Class 10 Chem: Eq. Types

Cheatsheet Content

### Introduction to Chemical Reactions & Equations A **chemical reaction** is a process that rearranges the atomic structure of substances, forming new substances with different properties. We represent these changes using **chemical equations**. **How to approach writing equations (Step-by-step):** 1. **Identify Reactants & Products:** What substances are starting (reactants) and what are formed (products)? 2. **Write Formulae:** Write correct chemical formulae for all reactants and products. (e.g., Water is H₂O, not HO). 3. **Skeleton Equation:** Write the unbalanced equation with reactants on the left, products on the right, separated by an arrow ($\rightarrow$). 4. **Balance Equation:** Ensure the number of atoms of each element is the same on both sides (Law of Conservation of Mass). 5. **Add State Symbols (Optional for beginners):** (s) solid, (l) liquid, (g) gas, (aq) aqueous solution. ### Combination Reaction * **Definition:** Two or more reactants combine to form a single product. It's like putting things together. * **General Form:** A + B $\rightarrow$ AB * **How to Identify:** Look for **two or more substances reacting to form only ONE new substance**. Keywords: "combines with", "reacts to form a single product". * **How to Form Equation:** 1. Identify the two combining reactants (A and B). 2. Identify the single product (AB). 3. Write correct formulae and balance. * **Solved Examples:** 1. **Magnesium ribbon burns in air (oxygen):** * Reactants: Magnesium (Mg), Oxygen (O₂) * Product: Magnesium Oxide (MgO) * Equation: $2Mg_{(s)} + O_{2(g)} \rightarrow 2MgO_{(s)}$ 2. **Quicklime reacts with water to form slaked lime:** * Reactants: Calcium Oxide (CaO), Water (H₂O) * Product: Calcium Hydroxide (Ca(OH)₂) * Equation: $CaO_{(s)} + H_2O_{(l)} \rightarrow Ca(OH)_{2(aq)}$ 3. **Formation of water from hydrogen and oxygen:** * Reactants: Hydrogen (H₂), Oxygen (O₂) * Product: Water (H₂O) * Equation: $2H_{2(g)} + O_{2(g)} \rightarrow 2H_2O_{(l)}$ ### Decomposition Reaction * **Definition:** A single reactant breaks down into two or more simpler products. It's like breaking something apart. Requires energy (heat, light, electricity). * **General Form:** AB $\rightarrow$ A + B (often with heat/light/electricity) * **How to Identify:** Look for **one reactant breaking down into multiple products**. Keywords: "decomposes", "breaks down", "heated", "electrolysis", "exposed to light". * **How to Form Equation:** 1. Identify the single reactant (AB). 2. Identify the two or more products (A and B). 3. Specify energy source if given (e.g., heat, light, electricity). 4. Write correct formulae and balance. * **Solved Examples:** 1. **Decomposition of Ferrous Sulphate on heating:** * Reactant: Ferrous Sulphate (FeSO₄) * Products: Ferric Oxide (Fe₂O₃), Sulphur Dioxide (SO₂), Sulphur Trioxide (SO₃) * Equation: $2FeSO_{4(s)} \xrightarrow{\text{heat}} Fe_2O_{3(s)} + SO_{2(g)} + SO_{3(g)}$ 2. **Electrolysis of Water:** * Reactant: Water (H₂O) * Products: Hydrogen gas (H₂), Oxygen gas (O₂) * Equation: $2H_2O_{(l)} \xrightarrow{\text{electricity}} 2H_{2(g)} + O_{2(g)}$ 3. **Decomposition of Silver Chloride in sunlight:** * Reactant: Silver Chloride (AgCl) * Products: Silver (Ag), Chlorine gas (Cl₂) * Equation: $2AgCl_{(s)} \xrightarrow{\text{sunlight}} 2Ag_{(s)} + Cl_{2(g)}$ ### Displacement Reaction * **Definition:** A more reactive element displaces a less reactive element from its compound. Think of it as a stronger person kicking out a weaker person. * **General Form:** A + BC $\rightarrow$ AC + B (where A is more reactive than B) * **How to Identify:** Look for an **element reacting with a compound**, and the element takes the place of another element in the compound. Keywords: "displaces", "reacts with metal salt solution". (Requires knowledge of reactivity series). * **How to Form Equation:** 1. Identify the element (A) and the compound (BC). 