Salts: Formation, Types & Neutralization - Tenth Grade Science
Salts are crystalline ionic compounds formed by the neutralization reaction between an acid and a base, consisting of an orderly lattice of positively charged cations derived from a base and negatively charged anions derived from an acid. Far from being restricted to table salt (sodium chloride), the chemical family of salts encompasses diverse minerals and synthetic reagents that drive metallurgical extraction, agricultural fertilization, physiological nerve transmission, and industrial synthesis. Depending on the relative strengths of their parent acids and bases, aqueous salt solutions can exhibit acidic, basic, or neutral behavior due to salt hydrolysis. In this chapter, you will investigate the synthetic pathways for preparing soluble and insoluble salts, analyze the four categories of salt hydrolysis, evaluate the role of water of crystallization, and explore the thermodynamics of neutralization.
The Chemistry of Neutralization Reactions
A neutralization reaction is a chemical process wherein an acid reacts quantitatively with an equivalent quantity of a base to produce a salt and water.
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| NEUTRALIZATION REACTION DYNAMICS |
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| MOLECULAR EQUATION: |
| HCl(aq) + NaOH(aq) -----> NaCl(aq) + H2O(l) |
| |
| COMPLETE IONIC EQUATION: |
| H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) -----> Na+(aq) + Cl-(aq) + H2O(l) |
| |
| NET IONIC EQUATION (Spectator ions canceled): |
| H+(aq) + OH-(aq) -----> H2O(l) Delta H = -57.1 kJ/mol |
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The Net Ionic Equation and Standard Enthalpy of Neutralization
In dilute aqueous solution, strong acids and strong bases exist completely dissociated into their constituent ions. When hydrochloric acid mixes with sodium hydroxide, sodium ions ($\text{Na}^+$) and chloride ions ($\text{Cl}^-$) remain unchanged throughout the reaction; they are spectator ions. Canceling spectator ions yields the fundamental net ionic equation of neutralization:
$$\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \longrightarrow \text{H}_2\text{O(l)}$$
Because this identical bond-forming process occurs whenever any strong monoprotic acid reacts with any strong monobasic alkali in dilute solution, the standard enthalpy of neutralization ($\Delta H^\circ_{\text{neut}}$) for all strong acid-strong base systems is a constant:
$$\Delta H^\circ_{\text{neut}} \approx -57.1\text{ kJ/mol} \quad (\text{or } -57.3\text{ kJ/mol})$$
When weak acids or weak bases participate in neutralization, the measured enthalpy release is lower because a portion of the evolved thermal energy is consumed to break covalent bonds and complete the ionization of the weak reactant.
Synthetic Pathways for Preparing Salts
The synthetic route chosen to produce a specific salt depends primarily on whether the target compound is soluble or insoluble in water.
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| SALT PREPARATION METHODOLOGIES |
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| SOLUBLE SALTS | INSOLUBLE SALTS |
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| Method 1: Titration | Method 4: Double Decomposition (Precipitation)|
| Alkali + Acid | Soluble Salt A + Soluble Salt B |
| NaOH(aq) + HCl(aq) -> NaCl + H2O | Pb(NO3)2(aq) + 2KI(aq) -> PbI2(v) + 2KNO3 |
| | |
| Method 2: Excess Insoluble Base | Method 5: Direct Combination (Synthesis) |
| Base + Acid | Element + Element |
| CuO(s) + H2SO4(aq) -> CuSO4 + H2O | 2Fe(s) + 3Cl2(g) -> 2FeCl3(s) |
| | |
| Method 3: Acid + Reactive Metal | |
| Zn(s) + H2SO4(aq) -> ZnSO4 + H2 | |
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Synthesis of Soluble Salts
Soluble salts are synthesized through three main techniques:
Titration Method: Used when both the acid and base are soluble liquids (such as aqueous sodium hydroxide and hydrochloric acid). A standardized volume of acid is delivered from a burette into the alkali containing an indicator until exact equivalence is reached. The indicator-free solution is then evaporated to crystallization point and allowed to cool.
Excess Insoluble Base or Carbonate Method: Used when preparing salts of heavy metals, such as copper(II) sulfate. Insoluble black copper(II) oxide ($\text{CuO}$) or copper(II) carbonate ($\text{CuCO}_3$) is added in small increments to warm dilute sulfuric acid until no more dissolves, ensuring all acid is consumed. The unreacted excess solid is removed by filtration, leaving a pure aqueous solution of copper(II) sulfate that is concentrated and crystallized.
Direct Action of Acid on Metal: Granular zinc, iron, or magnesium is reacted with dilute sulfuric or hydrochloric acid. Excess metal is filtered off, and the filtrate is cooled to yield hydrated salt crystals.
