Chemical Reactions and Equations · Class 10 Science: 66 flashcards
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1. What is a chemical reaction?
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A process in which one or more substances (reactants) are converted into new substances (products) with different properties, by the breaking and forming of chemical bonds.
2. List the four observations NCERT gives for deciding that a chemical reaction has taken place.
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Change of state, change of colour, change of temperature, evolution of a gas, and formation of a precipitate.
3. What are reactants and products?
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Reactants are the starting substances (written left of the arrow); products are the new substances formed (written right of the arrow).
4. What is a skeletal chemical equation?
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An equation with correct formulas of reactants and products but with unequal numbers of atoms on the two sides — an unbalanced equation.
5. State the law of conservation of mass.
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In a chemical reaction mass is neither created nor destroyed; the total mass of products equals the total mass of reactants because atoms are only rearranged.
6. Why must chemical equations be balanced?
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To satisfy the law of conservation of mass — the number of atoms of each element must be the same on both sides, since atoms are neither created nor destroyed.
7. What may be changed while balancing an equation, and what must never be changed?
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Only the coefficients (numbers written in front of formulas) may be changed; the subscripts within chemical formulas must never be altered.
8. How many atoms of each element are in 2Ca(OH)₂?
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2 calcium, 4 oxygen and 4 hydrogen atoms — the coefficient multiplies everything in the formula including the bracket.
9. Balance: H₂ + O₂ → H₂O.
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2H₂ + O₂ → 2H₂O. (H: 4=4, O: 2=2.)
10. Balance: Fe + H₂O → Fe₃O₄ + H₂.
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3Fe + 4H₂O → Fe₃O₄ + 4H₂.
11. Balance: CH₄ + O₂ → CO₂ + H₂O.
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CH₄ + 2O₂ → CO₂ + 2H₂O.
12. What do the state symbols (s), (l), (g) and (aq) mean?
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(s) solid, (l) pure liquid, (g) gas, (aq) aqueous — i.e., dissolved in water.
13. What is the difference between H₂O(l) and NaCl(aq)?
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H₂O(l) is pure liquid water; NaCl(aq) is common salt dissolved IN water. 'Aqueous' always means a water solution.
14. How are heat, light and catalysts shown in an equation?
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As conditions written above or below the arrow: Δ for heat, 'sunlight' for light, and the catalyst's symbol (e.g., Ni) for catalysed reactions.
15. Why is a magnesium ribbon cleaned with sandpaper before burning?
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To remove the protective magnesium oxide layer coating its surface, exposing fresh magnesium so it can ignite and burn.
16. Write the balanced equation for burning magnesium, with the observation.
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2Mg(s) + O₂(g) → 2MgO(s). Magnesium burns with a dazzling white flame, leaving white magnesium oxide powder.
17. What is a precipitate? What symbol marks it?
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An insoluble solid that separates out when two solutions react; marked with ↓ (or the state symbol (s)).
18. Give the word equation for zinc reacting with dilute sulphuric acid.
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Zinc + sulphuric acid → zinc sulphate + hydrogen. (Zn + H₂SO₄ → ZnSO₄ + H₂.)
19. Why is a word equation less useful than a balanced formula equation?
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A word equation gives no information about formulas, atom counts, quantities or states — a balanced formula equation shows the exact ratio of substances and satisfies the conservation of mass.
20. Classify: (a) melting of wax, (b) burning of wax.
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(a) Physical change — same substance in liquid form. (b) Chemical change — the wax combines with oxygen forming new substances (CO₂ and water) with heat and light.
21. Balance: Pb(NO₃)₂ → PbO + NO₂ + O₂ (on heating).
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2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. (Pb 2=2, N 4=4, O 12=2+8+2=12.)
22. In 3H₂SO₄, how many atoms of each element are present?
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6 hydrogen, 3 sulphur and 12 oxygen atoms.
23. Define a combination reaction with an example.
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A reaction where two or more reactants form a single product: A + B → AB. Example: CaO + H₂O → Ca(OH)₂ (quicklime + water → slaked lime, with much heat).
