ACIDS AND BASES

Acids (from the Latin acidus/acēre meaning sour)

Why learn about acids?

Guess the names of the following acids!

Used to ferment milk Named after the latin name of ants; often

found in ant stings Found in fizzy drinks Also called vitamin C Found in the nucleus of eukaryotic cells Found in car batteries Found in your stomach Found in oranges and lemons

What are acids? 3 definitions:

1. Arrhenius definition2. Bronsted-Lowry definition3. Lewis definition

Arrhenius definition Acids are substances which increase the

concentration of hydronium ions (H3O+) in solution (water).

In this definition, all acids must contain hydrogen.

Does it mean that all compounds which contain hydrogen are acids?

Name some examples of compounds containing hydrogen but are not acids.

The hydronium ions are responsible for the properties of acids!

Textbook definition Acids are substances which produce

hydrogen ions, H+, when dissolved in water.

Properties of acids Acids have a sour taste. It is the hydronium ions interacting with

the taste receptors that give the taste of sourness.

Acid molecules (Recall) Are HCl, HNO3 and H2SO4 covalent

molecules? Are they able to conduct electricity?

Properties of Acids Acids dissolve in water to form

solutions which conduct electricity. Take a look at HCl dissolved in water.

What are the ions present in the water?

HO

H

ClH

-+

Properties of Acids For simplicity’s sake, we often just say

that HCl dissociates in water to form H+

ions and Cl- ions. H Cl

H ClH+ Cl-

Properties of acids Dilute acids are irritants, and cause skin

to blister or redden. Concentrated acids are very corrosive.

Properties of acids What about HCl, HNO3 and H2SO4

dissolved in organic solvents like acetone?

H+

Cl-Cl-

H+

H+

Cl-

ClH

ClH

ClHAcetone Water

Properties of Acids Acids turn blue litmus paper red. What happens here? Litmus is a mixture of dyes that turns red

under acidic conditions and blue under basic conditions. What colour do you think neutral litmus paper is?

Properties of Acids Acids react with reactive metals to

form hydrogen and a salt. metal + acid → salt + hydrogen What is meant by a reactive metal?

Reactivity seriesPleaseStopCallingMeAZebraILoveHappyCatsSellingGoldfish

PotassiumSodiumCalciumMagnesiumAluminiumZincIronLeadHydrogenCopperSilverGold

Mor

e an

d m

ore

reac

tive

Reaction of metals with acids 3 widely used acids:

1. Hydrochloric acid – salts formed are chlorides2. Nitric acid – salts formed are nitrates3. Sulfuric acid – salts formed are sulfates

metal + acid → salt + hydrogen Examples

magnesium + hydrochloric acid → ? Mg(s) + HCl(aq) → MgCl2(aq) +H2(g)

How to test that the gas produced is hydrogen? Lighted splint test.

Reaction of metals with acids Copper, silver and gold do not react with dilute

acids. Copper reacts with concentrated nitric acid,

but no hydrogen is formed, hence it is not the typical kind of metal-acid reaction. Instead NO2 gas is formed.

Think: Lead is above hydrogen in the Reactivity series. Can you reason out why it shows no visible reaction with sulfuric acid and hydrochloric acid, but reacts with nitric acid?Cu(s)  +  4HNO3(aq)  ——>  Cu(NO3)2(aq)  +  2NO2(g)  +  2H2O(l)

Chemical equations calcium + hydrochloric acid → ? zinc + nitric acid → ? sodium + sulfuric acid → ?

Properties of acids Acids react with carbonates and

hydrogencarbonates to give a salt, carbon dioxide and water.

acid + carbonate → carbon dioxide + salt + water E.g. hydrochloric acid + sodium carbonate → ? 2HCl(aq) + Na2CO3(s)→ 2NaCl(aq) + CO2(g) + H2O(l) What happens if there is not enough HCl? HCl(aq) + Na2CO3(s)→ NaCl(aq) + NaHCO3(aq) NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l)

Properties of acids How to test for carbon dioxide? Limewater test – White precipitate

formed. What is this white precipitate?

Chemical equations nitric acid + calcium carbonate → ? hydrochloric acid + potassium carbonate → ? sulfuric acid + magnesium carbonate → ?

Properties of acids Acids react with metal oxides or metal

hydroxides to form a salt and water only.

acid + metal oxide/hydroxide → salt + water E.g. hydrochloric acid + sodium oxide → ? 2HCl(aq) + Na2O (s)→ 2NaCl(aq) + H2O(l) E.g. nitric acid + copper(II) hydroxide → ? 2HNO3(aq) + Cu(OH)2(aq)→ Cu(NO3)2(aq) + 2H2O(l)

Summary of properties of acids

Properties of AcidsAcids are sour

Acids are corrosiveAcids dissociate in water to form solutions that

conduct electricityAcids turn blue litmus red

Acids react with reactive metals to give salt and H2 gas

Acids react with carbonates/hydrogencarbonates to give CO2, salt and H2O

Acids react with metal oxides/ hydroxides to give salt and water

Basicity of acids The maximum number of hydrogen ions

produced by/ number of replaceable hydrogen atoms in a molecule of acid is called the basicity of the acid.