Synthesis of Insoluble Salts via Precipitation
Insoluble salts cannot be isolated by crystallization from solution. They are synthesized by ionic precipitation (double decomposition). Two soluble parent salts are dissolved separately in distilled water and combined. The target insoluble salt precipitates instantly:
BaCl2(aq) + Na2SO4(aq) -> BaSO4(s) + 2NaCl(aq)
The dense white precipitate of barium sulfate is separated by filtration, washed with distilled water to remove spectator sodium and chloride ions, and dried in a desiccating oven.
Salt Hydrolysis and pH Profiles of Aqueous Solutions
Although salts are neutralization products, dissolving a pure salt in water does not necessarily yield a neutral solution. Salt hydrolysis is the chemical interaction between salt ions and water molecules that perturbs the neutral $1:1$ ratio of hydronium to hydroxide ions.
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| HYDROLYSIS PROFILES OF SALTS |
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| Parent Acid & Base | Example Salt | Hydrolysis Type | Aqueous Solution pH |
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| Strong Acid + | NaCl, KNO3, | Neither ion | Neutral |
| Strong Base | Na2SO4 | hydrolyzes | (pH = 7.0) |
|---------------------+-------------------+-------------------+---------------------+
| Strong Acid + | NH4Cl, CuSO4, | Cationic | Acidic |
| Weak Base | FeCl3 | hydrolysis | (pH < 7.0) |
|---------------------+-------------------+-------------------+---------------------+
| Weak Acid + | CH3COONa, Na2CO3, | Anionic | Basic |
| Strong Base | KCN | hydrolysis | (pH > 7.0) |
|---------------------+-------------------+-------------------+---------------------+
| Weak Acid + | CH3COONH4, | Both ions | Depends on Ka |
| Weak Base | (NH4)2CO3 | hydrolyze | versus Kb values |
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Anionic Hydrolysis in Basic Salts
When sodium acetate ($\text{CH}_3\text{COONa}$) dissolves in water, it dissociates completely into sodium cations ($\text{Na}^+$) and acetate anions ($\text{CH}_3\text{COO}^-$). The sodium ion, derived from strong sodium hydroxide, does not interact with water. However, the acetate ion is the conjugate base of weak acetic acid and has a high affinity for protons. It abstracts a proton from water:
$$\text{CH}_3\text{COO}^-\text{(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{CH}_3\text{COOH(aq)} + \text{OH}^-\text{(aq)}$$
This interaction generates un-ionized acetic acid and liberates free hydroxide ions ($\text{OH}^-$), making the solution basic ($\text{pH} > 7.0$).
Cationic Hydrolysis in Acidic Salts
When ammonium chloride ($\text{NH}_4\text{Cl}$) dissolves, it yields ammonium ions ($\text{NH}_4^+$) and chloride ions ($\text{Cl}^-$). The chloride ion, derived from strong hydrochloric acid, does not hydrolyze. The ammonium ion, as the conjugate acid of weak ammonia, donates a proton to water:
$$\text{NH}_4^+\text{(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{NH}_3\text{(aq)} + \text{H}_3\text{O}^+\text{(aq)}$$
This reaction generates free hydronium ions, shifting the balance and making the solution acidic ($\text{pH} < 7.0$).
Water of Crystallization and Hydrated Crystals
Many salts incorporate a fixed number of water molecules into their definite crystalline structure during crystallization from aqueous solution. This water is known as water of crystallization.
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| WATER OF CRYSTALLIZATION PROFILES |
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| Salt Name | Chemical Formula | Crystal Hue |
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| Copper(II) Sulfate Pentahydrate | CuSO4.5H2O | Brilliant Azure Blue |
| Ferrous Sulfate Heptahydrate | FeSO4.7H2O | Pale Sea-Green |
| Sodium Carbonate Decahydrate | Na2CO3.10H2O | Transparent White |
| Calcium Sulfate Dihydrate | CaSO4.2H2O (Gypsum) | Monoclinic White |
| Anhydrous Copper(II) Sulfate | CuSO4 | Amorphous White |
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Reversible Thermal Dehydration of Copper Sulfate
Water of crystallization is chemically bonded to the central metal ion by coordinate covalent bonds and hydrogen bonds within the crystal lattice; it is not free liquid moisture. When azure blue copper(II) sulfate pentahydrate crystals are heated in a dry boiling tube, the coordinated water molecules are driven off as steam:
CuSO4.5H2O(s) [Blue] --[Heat]--> CuSO4(s) [White] + 5H2O(g)
The crystalline lattice collapses into an amorphous white powder of anhydrous copper(II) sulfate. If a few drops of water are added to the cooled white powder, hydration occurs immediately, releasing heat and restoring the azure blue color:
CuSO4(s) [White] + 5H2O(l) -> CuSO4.5H2O(s) [Blue] + Heat
This reversible color transition serves as a diagnostic chemical test for the presence of water.
