M6-S3 Acid-Base Reactions

Acid-Base Reactions

  • Acid-base reactions encompass a wide range of different reactions. They are more commonly referred to as neutralisation reactions because acids and bases neutralise each other’s chemical properties.

 

  • Acid-base reactions, in the Brønsted-Lowry theory, are reactions that involve transfer of protons or H+ This type of reaction is best exemplified by the dissociation of acid (or base) in water. In this reaction, water acts as both a solvent and base because it accepts a proton from the acid.

 $$HCl(aq) + H_2O(l) \rightarrow Cl^-(aq) + H_3O^+(aq)$$

 acid           base     conj. base    conj. acid

 

  • Acid-base reactions, in the Arrhenius theory always produce water (in addition to salt) because H+ ions (produced by acids) and OH ions (produced by bases) react in aqueous solution to form water.

 $$HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)$$

  acid            base              salt            water

 

  • Not all neutralisations produce water. For example, the reaction between ammonia (base) and hydrogen chloride (acid):

 

$$NH_3(g) + HCl(g) \leftrightharpoons NH_4Cl(s)$$

base          acid            salt

 

  • All reactions with acids form salts which are defined as an ionic compound that consists of an anion of acid and a cation of base. In other words, a salt consists of a conjugate base and a conjugate acid. The ions which make up the salt are spectator ions because they remain dissolved as ions.

 

Neutral species equation:

 $$2HNO_3(aq) + Ca(OH)_2(aq) \rightarrow Ca(NO_3)_2(aq) + 2H_2O(l)$$

Complete species equation:

 $$Ca^{2+}(aq) + 2OH^-(aq) + 2H^+ $$

Net ionic equation:

$$H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$$

 

Acids + metals → salt + hydrogen gas 

  • Reaction between acids and metals is a type of acid-base reaction.

Overall equation

 $$HCl(aq) + Na(s) \rightarrow NaCl(aq) + H_2(g)$$

  • Stepwise equations

Sodium metal reacts with water to form sodium hydroxide (strong base):

 

$$Na(s) + H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)$$

 

Sodium hydroxide then undergoes neutralisation with hydrochloric acid (HCl) to produce salt and water:

 $$NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)$$

 

  • The reaction between active metals (low ionisation energy) and dilute acids are extremely volatile. The products are, salt, hydrogen gas and heat – latter two react to produce explosions.

Metal

Observed Reaction with Acid

K, Na

Rapid Effervescence producing hydrogen gas which may ignite

Ca, Mg

Rapid bubbling leading to the release of hydrogen gas

Al, Zn, Fe, Sn, Pb

Moderate to very slow bubbling as hydrogen is released; reaction is faster in warm acid; lead stops reacting when coated with insoluble PbCl2 or PbSO4

Cu, Hg, Ag, Au

No Reaction

 

  • The production of hydrogen gas can be observed from bubbling and tested by conducting a ‘pop’ test. When hydrogen gas is lit in the presence of oxygen, they react to form water while producing a squeaky pop sound. The water condenses inside the test tube.

 $$H_2(g) + O_2(g) \rightarrow H_2O(l)$$

 

  • Some metals e.g. copper can react with acid to form gases other than hydrogen gas. For example, the reaction between copper metal and concentrated sulfuric acid produces sulfur dioxide.

 $$Cu(s) + H_2SO_4(aq) + 2H^+(aq) \rightarrow Cu^{2+}(aq) + SO_2(g) + 2H_2O(l)$$

 

  • Acids also undergo oxidation-reduction reactions with metals. The hydrogen ions from an acid gain electron from metals (reduction) to produce hydrogen gas. Conversely, metals lose electrons (oxidation) to produce cations.

 $$HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)$$

 

Acid + metal hydroxide → salt + water

  • Reaction between acid and metal hydroxide is considered an acid-base reaction in both the Arrhenius and Brønsted-Lowry definition.

 

Acid + metal oxide → salt + water

  • Metal oxides are considered basic oxides because they react with water to produce metal hydroxides which in turn produce OH ions in water.

 $$MgO(s) + H_2O(l) \rightarrow Mg(OH)_2(aq)$$

 $$Mg(OH)_2(aq) + 2HCl(aq) \rightarrow MgCl_2(aq)$$

Overall equation

 $$MgO(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2O(l)$$

 

Acid + metal carbonates/hydrogen carbonates → salt + water + carbon dioxide

  • Carbonates and hydrogen carbonates are polyatomic anionic bases in the Brønsted-Lowry theory because they do not dissociate into hydroxide ions in aqueous solution. Therefore, they are not considered as bases in the Arrhenius theory.

 

  • Reactions between acids and carbonates/hydrogen carbonates produce salts, water and carbon dioxide.

 

Acid + carbonate:

 $$2HCl(aq) + CaCO_3(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$$

Carbonate ions is hydrolysed by water to form hydrogen carbonate. In this step, carbonate ions receive protons from water. Thus, they are basic under the Brønsted-Lowry theory.

 $$CO_3^{2-}(aq) + H_2O(l) \leftrightharpoons OH^-(aq) + HCO_3^-(aq)$$

 

Hydrogen carbonate is hydrolysed by water to form carbonic acid

$$HCO_3^-(aq) + H_2O(l) \leftrightharpoons OH^-(aq) + H_2CO_3(aq)$$

 

Carbonic acid decomposes to produce carbon dioxide

 $$H_2CO_3(aq) \leftrightharpoons H_2O(l) + CO_2(g)$$

  

Acid + hydrogen carbonate:

 $$HCl(aq) + NaHCO_3(aq) \rightarrow NaCl(aq) + H_2O(l) + CO_2(g)$$

Hydrogen carbonate undergoes the same steps to produce carbon dioxide. HCO3 receives a proton from water. Thus, it is a base (Brønsted-Lowry theory)

 $$HCO_3^-(aq) + H_2O(l) \leftrightharpoons OH^-(aq) + H_2CO_3(aq)$$

 

$$H_2CO_3(aq) \leftrightharpoons H_2O(l) + CO_2(g)$$

 

 

  • Carbonate and hydrogen carbonate ions usually react with H2O alone but in the presence of an acid, the equilibrium of each hydrolysis reaction lies more to the right side. This is because H+ ions from the acid neutralises and reduces OH According to Le Chatelier’s principle, this shifts the equilibrium to the right side.

 

 

  • Carbon dioxide can be identified using the lime water test. Bubbling carbon dioxide in calcium hydroxide, Ca(OH)2 to form milky calcium carbonate, CaCO3.

 $$Ca(OH)_2(aq) + CO_2(aq) \rightarrow CaCO_3(aq) + H_2O(l)$$