Chemical Reactions Involving Acids

This is part of Year 11 HSC Chemistry course under the topic of Chemical Reactions

HSC Chemistry Syllabus

Conduct investigations to predict and identify the products of a range of reactions, for example:

– Acid/Base Reactions

– Acid/Carbonate Reactions

Acid-Base Reactions

This video discusses fundamentals of acid-base reactions including the reaction between acid and metal hydroxides and metal carbonate/hydrogen carbonate.

What are Acids and Bases?

In the context of the NSW HSC Chemistry syllabus, acids and bases are defined by two prominent theories: Arrhenius, and Brønsted - Lowry theories of acids and bases.

Arrhenius Theory

Arrhenius acid is a compound that dissociates in water to produce the hydrogen ion `H^+` in water. For instance, hydrochloric acid dissociates into hydrogen and chloride `Cl^–` ions and hence it is classified as an Arrhenius acid. Keep in mind that not all hydrogen containing substance are acids; only molecules that can dissociate in water to produce hydrogen ions are considered as acids.

Arrhenius base is a compound which dissociates to produce the hydroxide ion `OH^–` in water. An example of on such compound is sodium hydroxide which dissociates to form sodium `Na^+` and hydroxide ions.

Brønsted-Lowry Theory

The Brønsted-Lowry acid-base theory defines an acid as a proton donor i.e., a hydrogen ion donor, while a base acts as a proton acceptor.

The figure above demonstrates how hydrogen ions are simply protons. Hydrogen atoms consist of a proton and an electron, which when losing an electron to form an ion, leaves just a proton behind.

Arrhenius and Brønsted-Lowry bases are explored in further detail in Year 12 Module 6: Acids-base Reactions of the NSW HSC Chemistry Syllabus.

Neutralisation

A neutralisation reaction is a type of chemical reaction in which an acid and a base (metal hydroxide) react to produce a salt and water.

$$\text{acid} + \text{base} \rightarrow \text{salt} + \text{water}$$

$$\text{acid} + \text{metal hydroxide} \rightarrow \text{salt} + \text{water}$$

For example, the reaction between hydrochloride acid and sodium hydroxide:

$$HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H2O(l)$$

In the above reaction, sodium and chloride ions are spectator ions as they are aqueous before and after the neutralisation.

Therefore the net ionic equation is:

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

This net ionic equation represents Arrhenius' definition of an acid-base reaction. Arrhenius defined the reaction between an acid and a base (metal hydroxide) as the reaction between the hydrogen ion (from acid) and hydroxide ion (from base) to form water.

Under the model of the Brønsted-Lowry acid-base theory, neutralisation takes place when an acid donates a hydrogen ion to a base which accepts it. To illustrate this, consider the reaction between hydrochloric acid and ammonia.

$$HCl(aq) + NH_3(aq) \rightarrow NH_4^+(aq) + Cl^–(aq)$$

In this reaction, hydrochloric acid donates a proton to ammonia (which accepts a proton). This forms chloride ion and ammonium ion as the products.

Acid and Metal Reactions

These reactions can be generalised as:

$$\text{acid} + \text{metal} \rightarrow \text{salt} + \text{hydrogen gas}$$

Acid-metal reactions are still considered as acid-base reactions because of the formation of hydroxide when metals react with water.

The stepwise reactions between sodium metal and aqueous hydrochloric acid are demonstrated by the following equations.

Step 1: Sodium reacts with water to form sodium hydroxide and hydrogen gas:

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

Step 2: Sodium hydroxide then neutralises with hydrochloric acid (acid-base reaction):

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

Overall Equation:

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

Note: Consider sodium's reaction with water which forms sodium hydroxide. This then reacts with hydrochloric acid resulting in salt and water. The aqueous nature indicates that water is not included in the reaction.

Quick test: The pop test can be used to verify acid and metal reactions. Combusting small amounts of hydrogen gas results in a squeaky sound, signalling the production of water – a highly exothermic process.

Acids and Metal Oxides

Reactions between acids and metal oxides generally result in the formation of salt and water. Initially the reaction forms a hydroxide when the oxide reacts with water. This hydroxide then reacts with the acid to produce salt and water.

$$\text{acid} + \text{metal oxide} \rightarrow \text{salt} + \text{water}$$

E.g. reaction between calcium oxide and hydrochloric acid

$$CaO(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l)$$

Similar to acid and metal reactions, the reaction between acids and metal oxides occurs in steps:

Step 1: Calcium oxide reacts with water to form calcium hydroxide:

$$CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq)$$

Step 2: Calcium hydroxide then neutralises with hydrochloric acid (acid-base reaction):

$$Ca(OH)_2(aq) + 2HCl(aq) \rightarrow CaCl_2(aq) + 2H_2O(l)$$

Reactions of Acids with Metal Carbonates:

Acids and Metal Carbonate

An acid reacting with a carbonate gives rise to a salt, water, and carbon dioxide.

$$\text{acid} + \text{metal carbonate} \rightarrow \text{salt} + \text{water} + \text{carbon dioxide}$$

Consider the reaction between hydrochloric acid and calcium carbonate.

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

Acids and Metal Hydrogen Carbonate

The reaction between acid and metal hydrogen carbonate also produces a salt, water and carbon dioxide.

$$\text{acid} + \text{metal hydrogen carbonate} \rightarrow \text{salt} + \text{water} + \text{carbon dioxide}$$

Consider the reaction between hydrochloric acid and calcium hydrogen carbonate.

$$2HCl(aq) + Ca(HCO_3)_2(aq) \rightarrow CaCl_2(aq) + 2H_2O(l) + 2CO_2(g)$$

Comparing the two examples above, the main difference between the two reactions is the different stoichiometric ratios between the reactants and products.