Topic 8 : Acids and Bases

• The Brønsted-Lowry Theory defines acids and bases in terms of proton transfer between chemical compounds • A Brønsted-Lowry acid is a species that gives away a proton (H+) • A Brønsted-Lowry base is a species that accepts a proton (H+) using its lone pair of electrons • The Brønsted-Lowry Theory is not limited to aqueous solutions only and can also be applied to reactions that occur in the gas phase

• A conjugate acid-base pair is two species that are different from each other by a H+ ion • Conjugate here means related • In other words, the acid and base are related to each other by one proton difference

• Species that can act both as proton donors and acceptors are called amphiprotic • Eg. water as a Brønsted-Lowry acid

• A compound that is amphoteric means it has both basic and acidic character. When the compound reacts with an acid, it shows that it has basic character. When it reacts with a base, it shows that it’s acidic • An example of amphoteric is aluminium oxide • When a compound is amphiprotic, it means it can act as a proton donor and as a proton acceptor • Aluminium oxide is not amphiprotic, even though it is amphoteric

• Amphiprotic describes a substance that can accept and donate a proton or H+ while amphoteric means the ability to act as both an acid and a base • All amphiprotic substances are amphoteric but not all amphoteric substances are amphiprotic

• acid + metal → salt + hydrogen • the extent of reaction depends on the reactivity of the metal and the strength of the acid • Metals low in reactivity do not react at all, for instance copper does not react with dilute acids • Stronger acids will react more vigorously with metals than weak acids. • Using stronger acids will make the reaction more exothermic and the metal will dissolve faster

• acid + metal oxide → salt + water

acid + metal hydroxide → salt + water

• acid + metal carbonate → salt + water + carbon dioxide

acid + metal hydrogencarbonate → salt + water + carbon dioxide

• A neutralisation reaction is one in which an acid (pH <7) and a base/alkali (pH >7) react together to form water (pH = 7) and a salt • The proton of the acid reacts with the hydroxide of the base to form water • The spectator ions which are not involved in the formation of water, form the salt Acid + Base (alkali) → Salt + Water S

a chloride

A nitrate

a sulfate

• Acid-base titrations are used to find the unknown concentrations of solutions of acids and bases

• Acid-base indicators give information about the change in chemical environment. They change colour reversibly depending on the concentration of H+ ions in the solution • Indicators are weak acids and bases where the conjugate bases and acids have a different colour • Many acid-base indicators are derived from plants, such as litmus • A good indicator gives a very sharp colour change at the equivalence point

Acid - Pink Alkali - Blue

Acid - Red Alkali - Yellow

Acid - colourless Alkali - Pink

• The acidity of an aqueous solution depends on the number of H+ (H3O+) ions in solution • pH is defined as pH = -log10[H+] where [H+] is the concentration of H+ in mol dm–3 • The pH scale is a logarithmic scale with base 10 This means that each value is 10 times the value below it. For example, pH 5 is 10 times more acidic than pH 6. • It is a numerical scale that shows how acidic or alkaline a solution is • The scale goes from 0-14 (extremely acidic substances have values of below 0)

• All acids have pH values of below 7, all alkalis have pH values above 7 • The lower the pH then the more acidic the solution is • The higher the pH then the more alkaline the solution is

• Acidic solutions (strong or weak) always have more H+ than OH- ions • Since the concentration of H+ is always greater than the concentration of OH- ions, [H+] is always greater than 10-7 mol dm-3 • Using the pH formula, this means that the pH of acidic solutions is always below 7 • The higher the [H+] of the acid, the lower the pH

• Basic solutions (strong or weak) always have more OH- than H+ ions • Since the concentration of OH- is always greater than the concentration of H+ ions, [H+] is always smaller than 10-7 mol dm-3 • Using the pH formula, this means that the pH of basic solutions is always above 7 • The higher the [OH-] of the base, the higher the pH