ACIDS and BASES

Acid – Base theoriesNaming acids and basesOxidesReactions and properties of acids and basesStrengths of acids and bases

Acid and Base Theories1) Arrhenius Theory

• An acid is a substance that gives H+ ion, when dissolved in water.

For example, hydrochloric acid reacts with water to form hydrogen ions which are transferred to a water molecule to form a hydronium ion (H3O+).

But simply the reaction is: HCl H+ + Cl-

Acids which have one ionizable hydrogen atom per molecule are called monoprotic acids.

Example: HNO3 H+ + NO3

-

Acids which have two ionizable hydrogen atom per molecule are called diprotic acids.

Example: H2SO4 H+ + HSO4

−

HSO4− H⇌ + + SO4

2−

Acids which have three ionizable hydrogen atom per molecule are called triprotic acids.

Example:H3PO4   H⇌ + + H2PO4

–

H2PO4– H⇌ + + HPO4

2–       HPO4

2– H⇌ + +  PO43–       

• A base is a substance that gives OH- ion, when dissolved in water.

NaOH → Na+ + OH−

Ca(OH)2 → Ca2+ + 2OH-

Reaction of NH3 produce OH-:

NH3 + H2O → NH4+ + OH-

so it is a base.

Limitations of the Arrhenius theory

Hydrochloric acid is neutralized by both sodium hydroxide solution and ammonia solution. In both cases, you get a colourless solution which you can crystallize to get a white salt - either sodium chloride or ammonium chloride.

These are clearly very similar reactions. The full equations are:

NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) NH3(aq) + HCl(aq) NH4Cl(s)

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

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

You can get around this by saying that the ammonia reacts with the water, it is dissolved in to produce ammonium ions and hydroxide ions:

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

This is a reversible reaction, and in a typical dilute ammonia solution, about 99% of the ammonia remains as ammonia molecules. Nevertheless, there are hydroxide ions there, and we can squeeze this into the Arrhenius theory.

However, this same reaction also happens between ammonia gas and hydrogen chloride gas.

NH3(g) + HCl(g) NH4Cl(s) In this case, there aren't any hydrogen ions or

hydroxide ions in solution - because there isn't any solution. The Arrhenius theory wouldn't count this as an acid-base reaction, despite the fact that it is producing the same product as when the two substances were in solution.

Naming Acids and BasesA. Naming Acids:

The name of the acid is determined based on the name of the anion, specifically, based on the ending of the anion name.  The three possibilities are listed here:

Anion Name

Acid Name

-ide Hydro-ic acid

-ite -ous acid

-ate -ic acid

Fluoride F-

Chloride Cl-

Bromide Br-

Iodide I-

Sulfide S2-

Nitride N3-

Sulfite SO32-

Nitrite NO2-

Chlorite ClO2-

Hypochlorite OCl-

Phosphate PO43-

Hydrogen phosphate HPO42-

Dihydrogen phosphate H2PO4-

Nitrate NO3-

Sulfate SO42-

Hydrogen sulfate HSO4-

Perchlorate ClO4-

Chlorate ClO3-

Carbonate CO32-

Common Anions

B. Naming Bases

Simply use the normal rules for naming compounds; ionic or covalent depending on the elements in the compound.

Example:

NaOH: Sodium hydroxide

Ca(OH)2: Calcium hydroxide

NH3: Ammonia

Example:

a) Name the following acids and bases:

NaOH:

H2SO3:

H2S :

H3PO4:

NH3:HCN:

Ca(OH)2:

Fe(OH)3:

H3P:

Sodium hydroxide

Sulfurous acid

Hydrosulfuric acid

Phosphoric acid

Ammonia

Hydrocyanic acid

Calcium hydroxide

Iron (III) hydroxide

Hydrophosphoric acid

b) Write the formulas of the following acids and bases:

Hydrofluoric acid: Hydroselenic acid: Carbonic acid: Lithium hydroxide: Nitrous acid: Cobalt (II) hydroxide: Sulfuric acid: Beryllium hydroxide: Hydrobromic acid:

