Acids and Bases

Acids are substances that turn blue litmus red, and usually react with metals such as zinc, releasing hydrogen. Examples: hydrochloric acid (HCl), sulfuric acid (H 2 SO 4 ) and nitric acid (HNO 3 ) Bases are substances that turn red litmus blue, and neutralise acids. Examples: sodium hydroxide (NaOH), sodium carbonate (Na 2 CO 3 ) and ammonia (NH 3 )

An alkali is a base that dissolves in water. Sodium hydroxide is an alkali and a solution of this is said to be alkaline. A monobasic acid has one hydrogen atom per molecule that is removable by reaction with a base, e.g HCl A dibasic acid has two hydrogen atoms per molecule that are removable by reaction with a base, e.g. H 2 SO 4

Household acids include vinegar, lemon juice and dilute sulphuric acid (found in car batteries) Household bases include ammonia, sodium hydroxide and magnesium hydroxide When an acid reacts with a base a salt is formed. For example when hydrochloric acid reacts with sodium hydroxide, the salt sodium chloride is formed HCl + NaOH -> NaCl + H 2 O When sulphuric acid reacts with ammonia, the salt ammonium sulphate is formed: H 2 SO 4 + 2NH 3 -> (NH 4 )2SO 4

Neutralisation of an acid by a base has many everyday applications. For example magnesium hydroxide and sodium hydrogencarbonate are bases used in stomach powders to treat acid indigestion. The excess hydrochloric acid is neutralised: 2HCl + Mg(OH) 2 -> MgCl 2 + 2H 2 0 HCl + NaHCO 3 -> NaCl + H 2 O + CO 2

Arrhenius’ theory of acids and bases Arrhenius stated that: An acid is a neutral molecule (HX), which dissociates in water to form a hydrogen ion and an anion: HX -> H + + X - The acidic properties of the solution are due to the presence of H + ions

A base (MOH) dissociates in water to form a hydroxide ion and a cation: MOH -> OH - + M + The basic properties of the solution are due to the presence of OH - ions.

In solution strong acids and bases are fully dissociated In solution weak acids and bases are only slightly dissociated An Arrhenius acid is a substance that dissociates in aqueous solution, forming hydrogen ions. An Arrhenius base is a substance that dissociates in aqueous solution, forming hydroxide ions

According to the Arrhenius theory, neutralisation of a solution of a strong acid such as HCl by a solution of a strong base such as NaOH, occurs as follows: In solution the strong acid HCl is fully dissociated into H + and Cl - ions, while the strong base NaOH is fully dissociated into Na + and OH - ions. When the two solutions are mixed, the H + ions react with the OH - ions: H + + OH - -> H 2 O The other two ions present, Na + and Cl -, do not react.

The Arrhenius theory explains why strong acids only conduct electricity when dissolved in water, there are no ions present in a pure acid. A weak acid solution does not conduct electricity as well as a solution of a strong acid of a similar concentration as there are fewer ions in the weak acid solution. For similar reasons, a solution of a weak base does not conduct electricity as well as a solution of a strong base of a similar concentration. The Arrthenius theory was the first successful theory of acids and bases, but it has its limitations. In more recent times, more advanced theories of acids and bases have been developed.

Bronsted-Lowry theory of acids and bases According to the Bronsted-Lowry theory: An acid is proton donor A base is a proton acceptor An acid-base reaction involves transfer of a proton (H + ) from the acid to the base. The stronger the acid, the more readily it transfers a proton The stronger the base, the more readily it accepts a proton The weaker the acid, the less readily it transfers a proton The weaker the base, the less readily it accepts a proton

When HCl is added to water, the acid, HCl, transfers a proton to water, which in this case, being a proton acceptor is a base: HCl (g) + H2O (l) ⇌ H 3 O + (aq) + Cl - (aq)

The Bronsted-Lowry theory of acids and bases has a number of advantages compared to the Arrhenius theory, it broadens the range of reactions that can be regarded as acid-base reactions. (Arrhenius only dealt with reactions in water.) For example: ammonia reacts with hydrogen chloride gas in the absence of water NH 3(g) + HCl (g) ⇌ NH 4 + (g) + Cl - (g) NH 4 Cl (s) Ammonia acts as a Bronsted-Lowry base, because it accepts a proton from the Bronsted-Lowry acid, hydrogen chloride.

The Bronsted-Lowry theory also explains how substances can be amphoteric. For example when water reacts with ammonia it acts as an acid, donating a proton to ammonia. NH 3(g) + H 2 O (l) ⇌ NH 4 + (aq) + OH - (aq) When it reacts with nitric acid it acts as a base, accepting a proton from nitric acid HNO 3(aq) + H 2 O (l) ⇌ H 3 O + (aq) + NO 3 - (aq)

Conjugate acid-base pairs In the reaction of ammonia with water NH 3 + H 2 O ⇌ NH OH - Ammonia is a base and accepts a proton to form the ammonium ion, NH 4 +. Water acts as an acid, donating a proton to ammonia, the hydroxide ion, OH -, is formed. This is a reversible reaction, so the ammonium ion, NH4+, is an acid, since it can donate a proton. The hydroxide ion, OH-, is a base, since it can accept a proton. The acids and the bases in the above reaction are related as follows: NH 3 ⇌ NH 4 + H 2 O ⇌ OH -

Species such as these that differ by the presence or absence of a proton are called conjugate acid-base pairs. H 2 O and OH - form one conjugate pair, while NH 3 and NH 4 + form another. NH 4 + is called the conjugate acid of NH 3, while OH - is called the conjugate base of H 2 O A conjugate acid is formed when a proton (H+) is added to the Bronsted-Lowry base. A conjugate base is formed when a proton (H+) is removed from a Bronsted-Lowry acid.

A conjugate acid-base pair is an acid and base that differ by the presence or absence of a proton In the reaction: HNO 3 + H 2 O ⇌ H 3 O + + NO 3 - The product H 3 O + is acidic and the product NO 3 - is basic. An acid-base reaction such as this tends to lead preferentially to the formation of the weaker acid and base. HNO 3 is a stronger acid than H 3 O +, while H 2 O is a stronger base than NO 3 -. As a result, the reaction goes mainly to the right.

In the reaction: CH 3 COOH + H 2 O ⇌ CH 3 COO - + H 3 O + The reaction goes mainly to the left. This is because CH 3 COOH is a weaker acid than H 3 O +, and H 2 O is a weaker base than CH 3 COO -. In general, the stronger an acid, the weaker its conjugate base, and the stronger a base, the weaker its conjugate acid.