Chemical Changes: Acids, Bases, Salts, Reactivity, and Electrolysis

What is the pH scale, and what do its numerical values indicate about a solution?

The pH scale is a continuous scale from $0$ to $14$ measuring acidity and alkalinity. Values below $7$ indicate an acidic solution (high $[H^+]$), exactly $7$ is neutral, and values above $7$ indicate an alkaline solution (high $[OH^-]$).

What is the chemical definition of an acid?

An acid is any substance that dissociates in aqueous solution to release hydrogen ions ($H^+$), producing a solution with a pH less than $7$.

What is an alkali?

An alkali is a water-soluble base that dissolves in water to produce hydroxide ions ($OH^-$), yielding an aqueous solution with a pH greater than $7$.

What is the general ionic equation for an acid-alkali neutralisation reaction?

The ionic equation is: $H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)}$.

What distinguishes a strong acid from a weak acid in terms of ionisation?

A strong acid completely ionises and fully dissociates in water to release $H^+$ ions (e.g., $HCl$, $HNO_3$). A weak acid only partially ionises, establishing a dynamic equilibrium that lies far to the left (e.g., $CH_3COOH$, citric acid).

What is the difference between acid strength and acid concentration?

Acid strength refers to the proportion of acid molecules that ionise and dissociate into $H^+$ ions in water. Acid concentration refers to the total number of dissolved acid particles per unit volume ($mol\,dm^{-3}$).

How does a change of $1$ unit on the pH scale relate to hydrogen ion concentration ($[H^+]$)?

Every decrease of $1$ integer on the pH scale corresponds to a ten-fold ($10\times$) increase in $[H^+]$, calculated using the relationship: $\text{Factor} = 10^{-X}$, where $X = \text{final pH} - \text{initial pH}$.

What is an acid-base titration and why is it performed?

A titration is a quantitative volumetric analysis technique used to determine the exact concentration of an unknown acid or alkali by finding the precise volume needed to reach complete neutralisation.

What are concordant titres in a titration, and how are they used?

Concordant titres are titration volumes that agree within $0.1\,cm^3$ of each other. They are averaged together to calculate an accurate, reliable mean titre volume.

Why are single indicators used in titrations instead of wide-range indicators?

Single indicators provide a sharp, immediate colour change at the exact neutralisation end-point, whereas wide-range indicators transition colour gradually across a wide pH range, making precise end-point detection impossible.

What colour transitions occur for phenolphthalein and methyl orange?

Phenolphthalein is colourless in acid and turns bright pink in alkali. Methyl orange is red in acid and turns yellow in alkali.

What products are formed when an acid reacts with a metal carbonate?

An acid reacts with a metal carbonate to produce a salt, water, and carbon dioxide gas: $\text{Acid} + \text{Metal Carbonate} \rightarrow \text{Salt} + \text{Water} + CO_{2(g)}$.

What are the laboratory steps to prepare a pure, dry soluble salt from an insoluble base?

1. Gently heat dilute acid and stir in the insoluble base in excess until powder settles. 2. Filter out unreacted excess solid. 3. Heat the filtrate to evaporate water until the crystallisation point. 4. Allow the saturated solution to cool and crystallise, filter the solid, rinse with cold distilled water, and dry.

How is the reactivity of a metal defined in terms of atomic structure?

The reactivity of a metal is determined by the ease with which its atoms lose valence shell electrons to form positively charged cations.

What is the descending order of the reactivity series, including non-metal references?

$K > Na > Li > Ca > Mg > C > Zn > Fe > H > Cu > Au$.

Which metals can be extracted from their ores using carbon reduction, and why?

Metals situated below carbon in the reactivity series ($Zn$, $Fe$, $Cu$) can be extracted using carbon reduction because carbon is more reactive and can displace the metal from its oxide to form carbon dioxide and the elemental metal.

What is the mnemonic OIL RIG, and what does it define?

OIL RIG stands for Oxidation Is Loss of electrons, Reduction Is Gain of electrons.

What occurs during a metal displacement reaction?

A more reactive metal atom is oxidised (loses electrons) and displaces a less reactive metal ion from an aqueous salt solution, which is reduced (gains electrons) to form pure elemental metal.

What are spectator ions in a chemical equation?

Spectator ions are ionic species that remain in solution without changing charge, physical state, or participating in electron transfer during a chemical reaction.

What is electrolysis, and what constitutes an electrolyte?

Electrolysis is the process of chemically decomposing an ionic compound using direct electric current (DC). An electrolyte is a molten liquid or aqueous solution containing mobile ions that conduct electricity.

What occurs at the cathode and anode during electrolysis?

The cathode is the negative electrode where positive cations migrate, gain electrons, and undergo reduction. The anode is the positive electrode where negative anions migrate, lose electrons, and undergo oxidation.