24. Define a decomposition reaction with an example.
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A reaction where a single compound splits into two or more simpler substances: AB → A + B. Example: CaCO₃ --heat--> CaO + CO₂.
25. Name the three forms of energy that can drive decomposition, with one example each.
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Heat — CaCO₃ → CaO + CO₂; light — 2AgCl → 2Ag + Cl₂; electricity — 2H₂O → 2H₂ + O₂ (electrolysis).
26. What happens when ferrous sulphate crystals are heated?
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The green crystals lose water and decompose to brown ferric oxide with sulphur dioxide and sulphur trioxide gases (burning-sulphur smell): 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃.
27. Why does silver chloride turn grey in sunlight? Write the equation.
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It photodecomposes to finely divided grey silver metal and chlorine: 2AgCl --sunlight--> 2Ag + Cl₂.
28. Where is the light-decomposition of silver halides used?
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In black-and-white photography — light striking the film's AgCl/AgBr grains reduces them to dark silver, recording the image.
29. In the electrolysis of water, which gas collects at which electrode and in what ratio?
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Hydrogen at the cathode and oxygen at the anode; hydrogen's volume is double the oxygen's (2:1), matching H₂O's formula.
30. Why are a few drops of dilute acid added to water before electrolysis?
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Pure water is a very poor conductor of electricity; the acid supplies ions that carry the current.
31. Define a displacement reaction with an example.
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A reaction in which a more reactive element displaces a less reactive one from its compound: A + BC → AC + B. Example: Fe + CuSO₄ → FeSO₄ + Cu.
32. What two observations accompany an iron nail dipped in copper sulphate?
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(1) The nail gets a reddish-brown coating of copper. (2) The blue colour of the solution fades (toward the pale green of FeSO₄).
33. Can copper displace iron from iron sulphate? Why?
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No — copper is less reactive than iron. Only a MORE reactive metal can displace a less reactive one from its salt.
34. Define double displacement with an example.
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A reaction where two compounds exchange ions to form two new compounds: AB + CD → AD + CB. Example: Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl.
35. What is a precipitation reaction?
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A double displacement reaction in which one of the products is an insoluble solid (precipitate) — like the white BaSO₄ formed from barium chloride and sodium sulphate.
36. Why is neutralisation classed as double displacement?
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The acid's hydrogen ion and the base's metal ion exchange partners, giving salt and water: HCl + NaOH → NaCl + H₂O.
37. Define exothermic and endothermic reactions with one example each.
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Exothermic reactions release heat — e.g., burning of natural gas (CH₄ + 2O₂ → CO₂ + 2H₂O + heat). Endothermic reactions absorb heat — e.g., decomposition of CaCO₃ on heating.
38. Is respiration exothermic or endothermic? Why?
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Exothermic — glucose combines with oxygen releasing energy (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy) that powers the body.
39. Why does a white-washed wall become shiny after a few days?
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The slaked lime coating reacts slowly with atmospheric CO₂ to form a thin, hard, shiny layer of calcium carbonate: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.
40. Classify: Zn + H₂SO₄ → ZnSO₄ + H₂.
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A displacement reaction — the more reactive zinc displaces hydrogen from sulphuric acid.
41. Classify: 2Pb(NO₃)₂ --Δ--> 2PbO + 4NO₂ + O₂, and give the observations.
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Thermal decomposition. White lead nitrate crackles and gives off brown fumes of nitrogen dioxide, leaving yellow lead oxide.
42. Why are decomposition and combination called opposite reactions?
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Combination joins two or more substances into one product; decomposition splits one compound into two or more products — each pattern reverses the other.
43. Give one exothermic process used at home and one endothermic process in nature.
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Exothermic: burning LPG in the kitchen. Endothermic: photosynthesis in green plants (absorbing sunlight).