HFH2Se

H2CO3

LiOH

HNO2

Co(OH)2

H2SO4

Be(OH)2

HBr

Oxides

Nonmetal Oxides

Metal Oxides

CO2, SO2, SO3 etc. show acidic properties(acid anhydride)

CO, NO, N2O are neutral(have 1 oxygen atom in the formula)

Na2O, BaO etc. show basic properties(basic anhydrides)

Amphoteric metals show both basic and acidic properties such as Al and Zn

Acidic Property of Nonmetal Oxides• The oxides of nonmetals are usually acidic except

NO, N2O and CO (They are neutral)

CO2 + H2O H2CO3

SO2 + H2O H2SO3

SO3 + H2O H2SO4

N2O5 + H2O 2HNO3

Cl2O + H2O 2HOCl

P4O10 + H2O 4H3PO4

• Monoxides of halogens are acidic such as Cl2O, Br2O.• Oxides of some metals at high oxidation states show

acidic properties such as Mn2O7, CrO3.Acidic nonmetal oxides react with bases to form salts.

SO3 + 2KOH K2SO4 + H2O

ACID ANHYDRIDES Any oxygen-containing substance which will

produce an acid, when dissolved in water, is called an ACID ANHYDRIDE.

When sulfur dioxide (SO2) is dissolved in water, sulfurous acid is formed.

SO2 + H2O H2SO3

When sulfur trioxide (SO3) is dissolved in water, some sulfuric acid is formed.

SO3 + H2O H2SO4

ACID ANHYDRIDES

• Carbon dioxide dissolved in water is in equilibrium with carbonic acid:CO2 + H2O H⇌ 2CO3

• The hydration equilibrium constant at 25°C is Kh= 1.70×10−3: hence, the majority of the carbon dioxide is not converted into carbonic acid and stays as CO2 molecules.

Basic Properties of Metal Oxides• Oxides of metals are usually basic.

Na2O + H2O 2NaOH

BaO + H2O Ba(OH)2

• Some metal oxides can not dissolve in water but they can dissolve in acidic solutions.

MnO + 2HCl MnCl2 + H2O

CrO + 2HCl CrCl2 + H2O• Basic oxides react with acids to form salts.

CaO + H2SO4 CaSO4 + H2O

BASIC ANHYDRIDES

Any oxygen-containing substance which will produce a base, when dissolved in water, is called a BASIC ANHYDRIDE.

If sodium oxide (Na2O) is added to water, sodium hydroxide, a base, is formed.

Na2O + H2O 2 NaOH

ANHYDRIDES Anhydride means "without water", so anhydrides may

be classified as acids or bases with all the water removed.

It might be more accurate and understandable if we say anhydrides are acids or bases with all the hydrogen removed in the form of water.

Given a particular acid or base, we can determine its anhydride by removing two hydrogen atoms and one oxygen atom from its molecule (or molecules). For example:

2 HClO Cl2O + H2O

Ca(OH)2 CaO + H2O

The acid anhydride for hypochlorous acid, HClO, is Cl2O. The basic anhydride for calcium hydroxide is CaO.

In predicting anhydrides, ENOUGH H2O UNITS MUST BE REMOVED TO

LEAVE THE ANHYDRIDE WITHOUT ANY HYDROGEN.

To form the anhydride for an acid like H3PO4, remove three water molecules from two phosphoric acid molecules to produce the anhydride, P2O5.

2 H3PO4 P2O5+ 3 H2O

ANHYDRIDES

Amphoteric Oxides

Oxides amphoteric metals are also amphoteric.

Al2O3 + HCl AlCl3 + H2O

Al2O3 + 2NaOH + 3H2O 2NaAl(OH)4

(sodium tetrahydroxoaluminate)

Properties and Reactions of

Acids and Bases

A.Properties of Acids:• Are corrosive• They taste sour• They form solutions w/ pH less than 7 at 25°C.• They turn litmus dye from blue to red• They conduct electricity (electrolyte)

• They react with active metals to form salt and H2 gas.

