Introduction to Acids and Alkalis Study Guide

Common Acids in Daily Life and Laboratory Settings

Acids represent an essential group of chemicals that become acidic when they, or acidic substances, dissolve in water. In daily life, common acids include Hydrochloric acid ($HCl$), found in stomach acid (gastric juice) and toilet bowl cleaners; Sulphuric acid ($H_2SO_4$), utilized in drain cleaners and as the electrolyte in car batteries; and Nitric acid ($HNO_3$), used in drain cleaners and the manufacture of fertilizers. Phosphoric acid ($H_3PO_4$) is found in household cleansing agents. Organic acids are also prevalent: Methanoic acid ($HCOOH$), also known as formic acid, is found in the secretions of ants; Ethanoic acid ($CH_3COOH$), or acetic acid, is the primary component of vinegar and pickled food; Carbonic acid ($H_2CO_3$) is found in fizzy soft drinks; Lactic acid is in yoghurt and cosmetics; and Citric acid is found in citrus fruits like lemons.

Additional acids include Oxalic acid ($H_2C_2O_4$) found in spinach; Ascorbic acid, which is Vitamin C; Benzoic acid ($C_6H_5COOH$) and Sorbic acid, both used as food preservatives; Hypochlorous acid ($HOCl$) found in chlorine water; and Nitrous acid ($HNO_2$) and Sulphurous acid ($H_2SO_3$), both of which are present in acid rain. In the laboratory, mineral acids such as Hydrochloric acid, Sulphuric acid, and Nitric acid are the primary focus. Hydrochloric acid ($HCl$) is a gas at room temperature and pressure that is highly soluble in water. It is dissolved using an inverted funnel setup to prevent the "sucking back" of water and to increase the surface area for dissolution. Sulphuric and Nitric acids are manufactured via the Contact Process and Ostwald Process, respectively.

Characteristics and Properties of Dilute Acids

Dilute acids possess several defining characteristics. They have a sour taste and exhibit electrical conductivity because acid molecules dissolve in water to release mobile ions, making them electrolytes. At room temperature and pressure ($25^{\circ}C, 1\,atm$), acids have a pH < 7. They cause specific color changes in indicators: Litmus paper or solution turns red; Universal indicator or pH paper turns red or yellow; Methyl Orange turns red; and Phenolphthalein remains colourless. In terms of safety, very dilute acids are irritants, but as concentration increases, they become more corrosive.

Water plays a critical role in exhibiting these properties. For instance, citric acid crystals do not change the color of dry blue litmus paper or react with magnesium strips. However, an aqueous solution of citric acid turns blue litmus paper red and reacts with magnesium to release gas bubbles. This demonstrates that water is necessary for acids to ionize into $H^+$ ions and exhibit acidic behavior.

lonization and the Nature of the Hydrogen Ion

An acid is a hydrogen-containing covalent compound that produces hydrogen ions ($H^+(aq)$) as the only cations when dissolved in water. This process is known as ionization or dissociation. For example: $HCl(g) \xrightarrow{\text{water}} HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)$ $H_2SO_4(l) \xrightarrow{\text{water}} H_2SO_4(aq) \rightarrow H^+(aq) + HSO_4^-(aq) \rightleftharpoons 2\,H^+(aq) + SO_4^{2-}(aq)$ $HNO_3(l) \xrightarrow{\text{water}} HNO_3(aq) \rightarrow H^+(aq) + NO_3^-(aq)$

Remarkably, some acidic compounds like Sulphur dioxide ($SO_2$), Nitrogen dioxide ($NO_2$), and Chlorine ($Cl_2$) do not contain hydrogen themselves but are described as acidic because they react with water to produce hydrogen ions. For example, $SO_2(g) + H_2O(l) \rightleftharpoons H_2SO_3(aq)$, and the resulting sulphurous acid dissociates to give $H^+$ ions. In aqueous solution, a single proton ($H^+$) is unstable and combines with a water molecule via a dative covalent bond to form the hydroxonium ion ($H_3O^+$). This bond forms because the oxygen atom in water has a lone pair of electrons while the $H^+$ ion has a vacant electron shell. While $H_3O^+$ is the actual species, $H^+$ is commonly used for convenience.

