Acid / Base Theories Peter Jackson

Acids Have a sour taste React with metals to to form a salt plus hydrogen H2SO4 + Zn = ZnSO4 + H2 React with carbonates to form a salt plus water plus carbon dioxide CaCO3 + 2 HCl = CaCl2 + H2O + CO2 Turn blue litmus red Peter Jackson

Arrhenius Theory In 1887 Svante Arrhenius [a Swedish chemist] developed a theory of acids and bases based on two major observations. Peter Jackson

Acids contain at least one hydrogen atom per molecule. Pure acids such as glacial acetic acid and anhydrous sulphuric acid are non-conductors while their aqueous solutions are conductors [of electricity]. Peter Jackson

A substance that dissociates in water to produce H+ ions HCl → H+ + Cl- HNO3 → H+ + NO3- H2SO4 → 2 H+ + SO42- H3PO4 → 3 H+ + PO43- Hydrochloric and nitric acids are monobasic Sulphuric acid is dibasic Phosphoric acid is tribasic The above acids dissociate fully in water and are said to be strong acids Peter Jackson

Weak acids do not dissociate fully in water Carbonic acid Ethanoic acid Citric acid Ascorbic acid [ vitamin C] Are examples of weak acids Not all acids are dangerous Conc. Acids tend to be dangerous e.g. sulphuric acid in a car battery, formic acid in an ant bite or nettle sting is not dangerous but in a bottle they are Peter Jackson

Bases A base dissociates in water to produce OH- ions [hydroxide ions] Turn red litmus blue NaOH and KOH are strong bases - they dissociate fully in water Calcium hydroxide and Magnesium hydroxide are weak bases – they do not dissociate fully in water Bases that dissolve in water are called Alkalis Peter Jackson

Limitations of Arrhenius It is H3O+ not H+ that exists in water Restricted to aqueous solutions Not all acid base reactions require water HCl + NH3 = NH4Cl Peter Jackson

Brønsted Lowry Peter Jackson

Brønsted-Lowry Theory (1923) Both had same idea at the same time Acid is a proton [H+] donor Base is a proton [H+] acceptor acid-base reactions involve the transfer of a proton from the acid to the base. Stronger the acid - the more readily it donates the proton Stronger the base the more readily it accepts the proton Peter Jackson

Conjugate acid/base Pairs HCl + H2O = H3O+ + Cl- HCl gives away a proton [H+] to become Cl- so it is an acid H2O accepts a proton to become H3O+ so it is a base The reaction is reversible H3O+ gives away a proton [H+] to become H2O so it is an acid Cl- accepts a proton to become HCl so it is a base Peter Jackson

Conjugate acid/base Pairs Let us look at the water in the last equation [going right] accepts H+ H2O Base H3O+ Acid donates H+ [going left] Species which differ by H+ are called conjugate acid-base pairs Peter Jackson

Conjugate acid/base Pairs NH3 + H2O = NH4+ + OH- NH3 accepts a proton to become NH4+ so it is a base H2O donates a proton to become OH- so it is an acid In reverse NH4+ donates a proton to become NH3 so it is an acid OH- accepts a proton to become H2O so it is a base Peter Jackson

Strong Weak Acid Conjugate base Base Conjugate acid HClO4  ClO4- H2SO4  HSO4- HCl  Cl- HNO3  NO3- H3O+  H2O HSO4-  SO42- HF  F- CH3COOH  CH3COO- H2S  HS- NH4+  NH3 HCO3-  CO32- H2O  OH- HS-  S2- NH3  NH2- OH-  O2- O2-  OH- OH-  H2O CO32-  HCO3- NH3  NH4+ HS-  H2S CH3COO-  CH3COOH F-  HF H2O  H3O+ Cl-  HCl Weak Peter Jackson

Acid - Base reactions #1 These lead preferentially to the formation of the weaker acid and base HNO3 + H2O = H3O+ + NO3- H3O+ is a weaker acid than HNO3 NO3- is a weaker base than H2O so the reaction to the right is favoured Peter Jackson

Acid - Base reactions #2 These lead preferentially to the formation of the weaker acid and base CH3COOH + H2O = CH3COO- + H3O+ CH3COOH is a weaker acid than H3O+ H2O is a weaker base than CH3COO- so the reaction to the left is favoured Peter Jackson

Levelling Effect of Water HCl, HNO3, H2SO4 and HClO4 all fully dissociate in water to form H3O+ all appear equally strong they are NOT equally strong this is called the Levelling Effect of Water Their true strengths can be shown in ethanoic acid [less basic - more acidic] This makes it harder for them to dissociate Peter Jackson

Amphoteric behaviour Amphoteric = can be both acidic and basic Self Ionisation of Water H2O + H2O = H3O+ + OH- Base Acid Acid Base Another Example NH3 + NH3 = NH4+ + NH2- Base Acid Acid Base This only happens to a very limited extent Peter Jackson

Neutralisation Reactions HCl + NaOH = NaCl + H2O HCl dissociates in water to form H3O+ + Cl- H3O+ + Cl- + Na+ + OH- = 2 H2O + Na+ + Cl- the Cl- and the Na+ do not change therefore they have not reacted We can leave them out of the equation H3O+ + OH- = 2H2O this is called the Ionic or Half Equation The omitted ions are “spectator ions” Peter Jackson

Advantages of Bronsted-Lowry Not limited to reactions in water Allows more species to be regarded as acids and bases Explains the amphoteric nature of many substances - even relatively strong acids such as ethanoic acid CH3COOH + HCl = CH3COOH2+ + Cl- Base Acid Acid Base Peter Jackson