Acids & BasesChpt. 12

When we think of acids and bases we tend to think of chemistry lab acids and bases like

But we are surrounded by acids and bases in the world. Most of them are weak.

Lab Acids Lab BasesHCl – hydrochloric acid

NaOH – sodium hydroxide

H2 SO4 – sulphuric acid

Na2 CO3 – sodium carbonate

HNO3 – nitric acid

NH3 – ammonia

What is an Acid and a Base???Acidus – latin for sour

Properties of an AcidTastes sourTurns litmus paper redHas a pH of less than 7React with metals to produce hydrogen gas Examples:

Car battery – sulphuric acidVinegar – acetic acidOranges/Lemons – citric acidVitamin C – ascorbic acidSoap – stearic acidFood preservative – benzoic acidCarbonic acid – fizzy drinks

Acids cause:

•Lemons to be sour

•Acid rain to eat away at sculptures

•Framed paintings to be damaged

•Cavities in your teeth

•Digest food in your stomach

•Fight off other invading ants

Properties of a Base •Turns litmus paper blue•Has a pH greater than 7•Taste bitter and have a slippery feel*•Bases that are soluble (dissolve in water) are called alkalis•Examples:

Oven cleaners – caustic soda (NaOH)Milk of Magnesia – magnesium hydroxide

[Mg(OH)2]Limewater – calcium hydroxide [Ca(OH)2]

*Note: Many oven/drain cleaners contain soluble bases as degreasing agents which convert the oil/grease in your cooker/drains into soluble soaps which are easily washed away

*Note: you must know two examples of household uses for acids and bases

Svante Arrhenius:In 1887 put forward one of the first theories about acids and bases

Dissociation: is the process whereby a molecule is split into simpler fragments which may be smaller molecules, atoms, free radicals or ions

Arrhenius’ definition of an acid:

An acid is a substance that dissociates in water to produce Hydrogen ions (H+)

Arrhenius stated that an acid is a neutral molecule (HX) which dissociates in water to from a hydrogen ion and an anion:

HX H+ + X-

The acidic properties of the solution being due to the presence of H+ ions.

Examples of an Arrhenius acid:

When hydrogen chloride, nitric acid and sulphuric acid, phosphoric acid are added to water the following occurs:

HCl H+ + Cl-

HNO3 H+ + NO3-

H2SO4 2H+ + SO42-

H3PO4 3H+ + PO43-

Monobasic

Tribasic

Dibasic

Monobasic

Monobasic acid – donate one H+ ion in solution

Dibasic acid – donate two H+ ions in solution

Tribasic acid – donate three H+ ions in solution

Strong and Weak acids

Strong acids – dissociate fully in aqueous solution i.e. almost every molecule breaks up to give H+ ions

Examples include: HCl, HNO3, H2SO4

Weak acids – do not fully dissociate in aqueous solution

Examples include: ethanoic acid, methanoic acid

The H+ Ion

A H+ ion is simply a single proton. Arrhenius did not realise that it was impossible for a bare proton to exist independently in solution.

It is now known that the H+ ion reacts with water molecules to form the H3+ ion known as:

*hydronium ion*hydroxonium ionoxonium ion

Dative bond between the oxygen atom and the proton i.e. a bond where both electrons come from the one atom, in this case oxygen

More accurate account of what is happening when acids dissociate in water:

HA + H2 O A- + H3O+

HCl + H2 O Cl- + H3 O+

Arrhenius’ definition of a base:

A base is a substance that dissociates in water to produce Hydroxide ions (OH-)

Arrhenius stated that a base (MOH) dissociates in water to form a hydroxide ion and a cation:

MOH OH- + M+

The basic properties of the solution are due to the presence of OH- ions.

