Topic 8: Acids and Bases

Use this outline in conjunction with the IB syllabus.

8.1 Theories of acids and bases

8.1.1 

Bronsted-Lowry Definition

Acid – a proton donor

Base – a proton acceptor

HCl + NaOH → NaCl + H₂O

HCl donates the hydrogen ion, so is an acid, while NaOH accepts (its OH group does) the hydrogen ion.

Lewis Theory

Acid – an electron pair acceptor

Base – an electron pair donor

AMPHOTERIC species can act as both bases and acids. E.g. H₂PO₄^- or HCO₃^-

8.1.2

Species which transfer H⁺ ion on reaction act as Bronsted-Lowry base and acid. If there is not H⁺ transition, then we can refer to the neutralization only in terms of the Lewis theory – the electron pair transition. All Bronsted-Lowry acids are Lewis acids, but not all Lewis acids are Bronsted-Lowry acids. This works for the bases too.

Arrhenius Theory α Bronsted-Lowry Theory α Lewis Theory

8.1.3

The conjugate base will be an acid which lost a hydrogen proton, while conjugate acid will be a base which gained a hydrogen proton.

CH₃COOH + CH₃CH₂NH₂ → CH₂COO^- + CH₃CH₂NH₃⁺

Acid                  base           conj. base      conj. Acid

8.2 Properties of acids and bases

Acids

1. Taste sour
2. Burn skin
3. Corrode metals
4. React with metal to form hydrogen and salt
5. React with carbonates to form carbon dioxide, water and salt
6. Turn litmus paper red
7. pH lower than 7
8. Greater [H⁺] concentration than bases

Bases

1. Taste bitter
2. Feel soupy
3. Alkali is a base that dissolves in water
4. Neutralize acids
5. Usually visible by OH^- group
6. Turn litmus paper blue
7. Hydroxides, hydrogencarbonates, soluble carbonates and ammonia

          Ex: NaOH, NaHCO₃, Na₂CO₃, NH₃

 

Red – Yellow – Green – Light Blue – Dark Blue

Acids and bases react with each other to produce water and salt; this process is called neutralization. Neutralization is an exothermic reaction – we can feel a heat produced.

Experimental Properties:

1. Neutralization produces heat
2. If a lighted match is inserted into a container with hydrogen we can hear the “pop” sound
3. Carbon dioxide turns limewater milky
4. Phenolphthalein indicator turns red when in basic solution, otherwise colorless

8.3 Strong and Weak Acids and bases

 

8.3.1

Strong acids are those that almost completely dissociate in water. They have high electrical conductivity because more free charged ions are present to conduct the electrical current. [H⁺] is higher.

Weak acids are those that only partially dissociate in aqueous solution. They have low electrical conductivity because less charged particles are present. [H⁺] is lower.

Other species dissolvable in water also conduct electricity because provide the solution with freely moving ions.

8.3.2

Strong Acids: Hydrochloric acid – HCl, Nitric acid – HNO₃, Sulfuric acid – H­₂SO₄

Weak Acids: ethanoic acid - CH₃COOH (form of carboxylic acid), carbonic acid - H₂CO₃

Strong Base: group 1 hydroxides, Ba(OH)₂

Weak base: Ammonia – NH₃, CH₃CH₂NH₂ – amine (it contains the amine nitrogen group NH₂)

8.3.3

The strength of an acid or base can be measured with:

·         an indicator (universal)

·         pH meter.

·         the rate of reaction - hydrogen production with metals or CO2 with XCO3

·         electrical conductivity – conductivity probe

8.4 The pH scale

8.4.1

Acidic solutions have pH of less than seven. Neutral solutions have pH of seven. Alkaline solutions have pH of more than seven.

8.4.2

Weak acid will turn the universal indicator yellow, while a strong one red. Weak base will turn a universal indicator dark green or light blue, while a strong base dark blue.

8.4.3

Each change of one pH unit represents a tenfold change in the hydrogen ion concentration,