Maharashtra State Board Science-1-Chapter-3- Chemical Reactions And Equations-Can You Recall? : Welcome to the fascinating world of chemical reactions and equations! In this chapter, we explore how substances transform during chemical reactions and learn to represent these changes using chemical equations. A chemical reaction involves breaking old bonds and forming new ones, leading to the creation of new substances. By balancing chemical equations, we ensure the law of conservation of mass is upheld, reflecting that matter is neither created nor destroyed in a reaction. Get ready to delve into the core principles that explain how and why substances interact, react, and transform in the realm of chemistry!
1) What are the types of molecules of elements and compounds?
Molecules of Elements:
- Monatomic Molecules: Consist of a single atom. For example, noble gases like Helium (He), Neon (Ne), and Argon (Ar) exist as monatomic molecules.
- Diatomic Molecules: Consist of two atoms. Examples include Hydrogen (H₂), Oxygen (O₂), Nitrogen (N₂), and Halogens like Chlorine (Cl₂).
- Polyatomic Molecules: Consist of more than two atoms. For example, Ozone (O₃) is a triatomic molecule, and Phosphorus (P₄) and Sulfur (S₈) are examples of polyatomic molecules with more complex structures.
Molecules of Compounds:
- Binary Compounds: Composed of two different elements. Examples include Water (H₂O), Carbon Dioxide (CO₂), and Sodium Chloride (NaCl).
- Ternary Compounds: Composed of three different elements. Examples include Calcium Carbonate (CaCO₃) and Sulfuric Acid (H₂SO₄).
- Complex Compounds: Composed of more than three elements or more complex arrangements, such as organic molecules like Glucose (C₆H₁₂O₆) and biological macromolecules like DNA.
2) What is Meant by Valency of Elements?
Valency refers to the ability of an element to combine with other elements. It is determined by the number of electrons in the outermost shell (valence electrons) that an atom can lose, gain, or share to form a stable chemical bond. Valency is crucial in predicting how elements will interact in chemical reactions.
Key Points about Valency:
- Electrons in the Outer Shell: The valency of an element is typically the number of electrons in its outermost shell. For example, Sodium (Na) has one electron in its outer shell, giving it a valency of 1.
- Combining Power: Valency indicates the combining power of an element. For instance, Carbon (C) has a valency of 4 because it can form four bonds with other atoms.
3) What is the requirement for writing molecular formulae of different compounds? How are the molecular formulae of the compounds written?
Writing the molecular formulae of compounds requires understanding the composition and ratios of the elements involved. Here are the key requirements:
- Knowledge of Symbols: Each element is represented by a unique chemical symbol from the periodic table.
- Understanding Valency: Knowing the valency (combining power) of each element helps determine the ratios in which elements combine.
- Law of Conservation of Mass: The total mass of the reactants must equal the total mass of the products, ensuring balanced chemical equations.
- Chemical Rules: Following specific rules and conventions for combining elements.
How Molecular Formulae of Compounds are Written
To write the molecular formulae of compounds, follow these steps:
Step 1: Identify the Elements
Determine which elements are present in the compound.
Step 2: Determine the Valency of Each Element
Refer to the periodic table or other resources to find the valency of each element. For example:
Hydrogen (H) has a valency of 1.
Oxygen (O) has a valency of 2.
Sodium (Na) has a valency of 1.
Chlorine (Cl) has a valency of 1.
Step 3: Balance the Valencies
Combine the elements in such a way that the total number of valence electrons is balanced. This often involves using the “criss-cross” method where the valency of one element becomes the subscript of the other element.
Examples:
1.Sodium Chloride (NaCl):
- Sodium (Na) has a valency of 1.
- Chlorine (Cl) has a valency of 1.
- The valencies balance directly: Na₁Cl₁, which simplifies to NaCl.
2.Water (H₂O):
- Hydrogen (H) has a valency of 1.
- Oxygen (O) has a valency of 2.
- To balance the valencies: (H₁)₂O₁, which simplifies to H₂O.
3. Carbon Dioxide (CO₂):
- Carbon (C) has a valency of 4.
- Oxygen (O) has a valency of 2.
- To balance: C₁(O₂)₂, which simplifies to CO₂.
4.Aluminum Oxide (Al₂O₃):
- Aluminum (Al) has a valency of 3.
- Oxygen (O) has a valency of 2.
- Criss-cross the valencies: Al₂O₃.
Additional Rules:
- Polyatomic Ions: For compounds containing polyatomic ions (ions composed of multiple atoms), use the known formula of the ion and balance it with the other element. For example, Calcium Nitrate (Ca(NO₃)₂):
- Calcium (Ca²⁺)
- Nitrate (NO₃⁻)
- Balance the charges: Ca²⁺ + 2(NO₃⁻) = Ca(NO₃)₂.
- Prefixes in Covalent Compounds: For covalent compounds, prefixes are used to denote the number of each type of atom. For example, Dinitrogen Tetroxide (N₂O₄):
- “Di-” indicates 2 Nitrogen atoms.
- “Tetra-” indicates 4 Oxygen atoms.
- Formula: N₂O₄.
By understanding the elements involved, their valencies, and following these steps, you can accurately write the molecular formulae for a wide range of compounds.
Maharashtra State Board Science-1-Chapter-3- Chemical Reactions And Equations-Can You Recall?
Is it possible to produce hydrogen by decomposition of water by means of heat, electricity or light ?
Yes, it is possible to produce hydrogen by decomposing water using heat, electricity, or light. Each method has its own principles and applications:
