Comprehensive Study Guide to Acids, Bases, and Salts

Introduction to Acids, Bases, and Salts

• Classification of Compounds: Elements combine in fixed ratios to form compounds, which are classified into three distinct categories: acids, bases, and salts.
• General Characteristics:
  • Acids: Identified by their sour taste.
  • Bases: Identified by their bitter taste and soapy touch.
  • Salts: Formed through the combination of acids and bases.
  • Safety Warning: Not all acids and bases should be tasted or touched, as many are harmful to the body.
• Scientific Background: Lavoisier and Davy established scientific definitions for these substances:
  • All acids must contain hydrogen as their constituent element in the form of a proton ($H^+$). Examples include hydrochloric acid, sulphuric acid, and nitric acid.
  • All bases are metallic oxides ($O^{2-}$), metallic hydroxides ($OH^-$), or ammonia solutions. Examples include copper oxide, sodium hydroxide, and ammonium hydroxide.
  • All salts are chemical compounds formed by the interaction of an acid and a base, where the hydrogen ions in the acid are replaced by a metallic ion or non-metallic ammonium ions ($NH_4^+$). Examples include sodium chloride, calcium carbonate, and ammonium phosphate.

Detailed Study of Acids

• Etymology: The word "acid" is derived from the Latin word acidus, which translates to "sour."
• Naturally Occurring Acids and Their Sources:
  • Acetic acid: Vinegar
  • Citric acid: Citrus fruits (oranges and lemons)
  • Butyric acid: Rancid butter
  • Hydrochloric acid: Gastric juice
  • Formic acid: Sting of bees and ants
  • Lactic acid: Sour milk, curd
  • Malic acid: Apples
  • Oleic acid: Olive oil
  • Oxalic acid: Tomatoes
  • Stearic acid: Fats
  • Tartaric acid: Grapes, tamarind, and apples
  • Uric acid: Urine
• Definition of Acids: Acids are compounds containing one or more hydrogen atoms that, when dissolved in water, produce hydronium ions ($H_3O^+$) as the only positively charged ions.
• Mechanism of Hydronium Ion Formation:
  • An acid molecule furnishes a hydrogen ion (proton), $H^+$, in aqueous solution: $HCl \rightarrow H^+ + Cl^-$.
  • Because the $H^+$ ion cannot exist independently, it combines with a water molecule: $H^+ + H_2O \rightarrow H_3O^+$.
  • Reaction for Sulphuric Acid: $H_2SO_4 + 2H_2O \rightleftharpoons 2H_3O^+ + SO_4^{2-}$.
  • Reaction for Nitric Acid: $HNO_3 + H_2O \rightleftharpoons H_3O^+ + NO_3^-$.
  • Reaction for Acetic Acid: $CH_3COOH + H_2O \rightleftharpoons H_3O^+ + CH_3COO^-$.
• Acidity and Hydronium Ions: The acidic properties of an acid are specifically the properties of the hydronium ions present. Note that $H_3O^+$ can be written as $OH_3^+$ because the positive charge arises from the extra $H^+$ with the neutral water molecule.

