Revision: Chemical Reactions and Equations Science English Medium Class 10 CBSE

The reactions in which heat is absorbed are called endothermic reactions. The reactants absorb heat for form products.

A chemical equation is a balanced account of a chemical transaction. It is not merely a qualitative statement, but it also gives quantitative information of a chemical reaction.

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The representation of a chemical reaction in a condensed form using chemical formulae is called as the chemical equation.

A reaction in which a single product is formed from two or more reactants is known as a combination reaction.

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When two or more reactants combine in a reaction to form a single product, it is a combination reaction.

When an acid reacts with a base salt and water are formed and this is called Neutralisation.
Double-decomposition – Neutralisation reaction :
“Is the chemical reaction between two compounds (acid and base) to interchange radicals and produce salt and water.”

When a decomposition reaction is carried out by heating, it is called thermal decomposition.

A single reactant breaks down to give simpler products. This is a decomposition reaction.

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The chemical reaction in which two or more products are formed from a single reactant is a decomposition reaction.

The reaction in which the place of the ion of a less reactive element in a compound is taken by another more reactive element by formation of its own ions, is called displacement reaction.

Reactions in which there is an exchange of ions between the reactants are called double displacement reactions.

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The reaction in which the ions in the reactants are exchanged to form a precipitate is a double displacement reaction.

Redox reactions are those in which reduction and oxidation both takes place simultaneously i.e. one substance is reduced while the other gets oxidised.

The chemical reaction in which a reactant combines with oxygen or loses hydrogen to form the product is called oxidation reaction.

Chemical substances which bring about an oxidation reaction by making oxygen available are called oxidants or oxidizing agents.

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Substances that can give oxygen to other substances are called oxidants or oxidizing agents.

The chemical reaction in which reactants gain oxygen to form the corresponding oxide. Also, A chemical reaction in which reactants lose hydrogen to form a product is called oxidation.

Example: \[\ce{H2S + Cl -> 2HCl + S}\]

When the surface of a metal is attacked by air, moisture or any other substance around it, the metal is said to corrode and the phenomenon is known as corrosion.

Corrosion is an electrochemical process in which a metal is oxidised to a metal oxide or another salt of the metal by losing electrons to oxygen or another electronegative element, forming a coating on the metal surface.

Those reactions in which hydrogen combines with a substance or oxygen is removed from a substance, are known as reduction reactions.

Example: \[\ce{2HgO->[heat]2Hg +O2}\]

The oxidation number of an element is defined as the formal charge that an atom of that element appears to have when electrons are counted.

When fats and oils are oxidised, they become rancid and their smell and taste change.

• C + O₂ → CO₂
• 2H₂ + O₂ → 2H₂O
• 2Mg + O₂ → 2MgO

• Word equations use names; chemical equations use formulas.
• Reactants → Products, with arrow showing reaction direction.
• Use + between two or more reactants or products.
• Show states: (s), (l), (g), (aq); use ↑ for gas, ↓ for precipitate.
• Heat (Δ) or other conditions go above/below the arrow.

• Law of Conservation of Mass: In a chemical reaction, mass is neither created nor destroyed, so the number of atoms of each element must be equal on both sides.
• A skeletal (unbalanced) equation has unequal atoms of one or more elements on the LHS and RHS.
• Balancing is done using the hit-and-trial method, starting with the compound having the most atoms and balancing hydrogen and oxygen last.
• Only coefficients are changed while balancing; chemical formulas must not be altered.
• A balanced equation may also indicate physical states (s, l, g, aq) and reaction conditions, such as temperature, pressure, or a catalyst.

• A more reactive metal can replace a less reactive metal from its compound in solution.
• Zinc, iron, and lead can displace copper from copper sulphate or copper chloride solutions.
• During displacement, the displaced metal appears in elemental form, while the replacing metal forms its salt.
• In the reaction Fe + CuSO₄ → FeSO₄ + Cu, the iron nail turns brownish due to deposited copper, and the blue solution fades.
• Heat may be released during displacement reactions, indicating that the reaction is exothermic.