Chemical Reactions and Equations

Introduction

A chemical reaction is a process in which one or more substances (called reactants) change to form new substances (called products). These changes may involve color change, gas formation, temperature change, or formation of a solid.

1. Chemical Equation
A chemical equation is the symbolic representation of a chemical reaction using chemical formulas.

Example:
Magnesium + Oxygen → Magnesium oxide
Mg + O₂ → MgO

This is a chemical equation showing the reactants and the product.

2. Balanced Chemical Equation
The law of conservation of mass says that mass is neither created nor destroyed in a chemical reaction.
So, the number of atoms of each element must be the same on both sides of the equation.

Balanced example:
2Mg + O₂ → 2MgO

3. Types of Chemical Reactions
Combination Reaction:--
Two substances combine to form one compound.
Example: CaO + H₂O → Ca(OH)₂

Decomposition Reaction:--
A single compound breaks down into simpler substances.
Example: 2H₂O₂ → 2H₂O + O₂

Displacement Reaction:--
One element replaces another in a compound.
Example: Zn + CuSO₄ → ZnSO₄ + Cu

Double Displacement Reaction:
Two compounds exchange their ions.
Example: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

4. Effects of a Chemical Reaction
Chemical reactions may show:

Change in color
Evolution of gas
Formation of a precipitate
Change in temperature

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Questions on Chemical Reactions and Equations

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1.A solution of potassium bromide is mixed with a solution of lead nitrate. What will be the observation, and what type of reaction is it? 2.Which type of reaction involves the breakdown of a single compound into two or more simpler substances? 3.How do you test for the presence of hydrogen gas? 4.What is the balanced equation for the reaction of hydrogen gas with chlorine gas to form hydrogen chloride gas? 5.If a student heats a blue crystalline solid, it turns white, and water droplets are observed on the cooler parts of the test tube. What is the blue crystalline solid? 6.In the reaction: FeSO4(aq) + Cu(s) → No reaction. Why does this happen? 7.Which gas is released when dilute hydrochloric acid reacts with calcium carbonate? 8.Explain why an iron gate needs to be painted regularly. 9.What are the reactants required for the process of photosynthesis? 10.Which of the following is an example of a precipitate forming reaction? 11.Consider the reaction: 2SO2(g) + O2(g) → 2SO3(g). If this reaction is carried out at high temperature and pressure, how would it affect the rate of reaction? 12.What is the common term for a reaction that involves both oxidation and reduction simultaneously? 13.Identify the reaction where electricity is used to break down a compound. 14.Why are antioxidants added to food items containing fats and oils? 15.A white solid on heating produces a gas that turns limewater milky and a residue that is yellow when hot but white when cold. Identify the white solid. 16.Which type of reaction is the thermite reaction (Fe2O3 + 2Al → Al2O3 + 2Fe)? 17.Assertion (A): Rusting of iron is a fast process. Reason (R): Rusting requires specific conditions of air and moisture to occur. 18.Balance the equation: C6H12O6 + O2 → CO2 + H2O. What is the coefficient of O2? 19.Why is a black coating formed on the surface of some utensils exposed to a gas flame for a long time? 20.What is the general name for a reaction that releases energy, often in the form of heat or light? 21.In the reaction: Fe2O3 + 2Al → Al2O3 + 2Fe. Which element is acting as the reducing agent? 22.Give a reason why balanced chemical equations are important for industrial processes. 23.What type of chemical reaction is involved in the digestion of food? 24.When a student observed that a certain metal tarnished over time, forming a green layer, what could be the metal? 25.Which of the following reactions is an example of electrolytic decomposition? 26.Why does food cook faster in a pressure cooker? Relate it to chemical reactions. 27.Assertion (A): White silver chloride turns grey in sunlight. Reason (R): Decomposition of silver chloride in the presence of sunlight forms silver metal and chlorine gas. 28.What is the purpose of adding iron filings to a solution of copper sulphate in a laboratory experiment? 29.A reaction is represented as A + B → C + D. If A and B are reactants, and C and D are products, what must be true about the total mass of A and B compared to C and D? 30.What is the term for a substance that slows down the rate of rancidity?