Classification of Acids

• Based on Sources:
  • Organic Acids: Derived from plants. They contain carbon and hydrogen. Examples: Oxalic acid ($(COOH)_2$), Acetic acid ($CH_3COOH$), and Formic acid ($HCOOH$). They are typically weak acids and do not ionise completely.
  • Inorganic (Mineral) Acids: Derived from minerals and usually do not contain carbon. Examples: Hydrochloric acid ($HCl$), Sulphuric acid ($H_2SO_4$), and Nitric acid ($HNO_3$). They are strong acids that ionise completely. Carbonic acid ($H_2CO_3$) is an exception; it contains carbon but is a weak mineral acid used in soft drinks.
• Based on Composition:
  • Oxy-acids: Contain oxygen, hydrogen, and another element (e.g., $HNO_3$, $H_2SO_4$).
  • Hydracids: Contain hydrogen and a non-metallic element but no oxygen (e.g., $HCl$, $HBr$).
• Based on Basicity:
  • Basicity Definition: The number of hydronium ions ($H_3O^+$) produced by the ionisation of one molecule of the acid in aqueous solution.
  • Monobasic Acids: Produce one $H_3O^+$ ion per molecule. Examples: $HCl$, $HBr$, $HNO_3$, $CH_3COOH$. They ionise in one step and form one normal salt (e.g., $NaOH + HCl \rightarrow NaCl + H_2O$).
  • Dibasic Acids: Produce two $H_3O^+$ ions per molecule. Examples: $H_2SO_4$, $(COOH)_2$, $H_2SO_3$, $H_2CO_3$. They ionise in two steps and can form one acid salt and one normal salt. For instance, $H_2SO_4$ can produce $NaHSO_4$ (acid salt) and $Na_2SO_4$ (normal salt).
  • Special Case - Phosphorous Acid ($H_3PO_3$): It is a dibasic acid. In oxyacids of phosphorus, only hydrogen atoms attached to oxygen atoms are replaceable; those bonded directly to phosphorus are not.
  • Tribasic Acids: Produce three $H_3O^+$ ions per molecule. Examples: Phosphoric acid ($H_3PO_4$), Citric acid ($C_3H_5O(COOH)_3$). They ionise in three steps and can form two different acid salts ($NaH_2PO_4$, $Na_2HPO_4$) and one normal salt ($Na_3PO_4$).
  • Tetrabasic Acids: Silicic acid ($H_4SiO_4$) has a basicity of four.

Strength and Concentration of Acids

• Concentration: Refers to the amount of acid present in a definite amount of its aqueous solution.
  • Concentrated Acid: Contains very small amount of water or no water.
  • Dilute Acid: Contains significantly more water than its own mass.
• Dilution Protocol: Acid must be poured into water in small amounts with constant stirring. Water must never be added to acid because the process is highly exothermic; the resulting heat can cause splashing or shatter the container, which is potentially fatal.
• Strength: A measure of the concentration of hydronium ions produced. Strength depends on the Degree of Ionisation ($\alpha$).
  • $\alpha = \frac{\text{No. of acid molecules ionised}}{\text{Total no. of acid molecules present}} \times 100$
  • Strong Acid: $\alpha > 30\%$
  • Weak Acid: $\alpha < 30\%$
  • Example: Dilute $HCl$ is a stronger acid than highly concentrated acetic acid.

Preparation of Acids

• Synthesis: Preparation of binary acids from components.
  • $H_2 + Cl_2 \rightarrow 2HCl$
  • $H_2 + S \rightarrow H_2S$
• Action of Water on Acidic Oxides (Anhydrides):
  • $SO_3 + H_2O \rightarrow H_2SO_4$
  • $P_2O_5 + 3H_2O \rightarrow 2H_3PO_4$
  • Note: $NO_2$ is a "mixed anhydride" because it forms two acids: $2NO_2 + H_2O \rightarrow HNO_2 + HNO_3$.
• Oxidation of Non-metals: Non-metals like sulphur or phosphorus are oxidised by concentrated nitric acid.
  • $S + 6HNO_3 \rightarrow H_2SO_4 + 2H_2O + 6NO_2$
• Displacement: A less volatile acid (like $H_2SO_4$) displaces more volatile acids ($HCl$, $HNO_3$) from their salts.
  • $NaCl + H_2SO_4 \rightarrow NaHSO_4 + HCl$

