Complete science revision notes 10th, Study notes of Chemistry

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CHEMICAL REACTION AND EQUATION

Q. Balance the following chemical equations and determine the values of A, B, C , D and E in each case: (i) A MnO₂ + B HCl → C MnCl₂ + D H₂O + E Cl₂ (ii) A NaOH + B Al₂O₃ → C NaAlO₂ + D H₂O (iii) A Al + B H₂O → C Al₂O₃ + D H₂ (iv) A Pb(NO₃)₂ —heat → B PbO + C NO₂ + D O₂ (v) CH₄ + AO₂ → CO₂ + BH₂O + Heat

(i) MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ A = 1, B = 4, C = 1, D = 2, E = 1 (ii) 2NaOH + Al₂O₃ → 2NaAlO₂ + H₂O A = 2, B = 1, C = 2, D = 1 (iii) 2Al + 3H₂O → Al₂O₃ + 3H₂ A = 2, B = 3, C = 1, D = 3 (iv) 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ A = 2, B = 2, C = 4, D = 1 (v) A = 2, B = 2

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CHEMICAL REACTION AND EQUATION

Q. (a) What happens when copper is heated in air? (b) What happens when lead nitrate is heated? (c) What happens when ferrous sulphate is heated? (d) What happens when quicklime (CaO) reacts with water? For each reaction, answer the following: Write the balanced chemical equation. Name the type of reaction. What are the observations during the reaction?

(a) Balanced chemical equation: 2Cu + O₂ → 2CuO (Combination and oxidation reaction) Observations: Reddish-brown copper turns black due to formation of copper(II) oxide. (b) Balanced chemical equation: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ (Thermal decomposition reaction) Observations: Brown fumes of NO₂ are evolved and yellow PbO residue remains. (c)Balanced chemical equation: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (Thermal decomposition reaction) Observations: Green crystals turn brown and pungent gases are evolved. (d) Balanced chemical equation: CaO + H₂O → Ca(OH)₂ + Heat (Combination and exothermic reaction.) Observations: Heat is produced and white slaked lime is formed.

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CHEMICAL REACTION AND EQUATION

Q. (a) Name the process depicted in the diagram in which water decomposes into hydrogen and oxygen using electricity. (b) Write the material used for the anode and the cathode in this process. (c) Write the balanced chemical equation for the reaction taking place. (d) Why does the reaction not occur if a few drops of dilute sulphuric acid are not added to water? (e) During electrolysis of water, if the volumes of oxygen and hydrogen evolved at the electrodes are V (^) O and V (^) H respectively, what is the ratio V :V ?O H (f) Electrolysis of water is a decomposition reaction. What is the mass ratio (MH:MO)( of hydrogen and oxygen gases liberated at the electrodes?

CHEMICAL REACTION AND EQUATION

Q. What happens when silver chloride is exposed to sunlight? For the above reaction: (a) Write the balanced chemical equation. (b) Name the type of reaction. (c) What observation is made during the reaction? (d) Why are silver chloride crystals stored in dark coloured bottles?

When silver chloride is exposed to sunlight, it decomposes to form silver and chlorine gas. (a) Balanced chemical equation: 2AgCl(s) ⟶ 2Ag(s) + Cl₂(g) (in presence of sunlight) (b) Type of reaction: Photochemical decomposition reaction (also a decomposition reaction). (c) Observation: White silver chloride turns grey due to the formation of metallic silver. (d) Reason for storing in dark bottles: Silver chloride is photosensitive and decomposes in the presence of light, so it is stored in dark coloured bottles to prevent decomposition.

CHEMICAL REACTION AND EQUATION

Q. A student places an iron nail in blue copper sulphate solution. After some time, the solution turns green and a reddish-brown substance is deposited. (a) Identify the substance deposited. (b) Why does the solution change colour? (c) Write the balanced equation. (d) Identify the type of reaction.

(a) The reddish-brown substance deposited is copper (Cu). (b) The solution changes colour from blue to green because iron displaces copper from copper sulphate solution, forming iron sulphate (FeSO₄), which is green in colour. (c) Balanced chemical equation: Fe + CuSO₄ → FeSO₄ + Cu (d) The reaction is a displacement reaction (also a redox reaction).

