1. Bronsted/Lowry Theory of Acids and Bases
1923 Danish chemist Bronsted + English chemist Lowry independently proposed new definitions for an acid and a base
These simply state that:
A base is a proton donor
An acid is a protons aceptor

2. Consider This
When hydrogen chloride is added to water the following reaction occurs
HCl H2O H3O+ + Cl-
Because this donates a protono it is a B/L acid
Because this accepts a proton it is a B/L base

3. Consider This
When ammonia gas dissolves in water the following reaction occurs
NH H2O NH4 + + OH-
Accepts a proton B/L base
Donates a proton B/L acid

4. Water can act as either an acid or a base and for this reason is known as amphoteric or amphiprotic

5. Bronsted Lowry theory does not only refer to reactions where water is the solvent
In the following reaction between Hydrogen chloride and Ammonia
HCl + NH NH4+ + Cl-

6. Advantages of the Bronsted/Lowry Theory over Arrhenius’ Theory
Arrhenius’ theory was confined to aqueous solutions
Bronsted/Lowry’s theory broadens the range of species that can be defined as acids and bases
Substances that are not classified as acids or bases under Arrhenius’ theory are classified as acids or bases in the Bronsted/Lowry theory

7. Note
An acid will only donate a proton when there is something there to accept it
A base will only accept a proton if there is something there to donate it

8. Conjugate acid/base pairs
Certain reactions are reversible and can happen in both directions
We use the symbol to show this

9. Consider This If the following reaction is written as
CH3COOH + H2O CH3COO- + H3O+
Ethanoic acid Water
This means the reaction
And the reaction
CH3COO- + H3O CH3COOH + H2O
Can take place

10. Identify the B/L acid in the following Equations
CH3COOH + H2O CH3COO- + H3O+
CH3COO- + H3O CH3COOH + H2O

11. An acid changes to a conjugate base when it donates a proton
A base changes to a conjugate acid when it accepts a proton

12. CH3COO- is the conjugate base of CH3COOH
CH3COOH is the conjugate acid of CH3COO-
Every base has a conjugate acid and every acid has a conjugate base
Since CH3COO- and CH3COOH only differ by a proton we refer to them as a conjugate acid base pair

13. Why can H2O and H3O+ be called a conjugate acid base pair?
NB Study examples 12.1 and 12.2 on pages 140 and 141 of your text book

14. Neutralisation
When acids and bases react with each other in the right proportions they cancel each other out
When this happens both the acid and the base lose their characteristic properties
The solution formed is neutral and has no effect on litmus paper
This type of reaction is known as neutralisation

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

16. Reaction between HCl and NaOH
HCl + NaOH H2O + NaCl
Acid + Base Water + Salt
The word salt is a general term used to describe the substance formed when the hydrogen in an acid is replaced by a metal or an ammonium ion

17. Understanding what happens
HCl + NaOH H2O + NaCl
In this equation the following ions are all present in solution
H+ + Cl- + Na+ + OH Na+ + Cl- + H2O
The spectator ions are crossed out as what is important is the reaction where the H+ (Or more correctly Hydronium ions )and OH- ions react to form water
H + + OH H2O

18. Everyday examples of Neutralisation
Medicine – Hydrochloric acid in stomachs helps digestion. Over eating and stress can produce too much acid. Antacids such as Alka Seltzer, Bisodol and Milk of Magnesia may be used to try to neutralise excess acid

19. 2. Agriculture – If soil is too acidic crop yields can be low, therefore farmers often spread lime (CaO) on soil to neutralise the acidity 3. Environmental Protection- In areas affected by acid rain limestone is often added to lakes to neutralise acidity

20. Toothpaste is slightly basic to neutralise acids that cause tooth decay
Baking soda may be used to neutralise the acid of bees
Vinegar is used to neutralise the alkaline sting of wasps
Shampoo is slightly basic and can cause scales on hair to stick out, conditioner is slightly acidic so neutralises the base flattening the scales and making hair more shiny and manageable