Properties of Acids

• Physical Properties:
  • Taste: Sour.
  • Physical State: Solids (Boric, Oxalic, Tartaric, Citric, Phosphoric) or Liquids (Acetic, Formic, Carbonic, Hydrochloric, Nitric, Sulphurous, Sulphuric).
  • Volatility: Many liquid acids vaporise at room temperature or at $100^{\circ}C$. Sulphuric acid is non-volatile.
  • Effect on Skin: Mineral acids cause painful burns and blisters. $H_2SO_4$ chars skin black; $HNO_3$ stains it yellow; $HCl$ stains it amber. Carbonic and organic acids are non-corrosive.
• Indicators:
  • Litmus: Blue to Red.
  • Methyl orange: Orange to Pink.
  • Phenolphthalein: Remains colourless.
  • Red cabbage extract: Stays red (turns green in bases).
  • Olfactory Indicators: Substances like onion, vanilla, and clove oil whose smell changes. Onion smell disappears in basic solutions but remains in acidic solutions.
• Chemical Properties:
  • Reaction with Metals: $Active Metal + Dilute Acid \rightarrow Salt + H_2 \uparrow$. Note: $HNO_3$ is usually not used because it is a strong oxidising agent and produces water instead of $H_2$, except with $1\%$ dilute acid reacting with magnesium or manganese.
  • Neutralisation: $Base + Acid \rightarrow Salt + H_2O$.
  • Carbonates/Bicarbonates: Produce salt, water, and $CO_2$. Example: $CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2 \uparrow$. Reaction fails if the produced salt is insoluble (e.g., Lead carbonate with $HCl$).
  • Sulphites/Bisulphites: Produce salt, water, and $SO_2$.
  • Sulphides: Produce salt and $H_2S$ (hydrogen sulphide).
  • Chlorides/Nitrates: Generally require concentrated $H_2SO_4$ and heat to displace volatile acids.

Understanding Bases and Alkalis

• Base Definition: A metallic oxide, hydroxide, or ammonium hydroxide that reacts with hydronium ions of an acid to form salt and water only.
  • Basic Oxide: Contains $O^{2-}$ ion.
  • Basic Hydroxide: Contains $OH^-$ ion.
  • Exception: Lead (IV) oxide ($PbO_2$) is not a base because it reacts with hydrochloric acid to produce chlorine gas in addition to salt and water: $PbO_2 + 4HCl \rightarrow PbCl_2 + Cl_2 + 2H_2O$.
• Alkalis: Water-soluble bases that produce hydroxyl ($OH^-$) ions as the only negatively charged ions in aqueous solution.
  • Strong Alkalis: Completely dissociated (e.g., $NaOH$, $KOH$).
  • Weak Alkalis: Feebly ionised (e.g., $NH_4OH$, $Ca(OH)_2$).
  • Rule: All alkalis are bases, but not all bases are alkalis. Insoluble bases like $Fe(OH)_3$ and $Cu(OH)_2$ are not alkalis.

Classification and Preparation of Bases

• Classification by Acidity:
  • Acidity of a Base: The number of hydroxyl ions produced per molecule or the number of hydrogen ions from an acid a molecule will react with.
  • Monoacidic Base: Produces one $OH^-$ ion (e.g., $NaOH, KOH, NH_4OH$).
  • Diacidic Base: Produces two $OH^-$ ions (e.g., $Ca(OH)_2, Cu(OH)_2$).
  • Triacidic Base: Produces three $OH^-$ ions (e.g., $Al(OH)_3, Fe(OH)_3$).
• Preparation Methods:
  1. Metal + Oxygen: $4Na + O_2 \rightarrow 2Na_2O$.
  2. Reactive Metal + Water: $2Na + 2H_2O \rightarrow 2NaOH + H_2 \uparrow$.
  3. Soluble Metallic Oxide + Water: $Na_2O + H_2O \rightarrow 2NaOH$.
  4. Double Decomposition: $FeCl_3 + 3NaOH \rightarrow Fe(OH)_3 \downarrow + 3NaCl$.
  5. Oxygen + Metal Sulphides: $2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2$.
  6. Decomposition of Salts: $CaCO_3 \rightarrow CaO + CO_2 \uparrow$. (Note: Sodium and potassium carbonates do not decompose on heating).
  7. Dissolving Ammonia in Water: $NH_3 + H_2O \rightarrow NH_4OH$.

