Unit 5: Acids, Bases, Salts & Neutralisation
Understanding the fundamental definitions and reactions of acids, bases, and salts.
5.1 Introduction to Acids
Acids are a fundamental class of chemical compounds with distinct properties, most notably their sour taste (like in lemons). There are two key definitions used to describe them:
The Arrhenius definition states that an acid is a substance that produces hydrogen ions ($H^+$) when dissolved in water. These $H^+$ ions are responsible for the characteristic properties of acids. For example:
- Hydrochloric acid: $HCl(g) \rightarrow H^+(aq) + Cl^-(aq)$
- Nitric acid: $HNO_3(l) \rightarrow H^+(aq) + NO_3^-(aq)$
- Sulphuric acid: $H_2SO_4(l) \rightarrow 2H^+(aq) + SO_4^{2-}(aq)$
The Brønsted-Lowry definition provides a broader perspective. It defines an acid as a proton ($H^+$) donor. In any reaction involving an acid, the acid molecule or ion donates a proton to another substance (a base). This definition is more general because it is not limited to aqueous solutions. All Arrhenius acids are also Brønsted-Lowry acids.
Solved Examples:
- Identify the Arrhenius acid: $NaOH$, $HCl$, $NaCl$.
Solution: $HCl$ is the Arrhenius acid because it dissociates in water to produce $H^+$ ions. - Write the dissociation equation for phosphoric acid ($H_3PO_4$) in water.
Solution: $H_3PO_4(l) \rightarrow 3H^+(aq) + PO_4^{3-}(aq)$. It can donate three protons. - Is ammonia ($NH_3$) a Brønsted-Lowry acid?
Solution: No. In its typical reactions, ammonia accepts a proton to become $NH_4^+$, making it a Brønsted-Lowry base, not an acid. - When ethanoic acid ($CH_3COOH$) dissolves in water, it donates a proton to a water molecule. Write the equation.
Solution: $CH_3COOH(aq) + H_2O(l) \rightleftharpoons CH_3COO^-(aq) + H_3O^+(aq)$. Here, ethanoic acid acts as a Brønsted-Lowry acid. - Why is sulphuric acid considered a diprotic acid?
Solution: It is called diprotic because each molecule can donate two protons ($H^+$ ions) in solution. - Name the acid formed when hydrogen bromide ($HBr$) gas dissolves in water.
Solution: Hydrobromic acid. - Show how the ammonium ion ($NH_4^+$) can act as a Brønsted-Lowry acid.
Solution: The ammonium ion can donate a proton to a base, for example: $NH_4^+(aq) + OH^-(aq) \rightarrow NH_3(aq) + H_2O(l)$. - What is the anion produced when nitric acid ($HNO_3$) dissociates?
Solution: The nitrate ion, $NO_3^-$. - Is pure, gaseous hydrogen chloride ($HCl$) an Arrhenius acid?
Solution: No. According to the Arrhenius definition, a substance must be dissolved in water to produce $H^+$ ions. Pure, gaseous $HCl$ does not contain ions. - Identify the proton donor in the reaction: $H_2SO_4 + Cl^- \rightarrow HCl + HSO_4^-$.
Solution: $H_2SO_4$ is the proton donor (the Brønsted-Lowry acid) as it gives a proton to the $Cl^-$ ion.
5.2 Bases and Alkalis
Bases are substances that react with acids. Like acids, they have two common definitions:
The Arrhenius definition states that a base is a substance that produces hydroxide ions ($OH^-$) when dissolved in water. Common examples include metal hydroxides, oxides, and carbonates.
The Brønsted-Lowry definition is more general, defining a base as a proton ($H^+$) acceptor. A base reacts by accepting a proton from an acid. The most common types of species that act as bases are hydroxide ions ($OH^-$), oxide ions ($O^{2-}$), carbonate ions ($CO_3^{2-}$), and ammonia ($NH_3$).
An alkali is a specific type of base: it is a base that is soluble in water. All alkalis are bases, but not all bases are alkalis. For example, sodium hydroxide ($NaOH$) is an alkali because it is a base that dissolves in water. Copper(II) oxide ($CuO$) is a base because it reacts with acids, but it is not an alkali because it is insoluble in water.
Solved Examples:
- Is calcium carbonate ($CaCO_3$) a base or an alkali?
Solution: It is a base because it reacts with acids. It is not an alkali because it is insoluble in water. - Write an equation showing sodium hydroxide ($NaOH$) acting as an Arrhenius base.
Solution: $NaOH(s) \rightarrow Na^+(aq) + OH^-(aq)$. It dissolves in water to produce hydroxide ions. - Show how the oxide ion ($O^{2-}$) acts as a Brønsted-Lowry base.
Solution: It accepts two protons to form water: $O^{2-}(aq) + 2H^+(aq) \rightarrow H_2O(l)$. - Why is ammonia ($NH_3$) considered a Brønsted-Lowry base?
