilute refer to the amount of acid or base in a given volume of water.

Next >

Acids and Bases

7

What does the strength of an acid or base depends on?

Question

A) The degree of dissociation in which they ionize.

B) The amount of acid or base in a given volume.

C) Their degree of neutralization.

Acids and Bases

pH Scale

Acids and bases can be measured with the pH scale which specifies the concentration of hydrogen ions in a solution, [H+].

A neutral solution like pure water has a pH of 7, the strongest acids will have a pH of 0 and the stronger bases a pH of 14.

A change in the acidity or basicity of a solution of one whole pH unit, represents a change of actually 10 times in the concentration of hydrogen ions in the solution.

Household Lye

Bleach

Ammonia

BoraxBaking SodaSea WaterBloodDistilled WaterMilkCorn

Boric AcidOrange Juice

VinegarLemon Juice

Battery Acid

pH values of some common substances

H+ concentration moles per liter

Milk of Magnesia

Next >

Acids and Bases

8

The concentration of hydrogen ions in a solution with a pH of 1 is...?

Question

A) Twice the concentration of a solution with a pH 2.

B) Half the concentration of as a solution with a pH 2.

C) Ten times the concentration of a solution with a pH 2.

D) A tenth of the concentration of a solution with a pH 2.

Acids and Bases

Buffer Solutions

Buffers are solutions with a pair of substances that resist a change in pH when small amounts of acid or base are added so that they keep a nearly constant pH.

They contain a reservoir of the acidic component to neutralize OH- ions and a reservoir of basic component to neutralize H+ ions.

Thus, it is essential that these two components must be able to coexist in a solution without completely neutralizing each other.

Often buffers are made of weak acid-base conjugate pairs, for example, acetic acid, C2H4O2 with its acetate base C2H3O2

-. Acetic Acid

C2H4O2

BaseC2H3O2

Next >

Acids and Bases

9

What are buffer solutions?

Question

A) Solutions capable of maintain their pH when an acid or a base is added to them.

B) Solutions which change drastically their pH when an acid or a base is added to them.

Acids and Bases

Show knowledge and understanding about acids and bases and their models.

After completing this presentation you should be able to:

Summary

Show knowledge and understanding about the strength of acids and bases.

Show knowledge and understanding about neutralization.

Show knowledge and understanding of buffers.

Show knowledge and understanding of the pH scale.

End >