2. Check if A is more reactive than B (the element in the compound). If yes, displacement occurs. 3. Write the new compound (AC) and the displaced element (B). 4. Write correct formulae and balance. * **Solved Examples:** 1. **Iron nail dipped in Copper Sulphate solution:** * Reactant: Iron (Fe), Copper Sulphate (CuSO₄) * Products: Ferrous Sulphate (FeSO₄), Copper (Cu) * (Iron is more reactive than Copper) * Equation: $Fe_{(s)} + CuSO_{4(aq)} \rightarrow FeSO_{4(aq)} + Cu_{(s)}$ 2. **Zinc reacts with Dilute Sulphuric Acid:** * Reactant: Zinc (Zn), Sulphuric Acid (H₂SO₄) * Products: Zinc Sulphate (ZnSO₄), Hydrogen gas (H₂) * (Zinc is more reactive than Hydrogen) * Equation: $Zn_{(s)} + H_2SO_{4(aq)} \rightarrow ZnSO_{4(aq)} + H_{2(g)}$ 3. **Chlorine gas passed through Potassium Iodide solution:** * Reactant: Chlorine (Cl₂), Potassium Iodide (KI) * Products: Potassium Chloride (KCl), Iodine (I₂) * (Chlorine is more reactive than Iodine) * Equation: $Cl_{2(g)} + 2KI_{(aq)} \rightarrow 2KCl_{(aq)} + I_{2(s)}$ ### Double Displacement Reaction * **Definition:** Two compounds react by an exchange of ions to form two new compounds. It's like two couples swapping partners. Often results in a precipitate or water formation. * **General Form:** AB + CD $\rightarrow$ AD + CB * **How to Identify:** Look for **two compounds reacting to form two new compounds** by swapping their positive/negative ions. Keywords: "reacts to form precipitate", "exchange of ions". * **How to Form Equation:** 1. Identify the two compounds (AB and CD). 2. Identify the positive ions (A and C) and negative ions (B and D). 3. Swap the ions: A combines with D, C combines with B. 4. Write the new compounds (AD and CB). 5. Write correct formulae and balance. * **Solved Examples:** 1. **Sodium Sulphate reacts with Barium Chloride:** * Reactant: Sodium Sulphate (Na₂SO₄), Barium Chloride (BaCl₂) * Products: Barium Sulphate (BaSO₄ precipitate), Sodium Chloride (NaCl) * Equation: $Na_2SO_{4(aq)} + BaCl_{2(aq)} \rightarrow BaSO_{4(s)} + 2NaCl_{(aq)}$ 2. **Lead Nitrate reacts with Potassium Iodide:** * Reactant: Lead Nitrate (Pb(NO₃)₂), Potassium Iodide (KI) * Products: Lead Iodide (PbI₂ precipitate), Potassium Nitrate (KNO₃) * Equation: $Pb(NO_3)_{2(aq)} + 2KI_{(aq)} \rightarrow PbI_{2(s)} + 2KNO_{3(aq)}$ 3. **Silver Nitrate reacts with Sodium Chloride:** * Reactant: Silver Nitrate (AgNO₃), Sodium Chloride (NaCl) * Products: Silver Chloride (AgCl precipitate), Sodium Nitrate (NaNO₃) * Equation: $AgNO_{3(aq)} + NaCl_{(aq)} \rightarrow AgCl_{(s)} + NaNO_{3(aq)}$ ### Neutralization Reaction * **Definition:** A specific type of double displacement reaction where an acid reacts with a base to form salt and water. * **General Form:** Acid + Base $\rightarrow$ Salt + Water * **How to Identify:** Look for **an acid reacting with a base**. Keywords: "acid reacts with base", "neutralizes", "forms salt and water". * **How to Form Equation:** 1. Identify the acid and the base. 2. The H⁺ from the acid combines with OH⁻ from the base to form H₂O. 3. The remaining positive ion from the base and negative ion from the acid combine to form the salt. 4. Write correct formulae and balance. * **Solved Examples:** 1. **Hydrochloric acid reacts with Sodium Hydroxide:** * Reactant: Hydrochloric Acid (HCl), Sodium Hydroxide (NaOH) * Products: Sodium Chloride (NaCl), Water (H₂O) * Equation: $HCl_{(aq)} + NaOH_{(aq)} \rightarrow NaCl_{(aq)} + H_2O_{(l)}$ 2. **Sulphuric acid reacts with Potassium Hydroxide:** * Reactant: Sulphuric Acid (H₂SO₄), Potassium Hydroxide (KOH) * Products: Potassium Sulphate (K₂SO₄), Water (H₂O) * Equation: $H_2SO_{4(aq)} + 2KOH_{(aq)} \rightarrow K_2SO_{4(aq)} + 2H_2O_{(l)}$ 3. **Nitric acid reacts with Calcium Hydroxide:** * Reactant: Nitric Acid (HNO₃), Calcium Hydroxide (Ca(OH)₂) * Products: Calcium Nitrate (Ca(NO₃)₂), Water (H₂O) * Equation: $2HNO_{3(aq)} + Ca(OH)_{2(aq)} \rightarrow Ca(NO_3)_{2(aq)} + 2H_2O_{(l)}$ ### Redox Reaction (Oxidation & Reduction) * **Definition:** Reactions involving simultaneous oxidation and reduction. * **Oxidation:** Gain of Oxygen OR Loss of Hydrogen OR Loss of Electrons. * **Reduction:** Loss of Oxygen OR Gain of Hydrogen OR Gain of Electrons. * **General Form:** No single general form, as it covers many reaction types. * **How to Identify:** Look for **transfer of oxygen, hydrogen, or change in valency/charge**. Keywords: "oxidized", "reduced", "oxidizing agent", "reducing agent". * **How to Form Equation:** 1. Identify which substance gains oxygen/loses hydrogen (oxidized). 2. Identify which substance loses oxygen/gains hydrogen (reduced). 3. Write correct formulae and balance. * **Solved Examples:** 1. **Copper oxide reacts with Hydrogen (heating):** * Reactant: Copper Oxide (CuO), Hydrogen (H₂) * Products: Copper (Cu), Water (H₂O) * CuO loses oxygen (reduced to Cu), H₂ gains oxygen (oxidized to H₂O). * Equation: $CuO_{(s)} + H_{2(g)} \xrightarrow{\text{heat}} Cu_{(s)} + H_2O_{(l)}$ 2. **Zinc reacts with Copper Sulphate (Displacement):** * Reactant: Zinc (Zn), Copper Sulphate (CuSO₄) * Products: Zinc Sulphate (ZnSO₄), Copper (Cu) * Zn loses electrons (oxidized to Zn²⁺), Cu²⁺ gains electrons (reduced to Cu). * Equation: $Zn_{(s)} + CuSO_{4(aq)} \rightarrow ZnSO_{4(aq)} + Cu_{(s)}$ 3. **Manganese Dioxide reacts with HCl:** * Reactant: Manganese Dioxide (MnO₂), Hydrochloric Acid (HCl) * Products: Manganese Chloride (MnCl₂), Water (H₂O), Chlorine (Cl₂) * MnO₂ loses oxygen (reduced), HCl loses hydrogen (oxidized). * Equation: $MnO_{2(s)} + 4HCl_{(aq)} \rightarrow MnCl_{2(aq)} + 2H_2O_{(l)} + Cl_{2(g)}$ ### Combustion Reaction * **Definition:** A substance reacts rapidly with oxygen (often from air) to produce heat and light. Often involves hydrocarbons. * **General Form:** Fuel + O₂ $\rightarrow$ CO₂ + H₂O (for hydrocarbons) * **How to Identify:** Look for a substance **burning in air/oxygen**, producing **heat, light, carbon dioxide, and water**. Keywords: "burns", "ignites", "combustion". * **How to Form Equation:** 1. Identify the fuel and oxygen as reactants. 2. For hydrocarbons (compounds of C and H), products are always CO₂ and H₂O. 3. Write correct formulae and balance. * **Solved Examples:** 1. **Burning of Methane gas:** * Reactant: Methane (CH₄), Oxygen (O₂) * Products: Carbon Dioxide (CO₂), Water (H₂O) * Equation: $CH_{4(g)} + 2O_{2(g)} \rightarrow CO_{2(g)} + 2H_2O_{(g)}$ 2. **Burning of Ethanol:** * Reactant: Ethanol (C₂H₅OH), Oxygen (O₂) * Products: Carbon Dioxide (CO₂), Water (H₂O) * Equation: $C_2H_5OH_{(l)} + 3O_{2(g)} \rightarrow 2CO_{2(g)} + 3H_2O_{(g)}$ 3. **Burning of Coal (Carbon):** * Reactant: Carbon (C), Oxygen (O₂) * Product: Carbon Dioxide (CO₂) * Equation: $C_{(s)} + O_{2(g)} \rightarrow CO_{2(g)}$ ### How to Identify Acids, Bases, Salts, Metals, and Carbonates quickly * **Acids:** Usually start with H (e.g., HCl, H₂SO₄, HNO₃), except organic