E.g. HCl is monobasic, so it has only 1 replaceable hydrogen.

E.g. H2SO4 is dibasic, so it has 2 replaceable hydrogens.

E.g. H3PO4 is tribasic, so it has 3 replaceable hydrogens.

Write chemical equations for the reaction of sodium hydroxide with these 3 acids.

Basicity of acids Take a look at ethanoic acid. CH3COOH(aq) H+(aq) + CH3COO-(aq) How many hydrogen atoms does it contain? What is its basicity?

Importance of acid reactions in water

A piece of magnesium placed in a solution of HCl in a dry organic solvent shows no visible reaction.

When the same piece of magnesium is placed in a solution of HCl dissolved in water (what do you call this solution?), H2 gas is produced. Why?

Importance of acid reactions in water

When the hydrogen chloride molecule dissociates, it is the H+ ions that react with reactive metals and not the chloride ions, Cl-.

For instance, when HCl(aq) reacts with magnesium, 2H+(aq) + Mg(s) → H2(g)+ Mg2+(aq)

Importance of acid reactions in water

Similarly, it is the H+ ions that react with carbonate ions to give water and carbon dioxide gas.

For instance, when HCl(aq) reacts with sodium carbonate, 2H+(aq) + CO3

2-(aq) → CO2(g)+ H2O(l)

Alka Seltzer tablets These tablets contain sodium carbonate,

potassium carbonate and citric acid. Why is it that they can exist as a tablet without reacting?

Weird acids Aqua Regia – royal water Formed by mixing concentrated nitric

acid and concentrated hydrochloric acid in 1:3 ratio

Aqua Regia dissolves gold!

Uses of acids Industrial products

E.g. fertilizers Food preservation

E.g. ethanoic acid (vinegar) for pickling of vegetables

Sulfuric Acid Manufacturing of sulfuric acid via

Contact process. Sulfuric acid is used in:

Manufacturing detergents, paints, dyes, plastics

Manufacturing fertilisers e.g ammonium sulfate

Batteries for vehicles Making synthetic fibres, paints and

pigments

Contact Process 1. Sulfur is burnt in oxygen to form

sulfur dioxide. S(s) + O2(g) → SO2(g) 2. Sulfur dioxide is further reacted with oxygen to

form sulfur trioxide using vanadium (V) oxide, V2O5 as a catalyst at high temperature (400-500oC) and medium pressure (1-2 atm).

2SO2(g) + O2(g) → 2SO3(g)

Contact Process 3. The sulfur trioxide is then dissolved in

concentrated sulfuric acid to give a fuming liquid called oleum, H2S2O7.

SO3(g) + H2SO4(l) → H2S2O7(l) 4. Water is added to oleum to form concentrated

sulfuric acid. H2O(l) + H2S2O7(l) → 2H2SO4(l)

Uses of sulfuric acid Manufacture of fertilisers E.g. Ammonium sulfate. ammonia + sulfuric acid → ammonium sulfate + water 2NH3(aq) + H2SO4(aq) → (NH4)2SO4(aq) E.g. Superphosphate is manufactured by reacting calcium

phosphate with conc. sulfuric acid. Calcium phosphate + sulfuric acid → calcium

dihydrogen phosphate + calcium sulfate Ca3(PO4)2(s) + 2H2SO4(aq) → Ca(H2PO4)2(s) + 2CaSO4(s) The product of the combination of 2 calcium salts is called

superphosphate!

Uses of sulfuric acid Manufacture of detergents Treating hydrocarbons with concentrated

sulfuric acids give organic acids. These organic acids are then neutralized

with sodium hydroxide to give the detergent.

Uses of sulfuric acid As battery acids in cars Dilute sulfuric acid, lead plates, and

lead(IV) oxide plates are fitted in batteries.

When sulfuric acid, lead and lead(IV) oxide react, electrical energy is produced.

What are bases? 3 definitions:

1. Arrhenius definition2. Bronsted-Lowry definition3. Lewis definition

Textbook definition Bases are metal oxides or metal

hydroxides. Bases are substances which react with

acids to give a salt and water only. Bases which are soluble in water are

called alkali and they produce hydroxide ions (OH-) in water (Arrhenius definition).

Guess these alkalis/bases Drink this when you have indigestion Smells like urine Also called caustic soda Also called lime or quicklime Also called slaked lime Rust

Examples of basesBase FormulaSodium oxide Na2OAluminium hydroxide Al(OH) 3

Copper(II) oxide CuOMagnesium hydroxide Mg(OH)2

Zinc oxide ZnOFor instance, when HCl(aq) reacts with zinc oxide, 2HCl(aq) + ZnO(s) → H2O(l)+ ZnCl2(aq)

Properties of alkalisArabic al qalīy = the calcined ashes, referring to the original source of alkaline substance

Alkalis have a bitter taste and soapy feel.