44. What is observed when dilute HCl is added to zinc granules?
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Brisk effervescence — hydrogen gas bubbles form (Zn + 2HCl → ZnCl₂ + H₂) and the tube warms: a displacement, exothermic reaction.
45. Define oxidation in terms of oxygen and hydrogen.
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Oxidation is the gain of oxygen or the loss of hydrogen by a substance during a reaction.
46. Define reduction in terms of oxygen and hydrogen.
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Reduction is the loss of oxygen or the gain of hydrogen by a substance during a reaction.
47. What is a redox reaction?
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A reaction in which oxidation and reduction occur simultaneously — one substance is oxidised while another is reduced.
48. In CuO + H₂ → Cu + H₂O, which substance is oxidised and which is reduced?
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H₂ is oxidised (it gains oxygen, forming H₂O); CuO is reduced (it loses oxygen, forming Cu).
49. In the same reaction, name the oxidising and reducing agents.
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CuO is the oxidising agent (it supplies oxygen); H₂ is the reducing agent (it removes oxygen).
50. What is observed when copper powder is heated in air? Write the equation.
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The shiny brown powder becomes coated with a black substance — copper(II) oxide: 2Cu + O₂ --Δ--> 2CuO.
51. How can the black copper oxide be turned back to copper?
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By passing hydrogen gas over the heated black CuO: CuO + H₂ → Cu + H₂O — the black coating disappears and brown copper reappears.
52. In ZnO + C → Zn + CO, identify the oxidised and reduced substances.
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Carbon is oxidised (gains oxygen → CO) and is the reducing agent; ZnO is reduced (loses oxygen → Zn) and is the oxidising agent.
53. Define corrosion.
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The slow eating away of a metal's surface by the action of moisture, oxygen, acids and other substances in its surroundings.
54. What is rust chemically, and what colour is it?
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Hydrated iron(III) oxide, Fe₂O₃·xH₂O — a reddish-brown flaky substance.
55. State the two conditions BOTH necessary for iron to rust.
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Presence of air (oxygen) AND moisture (water) together — remove either and rusting stops.
56. Why does a silver ornament turn black over time?
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Silver reacts with traces of hydrogen sulphide (H₂S) in air, forming a black coating of silver sulphide, Ag₂S.
57. What is the green coating on old copper articles?
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Basic copper carbonate — formed by copper reacting with moist air's carbon dioxide and moisture.
58. List four methods of preventing corrosion of iron.
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Painting, applying oil/grease, galvanising (zinc coating), and electroplating with chromium/tin or alloying into stainless steel.
59. What is galvanisation and why does it protect even when scratched?
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Coating iron with a layer of zinc. Zinc, being more reactive, corrodes preferentially — it sacrifices itself and protects the iron even where the coat is broken.
60. Define rancidity.
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The oxidation of fats and oils present in food when exposed to air, which spoils the food's smell and taste.
61. List four ways to prevent rancidity.
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(1) Add antioxidants to fatty foods, (2) store food in airtight containers, (3) flush packets with nitrogen gas, (4) refrigerate (and keep food away from light).
62. Why are chips packets flushed with nitrogen?
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Nitrogen is unreactive and displaces the oxygen inside the packet, so the frying oil cannot be oxidised — the chips stay fresh (and the puffed packet also cushions them).
63. What are antioxidants?
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Substances added to oily/fatty foods that prevent or slow their oxidation, protecting the food from turning rancid.
64. Why doesn't an iron nail rust inside boiled water sealed with an oil layer?
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Boiling drives out the dissolved oxygen, and the oil layer prevents air from re-dissolving. With water present but oxygen absent, rusting cannot occur.
65. Corrosion and rancidity are both examples of which type of reaction?
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Oxidation — the metal (corrosion) or the fats/oils in food (rancidity) are oxidised by the oxygen and moisture of the surroundings.
66. Give the equation for hydrogen burning in oxygen, and identify what is oxidised.
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2H₂ + O₂ → 2H₂O. Hydrogen gains oxygen, so hydrogen is oxidised (oxygen is the oxidising agent).
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