CHEMICAL REACTION AND EQUATION

(i) MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ → (a) Type: Redox reaction (also displacement reaction) (b) Oxidised: HCl (Cl⁻ → Cl₂), Reduced: MnO₂ (MnO₂ → MnCl₂) (c) Oxidising agent: MnO₂, Reducing agent: HCl (ii) CuO + H₂ → Cu + H₂O → (a) Type: Redox reaction (b) Oxidised: H₂ (H₂ → H₂O), Reduced: CuO (CuO → Cu) (c) Oxidising agent: CuO, Reducing agent: H₂ (iii) 2Na + 2H₂O → 2NaOH + H₂ → (a) Type: Displacement and redox reaction (b) Oxidised: Na (Na → NaOH), Reduced: H₂O (H₂O → H₂) (c) Oxidising agent: H₂O, Reducing agent: Na

B. (a) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq) (b) Double displacement reaction (precipitation reaction) (c) The white precipitate formed is barium sulphate (BaSO₄). (d) Ionic equation: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

ACIDS, BASES AND SALTS

Q. (a) Match the following:

(b) An aqueous solution ‘A’ turns phenolphthalein solution pink. On adding another aqueous solution ‘B’, the pink colour disappears. (i) Identify the nature (acidic/basic) of solution ‘A’. (ii) Identify the nature of solution ‘B’. (iii) State the pH range of both solutions. (c) Why do turmeric stains turn red when washed with soap?

Column I Column II A. Vinegar i. Methanoic acid B. Lemon ii. Acetic acid C. Ant sting iii. Tartaric acid D. Tamarind iv. Citric acid

ACIDS, BASES AND SALTS

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Q. (a) Why do acids not show acidic behaviour in dry state? Explain with a suitable example. (b) Why does an aqueous solution of an acid conduct electricity but dry acid does not? (c) Write the chemical equation for the reaction of: (i) Zinc with dilute hydrochloric acid. (ii) Sodium carbonate with dilute hydrochloric acid. Identify the gas evolved in each case.

Ans.(a) Acids do not show acidic behaviour in the dry state because they do not produce hydrogen ions (H⁺) without water. Acids show acidic nature only in aqueous solution. Example: Dry HCl gas does not turn dry blue litmus red, but aqueous HCl does. (b) An aqueous solution of an acid conducts electricity because it ionises in water to produce ions (H⁺), which carry electric current. Dry acid does not conduct electricity because no ions are present in the absence of water. (c) (i) Zinc with dilute hydrochloric acid - Zn + 2HCl → ZnCl₂ + H₂↑ Gas evolved: Hydrogen gas (ii) Sodium carbonate with dilute hydrochloric acid - Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑ Gas evolved: Carbon dioxide gas

ACIDS, BASES AND SALTS

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Q. (a) You are given two solutions, A and B. The pH of solution A is 6, and the pH of solution B is 8. (i) Which solution is acidic and which is basic? (ii) Which solution has a higher concentration of H⁺ ions? (iii) Why is hydrochloric acid (HCl) considered a stronger acid than acetic acid (CH₃COOH)? (b) How do we differentiate between a strong acid and a weak base in terms of ion formation in aqueous solutions? (c) Why does dry HCl gas not change the color of dry litmus paper?

Answer:(a) Given solutions A (pH = 6) and B (pH = 8): (i) A is acidic (pH < 7), B is basic (pH > 7). (ii) A has a higher H⁺ ion concentration. (iii) HCl is a strong acid as it fully ionizes in water, while CH₃COOH is weak as it partially ionizes. (b) A strong acid completely ionises in aqueous solution and produces a large number of H⁺ ions, whereas a weak base partially ionises in aqueous solution and produces fewer OH⁻ ions. (c) Dry HCl gas & Litmus Paper: No water → No H⁺ ions → No color change.

