Properties of Acids and Bases - Tenth Grade Science
Acids and bases constitute two of the most ubiquitous and chemically consequential classes of chemical compounds known to science, governing phenomena ranging from industrial chemical synthesis to cellular homeostasis. From the tangy tartness of citric acid in citrus fruits and the stinging methanoic acid injected by ant bites to the slippery feel of alkaline sodium hydroxide in household soaps, their distinct chemical behaviors shape our physical world. At a fundamental molecular level, acids and bases are defined by how they ionize or dissociate in aqueous solutions to generate hydronium and hydroxide ions. In this chapter, you will investigate the Arrhenius and Bronsted-Lowry conceptual models, examine the characteristic chemical reactions of acids and bases with metals, carbonates, and oxides, and analyze the thermodynamic principles of acid dilution and ionization strength.
Molecular Definitions: Arrhenius and Bronsted-Lowry Theories
Understanding the behavior of acids and bases requires tracing the evolution of theoretical models from macroscopic taste and touch to subatomic proton transfer.
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| ACID-BASE THEORETICAL FRAMEWORKS |
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| ARRHENIUS MODEL (Aqueous Solution Only): |
| - Acid: Produces hydrogen ions (H+ / H3O+) in aqueous solution. |
| HCl(g) + H2O(l) -----> H3O+(aq) + Cl-(aq) |
| - Base: Produces hydroxide ions (OH-) in aqueous solution. |
| NaOH(s) --[H2O]--> Na+(aq) + OH-(aq) |
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| BRONSTED-LOWRY MODEL (Proton Transfer): |
| - Acid: Proton (H+) donor. |
| - Base: Proton (H+) acceptor. |
| NH3(aq) + H2O(l) <=====> NH4+(aq) + OH-(aq) |
| (Base) (Acid) (Conj Acid) (Conj Base) |
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The Arrhenius Definition and the Hydronium Ion
In 1884, Svante Arrhenius proposed that an acid is a hydrogen-containing substance that dissociates in water to yield hydrogen ions ($\text{H}^+$), whereas a base is a substance that dissociates in water to yield hydroxide ions ($\text{OH}^-$).
A bare hydrogen ion ($\text{H}^+$) is simply an unshielded proton with an exceptionally tiny ionic radius ($10^{-15}\text{ m}$) and an enormous positive charge density. A bare proton cannot exist independently in aqueous solution. Instead, it covalently coordinates with the lone electron pair of a water molecule to form the hydronium ion ($\text{H}_3\text{O}^+$):
$$\text{H}^+ + \text{H}_2\text{O} \longrightarrow \text{H}_3\text{O}^+$$
An acid cannot demonstrate acidic properties without water. If dry hydrogen chloride gas ($\text{HCl}$) is exposed to dry blue litmus paper, no color change occurs because un-ionized covalent $\text{HCl}$ molecules cannot transfer protons in the absence of water. Only upon dissolving in moisture does ionization occur, producing hydronium ions that turn blue litmus paper red.
Strong versus Weak Acids and Bases
Acids and bases are classified by their degree of ionization ($\alpha$) in aqueous solution. A strong acid, such as hydrochloric acid ($\text{HCl}$), nitric acid ($\text{HNO}_3$), or sulfuric acid ($\text{H}_2\text{SO}_4$), ionizes completely into ions when dissolved in water:
HCl(aq) + H2O(l) -> H3O+(aq) + Cl-(aq) (100% Complete Ionization)
A weak acid, such as acetic acid ($\text{CH}_3\text{COOH}$), carbonic acid ($\text{H}_2\text{CO}_3$), or citric acid, ionizes only partially, establishing a dynamic chemical equilibrium where un-ionized molecules outnumber ions:
CH3COOH(aq) + H2O(l) <===> H3O+(aq) + CH3COO-(aq) (Partial Ionization < 5%)
Similarly, strong bases like sodium hydroxide ($\text{NaOH}$) and potassium hydroxide ($\text{KOH}$) dissociate completely, whereas weak bases like ammonium hydroxide ($\text{NH}_4\text{OH}$) dissociate partially.
Chemical Properties and Reactions of Acids
Acids undergo predictable stoichiometric reactions with metals, metal carbonates, metal hydrogen carbonates, and metal oxides.
