Model Test Paper മാതൃകാ ചോദ്യപേപ്പർ
A full paper in the Kerala Class 9 Chemistry pattern, covering the whole of “Redox Reactions” – the law of conservation of mass with masses in u, balancing chemical equations, oxidation and reduction as loss and gain of electrons, oxidising and reducing agents, oxidation numbers in compounds and deciding whether a reaction is a redox reaction. Work on paper with a timer before you look at the solutions.
Standard IX · Chemistry
Chapter 4 · റിഡോക്സ് പ്രവർത്തനങ്ങൾ · Redox Reactions
- The first 15 minutes are cool-off time: read every question and plan your answers; do not write.
- Answer all questions. Marks for each question are shown on the right.
- Write oxidation numbers with their sign (+3, −2). Balance equations by changing only the coefficients.
- Atomic masses (u) you may need:
Element H C O Mg Ca Fe Atomic mass (u) 1 12 16 24 40 56
SECTION AAnswer all · 8 × 1 = 8 marks
1.
A 2.4 g piece of magnesium ribbon burns completely in air and forms 4.0 g of magnesium oxide. The mass of oxygen that combined with the magnesium is
(1)(a) 6.4 g(b) 1.6 g(c) 2.4 g(d) 4.0 g
2.
How many atoms in all are there in 3Ca(OH)2?
(1)3.
The oxidation number of phosphorus in a P4 molecule is
(1)(a) +4(b) −3(c) 0(d) +5
4.
Find the relation and fill in: Na → Na+ + e− : oxidation :: Zn2+ + 2e− → Zn :
(1)5.
In the reaction CuO + H2 → Cu + H2O, the reducing agent is
(1)(a) CuO(b) H2(c) Cu(d) H2O
6.
The oxidation number of nitrogen in ammonia, NH3, is .
(1)7.
Which of these is not a redox reaction?
(1)(a) 2Mg + O2 → 2MgO(b) 2Na + 2H2O → 2NaOH + H2(c) KOH + HNO3 → KNO3 + H2O(d) 2H2 + O2 → 2H2O
8.
Write the missing coefficient so that the equation is balanced: 2KClO3 → 2KCl + O2
(1)SECTION BAnswer all · 5 × 2 = 10 marks
9.
Sea shells, which are mostly calcium carbonate, are heated in kilns to make lime: CaCO3 → CaO + CO2. Using the atomic masses Ca = 40, C = 12, O = 16, show that this equation obeys the law of conservation of mass.
(2)10.
Balance the following equations. (a) Al + HCl → AlCl3 + H2 (b) C3H8 + O2 → CO2 + H2O (propane, one of the gases in LPG)
(2)11.
Potassium reacts with bromine: 2K + Br2 → 2KBr. (a) Write the equations showing the change that happens to a potassium atom and to a bromine atom. (b) Which of these changes is oxidation and which is reduction? Give the reason.
(2)12.
Find the oxidation number of (a) chlorine in sodium hypochlorite, NaClO (used in bleaching liquids) (b) phosphorus in phosphoric acid, H3PO4. Show the working.
(2)13.
In the thermite reaction, used to weld railway tracks, aluminium reacts with iron oxide: 2Al + Fe2O3 → Al2O3 + 2Fe. Using oxidation numbers, find (a) the element oxidised and the element reduced (b) the oxidising agent and the reducing agent.
(2)SECTION CAnswer all · 4 × 3 = 12 marks
14.
Iron is extracted from its oxide in a blast furnace: Fe2O3 + CO → Fe + CO2. (a) Balance the equation.
(b) Using the atomic masses Fe = 56, C = 12, O = 16, show that the total mass is the same on both sides of the balanced equation.
(c) Which substance is reduced in this reaction? Give the reason using oxidation numbers.
(3)(b) Using the atomic masses Fe = 56, C = 12, O = 16, show that the total mass is the same on both sides of the balanced equation.
(c) Which substance is reduced in this reaction? Give the reason using oxidation numbers.
15.
Find the oxidation number of (a) manganese in potassium manganate, K2MnO4
(b) nitrogen in dinitrogen trioxide, N2O3
(c) sulphur in aluminium sulphate, Al2(SO4)3.
(3)(b) nitrogen in dinitrogen trioxide, N2O3
(c) sulphur in aluminium sulphate, Al2(SO4)3.
16.
Write the oxidation number of every atom above its symbol and find out whether each reaction is a redox reaction. (a) CuO + 2HCl → CuCl2 + H2O
(b) 2FeCl2 + Cl2 → 2FeCl3
(c) For the redox reaction, name the oxidising agent and the reducing agent.
(3)(b) 2FeCl2 + Cl2 → 2FeCl3
(c) For the redox reaction, name the oxidising agent and the reducing agent.
17.
Anu puts about 20 mL of dilute hydrochloric acid in a conical flask. She places a few pieces of magnesium ribbon in a balloon, ties the balloon over the mouth of the flask without letting the magnesium fall in, and keeps the flask on an electronic balance. The reading is 86.40 g. She then lifts the balloon so that the magnesium falls into the acid. (a) Write two observations.
(b) Write the balanced equation of the reaction. What will the balance show after the reaction? Why?
(c) When she repeats the experiment in an open flask without the balloon, the reading becomes less. Does this go against the law of conservation of mass? Explain.
(3)(b) Write the balanced equation of the reaction. What will the balance show after the reaction? Why?
(c) When she repeats the experiment in an open flask without the balloon, the reading becomes less. Does this go against the law of conservation of mass? Explain.
SECTION DAnswer all · 2 × 5 = 10 marks
18.
Burning magnesium keeps burning even in carbon dioxide, forming white magnesium oxide and black specks of carbon: Mg + CO2 → MgO + C. (a) Balance the equation.
(b) Using the atomic masses Mg = 24, C = 12, O = 16, show that mass is conserved.
(c) Write the oxidation numbers of Mg, C and O before and after the reaction.
(d) Name the element oxidised, the element reduced, the oxidising agent and the reducing agent.
(e) Why should a carbon dioxide fire extinguisher not be used on burning magnesium?
(5)(b) Using the atomic masses Mg = 24, C = 12, O = 16, show that mass is conserved.
(c) Write the oxidation numbers of Mg, C and O before and after the reaction.
(d) Name the element oxidised, the element reduced, the oxidising agent and the reducing agent.
(e) Why should a carbon dioxide fire extinguisher not be used on burning magnesium?
19.
Read the passage and answer the questions.
(b) Find the oxidation number of iron before and after rusting. Write the equation for the change of an iron atom. Is iron oxidised or reduced?
(c) Which substance is the oxidising agent? Write the equation for the change of an oxygen atom.
(d) Does the increase in mass of the rusted anchor go against the law of conservation of mass? Explain.
(e) On the GI sheet, zinc reacts with oxygen: 2Zn + O2 → 2ZnO. Write the change in the oxidation number of zinc and explain how the zinc coating protects the iron.
(5)At Neendakara fishing harbour in Kollam, Rafeeq’s family owns a fishing boat. Its engine frame and anchor are made of iron. In the moist, salty sea air the iron parts are soon covered with a reddish-brown layer of rust, and the rusted anchor becomes heavier than when it was new. Rust forms when iron reacts with the oxygen and water of the air; in this question take rust as iron(III) hydroxide, Fe(OH)3. Rafeeq’s father paints the iron parts before every monsoon. The roof of their house is made of galvanised iron (GI) sheets – iron sheets coated with zinc. The zinc slowly forms a thin, white oxide layer (ഓക്സൈഡ് ആവരണം), and the sheets do not rust for many years.
(a) Balance the equation for rusting: Fe + O2 + H2O → Fe(OH)3.(b) Find the oxidation number of iron before and after rusting. Write the equation for the change of an iron atom. Is iron oxidised or reduced?
(c) Which substance is the oxidising agent? Write the equation for the change of an oxygen atom.
(d) Does the increase in mass of the rusted anchor go against the law of conservation of mass? Explain.
(e) On the GI sheet, zinc reacts with oxygen: 2Zn + O2 → 2ZnO. Write the change in the oxidation number of zinc and explain how the zinc coating protects the iron.
*** End of the question paper ***