CBSE Class 10 Chemistry Chemical Reactions and Equations Notes Set 26

Chemical Reactions and Equations

1. A chemical reaction changes substances into new substances. Reactants are on the left, products on the right, joined by an arrow. A change of state, colour, evolution of a gas or a change of temperature suggests a chemical reaction.
2. A good chemical equation must represent a real reaction, use correct formulae (gases such as H₂, O₂, N₂ and Cl₂ are written as molecules), have equal numbers of each kind of atom on both sides, and show states: (s), (l), (g), (aq). A ↓ beside a product shows a precipitate and an ↑ shows a gas.
3. A balanced equation tells the names of the substances, the ratio of molecules and moles, the ratio of masses and, for gases, of volumes. It does not tell the speed of the reaction, the conditions needed or the heat change unless these are added.
4. To balance by hit and trial: write the correct formulae, count atoms of each element, start with the compound that has the most atoms, change only coefficients (never subscripts), and check at the end. Example: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.

Types of Chemical Reactions

Decomposition reactions are the opposite of combination reactions. Reactivity order of common metals: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au; a metal displaces only metals below it, and only metals above hydrogen displace hydrogen from dilute acids.

Oxidation and Reduction

1. Old definition: oxidation is gain of oxygen or loss of hydrogen; reduction is loss of oxygen or gain of hydrogen. Modern definition: oxidation is loss of electrons (oxidation number increases) and reduction is gain of electrons (oxidation number decreases). Remember OIL RIG.
2. Oxidation and reduction always occur together, so such reactions are called redox reactions. The substance oxidised is the reducing agent; the substance reduced is the oxidising agent. Example: in CuO + H₂ → Cu + H₂O, CuO is reduced (oxidising agent) and H₂ is oxidised (reducing agent).
3. A species in an intermediate oxidation state can act as both: HNO₂, H₂O₂, SO₂. If the same substance is oxidised and reduced in one reaction, it is a disproportionation reaction (P₄ with alkali gives PH₃ and H₂PO₂^–).

Oxidation Number Rules

1. The oxidation number of an uncombined element is 0.
2. In a neutral compound the oxidation numbers add up to 0; in an ion they add up to the charge on the ion.
3. Alkali metals are +1, alkaline earth metals +2, fluorine is always –1, hydrogen is usually +1 (–1 in metal hydrides) and oxygen is usually –2 (–1 in peroxides, +2 in OF₂).
4. Oxidation numbers can be fractions (average value): Fe₃O₄ is +8/3, C₃O₂ is +4/3, YBa₂Cu₃O₇ has Cu at +7/3.
5. Examples: Cl in HClO₃ is +5, Mn in KMnO₄ is +7, S in H₂S₂O₈ is +6, C in CH₂O is 0.

Balancing Redox Equations

1. Oxidation number method: find the elements whose oxidation numbers change, make the total increase equal to the total decrease by choosing coefficients, then balance the remaining atoms (O with H₂O, H with H+ in acid).
2. Ion-electron (half-reaction) method: write the oxidation and reduction half-reactions, balance atoms other than O and H, then O with H₂O, H with H+ (add OH– in basic medium), then charge with electrons; multiply to equalise the electrons and add.
3. Example: Cr₂O₇^2– + 14H+ + 6e– → 2Cr³⁺ + 7H₂O is the reduction half-reaction of dichromate, and the full reaction with SO₂ is Cr₂O₇^2– + 3SO₂ + 2H+ → 2Cr³⁺ + 3SO₄^2– + H₂O.

Corrosion and Rancidity

1. Corrosion is the slow eating away of a metal by air, moisture or chemicals. Rust is hydrated iron(III) oxide, Fe₂O₃·xH₂O, and needs both oxygen and water. Prevention: painting, oiling, galvanising, alloying. Silver tarnishes (black Ag₂S), copper turns green, gold and silver resist corrosion.
2. Rancidity is the oxidation of fats and oils in food, which gives a bad smell and taste. Prevention: airtight containers, refrigeration, flushing with nitrogen, adding antioxidants.
3. Other useful facts: black CuO forms when brown copper is heated in air; whitewash is slaked lime from quick lime and water; Fe + CuSO₄ changes the blue solution to pale green; PbI₂ is a yellow precipitate; electrolysis of water gives H₂ and O₂ in the ratio 2 : 1.