Basicity of Acids

Basicity refers to the maximum number of hydrogen ions produced by one acid molecule when dissolved in water, or the number of ionizable hydrogen atoms in the molecule. Acids can be classified as monobasic (e.g., $HCl, HNO_3, HCOOH, CH_3COOH$), dibasic (e.g., $H_2SO_4, H_2C_2O_4, H_2CO_3, H_2SO_3$), or tribasic (e.g., $H_3PO_4$, Citric acid). Acids that produce more than one hydrogen ion are called polybasic acids and ionize in multiple steps. In organic acids, only the hydrogen atoms within the carboxyl group ($-COOH$) are ionizable due to the relatively weak $O-H$ bond; other hydrogen atoms in the molecule are not ionizable.

Reactions of Dilute Acids

Dilute acids react with various substances, with the hydrogen ion as the primary reactant. Acids react with metals more reactive than copper to form salt and hydrogen gas: $\text{Metal} + \text{Dilute acid} \rightarrow \text{Salt} + \text{Hydrogen}$. For example, $Mg(s) + 2\,HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$. Observable changes include the metal dissolving, gas bubbles forming, the solution becoming warmer (exothermic), and possible color changes if metal ions are colored.

Acids react with bases (metal oxides and metal hydroxides) in a neutralization reaction to form salt and water only: $\text{Base} + \text{Acid} \rightarrow \text{Salt} + \text{Water}$. Examples include: $MgO(s) + 2\,HCl(aq) \rightarrow MgCl_2(aq) + H_2O(l)$ $Mg(OH)_2(s) + H_2SO_4(aq) \rightarrow MgSO_4(aq) + 2\,H_2O(l)$ No gas bubbles are observed in neutralization. Acids also react with carbonates or hydrogencarbonates to release carbon dioxide gas (effervescence): $\text{Metal carbonate} + \text{Acid} \rightarrow \text{Salt} + \text{Water} + \text{CO}_2$. For instance, $CaCO_3(s) + 2\,HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$. Effervescent tablets, like Vitamin C tablets, utilize this reaction between sodium bicarbonate and solid organic acids (like citric acid) once dissolved in water.

Additionally, acids react with sulphites to release sulphur dioxide gas, which has a distinct choking smell: $\text{Metal sulphite} + \text{Acid} \rightarrow \text{Salt} + \text{Water} + \text{SO}_2$. Specific acids result in specific salts: phosphoric acid forms phosphates, ethanoic acid forms ethanoates (e.g., $CH_3COOK$ where the anion is written before the cation), and oxalic acid forms oxalates.

Common Bases and Alkalis

A base is a compound that reacts with an acid to form salt and water only. An alkali is a base that is soluble in water and produces hydroxide ions ($OH^-(aq)$). Common alkalis include Sodium hydroxide ($NaOH$, caustic soda), Potassium hydroxide ($KOH$, caustic potash), Calcium hydroxide ($Ca(OH)_2$, limewater), and Aqueous ammonia ($NH_3(aq)$). Insoluble bases include many metal oxides and hydroxides like Copper(II) oxide and Iron(III) hydroxide. Note that carbonates are not bases because they produce $CO_2$ with acids.

Alkalis are bitter, feel slippery, and conduct electricity as electrolytes. They have a pH > 7 and turn litmus blue, pH paper blue/purple, methyl orange yellow, and phenolphthalein pink. Preparation of laboratory alkalis involves dissolving pellets ($NaOH/KOH$) in water, reacting quicklime ($CaO$) with water ($CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq)$), or dissolving ammonia gas in water using an inverted funnel. Ammonia is highly soluble and only partially ionizes ($NH_3(g) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$). The ammonia fountain experiment demonstrates this high solubility and the resulting alkalinity.

Reactions of Alkalis

Alkalis undergo neutralization with acids: $OH^-(aq) + H^+(aq) \rightarrow H_2O(l)$. They also react with non-metal oxides like carbon dioxide and sulphur dioxide; for example, limewater turns milky when it reacts with $CO_2$ to form $CaCO_3(s)$. When heated with ammonium compounds, alkalis release ammonia gas: $\text{NH}_4^+(aq) + OH^-(aq) \rightarrow NH_3(g) + H_2O(l)$.