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| CORE CHEMICAL PATTERNS OF ACIDS |
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| 1. Acid + Reactive Metal -----> Salt + Hydrogen Gas (H2) |
| 2. Acid + Metal Carbonate -----> Salt + Water + Carbon Dioxide (CO2) |
| 3. Acid + Metal Bicarbonate -----> Salt + Water + Carbon Dioxide (CO2) |
| 4. Acid + Basic Metal Oxide -----> Salt + Water (Neutralization) |
| 5. Acid + Base (Hydroxide) -----> Salt + Water (Neutralization) |
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Reaction of Acids with Reactive Metals
When dilute mineral acids react with metals situated above hydrogen in the electrochemical reactivity series, single displacement occurs, evolving hydrogen gas:
Zn(s) + 2HCl(aq) -> ZnCl2(aq) + H2(g)
In the laboratory, the evolution of hydrogen gas is verified using the acoustic pop test: bringing a burning wooden splint near the mouth of the test tube causes the escaping gas to ignite with a sharp popping sound:
2H2(g) + O2(g) -> 2H2O(g)
However, concentrated nitric acid ($\text{HNO}_3$) does not evolve hydrogen gas when reacted with metals such as zinc or iron. Nitric acid is a potent oxidizing agent that oxidizes evolved hydrogen immediately into water, while the nitric acid itself is reduced to nitrogen oxides ($\text{NO}_2, \text{NO}$, or $\text{N}_2\text{O}$). Only extremely dilute nitric acid ($1\%$) reacts with magnesium or manganese to liberate hydrogen gas.
Reaction with Metal Carbonates and Metal Hydrogen Carbonates
When acids react with metal carbonates or metal hydrogen carbonates (bicarbonates), vigorous effervescence occurs, producing a salt, water, and carbon dioxide gas:
Na2CO3(s) + 2HCl(aq) -> 2NaCl(aq) + H2O(l) + CO2(g)
NaHCO3(s) + HCl(aq) -> NaCl(aq) + H2O(l) + CO2(g)
The presence of carbon dioxide gas is confirmed by bubbling the gas through freshly prepared limewater (aqueous calcium hydroxide). The limewater turns milky due to the precipitation of insoluble white calcium carbonate:
Ca(OH)2(aq) + CO2(g) -> CaCO3(s) + H2O(l) (Milky White Precipitate)
If excess carbon dioxide gas is bubbled into the milky suspension, the milkiness disappears completely, leaving a clear solution because insoluble calcium carbonate converts into soluble calcium bicarbonate:
CaCO3(s) + H2O(l) + CO2(g) -> Ca(HCO3)2(aq) (Soluble, Clear)
Reaction with Basic Metallic Oxides
Metallic oxides are basic anhydrides that react with acids to form salt and water, demonstrating their basic chemical nature:
CuO(s) + 2HCl(aq) -> CuCl2(aq) + H2O(l)
When black copper(II) oxide powder is warmed with dilute hydrochloric acid, the solid dissolves and the solution turns a vibrant bluish-green due to the formation of aqueous copper(II) chloride.
Chemical Properties and Reactions of Bases
Bases are substances that neutralize acids, taste bitter, feel slippery or soapy to the touch, and turn red litmus paper blue. Bases that are soluble in water are specifically designated as alkalis.
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| CORE CHEMICAL PATTERNS OF BASES |
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| 1. Base + Amphoteric Metal -----> Complex Salt + Hydrogen Gas (H2) |
| 2. Base + Acidic Non-Metal Oxide-----> Salt + Water |
| 3. Base + Acid (Neutralization) -----> Salt + Water |
| 4. Base + Ammonium Salt -----> Salt + Water + Ammonia Gas (NH3) |
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Alkalis versus Bases
All alkalis are bases, but not all bases are alkalis. Insoluble metallic oxides and hydroxides—such as cupric hydroxide ($\text{Cu(OH)}_2$) or ferric hydroxide ($\text{Fe(OH)}_3$)—are bases because they neutralize acids, but they do not dissolve in water to produce high concentrations of free hydroxide ions. Common alkalis include sodium hydroxide ($\text{NaOH}$), potassium hydroxide ($\text{KOH}$), and calcium hydroxide ($\text{Ca(OH)}_2$).
Reaction of Alkalis with Amphoteric Metals
Certain amphoteric metals, notably zinc and aluminium, react with strong alkalis at elevated temperatures to evolve hydrogen gas, forming complex oxy-salts:
Zn(s) + 2NaOH(aq) --[Heat]--> Na2ZnO2(aq) + H2(g) (Sodium Zincate)
2Al(s) + 2NaOH(aq) + 2H2O(l) --[Heat]--> 2NaAlO2(aq) + 3H2(g) (Sodium Aluminate)
Reaction with Non-Metallic Acidic Oxides
Non-metallic oxides, such as carbon dioxide ($\text{CO}_2$) and sulfur dioxide ($\text{SO}_2$), are acidic in nature and react with alkalis to yield salt and water:
Ca(OH)2(aq) + CO2(g) -> CaCO3(s) + H2O(l)
2NaOH(aq) + SO2(g) -> Na2SO3(aq) + H2O(l)
This neutralization reaction confirms that non-metallic oxides behave as acidic anhydrides.