It is the reaction of hydroxide with the fats on our skin that gives the soapy feel.

Properties of alkalis Alkalis turn red litmus paper blue.

Properties of alkalis Alkalis produce hydroxide ions when

dissolved in water. Note: The only hydroxides that are

soluble are: Group I hydroxides, barium hydroxide and calcium hydroxide (sparingly soluble).

Alkalis FormulaSodium hydroxide NaOHPotassium hydroxide KOHBarium hydroxide Ba(OH)2

Lithium hydroxide LiOHAmmonia (aqueous) NH3

Topics today Alkalis and properties (continued) Strength and concentration of acids pH scale pH indicators pH and soil (oxides)

Properties of alkalis Why is aqueous ammonia an alkali? ammonia + water → ammonium ion +

hydroxide ion NH3

(g) + H2O(l) → NH4+(aq) + OH-(aq)

Properties of alkalis Alkalis react with acids to form a salt

and water only. This reaction is called neutralization. This is represented as follows: H+(aq) + OH-(aq) → H2O(l) or H3O+(aq) + OH-(aq) → 2H2O(l) Neutralization can take place between

bases and acids too.

Properties of alkalis Consider these reactions: manganese(IV) oxide + hydrochloric acid

→ manganese(II) chloride + water + chlorine MnO2(s) + 4HCl(aq) → MnCl2(aq) + 2H2O(l) +

Cl2(g) manganese(II) oxide + hydrochloric acid

→ manganese(II) chloride + water MnO (s) + 2HCl(aq) → MnCl2(aq) + H2O(l) Which one is a neutralization reaction?

Properties of alkalis Alkalis heated with ammonium salts give

off ammonia gas. alkali + ammonium salt → ammonia + water

+ salt E.g. calcium hydroxide + ammonium chloride →

calcium chloride + water + ammonia Ca(OH)2(aq) + 2NH4Cl(aq) → CaCl2(aq) + 2H2O(l) +

2NH3(g) OH-(aq) + NH4

+(aq) → NH3(g) + H2O(l)

Properties of alkalis How to test for ammonia? Litmus test – turns moist red litmus

paper blue

Properties of alkalis Alkalis react with a solution of one metal salt

to give metal hydroxide and another metal salt.

Alkali (of metal B) + salt (of metal A) → metal A hydroxide + salt (of metal B)

E.g. sodium hydroxide + iron(II) sulfate → iron(II)

hydroxide + sodium sulfate 2NaOH(aq) + FeSO4(aq) → Fe(OH)2(s) + Na2SO4(aq) Useful for testing for cations in labs.

Properties of alkalisProperties of Acids

Alkalis are bitterAlkalis feel soapy/ slippery

Concentrated alkalis are causticAlkalis turn red litmus blue

Alkalis react with acids to give salt and water

Alkalis react with ammonium salts to give ammonia, salt and water

Alkalis react with a solution of one metal salt to give metal hydroxide and another

metal salt.

Uses of bases and alkalis Toothpastes Neutralizing acids

E.g. Liming soils/lakes Indigestion pills

E.g. milk of magnesia Cleaning agents

E.g. detergents, soap

Strength of Acids/Alkalis A strong acid is said to dissociate easily

in water whereas a weak acid hardly dissociates when dissolved in water. It ionizes completely in water.

H+ ions

Cl- ions

Ethanoate ions

Note: Ethanoate ions are not monatomic atoms! The ethanoate ion looks like this:

C

CH3

O

-O

Strong and weak acids Most of the mineral acids are strong:

Hydrochloric acid Nitric acid Sulfuric acid

Weak acids include: Ethanoic/acetic acid (vinegar) Citric acid Carbonic acid

Carbonic acid Formed when carbon dioxide dissolves in

water. CO2(g) + H2O(l) H2CO3(aq) Only a small portion of carbonic acid

dissociates. H2CO3(aq) 2H+(aq) + CO3

2-(aq) Found in your soft drinks.

Strong acid vs Weak acid Strong acids react more vigorously with

weak acids. So if I place a piece of magnesium in

hydrochloric acid vs in ethanoic acid, which will show more vigorous effervescence?

Strong acids Sulfuric acid is strong, but… Fluoroantimonic acid (HSbF6) is 2×1019

times stronger than sulfuric acid! Possibly the strongest acid in the world –

a superacid!

Strong and weak alkalis Group I hydroxides are strong:

Sodium hydroxide Potassium hydroxide

Weak alkalis include: Ammonia (why?) Magnesium hydroxide (why?)

Concentration A brief definition of concentration would

be: The amount of substance (no. of

molecules) that is dissolved in 1 dm3 of the solution/ the amount of substance per unit volume of the solution.

Concentration changes on addition of water or removal of water (e.g. evaporation). Strength of acid/alkali does not change!

Concentration Concentration can be applied to any

soluble or sparingly soluble substance, not just acids and alkalis. For instance, sugar: