Chemistry Chemical Reactions - Class 10 UP Board Chemistry

Chemical Reactions and Equations

Valency and Compound Formation

Understanding valency is essential for writing chemical formulas and equations. Valency indicates the combining capacity of an element or ion, and compounds are formed by balancing the valencies of participating ions.

• Valency Rules:
  • Elements with valency +1: e.g., Na, K, Ag, NH₄⁺, etc.
  • Elements with valency -1: e.g., Cl⁻, Br⁻, NO₃⁻, etc.
  • Elements with valency +2: e.g., Mg²⁺, Ca²⁺, Zn²⁺, etc.
  • Elements with valency -2: e.g., O²⁻, SO₄²⁻, CO₃²⁻, etc.
  • Elements with variable valency: Fe (+2, +3), Pb (+2), Al (+3).
• Compound Formation…

Chemical Reactions and Equations

Valency and Compound Formation

Understanding valency is essential for writing chemical formulas and equations. Valency indicates the combining capacity of an element or ion, and compounds are formed by balancing the valencies of participating ions.

• Valency Rules:
  • Elements with valency +1: e.g., Na, K, Ag, NH₄⁺, etc.
  • Elements with valency -1: e.g., Cl⁻, Br⁻, NO₃⁻, etc.
  • Elements with valency +2: e.g., Mg²⁺, Ca²⁺, Zn²⁺, etc.
  • Elements with valency -2: e.g., O²⁻, SO₄²⁻, CO₃²⁻, etc.
  • Elements with variable valency: Fe (+2, +3), Pb (+2), Al (+3).
• Compound Formation (Criss-Cross Method):
  • Write symbols and valencies.
  • Cross the valencies to the opposite ion.
  • Example: Sodium sulfate: Na (+1), SO₄ (-2) → Na₂SO₄.
  • Example: Aluminium chloride: Al (+3), Cl (-1) → AlCl₃.
  • For polyatomic ions needing more than one, use brackets: Lead nitrate: Pb (+2), NO₃ (-1) → Pb(NO₃)₂.

Types of Changes: Physical and Chemical

Changes in matter are classified as physical or chemical. Physical changes affect only the state or appearance, while chemical changes produce new substances with different properties.

• Physical Change: Only state or shape changes (e.g., melting ice).
• Chemical Change: New substances form with different properties (e.g., burning methane forms CO₂ and H₂O).
• Key Point: Chemical changes involve breaking old bonds and forming new ones, resulting in substances with new chemical properties.

Chemical Reactions: Definition and Indicators

A chemical reaction is a process where one or more substances (reactants) are transformed into new substances (products) with different properties.

• Definition: Transformation of chemical substances into other chemical substances.
• Indicators of Chemical Reaction:
  • Evolution of gas (e.g., bubbles in spoiled food).
  • Change in color (e.g., iron nail in copper sulfate turns solution from blue to green).
  • Change in physical state (e.g., wax melting and vaporizing).
  • Change in temperature (e.g., test tube becomes hot during reaction).
  • Formation of precipitate (e.g., mixing sodium sulfate and barium chloride forms solid barium sulfate).

Representation of Chemical Reactions

Chemical reactions can be represented in different forms for clarity and brevity.

• Word Equation: Uses names of reactants and products (e.g., Hydrogen + Oxygen → Water).
• Chemical Equation: Uses symbols and formulas (e.g., $H_2 + O_2 \rightarrow H_2O$).
• Balanced Equation: Number of atoms of each element is equal on both sides, satisfying the law of conservation of mass.
• Skeletal Equation: Unbalanced equation, also called skeletal chemical equation.

Balancing Chemical Equations

Balancing ensures the number of atoms for each element is equal on both sides, in accordance with the law of conservation of mass.

• Steps to Balance:
  1. Count atoms of each element on both sides.
  2. Multiply by suitable coefficients to equalize atom numbers.
  3. Only multiplication is allowed; do not alter formulas.
• Example:
  • $H_2 + O_2 \rightarrow H_2O$ (unbalanced)
  • Balance O: $H_2 + O_2 \rightarrow 2H_2O$
  • Balance H: $2H_2 + O_2 \rightarrow 2H_2O$
• Key Rule: Only elements present in reactants can appear in products.

Types of Chemical Reactions

Chemical reactions are classified based on the nature of reactants and products.

