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

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.

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.

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

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.

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

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

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

OR

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.

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.

OR

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 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.

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.

The rate of a chemical reaction may be defined as the change in concentration of any of the reactants or any of the products per unit time.

Rate of Reaction = `"Change in concentration of a reactant or a prodect"/"Time taken for the change"`

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

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

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.

When an iron nail is kept in copper sulphate solution, a reddish brown coating is seen on the iron nail after sometime. Also, the colour of the copper sulphate solution changes from blue to green. This is observed because iron displaces copper from copper sulphate solution, as iron is more reactive than copper.

\[\ce{Fe_{(s)} + CuSO4_{(aq)} -> FeSO4_{(aq)} + Cu_{(s)}}\]

\[\mathrm{Rate}=\frac{\text{Decrease in concentration of Reactant}}{\text{Time interval}}\]

\[=-\frac{\Delta[R]}{\Delta T}\]

\[\mathrm{Rate}=\frac{\text{Increase in concentration of Product}}{\text{Time interval}}\]

\[=+\frac{\Delta\left[P\right]}{\Delta T}\]

For a general reaction aA + bB → cC + dD:

\[\frac{dx}{dt}=-\frac{1}{a}\frac{d[A]}{dt}=-\frac{1}{b}\frac{d[B]}{dt}=+\frac{1}{c}\frac{d[C]}{dt}=+\frac{1}{d}\frac{d[D]}{dt}\]

• 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).