Chemical Reations And Equations

CloseWhen copper is exposed to moist air for a long time, it reacts with carbon dioxide and water to form a green coating of basic copper carbonate. This gradual destruction of metal due to environmental attack is called corrosion.

CloseRusting is the corrosion of iron. Iron reacts with oxygen and moisture to form hydrated iron(III) oxide (Fe₂O₃·xH₂O), which is reddish-brown rust. Corrosion is the general term for deterioration of metals by environmental reaction.

CloseSilver reacts with hydrogen sulphide (H₂S) present in air to form a black layer of silver sulphide (Ag₂S). This is a form of corrosion, though silver does not rust like iron.

CloseNitrogen is an inert gas that does not react with food. It displaces oxygen inside the packet, preventing oxidation of fats and oils (rancidity) and keeping chips fresh for longer.

CloseRancidity occurs when unsaturated fats react with atmospheric oxygen, breaking down into smaller, smelly compounds like aldehydes and carboxylic acids. This oxidation process spoils the taste and smell of food.

CloseWhen fats and oils are oxidized, they produce volatile organic compounds that have an unpleasant, sharp smell and bitter taste. The physical shape and colour may remain unchanged initially.

CloseAntioxidants are substances that prevent oxidation by getting oxidized themselves in place of the food. Examples include vitamin C (ascorbic acid), vitamin E, and BHA (butylated hydroxyanisole) in packaged foods.

CloseCopper in CuSO₄ has a +2 oxidation state. It gains 2 electrons to become Cu(0) metal. Gain of electrons is reduction. Iron is oxidized (loses electrons to become Fe²⁺), so this is a redox reaction.

CloseThe classical definition of oxidation is the addition of oxygen to a substance. For example, 2Mg + O₂ → 2MgO, magnesium gains oxygen and is oxidized. Alternatively, loss of hydrogen is also oxidation.

CloseThe classical definition of reduction is the removal of oxygen from a substance. For example, in CuO + H₂ → Cu + H₂O, copper(II) oxide loses oxygen to become copper metal, so copper is reduced.

CloseCarbon (C) gains oxygen to form carbon dioxide (CO₂). Gain of oxygen is oxidation. Lead oxide (PbO) loses oxygen to become lead (Pb), so lead is reduced. This is a redox reaction used in metal extraction.

CloseRedox is short for reduction-oxidation. In any such reaction, one species loses electrons (oxidation) and another gains electrons (reduction). They cannot occur separately; they happen together.

CloseWhen magnesium burns in air (oxygen), it forms a white powder of magnesium oxide (MgO). The reaction is 2Mg + O₂ → 2MgO. It is a combination reaction and is highly exothermic.

CloseThe bright white light and white powder are characteristic of magnesium burning. The product is magnesium oxide (MgO). It is basic in nature and turns red litmus blue when dissolved in water.

CloseZinc is more reactive than iron according to the reactivity series (K > Na > Ca > Mg > Al > Zn > Fe > Pb > Cu > Ag > Au). A more reactive metal displaces a less reactive metal from its salt solution. Zn + FeSO₄ → ZnSO₄ + Fe.

CloseThis is a double displacement reaction: 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄↓ + 2AlCl₃. Barium sulphate (BaSO₄) is a white precipitate that is insoluble in water, confirming the reaction.

CloseAluminium displaces iron from iron(III) oxide. Since aluminium is more reactive than iron, it is a single displacement reaction. This reaction is also called the thermite reaction and produces enormous heat.

CloseThe law of conservation of mass states that matter can neither be created nor destroyed in a chemical reaction. A balanced equation has the same number of atoms of each element on both sides, obeying this law.

Close(s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous solution (substance dissolved in water). These physical state symbols are written in brackets after each chemical formula.

CloseThis is a neutralization reaction (acid + base → salt + water). 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O. Sodium sulphate (Na₂SO₄) is the salt formed.

CloseThis is a double displacement reaction: AgNO₃ + NaCl → AgCl↓ + NaNO₃. Silver chloride (AgCl) is a white precipitate that turns grey in sunlight due to photolytic decomposition.

CloseDecomposition reaction is one in which a single compound breaks down into two or more simpler substances. Here, potassium chlorate decomposes into potassium chloride and oxygen gas upon heating.

ClosePhotolytic decomposition uses light energy. 2AgCl (white) → 2Ag (grey) + Cl₂ (gas) in sunlight. This is why silver chloride turns grey in sunlight and is used in black-and-white photography.

CloseExothermic reactions release heat. Respiration (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy) releases energy. Photosynthesis is endothermic, decomposition of CaCO₃ requires heat, and electrolysis requires electrical energy.

Close2K + 2H₂O → 2KOH + H₂↑. Potassium is an extremely reactive metal. The reaction is violent, produces heat (exothermic), and hydrogen gas catches fire immediately. Potassium hydroxide (KOH) is a strong base.

CloseFe + 2HCl → FeCl₂ + H₂↑. Iron displaces hydrogen from hydrochloric acid because iron is more reactive than hydrogen. Iron(II) chloride (FeCl₂) is formed, and hydrogen gas is released as bubbles.

CloseRust is hydrated iron(III) oxide (Fe₂O₃·xH₂O). The reddish-brown colour is characteristic of iron(III) compounds. Green coating is for copper, black for silver sulphide.

CloseCorrosion affects any metal structure exposed to moisture and air. Car bodies, iron railings, bridges, pipelines, ships, and monuments all suffer degradation, costing billions in maintenance and replacement annually.

CloseEndothermic reactions absorb heat. CaCO₃ (limestone) requires continuous heating to decompose into CaO (quicklime) and CO₂. Burning magnesium, respiration, and neutralization all release heat (exothermic).

CloseBaCl₂ + H₂SO₄ → BaSO₄↓ + 2HCl. Barium sulphate forms a white precipitate that is insoluble in water and even in strong acids. This reaction is used as a test for sulphate ions.

CloseN₂ + 3H₂ ⇌ 2NH₃. This is the Haber process for manufacturing ammonia. The reaction is reversible and requires high pressure (200 atm), high temperature (400-500°C), and an iron catalyst.

Close2H₂S + 3O₂ → 2H₂O + 2SO₂. Hydrogen sulphide burns in excess oxygen to give sulphur dioxide and water. It has a characteristic smell of rotten eggs and is a toxic gas.

CloseIn a single displacement reaction (also called substitution reaction), one element displaces another from its compound. Example: Zn + CuSO₄ → ZnSO₄ + Cu. The reactivity series determines whether displacement occurs.

CloseGold is a noble metal and is at the bottom of the reactivity series. It does not react with oxygen, moisture, or most acids. That is why gold artifacts remain shiny for thousands of years and it is used in jewellery.

CloseAntioxidants prevent oxidation of fats and oils. Other prevention methods include flushing food packets with nitrogen gas, storing food in airtight containers, refrigeration, and keeping food away from light.