Efflorescence and Deliquescence
Efflorescence is the spontaneous loss of water of crystallization from a hydrated salt to the surrounding atmosphere when the vapor pressure of the hydrated crystal exceeds the partial pressure of water vapor in ambient air. For example, washing soda ($\text{Na}_2\text{CO}_3\cdot 10\text{H}_2\text{O}$) effloresces when exposed to dry air, losing nine water molecules to become a white monohydrate powder:
Na2CO3.10H2O(s) -> Na2CO3.H2O(s) + 9H2O(g)
Conversely, deliquescence is the phenomenon where a substance absorbs moisture from atmospheric air until it completely dissolves in that absorbed water to form a concentrated aqueous solution. Deliquescent substances, such as solid calcium chloride ($\text{CaCl}_2$) and solid sodium hydroxide pellets ($\text{NaOH}$), have extremely low surface vapor pressures relative to atmospheric humidity.
Chapter Exercises
Exercise 1: Classify aqueous solutions of potassium nitrate ($\text{KNO}_3$), sodium carbonate ($\text{Na}_2\text{CO}_3$), and aluminium chloride ($\text{AlCl}_3$) as acidic, basic, or neutral, writing chemical equations for any hydrolysis reactions that occur.
Exercise 2: Outline the detailed experimental procedure for preparing pure, dry crystals of copper(II) sulfate pentahydrate starting from solid copper(II) oxide and dilute sulfuric acid, explaining why excess copper(II) oxide is used.
Exercise 3: Explain why the heat of neutralization of hydrofluoric acid ($\text{HF}$) with sodium hydroxide ($\text{NaOH}$) is $-68.6\text{ kJ/mol}$, which is significantly higher than the standard $-57.1\text{ kJ/mol}$ observed for hydrochloric acid with sodium hydroxide.
Solutions and Explanations
Solution 1: An aqueous solution of potassium nitrate ($\text{KNO}_3$) is neutral ($\text{pH} = 7.0$) because it is derived from a strong base ($\text{KOH}$) and a strong acid ($\text{HNO}_3$); neither the potassium cation nor the nitrate anion has a tendency to react with water. An aqueous solution of sodium carbonate ($\text{Na}_2\text{CO}_3$) is basic ($\text{pH} > 7.0$) because it is derived from a strong base ($\text{NaOH}$) and a weak acid ($\text{H}_2\text{CO}_3$). The carbonate anion undergoes anionic hydrolysis: $\text{CO}_3^{2-}\text{(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{HCO}_3^-\text{(aq)} + \text{OH}^-\text{(aq)}$, generating free hydroxide ions. An aqueous solution of aluminium chloride ($\text{AlCl}_3$) is acidic ($\text{pH} < 7.0$) because it is derived from a weak base ($\text{Al(OH)}_3$) and a strong acid ($\text{HCl}$). The small, highly charged aluminium cation coordinates with water molecules to form a hydrated complex $[\text{Al(H}_2\text{O)}_6]^{3+}$, which undergoes cationic hydrolysis: $[\text{Al(H}_2\text{O)}_6]^{3+}\text{(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons [\text{Al(H}_2\text{O)}_5\text{(OH)}]^{2+}\text{(aq)} + \text{H}_3\text{O}^+\text{(aq)}$, liberating hydronium ions into solution.
Solution 2: Measure approximately $50\text{ cm}^3$ of dilute sulfuric acid into a glass beaker and warm it gently over a wire gauze using a Bunsen burner. Add black copper(II) oxide powder in small spatulas while stirring continuously with a glass rod. The black powder dissolves, turning the solution blue as copper(II) sulfate forms: $\text{CuO(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{CuSO}_4\text{(aq)} + \text{H}_2\text{O(l)}$. Continue adding copper(II) oxide until a small quantity of black powder remains unreacted at the bottom of the beaker, ensuring that all sulfuric acid has been completely neutralized. Filter the hot mixture through filter paper in a funnel to separate the unreacted excess copper(II) oxide. Transfer the clear blue filtrate into an evaporating dish and heat gently to concentrate the solution until crystallization point is reached (tested by dipping a glass rod into the solution and observing crust formation on cooling). Allow the saturated solution to cool slowly at room temperature, promoting the formation of large azure blue crystals of $\text{CuSO}_4\cdot 5\text{H}_2\text{O}$. Finally, separate the crystals by filtration and dry them between sheets of filter paper.
Solution 3: While hydrofluoric acid ($\text{HF}$) is a weak acid that ionizes only partially in dilute solution, its neutralization by sodium hydroxide releases $-68.6\text{ kJ/mol}$, which is significantly more exothermic than the $-57.1\text{ kJ/mol}$ standard. Although energy is absorbed to dissociate the remaining covalent $\text{H}-\text{F}$ molecules, the resulting fluoride ion ($\text{F}^-$) is exceptionally small with an extremely high negative charge density. When fluoride ions are released into water, water dipoles orient around them, forming a tight hydration shell that releases a large quantity of hydration enthalpy. The hydration enthalpy released by the fluoride ion far exceeds the energy consumed in breaking the hydrogen-fluorine covalent bonds, making the overall net neutralization enthalpy substantially more exothermic than that of other mineral acids.