Thermodynamics of Acid Dilution
The process of dissolving a concentrated mineral acid or alkali in water is an intensely exothermic physical transformation, releasing large quantities of hydration enthalpy.
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| DILUTION SAFETY MANDATE |
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| CORRECT PROTOCOL DANGEROUS PROTOCOL |
| Acid into Water Slowly Water into Concentrated Acid |
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| [Acid Dripping] [Water Dropping] |
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| v v |
| +--------------+ +--------------+ |
| | Water (Bulk) | | Acid (Bulk) | |
| +--------------+ +--------------+ |
| Heat safely absorbed by Violent localized steam burst |
| large volume of water. splatters corrosive acid! |
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Enthalpy of Hydration and Specific Heat Capacity
When water interacts with concentrated sulfuric acid, strong hydrogen bonds and coordinate covalent bonds form between water molecules and protons, accompanied by an enthalpy change ($\Delta H < 0$) exceeding $-880\text{ kJ/mol}$. Water possesses an exceptionally high specific heat capacity ($4.184\text{ J/g}\cdot^\circ\text{C}$). Adding acid slowly to a large volume of water allows the water to absorb and dissipate the released heat safely without boiling.
If water is poured into concentrated acid, the lower density of water causes it to float on top of the acid. The intense heat of hydration generated at the narrow contact interface instantly flash-boils the localized water into steam, ejecting corrosive acid droplets outward.
Chapter Exercises
Exercise 1: A gas evolved during a reaction turns moist blue litmus paper red and extinguishes a burning splint without any sound. When passed through clear limewater, it renders the solution milky; however, continued bubbling of the gas clarifies the milky suspension. Identify the gas, explain the chemical reactions taking place with balanced chemical equations, and identify the compounds responsible for the appearance and disappearance of milkiness.
Exercise 2: Write balanced chemical equations for the following transformations: first, the reaction of granular zinc with dilute sulfuric acid; second, the reaction of granular zinc with hot, concentrated aqueous sodium hydroxide; and third, the reaction of solid calcium carbonate with aqueous hydrochloric acid.
Exercise 3: Explain why an aqueous solution of glucose ($\text{C}*6\text{H}*{12}\text{O}_6$) or ethanol ($\text{C}_2\text{H}_5\text{OH}$) does not conduct an electric current and does not alter the color of litmus paper, even though their molecular structures contain multiple hydrogen atoms.
Solutions and Explanations
Solution 1: The gas evolved is carbon dioxide ($\text{CO}_2$). Carbon dioxide dissolves in water to form weak carbonic acid ($\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3$), which ionizes partially to produce hydronium ions that turn blue litmus paper red. Being non-flammable and an inert blanket, it extinguishes a burning splint. When passed into limewater (aqueous calcium hydroxide), carbon dioxide reacts to form an insoluble white precipitate of calcium carbonate, turning the liquid milky: $\text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}$. When excess carbon dioxide is bubbled through the suspension, the insoluble calcium carbonate reacts with dissolved carbon dioxide and water to form soluble calcium hydrogen carbonate: $\text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} \rightarrow \text{Ca(HCO}_3)_2\text{(aq)}$. Because calcium hydrogen carbonate is completely soluble in water, the white precipitate dissolves and the solution becomes completely clear.
Solution 2: For granular zinc with dilute sulfuric acid: $\text{Zn(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{H}_2\text{(g)}$. For granular zinc with hot concentrated sodium hydroxide: $\text{Zn(s)} + 2\text{NaOH(aq)} \rightarrow \text{Na}_2\text{ZnO}_2\text{(aq)} + \text{H}_2\text{(g)}$. For solid calcium carbonate with hydrochloric acid: $\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$.
Solution 3: Electrical conductivity in an aqueous solution requires mobile ions that move toward oppositely charged electrodes when an electric potential is applied. While glucose ($\text{C}*6\text{H}*{12}\text{O}_6$) and ethanol ($\text{C}_2\text{H}_5\text{OH}$) contain hydrogen atoms, these hydrogen atoms are held to carbon and oxygen atoms by stable covalent bonds. Water cannot overcome the bond energies of these covalent bonds, meaning neither glucose nor ethanol ionizes in water to liberate free hydrogen ions ($\text{H}^+$) or hydronium ions ($\text{H}_3\text{O}^+$). Because they remain intact, neutral covalent molecules without mobile charge carriers, their aqueous solutions cannot conduct an electric current. Furthermore, because acidity is defined by the presence of excess hydronium ions, the absence of hydronium ions means these solutions are neutral and cannot alter the color of litmus paper.