acids (e.g., CH₃COOH). Taste sour, turn blue litmus red. * **Bases:** Usually end with OH (e.g., NaOH, KOH, Ca(OH)₂), or metal oxides (e.g., CuO, MgO). Taste bitter, turn red litmus blue. Ammonia (NH₃) is also a base. * **Salts:** Formed from acid + base. Typically metal + non-metal (e.g., NaCl, CuSO₄, KNO₃, CaCO₃). * **Metals:** Elements like Na, K, Mg, Fe, Zn, Cu, Ag, Au. Shiny, good conductors. * **Carbonates:** Compounds containing the CO₃²⁻ ion (e.g., CaCO₃, Na₂CO₃, MgCO₃). Often react with acids to produce CO₂ gas. ### Top Reaction Patterns to Memorize 1. **Metal + Oxygen $\rightarrow$ Metal Oxide** (Combination) * $2Mg + O_2 \rightarrow 2MgO$ 2. **Non-metal + Oxygen $\rightarrow$ Non-metal Oxide** (Combination) * $C + O_2 \rightarrow CO_2$ 3. **Metal + Water $\rightarrow$ Metal Hydroxide/Oxide + Hydrogen gas** (Displacement) * $2Na + 2H_2O \rightarrow 2NaOH + H_2$ * $Fe + 4H_2O_{(steam)} \rightarrow Fe_3O_4 + 4H_2$ 4. **Metal + Dilute Acid $\rightarrow$ Salt + Hydrogen gas** (Displacement) * $Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2$ 5. **Acid + Base $\rightarrow$ Salt + Water** (Neutralization / Double Displacement) * $HCl + NaOH \rightarrow NaCl + H_2O$ 6. **Metal Carbonate/Bicarbonate + Acid $\rightarrow$ Salt + Water + Carbon Dioxide** (Double Displacement & Decomposition of carbonic acid) * $CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2$ 7. **Metal Oxide + Acid $\rightarrow$ Salt + Water** (Neutralization) * $CuO + 2HCl \rightarrow CuCl_2 + H_2O$ 8. **Non-metal Oxide + Base $\rightarrow$ Salt + Water** (Neutralization) * $CO_2 + 2NaOH \rightarrow Na_2CO_3 + H_2O$ 9. **Precipitation Reactions:** (Double Displacement) Two aqueous solutions react to form an insoluble solid (precipitate). * $AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)$ 10. **Thermal Decomposition:** (Decomposition) * **Carbonates:** Metal Carbonate $\xrightarrow{\text{heat}}$ Metal Oxide + Carbon Dioxide (e.g., $CaCO_3 \rightarrow CaO + CO_2$) * **Nitrates:** Metal Nitrate $\xrightarrow{\text{heat}}$ Metal Oxide + Nitrogen Dioxide + Oxygen (e.g., $2Pb(NO_3)_2 \rightarrow 2PbO + 4NO_2 + O_2$) * **Hydroxides:** Metal Hydroxide $\xrightarrow{\text{heat}}$ Metal Oxide + Water (e.g., $2Fe(OH)_3 \rightarrow Fe_2O_3 + 3H_2O$) ### Balancing Chemical Equations – Simple Method for Beginners (Hit & Trial) **Goal:** Equal number of atoms of each element on both sides. **Steps:** 1. **Write the unbalanced equation.** 2. **List elements:** Write down all elements present on both sides. 3. **Count atoms:** Count the number of atoms of each element on both reactant and product sides. 4. **Balance one element at a time:** * Start with the element that appears least often, or in the largest molecule. * Often, balance metals first, then non-metals (other than O and H), then Oxygen, then Hydrogen. * Use coefficients (numbers in front of formulae) to balance atoms. NEVER change subscripts! * If a polyatomic ion (like SO₄²⁻, NO₃⁻) remains unchanged, treat it as a single unit. 5. **Recount and repeat:** After balancing one element, recount all atoms and continue until all elements are balanced. 