RowQ
The Vault
RowQ
The Vault
CBSE Class 10 Chemistry · 12 questions · 28 marks
Every rusting nail, digesting meal, and burning matchstick is a chemical reaction in disguise. This chapter teaches you to write balanced chemical equations and recognise the six major reaction types so you can predict what happens when substances meet.
Which of the following is a decomposition reaction?
Answer
2AgCl -> 2Ag + Cl2 (in sunlight) — a single compound (silver chloride) breaks down into two simpler substances under light, which is the defining feature of decomposition.
In the reaction Zn + CuSO4 -> ZnSO4 + Cu, which substance is reduced?
Answer
CuSO4 (the Cu2+ ion) is reduced, because it gains electrons to form neutral copper metal while zinc is oxidised by losing electrons.
The white colour of freshly precipitated barium sulphate formed by mixing sodium sulphate and barium chloride solutions is evidence of which type of reaction?
Answer
Double displacement, because the Ba2+ and SO4 2- ions exchange partners with Na+ and Cl- to form insoluble BaSO4 and soluble NaCl.
Which statement about rancidity is correct?
Answer
It is caused by oxidation of fats and oils, which can be slowed with antioxidants — oxidation changes the smell and taste of fatty food, and antioxidants or vacuum-sealed packaging (flushing with nitrogen) delay it.
Assertion (A): Silver articles turn black over time when exposed to air. Reason (R): Silver reacts with sulphur compounds present in air to form a layer of silver sulphide, a process called corrosion.
Answer
Both A and R are true and R is the correct explanation of A. Silver reacts with trace sulphur compounds in the atmosphere to form a black coating of Ag2S, which is a form of corrosion.
Balance the following equation and identify its type: Al + O2 -> Al2O3.
Answer
The balanced equation is 4Al + 3O2 -> 2Al2O3. It is a combination reaction because two reactants (aluminium and oxygen) combine to form a single product (aluminium oxide); it is also an oxidation reaction since aluminium gains oxygen.
Why is it necessary to balance a chemical equation? Give one method used to balance equations.
Answer
A chemical equation must be balanced to obey the law of conservation of mass, which states that mass can neither be created nor destroyed in a chemical reaction, so atoms of each element must be equal on both sides. This is done by the hit-and-trial method, adjusting whole-number coefficients in front of formulae (without changing the formulae themselves) until the atom counts match on both sides.
Distinguish between exothermic and endothermic reactions with one example of each.
Answer
Exothermic reactions release energy (usually as heat) to the surroundings, so the temperature of the surroundings rises; for example, burning of natural gas: CH4 + 2O2 -> CO2 + 2H2O + heat. Endothermic reactions absorb energy from the surroundings, so the temperature of the surroundings falls; for example, decomposition of calcium carbonate on heating: CaCO3 -> CaO + CO2 (requires continuous heat supply).
What is a redox reaction? Using the reaction between hydrogen and copper(II) oxide, explain oxidation and reduction, and identify the oxidising and reducing agents.
Answer
A redox reaction is one in which oxidation and reduction occur simultaneously — one reactant loses electrons (or gains oxygen/loses hydrogen) while another gains electrons (or loses oxygen/gains hydrogen). In the reaction CuO + H2 -> Cu + H2O, copper(II) oxide loses oxygen to form copper, so CuO is reduced; hydrogen gains oxygen to form water, so H2 is oxidised. Since CuO causes the oxidation of hydrogen by supplying oxygen, CuO is the oxidising agent; since H2 causes the reduction of CuO by removing its oxygen, H2 is the reducing agent. This reaction is used industrially to extract metals from their oxides and illustrates how oxidation and reduction are two halves of the same process, which is why the term redox (reduction + oxidation) is used.
Explain, with a balanced equation for each, the four main types of chemical reactions other than combination reactions: decomposition, displacement, double displacement, and precipitation.
Answer
Decomposition reaction: a single compound splits into two or more simpler substances, usually needing heat, light, or electricity, e.g. 2H2O -> 2H2 + O2 (electrolysis, electrical energy). Displacement reaction: a more reactive element displaces a less reactive element from its salt solution, e.g. Zn + CuSO4 -> ZnSO4 + Cu, since zinc is more reactive than copper. Double displacement reaction: the positive and negative ions of two different compounds exchange partners, e.g. Pb(NO3)2 + 2KI -> PbI2 (down arrow) + 2KNO3. Precipitation reaction: a specific case of double displacement where an insoluble solid (precipitate) forms and settles out, as seen in the yellow PbI2 precipitate above, which is used to test for the presence of lead or iodide ions in solution.
A student places a few pieces of zinc granules in a test tube containing copper sulphate solution, which is initially blue. After some time, the blue colour of the solution starts fading and a reddish-brown deposit appears on the zinc granules. Based on this observation, answer: (a) Name and write the balanced equation for the reaction taking place. (b) Why does the blue colour of the solution fade? (c) Identify the type of reaction and explain why it occurs in terms of reactivity.
Answer
(a) This is a displacement reaction: Zn + CuSO4 -> ZnSO4 + Cu. (b) The blue colour fades because blue copper sulphate is converted into colourless zinc sulphate as copper ions are replaced by zinc ions in solution. (c) It is a displacement reaction, occurring because zinc is more reactive than copper (zinc lies above copper in the reactivity series), so zinc can displace copper from its salt solution, depositing reddish-brown copper metal on the granules.
Write the balanced equation for the decomposition of ferrous sulphate crystals on heating, and state one observation a student would make.
Answer
2FeSO4 -> Fe2O3 + SO2 + SO3 (on heating). On heating green ferrous sulphate crystals, the green colour changes to a reddish-brown residue (ferric oxide), and pungent-smelling gases (SO2 and SO3) are released, which is characteristic of a thermal decomposition reaction.
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