Revision: Chemical Reactions and Equations Science and Technology 1 SSC (English Medium) 10th Standard Maharashtra State Board

The cell which converts electrical energy into chemical energy is called an electrolytic cell and the process which converts electrical energy into chemical energy is called electrolysis.

A chemical reaction that occurs in the presence of light is called a photochemical reaction.

A catalyst is a substance that either increases or decreases the rate of a chemical reaction without itself undergoing any chemical change during the reaction.

Any chemical change in matter that involves its transformation into one or more new substances is called a chemical reaction.

An electrochemical reaction is a chemical reaction involving the transfer of electrons, where oxidation and reduction occur simultaneously.

Chemical reactions which are characterised by the formation of insoluble solid substances are called precipitates.

A chemical reaction in which two compounds in their aqueous state react to form an insoluble salt as one of the products.

A chemical change is a permanent change in which the chemical composition of a substance is changed and one or more new substances with different chemical compositions and different properties are formed.

The substances formed as a result of a chemical reaction by formation of new bonds are called products.

A chemical change is a process in which the composition of matter changes, resulting in the formation of one or more new substances.

The substances taking part in chemical reaction are called reactants.

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.

OR

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

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.”

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 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.

OR

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.

A chemical reaction in which heat is released when reactants are converted to products is called an exothermic reaction.

The process in which heat is absorbed from the outside, is called endothermic process.

A chemical reaction in which heat is absorbed from the surroundings or must be continuously supplied is called an endothermic reaction.

The processes in which heat is given out are called exothermic processes.

“The substance in whose presence the rate of a chemical reaction increases, without causing any chemical change to it, is called a catalyst.”

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.

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 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.

OR

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

A reducing agent is one that reduces other substances by providing electrons, or by providing hydrogen or an electropositive ion, (or by removing oxygen or an electronegative ion). Thus, in the above examples, hydrogen, carbon, hydrogen sulphide and ammonia are the reducing agents.

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The substance that brings about reduction is called a reductant, or a reducing agent.

In the electronic concept, reduction is defined as a process in which an atom or an ion gains electron(s).

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Reduction is also defined as a chemical process which involves removal of oxygen or addition of hydrogen.

OR

The chemical reactions in which reactants gain hydrogen are called ‘reduction’ reactions. Similarly, the reaction in which a reactant loses oxygen to form the product is also called reduction 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.

Rancidity is the condition in which oils and fats get oxidized, producing an unpleasant smell and taste. This definition is precise and suitable for exams.

Corrosion is the gradual damage of metals caused by their reaction with components of the atmosphere, such as oxygen and moisture.

• 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.

• Nature of Reactants: More reactive substances (e.g., Al over Zn) react faster, affecting the reaction rate.
• Particle Size: Smaller particles have more surface area, leading to a faster reaction (e.g., powder reacts faster than pieces).
• Concentration: A higher concentration of reactants increases the rate of reaction (e.g., concentrated HCl reacts faster than dilute HCl).
• Temperature: Higher temperature increases particle energy, leading to a faster reaction rate.
• Catalyst: A catalyst speeds up a reaction without undergoing any chemical change itself (e.g., MnO₂ in decomposition reactions).