6. **Check:** Ensure coefficients are the smallest whole numbers. **Example: Burning of Propane (C₃H₈)** 1. **Unbalanced:** $C_3H_{8(g)} + O_{2(g)} \rightarrow CO_{2(g)} + H_2O_{(g)}$ 2. **Elements:** C, H, O 3. **Counts:** * Reactant: C=3, H=8, O=2 * Product: C=1, H=2, O=3 4. **Balance C:** (3 on left, 1 on right) * $C_3H_8 + O_2 \rightarrow 3CO_2 + H_2O$ * Counts: C=3, H=8, O=2 (left); C=3, H=2, O=(3x2)+1 = 7 (right) 5. **Balance H:** (8 on left, 2 on right) * $C_3H_8 + O_2 \rightarrow 3CO_2 + 4H_2O$ (since 4x2 = 8) * Counts: C=3, H=8, O=2 (left); C=3, H=8, O=(3x2)+(4x1) = 6+4 = 10 (right) 6. **Balance O:** (2 on left, 10 on right) * $C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$ (since 5x2 = 10) * Counts: C=3, H=8, O=10 (left); C=3, H=8, O=10 (right) 7. **Check:** All balanced! ### Common Mistakes Students Make (and How to Avoid Them) 1. **Changing Subscripts:** NEVER change the small numbers (subscripts) in a chemical formula (e.g., H₂O is water, HO is not). Only change coefficients. * *Avoid:* Understand that subscripts define the compound, coefficients define the amount. 2. **Incorrect Formulae:** Using wrong chemical formulae (e.g., Na for Sodium, but Na₂ for Sodium if it were diatomic, or writing CaOH for Calcium Hydroxide instead of Ca(OH)₂). * *Avoid:* Memorize common valencies/ions and practice writing formulae. 3. **Forgetting Diatomic Molecules:** Elements like H₂, O₂, N₂, Cl₂, F₂, Br₂, I₂ exist as diatomic molecules. * *Avoid:* Memorize "Have No Fear Of Ice Cold Beer" (H₂, N₂, F₂, O₂, I₂, Cl₂, Br₂). 4. **Not Balancing All Elements:** Sometimes students balance most elements but miss one, especially oxygen or hydrogen if they appear in multiple products/reactants. * *Avoid:* Always make a final check of ALL elements on both sides. 5. **Not Identifying Reaction Type:** This makes forming products difficult. * *Avoid:* Practice identifying keywords and general forms for each reaction type. 6. **Ignoring Reactivity Series:** In displacement reactions, if the element is less reactive, no reaction occurs! * *Avoid:* Learn the basic reactivity series (K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au). ### Practice Questions **Write balanced chemical equations for the following reactions, and identify the type of reaction:** 1. Hydrogen gas combines with Chlorine gas to form Hydrogen Chloride. 2. Barium Chloride solution reacts with Sodium Sulphate solution to give Barium Sulphate precipitate and Sodium Chloride solution. 3. Zinc metal reacts with Silver Nitrate solution to produce Zinc Nitrate solution and Silver metal. 4. Potassium Bromide solution reacts with aqueous Lead Iodide to form Potassium Iodide solution and Lead Bromide precipitate. 5. Ferric Oxide reacts with Aluminium to form Aluminium Oxide and Iron. 6. Carbon dioxide gas is passed through Calcium Hydroxide solution. 7. Methane (CH₄) burns in oxygen. 8. Limestone (Calcium Carbonate) is heated. 9. Sodium metal reacts with water. 10. Sulphur Dioxide gas reacts with Oxygen gas to form Sulphur Trioxide. --- ### Answers 1. **Combination:** $H_{2(g)} + Cl_{2(g)} \rightarrow 2HCl_{(g)}$ 2. **Double Displacement (Precipitation):** $BaCl_{2(aq)} + Na_2SO_{4(aq)} \rightarrow BaSO_{4(s)} + 2NaCl_{(aq)}$ 3. **Displacement:** $Zn_{(s)} + 2AgNO_{3(aq)} \rightarrow Zn(NO_3)_{2(aq)} + 2Ag_{(s)}$ 4. **No Reaction (Incorrect Question - Lead Iodide is insoluble, implies a product not reactant. Assuming Lead Nitrate and Potassium Bromide):** * Corrected (Double Displacement): $Pb(NO_3)_{2(aq)} + 2KBr_{(aq)} \rightarrow PbBr_{2(s)} + 2KNO_{3(aq)}$ 5. **Displacement:** $Fe_2O_{3(s)} + 2Al_{(s)} \rightarrow Al_2O_{3(s)} + 2Fe_{(s)}$ 6. **Combination (Acidic oxide + Base):** $CO_{2(g)} + Ca(OH)_{2(aq)} \rightarrow CaCO_{3(s)} + H_2O_{(l)}$ 7. **Combustion:** $CH_{4(g)} + 2O_{2(g)} \rightarrow CO_{2(g)} + 2H_2O_{(g)}$ 8. **Decomposition:** $CaCO_{3(s)} \xrightarrow{\text{heat}} CaO_{(s)} + CO_{2(g)}$ 9. **Displacement:** $2Na_{(s)} + 2H_2O_{(l)} \rightarrow 2NaOH_{(aq)} + H_{2(g)}$ 10. **Combination:** $2SO_{2(g)} + O_{2(g)} \rightarrow 2SO_{3(g)}$