ACIDS, BASES AND SALTS

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Q. (a) Write the balanced chemical equation for the chlor-alkali process. (b) Identify the products formed and mention their uses. (c) Name the gas liberated at the anode. Write one of its uses. (d) Why is the solution formed during the chlor-alkali process slippery in nature? (e) What would happen if the hydrogen gas produced is not handled properly? (f) Why is chlorine gas collected at the anode while hydrogen gas is collected at the cathode?

Answer (a) Balanced Chemical Equation: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂ (b) Sodium hydroxide (NaOH): Used in soap and paper industries. Chlorine (Cl₂): Used for water purification and manufacturing PVC. Hydrogen (H₂): Used in making ammonia and fuels. (c) Gas at the Anode: Chlorine (Cl₂), Use: Disinfection of drinking water. (d) The solution formed contains sodium hydroxide (NaOH), which is a strong base and makes the solution slippery to touch. (e) Hydrogen gas is highly flammable, so it must be handled with care to prevent explosions. (f) During electrolysis, Cl₂ is formed at the anode (oxidation) and H₂ at the cathode (reduction) due to the movement of ions in the electrolyte.

ACIDS, BASES AND SALTS

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Q. (a) What is the chemical name of common salt? How is it obtained? (b) Common salt is an important raw material for many chemicals. Write balanced chemical equations for the formation of: (i) Sodium Hydroxide (ii) Washing Soda (iii) Baking Soda (iv) Bleaching Powder (c) How does sodium carbonate react with water to form washing soda? Write the equation. (d) Why is baking soda used in antacids? Write the reaction involved. (e) What happens when bleaching powder is exposed to air for a long time? Ans. (a) Chemical Name: Sodium Chloride (NaCl), Obtained from: Sea water by evaporation or from underground deposits (rock salt). (b) (i) Formation of Sodium Hydroxide (NaOH) – Chlor-Alkali Process: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂ (ii) Step 1: Formation of Sodium Carbonate (Na₂CO₃) NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O Step 2: Conversion to Washing Soda → Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O (iii) Formation of Baking Soda (NaHCO₃)

ACIDS, BASES AND SALTS

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Q. A. On heating X at 373 K, it loses water molecules and becomes Y. Y is a substance which doctors use for supporting fractured bones in the right position. (i) Identify X and V. (ii) How can X be reobtained from Y? B. Consider the following salts: (i) yCI (ii) NH4X (iii) ZCO (a) What would be the pH of the salt solution if in yCI, y is sodium? Give a reason for your answer. (b) If in salt NH4X, X is nitrate, then its solution will give what colour with a universal indicator? Why? (c) What would be the change in colour in a blue litmus solution if ZCO3 is added to it and Z is potassium? (d) A metal carbonate X on reacting with an acid gives a gas which, when passed through a solution Y, gives the carbonate back. On the other hand, a gas G that is obtained at the anode during electrolysis of brine is passed on dry Y, it gives a compound Z, used for disinfecting drinking water. Identify X, Y, G and Z.

ACIDS, BASES AND SALTS

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Ans: A. (i) X is gypsum (CaSO4 · 2H2O) and Y is Plaster of Paris (CaSO4 · 0.5H2O). V is the substance doctors use for supporting fractured bones in the right position. (ii) X can be reobtained from Y by: Adding water to Plaster of Paris. This process converts it back to gypsum. The reaction is: CaSO4 · 0.5H2O + 1.5H2O → CaSO4 · 2H2O

B. (a) If Y in YCl is sodium, the salt formed is NaCl. NaCl is formed from a strong acid (HCl) and a strong base (NaOH). Therefore, it is a neutral salt and its pH is 7. (b) If X in NH₄X is nitrate, the salt formed is NH₄NO₃. It is formed from a weak base (NH₄OH) and a strong acid (HNO₃). Hence, it is an acidic salt (pH < 7) and gives orange-yellow colour with universal indicator. (c) K₂CO₃ is a basic salt formed from a strong base (KOH) and a weak acid (H₂CO₃). It produces OH⁻ ions in solution, making it basic (pH > 7). Therefore, blue litmus remains blue. (d X = Calcium carbonate (CaCO₃) Y = Calcium hydroxide (Ca(OH)₂) G = Chlorine gas (Cl₂) Z = Bleaching powder (CaOCl₂)