Acid/Base Reactions

This is part of the HSC Chemistry course under the topic Properties of Acids and Bases.

HSC Chemistry Syllabus

Predict the products of acid reactions and write balanced equations to represent:
- acids and bases (neutralisation)
- acids and carbonates
- acids and metals (ACSCH067)

What type of reactions involve acids and bases?

This video explores various reactions involving acids and bases including reactions between:

acid and metal hydroxide (neutralisation)
acid and metal
acid and metal oxide
acid and metal (hydrogen) carbonate

Acid and 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.

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

acid base conj. base conj. acid

General equation: 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-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

Net ionic equation of an Arrhenius acid-base reaction:

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

The reaction between an acid and an Arrhenius base can be simply understood as the neutralisation between hydrogen ions and hydroxide ions.

Brønsted-Lowry Acid-base Reactions

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.

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.

Acid and Metal Reactions

General equation: acids + metal → salt + hydrogen gas

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

Overall equation

$$2HCl(aq) + 2Na(s) \rightarrow 2NaCl(aq) + H_2(g)$$

Stepwise equations

Sodium metal reacts with water to form sodium hydroxide and hydrogen gas:

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

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 and Metal Oxide Reactions

General equation: acid + metal oxide → salt + water

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

Stepwise equations

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

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

The resultant metal hydroxide neutralises with acid to produce salt and water

$$Mg(OH)_2(aq) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2O(l)$$

Acid and Metal (Hydrogen) Carbonate Reactions

General equation: acid + metal carbonates/hydrogen carbonates → salt + water + carbon dioxide

Carbonates and hydrogen carbonates are Brønsted-Lowry bases because they are able to accept proton(s).

Carbonates and hydrogen carbonates are not considered as bases in the Arrhenius theory because they do not dissociate into hydroxide ions in aqueous solution.

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 accepts a proton from HCl to form hydrogen carbonate.

$$CO_3^{2-}(aq) + HCl(aq) \leftrightharpoons Cl^-(aq) + HCO_3^-(aq)$$

Hydrogen carbonate accepts another proton from a second HCl molecule to form carbonic acid.

$$HCO_3^-(aq) + HCl(aq) \leftrightharpoons Cl^-(aq) + H_2CO_3(aq)$$

Carbonic acid then decomposes to produce water and 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 HCl.

$$HCO_3^-(aq) + HCl(aq) \leftrightharpoons Cl^-(aq) + H_2CO_3(aq)$$

Carbonic acid then decomposes to form water and carbon dioxide.