Why is cryolite used in the industrial extraction of aluminium from bauxite?

Purified aluminium oxide ($Al_2O_3$) has a melting point exceeding $2000\,^\circ\text{C}$. Dissolving it in molten cryolite lowers the working melting point to approximately $900\,^\circ\text{C}$, conserving large amounts of thermal energy and reducing operating costs.

Why must carbon anodes be replaced regularly during aluminium electrolysis?

At operating temperatures above $900\,^\circ\text{C}$, the oxygen gas discharged at the positive carbon anodes reacts with the carbon blocks to form carbon dioxide gas ($C_{(s)} + O_{2(g)} \rightarrow CO_{2(g)}$), gradually eroding them.

What rule governs which substance is discharged at the cathode during aqueous electrolysis?

$H^+$ ions from water are discharged to produce hydrogen gas ($H_{2(g)}$) unless the metal in the dissolved salt is less reactive than hydrogen ($Cu$, $Ag$, $Au$), in which case the pure metal is deposited.

What rule governs which substance is discharged at the anode during aqueous electrolysis?

If halide ions ($Cl^-$, $Br^-$, $I^-$) are present, they are preferentially oxidised to form halogen molecules ($Cl_2$, $Br_2$, $I_2$). If no halide ions are present, $OH^-$ ions from water are oxidised to produce oxygen gas and water.

What are the standard diagnostic chemical tests for hydrogen, oxygen, and chlorine gases?

Hydrogen ($H_2$) burns with a squeaky pop when exposed to a lit wooden splint; Oxygen ($O_2$) relights a glowing wooden splint; Chlorine ($Cl_2$) turns damp blue litmus paper temporarily red before bleaching it white.

Household and Biological pH Values

Car battery acid and stomach acid have pH $1$ to $2$; lemon juice and vinegar have pH $2$ to $3$; acid rain has pH $4$; human skin has pH $5.5$; unpolluted rain has pH $6$; pure water has pH $7$; pancreatic secretions have pH $8$; bleach has pH $12$; and caustic soda has pH $13$ to $14$.

Universal Indicator Colour Transitions

A wide-range indicator that changes continuously across the pH spectrum: deep red in strong acid, orange and yellow in weak acid, green at neutrality (pH $7$), and blue to deep purple in alkaline solutions.

Proton Nature of the Hydrogen Ion

A neutral hydrogen atom contains one proton and one electron. When it loses its valence electron to become an aqueous hydrogen ion ($H^+$), it is fundamentally just a bare proton.

Dynamic Equilibrium in Weak Acid Dissociation

Weak acids ionise reversibly (e.g., $CH_3COOH_{(aq)} \rightleftharpoons H^+_{(aq)} + CH_3COO^-_{(aq)}$). Because only a small fraction of molecules dissociate into ions, the position of equilibrium lies far to the left.

Reaction Rates of Strong vs. Weak Acids

At equal molar concentrations, a strong acid reacts at a significantly faster rate than a weak acid because complete dissociation provides a much greater concentration of active hydrogen ions ($H^+$).

Effect of Dilution on Acid pH

Diluting any acid lowers the number of dissolved $H^+$ ions per unit volume of solution. This reduction in concentration causes the pH value to increase towards neutral pH $7$.

Burette Filling Safety Procedures

Always fill the burette below eye level using a small funnel while wearing safety glasses to prevent facial splashes. Remove the funnel immediately after filling so stray droplets do not alter the titre reading.

Rough Titration Purpose

An initial rapid run used to find the approximate neutralization end-point. It acts as a benchmark so subsequent accurate titrations know when to slow to dropwise addition.

Single Indicator: Litmus

A single-transition indicator used in titrations that turns red in acidic solutions and blue in alkaline solutions.

Single Indicator: Methyl Orange

A single-transition indicator used in titrations that appears red in acidic solutions and turns yellow in alkaline solutions.

Single Indicator: Phenolphthalein

A single-transition indicator used in titrations that remains colourless in acidic solutions and turns bright pink in alkaline solutions.

Salt Anions Formed from Common Laboratory Acids

Hydrochloric acid ($HCl$) consistently forms chloride salts ($Cl^-$), sulfuric acid ($H_2SO_4$) forms sulfate salts ($SO_4^{2-}$), and nitric acid ($HNO_3$) forms nitrate salts ($NO_3^-$).

Excess Insoluble Base Identification

When preparing a soluble salt, the base is known to be in excess when unreacted solid powder settles and remains visible at the bottom of the beaker even after continuous stirring.

Evaporation Safety in Salt Crystallisation

Heating the filtered salt solution over an electric hotplate or water bath rather than directly over a naked Bunsen flame prevents thermal decomposition and destruction of the hydrated salt crystals.