Reaction with metal ions in solution results in metal hydroxide precipitates. With excess $NaOH$, precipitates of $Zn^{2+}, Al^{3+},$ and $Pb^{2+}$ redissolve to form soluble complex salts (e.g., $[Zn(OH)_4]^{2-}(aq)$). With excess aqueous ammonia, only $Zn^{2+}$ (yielding a colourless solution) and $Cu^{2+}$ (yielding a deep blue solution) redissolve. Specifically, $Cu(OH)_2(s) + 4\,NH_3(aq) \rightarrow [Cu(NH_3)_4]^{2+}(aq) + 2\,OH^-(aq)$.

Qualitative Analysis and Chemical Tests

Qualitative analysis identifies substances based on observable changes. Tests for gases include: Hydrogen (burning splint, 'pop' sound), Oxygen (glowing splint relights), Carbon dioxide (limewater turns milky), Chlorine (moist blue litmus turns red then white), and Ammonia (moist red litmus turns blue, or dense white fumes with conc. $HCl$). Water is tested using dry blue cobalt(II) chloride paper (turns pink) or anhydrous copper(II) sulphate (turns white to blue).

Flame tests identify cations: Potassium (lilac), Sodium (golden yellow), Calcium (brick-red), and Copper(II) (bluish green). Ammonium ions are tested by heating with $NaOH$ to release pungent ammonia gas. For anions: Halides ($Cl^-, Br^-, I^-$) are tested with acidified silver nitrate to form white, creamy, and yellow precipitates, respectively. Sulphate ($SO_4^{2-}$) is tested with acidified barium chloride to form a white precipitate. Carbonate ($CO_3^{2-}$) is tested with dilute $HCl$ to release $CO_2$ gas.

Concentrated Acids and Alkalis

Concentrated acids (Conc. $HCl$ ~35%, Conc. $HNO_3$ ~70%, Conc. $H_2SO_4$ ~98%) have unique properties. Conc. $HCl$ is volatile and forms acid mist. Conc. $HNO_3$ is a strong oxidizing agent; it reacts with metals like copper to produce nitrogen dioxide ($NO_2$, brown gas) or nitrogen monoxide ($NO$, colourless). It decomposes under sunlight (photolysis) and must be stored in brown bottles.

Conc. $H_2SO_4$ ($H_2SO_4(l)$) is a strong oxidizing agent (releasing $SO_2$ with metals), a dehydrating agent (removing water from compounds like sugar, leaving black carbon), and is hygroscopic (absorbing air moisture). Safety in dilution is paramount: always add concentrated acid/alkali to a large amount of water slowly with continuous stirring to manage the intense heat released. Concentrated alkalis are highly corrosive and can dissolve skin; potassium hydroxide is used in biocremation to break down human remains into mineral ash.

Drying Agents

Drying agents are hygroscopic substances used to dry chemicals without reacting with them. Common agents include Silica gel, anhydrous calcium chloride (cannot dry alkaline ammonia), Concentrated sulphuric acid (cannot dry alkaline gases), and Calcium oxide (cannot dry acidic gases like $SO_2$). Laboratory setups for drying include desiccators, U-tubes, and drying tubes.

Questions & Discussion

Q: Why was sulphur dioxide released only after water was added to a mixture of solid sodium sulphite and solid oxalic acid?A: Solid oxalic acid cannot ionize to release $H^+$ ions without water. Once water is added, the acid dissolves and ionizes, allowing the $H^+$ ions to react with the sodium sulphite to produce sulphur dioxide gas.

Q: How is the dative covalent bond formed in the hydroxonium ion ($H_3O^+$)?A: There is a lone pair of electrons on the oxygen atom in the water molecule. The $H^+$ ion has an empty electron shell (vacant site). The dative covalent bond is formed by the oxygen atom sharing its lone pair of electrons with the $H^+$ ion.

Q: Why do more gas bubbles appear in carbonated water when magnesium reacts with acid in an immersed test tube?A: The reaction between magnesium and acid is exothermic and releases heat. Carbon dioxide is less soluble in carbonated water as temperature increases, causing more gas to evolve and form bubbles.

Q: Describe a chemical test to distinguish ZnCl2 and MgCl2.A: Add excess $NaOH$ or ammonia solution to both. Both form a white precipitate initially, but only the precipitate formed by $ZnCl_2$ redissolves in excess reagent to form a colourless solution.