Solution: It accepts a proton ($H^+$) to form the ammonium ion ($NH_4^+$). For example: $NH_3(g) + H^+(aq) \rightarrow NH_4^+(aq)$. - Classify potassium oxide ($K_2O$) as a base or alkali.
Solution: It is an alkali. It is a base that is soluble in water, reacting to form potassium hydroxide: $K_2O(s) + H_2O(l) \rightarrow 2KOH(aq)$. - Identify the proton acceptor in the reaction: $HCl + NH_3 \rightarrow NH_4Cl$.
Solution: $NH_3$ is the proton acceptor (the Brønsted-Lowry base). - Which of these is NOT a base: $CuO$, $SO_2$, $MgO$?
Solution: $SO_2$ (sulfur dioxide) is an acidic oxide, not a basic one. $CuO$ and $MgO$ are basic oxides. - Write the equation for the reaction of carbonate ions ($CO_3^{2-}$) with hydrogen ions.
Solution: $CO_3^{2-}(aq) + 2H^+(aq) \rightarrow H_2O(l) + CO_2(g)$. The carbonate ion accepts two protons. - Name a common laboratory alkali.
Solution: Sodium hydroxide ($NaOH$) or potassium hydroxide ($KOH$). - Explain the difference between a base and an alkali using the examples of iron(III) hydroxide and sodium hydroxide.
Solution: Both are bases as they neutralize acids. However, sodium hydroxide is an alkali because it dissolves in water, while iron(III) hydroxide is not an alkali because it is insoluble.
5.3 Salts and Neutralisation Reactions
A salt is an ionic compound formed when the hydrogen ion ($H^+$) of an acid is replaced by a metal ion or an ammonium ion ($NH_4^+$). For example, in the reaction between hydrochloric acid ($HCl$) and sodium hydroxide ($NaOH$), the $H^+$ from the acid is replaced by the $Na^+$ from the base to form the salt sodium chloride ($NaCl$).
The reaction between an acid and a base is called a neutralisation reaction. In this reaction, the acidic and basic properties are cancelled out, or 'neutralised', to form a salt and, in most cases, water.
There are four general types of neutralisation reactions:
- Acid + Metal Hydroxide → Salt + Water
Example: $2HCl(aq) + Ca(OH)_2(aq) \rightarrow CaCl_2(aq) + 2H_2O(l)$ - Acid + Metal Oxide → Salt + Water
Example: $H_2SO_4(aq) + CuO(s) \rightarrow CuSO_4(aq) + H_2O(l)$ - Acid + Metal Carbonate → Salt + Water + Carbon Dioxide
Example: $2HNO_3(aq) + Na_2CO_3(s) \rightarrow 2NaNO_3(aq) + H_2O(l) + CO_2(g)$ - Acid + Ammonia → Ammonium Salt
Example: $HCl(aq) + NH_3(aq) \rightarrow NH_4Cl(aq)$
Solved Examples:
- What salt is produced when sulphuric acid ($H_2SO_4$) reacts with potassium hydroxide ($KOH$)?
Solution: Potassium sulphate ($K_2SO_4$). The equation is $H_2SO_4 + 2KOH \rightarrow K_2SO_4 + 2H_2O$. - Write a balanced equation for the reaction between nitric acid and magnesium oxide.
Solution: $2HNO_3(aq) + MgO(s) \rightarrow Mg(NO_3)_2(aq) + H_2O(l)$. - Identify the products of the reaction between hydrochloric acid and zinc carbonate.
Solution: Zinc chloride ($ZnCl_2$), water ($H_2O$), and carbon dioxide ($CO_2$). - Name the salt formed when sulphuric acid reacts with ammonia.
Solution: Ammonium sulphate, $(NH_4)_2SO_4$. - Complete and balance the equation: $H_3PO_4 + NaOH \rightarrow$
Solution: $H_3PO_4(aq) + 3NaOH(aq) \rightarrow Na_3PO_4(aq) + 3H_2O(l)$. - What gas is produced when an acid reacts with a metal carbonate?
Solution: Carbon dioxide ($CO_2$). - A student mixes hydrochloric acid and potassium hydroxide. What type of reaction is this?
Solution: A neutralisation reaction. - What are the spectator ions in the reaction between $HNO_3(aq)$ and $KOH(aq)$?
Solution: The spectator ions are $K^+(aq)$ and $NO_3^-(aq)$. The net ionic equation is $H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$. - Write the formula for the salt calcium nitrate.
Solution: $Ca(NO_3)_2$. It is formed from the $Ca^{2+}$ ion and the $NO_3^-$ ion. - Predict the products of the reaction between ethanoic acid ($CH_3COOH$) and sodium hydroxide ($NaOH$).
Solution: Sodium ethanoate ($CH_3COONa$) and water ($H_2O$).