CBSE Notes Class 10 Science Chapter 1 - Chemical Reactions and Equation

Chemical reactions are a fundamental concept in chemistry that describes the transformation of substances. The substances that undergo change are called reactants, and the new substances formed are known as products. This transformation can be expressed using chemical equations, where the symbols and formulas of the substances involved are written to represent a chemical change.

1.0What is a Chemical Reaction?

A chemical reaction is a process in which one or more substances (reactants) are transformed into different substances (products). This occurs when chemical bonds between atoms are broken and new bonds are formed. The change can be observed in various ways, such as color change, gas formation, precipitation, or temperature change. Here is an image representing balancing chemical equation-

Other Examples of Chemical Reactions:

• Rusting of iron: 

4Fe + 3O₂ → 2Fe₂O₃

• Burning of magnesium in the air: 

2Mg + O₂ → 2MgO

2.0Writing Chemical Equations

A chemical equation is the symbolic representation of a chemical reaction. It involves the chemical formulas of the reactants on the left side and the products on the right, separated by an arrow (→), which shows the direction of the reaction.

Example: The reaction of hydrogen with oxygen to form water can be written as:

                            2H₂ + O₂ → 2H₂O 

In a chemical equation:

• Reactants are written on the left side.
• Products are written on the right side.
• An arrow points from reactants to products, indicating the direction of the reaction.

3.0Balancing Chemical Equations

One of the most important skills in chemistry is balancing chemical equations. A balanced equation has the same number of atoms of each element on both sides. This is essential because matter cannot be created or destroyed (the Law of Conservation of Mass).

Steps to Balance a Chemical Equation:

1. Write the unbalanced equation with the correct chemical formulas for all reactants and products.
2. Count the number of atoms of each element on both sides.
3. Adjust the coefficients (the numbers before the formulas) to balance the atoms.
4. Ensure the coefficients are in the simplest whole number ratio.

Example: Balancing the equation for the formation of water:

                          H₂ + O₂ → H₂O (unbalanced) 

Step 1: There are 2 hydrogen atoms on both sides but only 1 oxygen atom on the right side.

Step 2: Adjust the coefficient of water:

                         2H₂ + O₂ → 2H₂O (balanced)

Now, both sides have 4 hydrogen atoms and 2 oxygen atoms.

4.0Types of Chemical Reactions

Chemical reactions can be classified into several types based on how the reactants transform into products. In Class 10, you’ll learn about five main types of reactions:

1. Combination Reaction: In this type, two or more reactants combine to form a single product. Example: 2H₂ + O₂ → 2H₂O
2. Decomposition Reaction: A single compound breaks down into two or more simpler substances. Example: 2H₂O₂  → 2H₂O + O2
3. Displacement Reaction: In this reaction, one element replaces another in a compound. Example: Zn + CuSO₄ → ZnSO₄ + Cu
4. Double Displacement Reaction: Two compounds exchange ions to form new compounds. Example: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
5. Oxidation-Reduction or Redox Reactions: In an oxidation-reduction (redox) reaction, one substance gains oxygen or loses electrons (oxidation), while another loses oxygen or gains electrons (reduction). Example: ZnO + C → Zn + CO

• Zinc oxide (ZnO) is reduced to zinc (Zn) because it loses oxygen.
• Carbon (C) is oxidized to carbon monoxide (CO) because it gains oxygen.

5.0Observations of Chemical Reactions

Certain observable changes indicate that a chemical reaction has taken place. These include:

Corrosion: Corrosion is the gradual destruction of metals due to reactions with substances like moisture or acids. Rusting of iron, blackening of silver, and greening of copper are common examples.

Rancidity: Rancidity happens when fats and oils oxidize, changing their taste and smell. To prevent it, antioxidants are added, and foods are kept in airtight containers. Chips bags are flushed with nitrogen to slow oxidation.

6.0Solved Examples of Chemical Reactions and Equations

Q. Why does the color of copper sulfate solution change when an iron nail is dipped in it?

Ans: The color changes because iron displaces copper from copper sulfate solution, forming iron sulfate (greenish) and copper metal (brownish-red) is deposited on the nail.

Q. Identify the substances oxidized and reduced:

Ans:

i) 4Na(s) + O₂(g) → 2Na₂O(s)

• Oxidized: Sodium (Na) – it gains oxygen.
• Reduced: Oxygen (O₂) – it gains electrons.

ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)

• Oxidized: Hydrogen (H₂) – it gains oxygen.
• Reduced: Copper oxide (CuO) – it loses oxygen.

Q. Why should a magnesium ribbon be cleaned before burning in the air?

Ans: Magnesium ribbon is cleaned to remove the oxide layer that forms on its surface, allowing it to burn more easily.

Q. Write balanced chemical equations for given statements:

i) Barium chloride and Sodium sulfate reaction:

Sol.    BaCl₂(aq) + Na₂SO₄(aq)   →   BaSO₄(s) + 2NaCl(aq)

(Barium sulfate precipitate and sodium chloride solution form.)

ii) Sodium hydroxide and Hydrochloric acid reaction:

Sol.   NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

(Sodium chloride solution and water form.)