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

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

Consider This When ammonia gas dissolves in water the following reaction occurs NH3 + H2O NH4 + + OH- Accepts a proton B/L base Donates a proton B/L acid

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

Bronsted Lowry theory does not only refer to reactions where water is the solvent In the following reaction between Hydrogen chloride and Ammonia HCl + NH3 NH4+ + Cl-

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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