Fundamental Basis of Metal Reactivity

A metal's position in the reactivity series is determined by the ease with which its atoms lose valence electrons to form positive cations: elements that lose electrons more readily react with greater vigour.

Acid-Metal Reaction Observations Across the Series

Potassium, sodium, and calcium react violently and explosively; magnesium reacts vigorously with abundant effervescence; zinc and iron react slowly at room temperature; copper does not react.

Reaction of Metals with Cold Water

Metals high in the series react with liquid water at room temperature to form a metal hydroxide and hydrogen gas ($\text{Metal} + \text{Water} \rightarrow \text{Metal Hydroxide} + H_{2(g)}$).

Calorimetric Assessment of Metal Reactivity

Acid-metal displacement reactions release heat. Under strict controls (equal mass, surface area, and acid concentration), a greater temperature rise ($\Delta T$) quantitatively indicates a more reactive metal.

Native Metals

Extremely unreactive metals such as gold ($Au$) that exist in the Earth's crust in their pure, elemental form because they do not spontaneously oxidise in the presence of atmospheric oxygen.

Smelting Iron Ore with Carbon

Iron(III) oxide is reduced by heating with carbon (coke) in a blast furnace: $2Fe_2O_{3(s)} + 3C_{(s)} \rightarrow 4Fe_{(l)} + 3CO_{2(g)}$, where iron loses oxygen and carbon gains oxygen.

Role of Non-Metals in the Reactivity Series

Carbon and hydrogen are included as reference elements: carbon determines whether a metal oxide can be reduced by heating with carbon, and hydrogen predicts if a metal will react with dilute acids.

Net Ionic Equation and Spectator Ions

An equation showing only the species directly changing in charge or state during a reaction. Spectator ions remain identical in oxidation state and solution before and after, so they are omitted.

Balancing Charges in Redox Half Equations

Electrons ($e^-$) are added to whichever side of the half equation has the more positive total charge so that both mass and net electrical charge balance on either side.

Non-Conductivity of Solid Ionic Lattices

Solid ionic compounds cannot undergo electrolysis because their ions are held in fixed positions within a rigid 3D ionic lattice, preventing them from moving toward charged electrodes.

Ion Discharge

The process by which an ion gains or loses electrons at an electrode surface, neutralising its electrical charge to become an uncharged elemental atom or molecule.

Electrolysis of Molten Lead(II) Bromide ($PbBr_2$)

Cathode ($-\text{ve}$): Lead ions gain electrons to form liquid lead ($Pb^{2+}_{(l)} + 2e^- \rightarrow Pb_{(l)}$). Anode ($+\text{ve}$): Bromide ions lose electrons to form bromine gas ($2Br^-_{(l)} \rightarrow Br_{2(g)} + 2e^-$).

Function of Cryolite in Aluminium Smelting

Purified bauxite alumina ($Al_2O_3$) is dissolved in molten cryolite ($Na_3AlF_6$) to reduce the melting point from over $2000\,^\circ\text{C}$ to approximately $900\,^\circ\text{C}$, saving massive amounts of electrical and thermal energy.

Degradation of Carbon Anodes in Aluminium Extraction

At temperatures above $900\,^\circ\text{C}$, oxygen gas discharged at the positive anode reacts with the graphite carbon rods ($C_{(s)} + O_{2(g)} \rightarrow CO_{2(g)}$), causing them to burn away and require replacement.

Ions Present in Aqueous Solutions

In addition to the dissolved solute ions, all aqueous electrolytes contain hydrogen ions ($H^+$) and hydroxide ions ($OH^-$) due to the reversible auto-ionisation of water: $H_2O_{(l)} \rightleftharpoons H^+_{(aq)} + OH^-_{(aq)}$.

Aqueous Electrolysis: Cathode Discharge Rule

Hydrogen ions ($H^+$) are discharged to evolve hydrogen gas ($2H^+_{(aq)} + 2e^- \rightarrow H_{2(g)}$) unless the metal cation is less reactive than hydrogen (such as copper), in which case the metal deposits.

Aqueous Electrolysis: Anode Discharge Rule

If halide ions ($Cl^-$, $Br^-$, $I^-$) are present, they are oxidised to halogen molecules. If no halides are present, hydroxide ions ($OH^-$) are discharged to produce oxygen gas and water: $4OH^-_{(aq)} \rightarrow O_{2(g)} + 2H_2O_{(l)} + 4e^-$.

Diagnostic Test: Hydrogen Gas ($H_2$)

Exposing a burning wooden splint to the collected gas produces a distinct squeaky pop sound.

Diagnostic Test: Oxygen Gas ($O_2$)

Introducing a glowing wooden splint into the gas causes the splint to relight.

Diagnostic Test: Chlorine Gas ($Cl_2$)

Placing damp blue litmus paper into