Mg + 2HCl MgCl2 + H2

• The acids which do not contain oxygen in their structures can not react with semi noble metals Cu, Hg, Ag.The oxy acids react with these metals producing gases other than H2.

Cu + 2H2SO4 CuSO4 + SO2 + 2H2O

3Ag + 4HNO3 3AgNO3 + NO + 2H2O

• They react with metal carbonates and hydrogen carbonates to give a salt, water and carbon dioxide, which appears as effervescence (bubbles).

Na2CO3 + 2HCl NaCl + H2O + CO2

CH3COOH (aq)+NaHCO3 (aq)NaCH3COO(aq) +H2O (l) + CO2

ethanoic acid metalh hydrogen salt water carbon carbonate

dioxide

• They react with bases to form salts and water.

HCl + NaOH NaCl + H2O (neutralization)

H+ (aq) + OH- (aq) H2O(l) (net ionic equation)

B. Properties of Bases• They have bitter taste• Aqueous solutions of bases, known as alkalis (for

example aqueous sodium hydroxide), have a slippery feel.

• They turn the litmus dye from red to blue• They react with fats in the skin to form soaps• They conduct electricity (electrolyte)• The most common bases are the oxides,

hydroxides and carbonates of metals, but a number of other compounds, such as ammonia and amines also act as bases.

• They only react with amphoteric metals: Zn, Al

Zn + 2NaOH Na2ZnO2 + H2

2Al + 6 NaOH 2Na3AlO3 + 3H2

• If they are soluble in water they give a solution with pH>7

• They react with acids to form a salt. For example, calcium oxide will react with hydrochloric acid to form calcium chloride and water:

CaO (s) + 2 HCl (aq) CaCl2 (aq) + H2O (l) base acid salt water

• Amphoteric metals have both acidic and basic properties such as Al, Zn, Sn, Pb, Cr

Al + 6HCl AlCl3 + 3H2

2Al + 6NaOH 2Na3AlO3 + 3H2

• Oxides and hydroxides of amphoteric metals are also amphoteric.

Al2O3 + HCl AlCl3 + H2O

Al2O3 + 2NaOH + 3H2O 2NaAl(OH)4

ZnO + 2 HCl ZnCl2 + H2O

ZnO + 2NaOH + H2O Na2Zn(OH)4

Neutralization

Examples of Acids & Bases

Acids

HCl

H2SO4

HNO3

Juices, Soda

NaOHCa(OH)2

KOHSoap, Ammonia, Baking Soda

Bases

2.Bronsted-Lowry Theory A Bronsted-Lowry (BL) acid is defined as any

substance that can donate a hydrogen ion (proton) and a Bronsted-Lowry base is any substance that can accept a hydrogen ion (proton). Thus, according to the BL definition, acids and bases must come in conjugate pairs. For example, consider acetic acid dissolved in water: CH3COOH + H2O CH3COO- + H3O+

Conjugate acid-base pairs:

1. CH3COOH and CH3COO-

2. H2O and H3O+

Act as an acid Act as a base Act as an acidAct as a base

Label Bronsted-Lowry acids and bases in the following reactions and show the direction of proton transfer.

1. H2O + H2O OH- + H3O+

Acid Base Base Acid

H+

2. NH3 + H2O NH4+ + OH-

Acid BaseAcidBase

H+

When a Bronsted-Lowry acid has given up its proton, it is capable of getting back that proton and acting as a base. Conjugate base is what is left after an acid gives up a proton. The stronger the acid, the weaker the conjugate base. The stronger the base, the weaker the conjugate acid.

The relationship between the Bronsted-Lowry theory and the Arrhenius theory

The Bronsted-Lowry theory doesn't go against the Arrhenius theory in any way - it just adds to it.

Hydroxide ions are still bases because they accept hydrogen ions from acids and form water.