Examples of an Arrhenius base:

When sodium hydroxide, magnesium hydroxide and calcium hydroxide are added to water the following occurs:

NaOH Na+ + OH-

Mg(OH)2 Mg2+ + 2OH-

Ca(OH)2 Ca2+ + 2OH-

Strong and Weak bases

Strong bases – dissociate fully in aqueous solution

Examples include: NaOH

Weak bases – do not fully dissociate in aqueous solution

Examples include: Ca(OH)2 , Mg(OH)2

An alkali: is a base that dissolves in water

Neutralisation according to Arrhenius Theory

Neutralisation of a strong acid (HCl) with a strong base (NaOH) results in the following:

In solution:• Strong acid, HCl, is fully dissociated into H+ and Cl- ions• Strong base, NaOH, is fully dissociated into Na+ and OH- ions• When both solutions are mixed, the H+ ions react with the OH- ions:

H+ + OH- H2O (Half eqn.)

• Na+ and Cl- ions do NOT react

Limitations of Arrhenius Theory

• Hydronium ions exist in solution not H+ ions as suggested by Arrhenius

• Arrhenius restricted his definitions to aqueous (water based) solutions

• Not all acid-base reactions require water as suggested by Arrhenius e.g. rxn of ammonia gas with hydrogen chloride gas

The Bronsted

-Lowry Model

In 1923, Bronsted and Lowry proposed a broader theory of acids and bases:

Bronsted-Lowry definition of an acid:

An acid is a proton donor

Bronsted-Lowry definition of a base:

A base is a proton acceptor

Strengths of Acids and Bases

Acids:• Stronger acid more readily donates a proton• Weaker acid less readily donates a proton

Bases:• Stronger base more readily accepts a proton• Weaker base less readily accepts a proton

Acid-Base reactions according to the Bronsted-Lowry Theory

An acid is a proton donor (H+)In general:HA (aq) + H2O (l) H3O+ (aq) + A- (aq)

B/L acid B/L base

Example A:HCl (aq) + H2O (l) H3O+ (aq) + Cl- (aq)

B/L acid B/L base

A base is proton acceptorIn general:

A- (aq) + H2O (l) HA (aq) + OH- (aq)

B/L base B/L acid

Example B:

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

B/L base B/L acid

Amphoteric (amphiprotic) nature of water

In equation A the water is acting as a base because it accepts a proton, but in equation B the water is acting as an acid as it donates a proton.

Thus water is amphoteric, it can act as either an acid or a base depending on circumstances.

A substance which can act as either an acid or a base is said to be amphoteric

Bronsted-Lowry Vs. Arrhenius

The Bronsted-Lowry theory of acids and bases may be applied to acid-base reactions that do not involve water as the solvent:

HCl + NH3 NH4+ + Cl-

B/L acid B/L base

No water molecules present therefore no hydronium ions formed

The Bronsted-Lowry theory broadens the range of species that can be defined as acids and bases. Substances that would not be classified as acids or bases in the Arrhenius theory e.g. NH3 are classified as acids or bases in the Bronsted-Lowry theory.

Conjugate Acid-Base PairsEthanoic acid, CH3COOH, is the acid found in vinegar. Let us look more closely at the reaction that occurs when ethanoic acid is added to water:

CH3COOH + H2O H3O+ + CH3COO-

This reaction can also take place in the reverse direction:Reverse Rxn.:

H3O+ + CH3COO- CH3COOH + H2O

Forward Rxn.:

This symbol is used to indicate that a reaction can take place in both directions – forward and reverse:

CH3COOH + H2O H3O+ + CH3COO-

B/L acid B/L base

CH3COOH + H2O H3O+ + CH3COO-

Forward Rxn:

Reverse Reaction:

B/L acid B/L base

When you dissolve an acid in water, the acid donates a proton to a water molecule to form a new acid (conjugate acid) and a new base(conjugate base)

CH3COOH + H2O H3O+ + CH3COO-

Acid Base Conjugate Conjugate Acid Base

An acid changes into a conjugate base when it donates a proton (H+)

A base changes into a conjugate acid when it accepts a proton (H+)

A conjugate acid-base pair is any pair consisting of an acid and a base that differ by one proton (H+):

CH3COOH and CH3COO-

H2O and H3O+

In each of these pairs the acid and base only differ by a proton i.e. conjugate acid-base pairs

Which is the acid and which is the base in these equations?

HCl (aq) + H2O (l) H3O+ (aq) + Cl- (aq)

acid base Conjugate acid

Conjugate base

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

base acid Conjugate acid

Conjugate base

HCl + NH3 NH4+ + Cl-

Acid Base Conjugate acid

Conjugate base

Write conjugate acid / base pairs for the following:a. HClO4

b. H3PO4

c. H2SO4

d. CH3NH3+

Which of the following are acid-base pairs?

a. H2O, H3O+

b. OH- ,HNO3

c. HC2H3O2 , C2H3O2-

d.* H2SO4, SO4 2-

ClO4 -

H2PO4 -

HSO4 -

CH3NH2

Yes

No

Yes

No

Example 1:What is

i) the conjugate acid of HC2O4-

ii) the conjugate base of HC2O4-

Example 2:Indicate which species acting as acids and which are acting as bases:

HNO3 + H2F2 H2NO3+ + HF2

-

Also indicate the conjugate acid-base pairs

Conjugate Acid-Base Pairs

Reactions lead preferentially to the production of the weaker acid and base:

HNO3 + H2O H3O+ + NO3-

the above reaction goes mainly to the right

The stronger an acid the weaker its conjugate base

The stronger a base the weaker its conjugate acid

Stronger Acid

Weaker Base

Stronger Base

Weaker Acid

Complete the following questions:

Book: 12.2 – 12.6

Neutralisation

When an acid and a base react with each other in the right proportions they cancel each other out.

Since the solution formed has no effect on litmus the process is called neutralisation

Neutralisation is the reaction between an acid and a base to form a salt and water

ACID + BASE SALT + WATER

A salt: is the substance formed when the hydrogen in an acid is replaced by a metal or an

ammonium ion.

A closer look: Let us consider the rxn between HCl and NaOH:

HCl + NaOH NaCl + H2OIn terms of the ions formed:

H+ + Cl- + Na+ + OH- Na+ + Cl- + H2OIf we remove ions common to both sides of eqn. (spectator ions):

We can now see that neutralisation actually occurs because the hydrogen ions from the acid react with the hydroxide ions from the base to form water

H+ + OH- H2O

Examples of Neutralisation

1. Medicine

2. Agriculture

3. Environmental protection

4. Miscellaneous

1. Medicine:

Hydrochloric acid is produced by the stomach to help digest food. In times of stress etc. there may be an overproduction of HCl which can lead to damaging of the lining of the stomach and ulcers.Relief from excess HCl may be obtained by taking an antacid which contains a base to neutralise the excess HCl:

Alka-Seltzer, Bisodol – contain sodium hydrogencarbonate

(NaHCO3)Milk of Magnesia – contains magnesium

hydroxide(Mg(OH)2)

2. Agriculture: If soil is too acidic (H2SO4) crop yields tend to be low, therefore farmers often spread lime on soil to neutralise the acidity in the soil:

Lime is called calcium oxide, CaO, which reacts with water to form calcium hydroxide (Ca(OH)2), a basic solution

Calcium hydroxide neutralises the acidity in the soil.

3. Environmental Protection

In areas that suffer from acid rain limestone (CaCO3) is added to lakes to neutralise acidity.

In coal burning power plants limestone is often added to the chimneys to neutralise the acidic substances released from the burning of coal

4. Miscellaneous Toothpaste is slightly basic to neutralise the acids in food.

Baking soda is used to neutralise the acidic sting of bees.

Vinegar is used to neutralise the alkaline sting of wasps.

Shampoo is slightly basic which makes hair difficult to manage. Conditioner is slightly acidic to neutralise the base in shampoo making the hair easier to manage.

*Must know two examples of

neutralisation in everyday life