Properties and Uses of Bases

• Physical Properties:
  • Sharp, bitter taste.
  • Soapy/slippery touch (due to reaction with skin oils to form soap).
  • Strong electrolytes and mildly corrosive to the skin.
  • Indicators: Litmus (Red to Blue); Methyl orange (Orange to Yellow); Phenolphthalein (Colourless to Pink).
• Chemical Properties:
  • Absorbing $CO_2$: Strong alkalis form carbonates: $2NaOH + CO_2 \rightarrow Na_2CO_3 + H_2O$.
  • Neutralisation: React with acids to form salt and water.
  • Precipitation: Form insoluble metallic hydroxides from heavy metal salt solutions (e.g., $CuSO_4 + 2NH_4OH \rightarrow (NH_4)_2SO_4 + Cu(OH)_2 \downarrow$ pale blue precipitate).
  • Ammonia Evolution: Alkali + Ammonium salt + heat produces ammonia gas: $NH_4Cl + NaOH \rightarrow NaCl + H_2O + NH_3 \uparrow$.
  • Amphoteric Nature: Zinc, aluminium, and lead hydroxides dissolve in excess $NaOH$ or $KOH$. Example: $Zn(OH)_2 + 2NaOH \rightarrow Na_2ZnO_2 + 2H_2O$ (sodium zincate).
• General Uses:
  • Sodium hydroxide: Soap manufacture.
  • Potassium hydroxide: Batteries, soaps.
  • Calcium hydroxide: Bleaching powder, softening hard water.
  • Magnesium hydroxide: Antacid.
  • Aluminium hydroxide: Foaming agent in fire extinguishers.
  • Ammonium hydroxide: Grease stain removal.
  • Magnesia ($MgO$): Refractory bricks.
  • Calcium oxide: Drying agent, basic flux.

The pH Scale and Its Importance

• Ionic Product of Water ($K_w$): At $25^{\circ}C$, both $[H_3O^+]$ and $[OH^-]$ in pure water are $1 \times 10^{-7}\,mol/L$. The product is $1 \times 10^{-14}\,mol^2/L^2$.
• pH Scale Definition: Devised by Sorensen in 1909. pH stands for "Potenz" (power) of hydrogen.
  • $pH = -\log_{10}[H^+] = \log_{10}\frac{1}{[H^+]}$
  • A solution with $[H^+] = 10^{-12}\,mol/L$ has a $pH$ of $12$.
• Scale Range (0 to 14):
  • $pH = 7$: Neutral.
  • $pH < 7$: Acidic (acidity increases as pH decreases).
  • $pH > 7$: Alkaline (alkalinity increases as pH increases).
• Universal Indicator: A mixture of dyes showing a spectrum of colors:
  • Neutral ($pH = 7$): Green.
  • Alkaline ($pH 7 \rightarrow 14$): Blue to Indigo to Violet.
  • Acidic ($pH 7 \rightarrow 1$): Yellow to Pink to Red.
• Daily Life Implications:
  • Human Body: Functions within $pH$ $7.0$ to $7.8$.
  • Agriculture: Specific crops require specific ranges (Rice/Acidic, Sugarcane/Neutral, Citrus/Alkaline).
  • Acid Rain: Rainwater with $pH < 5.6$ containing $H_2SO_4$, $HNO_3$, and $H_2CO_3$ from pollutants. It reduces soil fertility and damages monuments.
  • Digestion: Stomach naturally produces $HCl$. Excess $HCl$ causes pain, treated with antacids like milk of magnesia ($Mg(OH)_2$).
  • Tooth Decay: Bacteria degrade sugar, dropping $pH$ below $5.5$. This corrodes enamel (calcium phosphate). Saliva and toothpaste (alkaline) help neutralise this.
  • Bee Stings: These inject acid; applying baking soda (base) provides relief.

Questions & Discussion

• Basicity of $CH_3COOH$: Why is acetic acid monobasic despite having four hydrogen atoms? Because only one hydrogen atom is ionisable and can be replaced when forming salts.
• Strength vs Concentration: Why is dilute $HCl$ stronger than concentrated acetic acid? Because strength refers to the degree of ionisation ($\alpha$); $HCl$ ionises almost completely, while acetic acid remains largely as molecules.
• Preparation from Volatile Acids: How is a non-volatile acid used to prepare a volatile one? For example, concentrated sulphuric acid is used to prepare hydrochloric acid or nitric acid from their salts through displacement.
• Nitrogen Dioxide Anhydride: Why is $NO_2$ called a double acid anhydride? Because when it reacts with water, it produces two distinct acids ($HNO_2$ and $HNO_3$).