An acid produces hydrogen ions in solution because it reacts with the water molecules by giving a proton to them.

The hydrogen chloride / ammonia problemThis is no longer a problem using the

Bronsted-Lowry theory. Whether you are talking about the reaction in solution or in the gas state, ammonia is a base because it accepts a proton (a hydrogen ion).

If it is in solution, the ammonia accepts a proton from a hydronium ion:

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

If the reaction is happening in the gas state, the ammonia accepts a proton directly from the hydrogen chloride:

NH3(g) + HCl(g) NH4Cl(s)

3. Lewis Theory

A Lewis acid is a chemical compound, A, that can accept a pair of electrons from a Lewis base, B, that acts as an electron-pair donor, forming an adduct, AB.

A + :B → A—B

A Lewis base is also a Brønsted-Lowry base.

The Bronsted-Lowry theory says that they are acting as bases because they are combining with hydrogen ions. The reason they are combining with hydrogen ions is that they have lone pairs of electrons - which is what the Lewis theory says. The two are entirely consistent.

But what about other similar reactions of ammonia or water, for example?

• Ammonia reacts with BF3 by using its lone pair to form a co-ordinate bond with the empty orbital on the boron.

Co-ordinate (dative covalent) bonding

A covalent bond is formed by two atoms sharing a pair of electrons. The atoms are held together because the electron pair is attracted by both of the nuclei.

In the formation of a simple covalent bond, each atom supplies one electron to the bond.

A co-ordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom.

The reaction between ammonia and hydrogen chloride

Representing co-ordinate bonds

In simple diagrams, a co-ordinate bond is shown by an arrow. The arrow points from the atom donating the lone pair to the atom accepting it.

Dissolving hydrogen chloride in water to make hydrochloric acid

Lewis acidsLewis acids are electron pair acceptors. In

the above example, the BF3 is acting as the Lewis acid by accepting the nitrogen's lone pair. On the Bronsted-Lowry theory, the BF3 has nothing about it.

Then what makes HCl a Lewis acid?Chlorine is more electronegative than

hydrogen, and that means that the hydrogen chloride will be a polar molecule. The electrons in the hydrogen-chlorine bond will be attracted towards the chlorine end, leaving the hydrogen slightly positive and the chlorine slightly negative.

The lone pair on the nitrogen of an ammonia molecule is attracted to the slightly positive hydrogen atom in the HCl. As it approaches it, the electrons in the hydrogen-chlorine bond are repelled still further towards the chlorine.

Eventually, a co-ordinate bond is formed between the nitrogen and the hydrogen, and the chlorine breaks away as a chloride ion.

Relative Strengths of acids and Bases

The strength of an acid depends on how easily the proton H+ is lost or removed from an acid

Two factors determine the acid strength:

1. The polarity of H atom: The more polarized the bond is, the more easily the proton is removed and greater the acid strength.

2. The size of the atom X (in HX): The greater the atom X, the weaker is the bond and greater the acid strength.

• Periodic Trends for Binary Acids:Down a group: Sizes of the atoms increase.HFHCl Acidity increasesHBrHI

Across a period: Polarity of the bond increases.

CH4 NH3 H2O HF

Acidity inreases.

• Oxyacids:

HOF

HOCl Acidity decreases. H-O bond

HOBr ionizes more easily when the

HOI oxygen atom is bonded to a

more electronegative atom.

• For a series of oxyacids:

HClO HClO2 HClO3 HClO4

Acidity increases

As the number of oxygen atoms increases,The oxidation number of central atom (Cl) increases. This increases the ionization ofO-H bond. Therefore, acid strength increases.

• Polyprotic Acids and Their Anions:

H3PO4 H2PO4- HPO4

2-

H2CO3 HCO3- Acidity decreases

H2SO4 HSO4-

Organic Acids

Organic acids have carboxyl group (COOH). They are weak acids.

Example:

HCOOH: Formic acid

CH3COOH: Acetic acid