Focused Chemistry chapters — Chemical Reactions, Acids, Metals, Carbon Compounds.
Types, Balancing, Oxidation-Reduction
pH scale, Neutralisation, Common salts
Reactivity, Extraction, Alloys
Covalent bonding, Functional groups, Soaps
Answers: (a) and (b) are incorrect.
• PbO is getting reduced to Pb (lead oxide losing oxygen = reduction)
• C is getting oxidised to CO₂ (carbon gaining oxygen = oxidation)
• (a) is wrong because Pb is a product, not being reduced
• (b) is wrong because CO₂ is a product, not being oxidised
Iron reacts with dilute HCl to form iron(II) chloride and hydrogen gas.
This is a displacement reaction. Iron displaces hydrogen from the acid.
A balanced chemical equation has an equal number of atoms of each element on both sides. Equations must be balanced to satisfy the Law of Conservation of Mass — matter can neither be created nor destroyed in a chemical reaction.
Exothermic: Reactions that release heat. Examples: burning of natural gas (CH₄ + 2O₂ → CO₂ + 2H₂O), respiration, neutralisation.
Endothermic: Reactions that absorb heat. Examples: decomposition of calcium carbonate, photosynthesis, melting of ice.
Oxidation: Gain of oxygen or loss of hydrogen
Reduction: Loss of oxygen or gain of hydrogen
A redox reaction is one in which both oxidation and reduction occur simultaneously.
Example: CuO(s) + H₂(g) → Cu(s) + H₂O(l)
CuO is reduced (loses O), H₂ is oxidised (gains O).
ZnO is reduced to Zn (loss of oxygen = reduction)
C is oxidised to CO (gain of oxygen = oxidation)
Decomposition reactions are reactions in which a single compound breaks down into two or more simpler substances.
Example: 2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g)
Types: Thermal, electrolytic, photolytic decomposition.
Zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas. The gas burns with a pop sound when a burning matchstick is brought near it.
The pH scale measures how acidic or basic a solution is. It ranges from 0 to 14.
• pH < 7: Acidic
• pH = 7: Neutral
• pH > 7: Basic
pH = 7 indicates a neutral solution (like pure water).
Neutralisation is the reaction between an acid and a base to form salt and water.
Examples:
• HCl + NaOH → NaCl + H₂O
• H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
• Antacids neutralise excess stomach acid
Acids:
• Turn blue litmus red
• Have sour taste
• React with metals to produce H₂
• React with bases to form salt and water
Bases:
• Turn red litmus blue
• Have bitter taste
• Feel slippery to touch
• React with acids to form salt and water
Deliquescent: Substances that absorb moisture from air and dissolve in it, forming a solution. Examples: CaCl₂, NaOH, P₂O₅.
Efflorescent: Substances that lose their water of crystallisation to air, becoming powdery. Examples: Na₂CO₃·10H₂O, CuSO₄·5H₂O.
Washing soda = Sodium carbonate decahydrate = Na₂CO₃·10H₂O
Uses:
• In cleaning clothes (removes grease)
• In glass, soap, and paper industries
• In removing permanent hardness of water
• As a household清洁 agent
The hydrogen gas can be tested by bringing a burning matchstick near it — it burns with a pop sound.
Strong acids: Completely ionise in water. Examples: HCl (hydrochloric acid), H₂SO₄ (sulphuric acid), HNO₃ (nitric acid).
Weak acids: Partially ionise in water. Examples: CH₃COOH (acetic acid/vinegar), H₂CO₃ (carbonic acid), H₃PO₄ (phosphoric acid).
Bleaching powder (calcium oxychloride) is prepared by passing chlorine gas through dry slaked lime (calcium hydroxide). It is used for bleaching clothes, as a disinfectant, and in water purification.
Properties of metals:
• Lustre: Shiny appearance
• Hardness: Generally hard (except sodium, potassium)
• Conductivity: Good conductors of heat and electricity
• Malleability: Can be beaten into thin sheets
• Ductility: Can be drawn into wires
• Sonorous: Produce ringing sound when struck
• High melting point
(i) Mercury (Hg) — liquid at room temperature
(ii) Sodium (Na) — soft, can be cut with a knife
(iii) Silver (Ag) — best conductor of heat
(iv) Lead (Pb) — poor conductor of heat (though it is a metal)
Reactivity series (most reactive to least reactive):
K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au
• Metals above hydrogen displace it from acids
• More reactive metals displace less reactive metals from their salt solutions
• K, Na, Ca are so reactive they react with cold water
Iron displaces copper from copper sulphate solution because iron is more reactive than copper. The blue colour of the solution fades and reddish-brown copper deposits on the iron nails.
Corrosion is the slow degradation of metals due to reaction with air, moisture, and chemicals.
Prevention methods:
• Painting the metal surface
• Oiling and greasing
• Galvanisation — coating with zinc
• Electroplating with chromium or tin
• Alloying — mixing with other metals
• Chrome plating
Ionic compounds: Formed by transfer of electrons (metal + non-metal). High melting point, conduct electricity in solution. Example: NaCl.
Covalent compounds: Formed by sharing of electrons (non-metal + non-metal). Low melting point, do not conduct electricity. Example: H₂O, CO₂.
Alloys are homogeneous mixtures of two or more metals (or metal and non-metal).
Examples:
• Brass: Cu + Zn — used in decorative items, instruments
• Bronze: Cu + Sn — used in statues, medals
• Steel: Fe + C — used in construction, bridges
• Solder: Pb + Sn — used in electrical wiring
• Amalgam: Hg + other metal — used in dental fillings
Sodium is extracted by electrolytic reduction (Down's process) of molten NaCl.
It cannot be extracted by heating with carbon because sodium is more reactive than carbon. Carbon cannot reduce sodium oxide or sodium chloride. Electrolysis is required because sodium is a very reactive metal.
Copper is used because:
• It is an excellent conductor of electricity
• It is malleable and ductile — can be drawn into wires
• It is relatively cheap compared to silver
• It has good tensile strength
• It is resistant to corrosion
Catenation is the ability of an element to form long chains, branched chains, and ring structures by self-linking of atoms.
Carbon shows catenation because:
• Carbon-carbon bonds are very strong
• Carbon has small atomic size, allowing close packing
• Carbon forms stable covalent bonds with other carbon atoms
• This leads to millions of carbon compounds
Saturated hydrocarbons: Have only single bonds (C-C). General formula: C₍H₂₍+₂. Example: Methane (CH₄), Ethane (C₂H₆). Called alkanes.
Unsaturated hydrocarbons: Have double or triple bonds. Alkenes (C=C, C₍H₂₍), Alkynes (C≡C, C₍H₂₍-₂). Example: Ethene (C₂H₄), Ethyne (C₂H₂).
A functional group is an atom or group of atoms that determines the chemical properties of a compound.
• Ethanol: -OH (hydroxyl/alcohol group)
• Ethanoic acid: -COOH (carboxyl group)
Other functional groups: -CHO (aldehyde), -CO- (ketone), -COO- (ester)
A homologous series is a group of compounds with the same functional group, similar chemical properties, and where successive members differ by a -CH₂ group.
Example: Alkanes: CH₄, C₂H₆, C₃H₈, C₄H₁₀...
Each member differs by CH₂ and has similar properties.
The IUPAC name is Ethanoic acid (common name: acetic acid / vinegar).
Preparation:
It can be prepared by oxidation of ethanol using alkaline potassium permanganate or acidified potassium dichromate.
Properties of ethanoic acid:
• Weak acid (partially ionises in water)
• Vinegar is 5-8% solution of ethanoic acid
• Freezes at 290 K (called glacial acetic acid when pure)
• Reacts with bases to form salts and water
• Reacts with alcohols to form esters (sweet smelling)
• Turns blue litmus red
Saponification is the process of making soap by reacting a fat or oil with sodium hydroxide (NaOH).
The reaction produces soap and glycerol. The soap is separated by adding common salt (NaCl) — this is called salting out.
Soaps:
• Sodium salts of long chain fatty acids
• Made by saponification
• Do not work in hard water (form scum)
• Biodegradable
Detergents:
• Sodium salts of sulphonic acids
• Made chemically
• Work in hard water
• May not be biodegradable
An ester is a sweet-smelling organic compound formed by the reaction of an acid with an alcohol.
Esters are used in perfumes and as solvents. They have a fruity smell.
Ethanol:
• As a solvent in medicines
• In alkoholic drinks
• As fuel and fuel additive
• In antiseptics (70% solution)
• In industry as a solvent
Ethanoic acid:
• In vinegar (food preservative)
• In making esters for perfumes
• In dyeing industry
• As a reagent in laboratories