Acids and Bases

Arrhenius Theory

Acids produced H+ ions, bases produce OH- ions

IMPORTANT: Brønsted-Lowry Theory

Acids are H+ donors, bases are H+ accepters

Conjugate system of an acid

Acid ⇌ proton + conjugate base

Conjugate system of a base

Base + proton ⇌ conjugate acid

What does an acid need to behave as one?

A present base to accept a proton

Formula to represent this

HA + B ⇌ BH+ + A-

HA is acid, B is base, BH+ is conjugate acid, A- is conjugate base

What is a conjugate system?

Any pair of substances whose formulas differ by 1 H+ ion

Eg: NH4+ + OH- ⇌ NH3 + H2O

What is the rule about acids and conjugate bases?

The more readily an acid donates a H+, the less likely the conjugate base will accept H+

“A strong acid has a very weak conjugate base”

What is the rule about bases and conjugate acids?

The more readily a base accepts an OH-, the less likely the conjugate acid will donate H+

“A strong base has a very weak conjugate acid”

CH3COO- + H2O ⇌ CH3COOH + OH-

What is the acid, base, conjugate acid and conjugate base?

CH3COO- : base (accepts H+)

H2O: acid (donates H+)

CH3COOH: conjugate acid (now has H+)

OH- : conjugate base (is now basic)

What is the conjugate acid for the following bases:

• NH3

• HS-

• HCO3-

• NH4+

• H2S

• H2CO3

What is the conjugate base for the following acids:

• H3PO4

• HNO3

• HSO4-

• H2PO4-

• NO3-

• SO4(2-)

What is the conjugate acid and base for the successive ionisation of H3PO4 + H2O?

Conjugate base: H3O+ (accepts H+)

Conjugate acid: H2PO4- , HPO4(2-), PO4(3-) (donates H+)

What did Humphrey Davy discover/propose?

• Basic compounds of group 1 + 2 were all oxides

• Proved that muriatic acid (HCl) produced H+ and Cl- ions instead of O2- ions

• Theorised that presence of H+ gave it its acidic properties

What did Van Liebig discover/propose?

• Metals reacted with acids to produce H2 gas

• Proposed that acids had replaceable hydrogen, did not account for HNO3 producing NO2(g)

What did Svance Arrhenius discover/propose?

• Theorised the role of H+ and OH- ions in acid-base behaviour

• Acid ionises to form H+ ions

• Base ionises to form OH- ions

• Neutralised when mixed

• Only applies in aqueous solutions

What did the Brønsted-Lowry theory propose?

• Proton transfer occurs between two species

• Acid donates H+, base accepts OH-

What can happen to a conjugate acid of a base?

As it has accepted an H+, it has a likelihood of giving back this H+

This is why it is a conjugate acid (can donate H+ ion)

Why can a conjugate base also be an acid?

Bronsted-Lowry states that in the following, HSO3- is a conjugate base:

H2SO3 + H2O ⇌ H3O+ + HSO3-

However in the following, it can be considered an acid:

HSO3- + H2O ⇌ SO3(2-) + H3O+

It varies depending on whether it receives or donates H+ ions

What is Ka and what does it measure?

The acidity constant, which measures the extent to which proton transfer goes to completion

A small Ka value signifies weak dissociation, and a larger Ka value signifies strong dissociation hence a strong acid/base

What does a high Ka value signify (acid)?

Strong acid, and it has an extremely weak conjugate base

What happens as Ka gets smaller?

Acid becomes weaker, and conjugate base becomes stronger

What is a polyprotic acid?

An acid with two or more H+ ions

Undergoes successive ionisation reactions

What are the predicted properties of polyprotic acids?

• Donate more than one H+ per acid molecule

• Conjugate base of polyprotic acid is USUALLY (not always) more acidic than water

• Subsequent acidity constants decrease with each proton transfer

What happens when an acid reacts with water?

Salt and water are always produced

What determines if a salt is acidic?

If one of the ions present in a salt reacts with water acting as a proton donor (acid) then its solutions will be acidic

What determines if a salt is basic?

If a salt contains an ion that hydrolyses by acting as a proton acceptor (base) then its solutions will be basic

What happens if a salt has both acidic and basic ions?

May be neutral, or its acidity is governed by the ion with the strongest acid/base characteristics

What is a buffer?

A solution that resists changes in pH when a small amount of acid or base is added

What does a buffer do?

Prevents the pH from changing - when an acid is added to a system, the buffer opposes the change and neutralises it by forming OH-

Why does a buffer have to have a weak acid?

For equilibrium to be present, there must be a weak acid so ions can be removed + produced.

Why can’t a strong acid be used in a buffer solution?

It is fully ionised, and H+ cannot be removed

What happens when adding acid to an acidic buffer solution, CH3COOH?

Reaction: CH3COOH ⇌ CH3COO- + H+

• Adding acid increase H3O+, which reacts with CH3COO- to form CH3COOH

• Equilibrium moves to the left

• [CH3COO-] is very small, only a few H+ ions can be consumed, larger [CH3COO-] needed

• NaCH3COO or KCH3COO is added to increase [CH3COO-]

What happens when adding base to an acidic buffer solution, CH3COOH?

Neutralises some of the H3O+, which means more CH3COOH will ionise to replace ions

• Equilibrium moves to the right

• Must have a large concentration of unionised acid molecules available

• Much of the excess acid/base added to buffer solution can react without a great alteration of [H3O+] in the original mixture

• pH of buffer solution only drops slightly compared to the original pH

What happens when adding acid to a basic buffer solution, NH3?

Reaction: NH3 + H2O ⇌ OH- + NH4+

• Needs large [OH-] to react with any added H+, large NH4+ to react with any added OH-

• Enough NH3 exists to act as a source of OH-, but need to increase [NH4+] by adding ammonium salt

• Use NH4Cl + NH3

• Adding acid decreases [OH-] and shifts equilibrium right to replace [OH-]

• pH only drops slightly compared to original pH

What do you need for an acidic buffer solution? (concentration)

The concentration must be significant for

• H+ to be ionised when the base is added

• removing H+ as it is added

What is the buffering capacity?

The amount of acid or base that can be added before the pH changes by more than one unit of pH

What does buffering capacity depend on?

• Relative concentrations of weak acid/conjugate base - greatest capacity achieved when concs. of acid/conjugate base are equal

• Increases with an increase in concentration of conjugate pair - want a high concentration of weak acid and weak base for buffer solution to be effective