Acid-Base Equilibria L.O.: To know what the Brønsted-Lowry theory is and what is explains Humprey Davy Jabir ibn-Hayyan Jabir ibn-Hayyan discovered mineral acids around 722-815 AD. Pioneered experimental chemistry and used techniques such as crystallisation and distillation to discover sulfuric, hydrochloric and nitric acid. Acids originally were recognised by their properties: sour taste, ability to corrode metals and changes they inflicted on indicators. Lavoisier (1743-1794) – developed the oxygen-theory of burning and gave the gas the name “acid former” as he thought all acids contained oxygen. This was disproved however by Davy around 1809-1810. He heated HCl to very high temperatures in the presence of several different metals and non-metals and found no residue of oxygen in the resultant compounds. Around 1816, it was Davy who proposed that all acids must contain hydrogen. Antoine Lavoisier

Proton donors and proton acceptors The story so far…… Johannes Brønsted Thomas Lowry Svante Arrhenius 19th century around 1884, Svante Arrhenius proposed that some compounds exist as ions in solution at all times. This formed the ionic theory we use today. At the time though it was controversial and was not accepted. Arrenhius was later vindicated and awarded the nobel prize for chemistry (1903). He then used the ionic theory to explain the properties of acids when dissolved in water. He was able to explain the difference between strong and weak acids and also what is happening in neutralisation reactions. We do still use Arrehius’ ionic theory, however, it does have problems because it is limited to aqueous solutions. 20th century – Bronsted and Lowry independently came up with what is known today as the Bronsted-Lowry theory (1923). This discribes acids as proton donors and bases as proton acceptors. Remember, alkalis are soluble bases so this theory can account for those substances which are basic but which do not dissolve in water (problem with Arrenhius’ ideas). Strong and weak acids? Proton donors and proton acceptors

Brønsted-Lowry theory Lone pair acceptor OR proton donor ACID proton donor HCl H+(aq) + Cl¯(aq) BASE proton acceptor NH3 (aq) + H+(aq) NH4+(aq) Acids act as proton donors, so in the case of HCl, when in water the HCl readily gives a proton to water when it dissolves, producing oxonium ions (H30+ commonly written as H+) and Cl- ions. In the case of HCl because it is a strong acid the position of the eqilibrium is well over to the right, favouring the forward dissociation reaction. Bases act as proton acceptors, they have the ability to accept a proton from the acid. This is because they have a LONE PAIR of electrons, capable of forming a DATIVE COVALENT BOND with the proton Lone pair donor OR proton acceptor Lone pair acceptor OR proton donor Biochemistry bases??

Conjugate systems Two conjugate acid-base pairs here: NH4+ and NH3 H3O+ and H2O Acid-base reactions involve competition for protons occurs. Example: NH4+ (aq) + H2O (l) NH3 (aq) + H3O+ (aq) In the above example the ammonium ion is acting as an acid and the water the base. On the right, the ammonia is the base and the oxonium ion the acid. Competition occurs between ammonia and water. Protons are held by the lone pairs on the ammonia molecules on the left side and by the water molecules on the right. The strength of the proton hold by either of the two molecules determines the position of the equilibrium. Acids are related to bases Acid Proton + Conjugate base Bases are related to acids Base + Proton Conjugate acid For an acid to behave as an acid, it must have a base present to accept a proton... HA + B A¯ + BH+ acid base conjugate conjugate base acid Example: NH4+ (aq) + H2O (l) NH3 (aq) + H3O+ (aq) acid base conjugate base conjugate acid

Can you? Identify which compound is acting as an acid and which the base when H2SO4 reacts with NaCl. Explain why the nucleic acids making up DNA and RNA referred to as bases. Explain why Arrenhius’ ionic theory falls down when explaining acid-base equilibria. Name the conjugate bases of these acids: CH3COOH, HNO3 Name the conjugate acids of these bases: O2-, OH-