• Combination Reaction: Two or more reactants combine to form a single product.
  • Pattern: $A + B \rightarrow AB$
  • Examples: $Mg + O_2 \rightarrow MgO$, $C + O_2 \rightarrow CO_2$
• Decomposition Reaction: A single reactant breaks down into two or more products, usually requiring energy.
  • Pattern: $AB \rightarrow A + B$
  • Always endothermic (energy required).
  • Types:
    • Thermal decomposition (heat): $CaCO_3 \xrightarrow{heat} CaO + CO_2$
    • Electrolytic decomposition (electricity): $2H_2O \xrightarrow{electricity} 2H_2 + O_2$
    • Photolytic decomposition (light): $2AgCl \xrightarrow{sunlight} 2Ag + Cl_2$
• Displacement Reaction: A more reactive element displaces a less reactive element from its compound.
  • Pattern: $A + BC \rightarrow AC + B$ (if $A$ is more reactive than $B$)
  • Example: $Fe + CuSO_4 \rightarrow FeSO_4 + Cu$
• Double Displacement Reaction: Exchange of ions between two compounds to form new compounds.
  • Pattern: $AB + CD \rightarrow AD + CB$
  • Example: $BaCl_2 + Na_2SO_4 \rightarrow BaSO_4 + 2NaCl$
  • If a solid (precipitate) forms, it is called a precipitation reaction.
  • If salt and water form, it is a neutralization reaction.
• Redox Reaction: Both oxidation and reduction occur simultaneously.

Oxidation and Reduction (Redox Reactions)

Oxidation and reduction are processes involving the gain or loss of oxygen or hydrogen.

• Oxidation: Addition of oxygen or removal of hydrogen.
• Reduction: Addition of hydrogen or removal of oxygen.
• Redox Reaction: Both oxidation and reduction occur together.
• Example: $CuO + H_2 \rightarrow Cu + H_2O$
  • $CuO$ is reduced to $Cu$ (loss of oxygen).
  • $H_2$ is oxidized to $H_2O$ (gain of oxygen).
• Oxidizing Agent: Substance that gets reduced (e.g., $CuO$).
• Reducing Agent: Substance that gets oxidized (e.g., $H_2$).

Reactivity Series and Displacement

The reactivity series ranks elements by their reactivity, determining which can displace others in reactions.

• Order (from most to least reactive):
  • K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Au, Pt
• Rule: A more reactive element can displace a less reactive one from its compound.

Corrosion and Rancidity

Corrosion is the slow oxidation of metals, while rancidity is the spoilage of food due to oxidation of fats and oils.

• Corrosion: Unwanted reaction of metals with atmospheric oxygen, water, or acids, forming undesirable compounds.
  • Examples:
    • Rusting of iron: $Fe$ reacts with $O_2$ and $H_2O$ to form hydrated iron oxide.
    • Tarnishing of silver: $Ag$ reacts with $H_2S$ to form $Ag_2S$ (black).
    • Green coating on copper: $Cu$ reacts with $CO_2$, $O_2$, and $H_2O$ to form copper carbonate and hydroxide.
• Prevention of Corrosion:
  • Painting, greasing, oiling, galvanization (coating with zinc).
• Rancidity: Oxidation of fats and oils in food, leading to bad taste and smell.
  • Prevention: Store food in airtight containers, refrigeration, filling packets with nitrogen, use of antioxidants.

Key Exam Points and Common Pitfalls

Certain rules and observations are critical for exams and must be remembered to avoid common mistakes.

• Only elements present in reactants can appear in products.
• Balancing equations is mandatory for full marks.
• Precipitate: An insoluble solid formed in a reaction (e.g., $BaSO_4$ is white).
• Color Indicators:
  • Copper sulfate solution: Blue
  • Ferrous sulfate: Green
  • Copper metal: Shiny brown
  • Silver chloride: White
• Practice is essential for mastering equation balancing and identifying reaction types.
• In double displacement, not all reactions form precipitate or water; check products.
• In displacement, check reactivity series to predict if reaction will occur.
• For compound formation, always use correct valency and criss-cross method.

Sample Balanced Equations and Patterns

Familiarity with common equations and their balancing patterns is crucial for exams.

• $2H_2 + O_2 \rightarrow 2H_2O$
• $Zn + 2HCl \rightarrow ZnCl_2 + H_2$
• $CaCO_3 \xrightarrow{heat} CaO + CO_2$
• $2AgCl \xrightarrow{sunlight} 2Ag + Cl_2$
• $Fe + CuSO_4 \rightarrow FeSO_4 + Cu$
• $BaCl_2 + Na_2SO_4 \rightarrow BaSO_4 + 2NaCl$

Practice balancing and identifying the type of each reaction for exam readiness.