Edexcel Jan 2010 Paper 2 Q16 (with explained solutions) |

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Edexcel Jan 2010 Paper 2 Q16

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PMT Answer ALL the questions. Write your answers in the spaces provided. SECTION B 16 Magnesium nitrate, Mg(NO3)2, decomposes when it is heated. One product is the brown gas, nitrogen dioxide. (a) (i) Write an equation for this reaction. State symbols are not required. (2) (2) (ii) Calcium nitrate decomposes in a similar way to magnesium nitrate, but at a higher temperature. Explain why the two nitrates have different stability to heat.(b) Sodium nitrate decomposes to give different products to magnesium nitrate. Write an equation for the decomposition of sodium nitrate. State symbols are not required. (1) *N35692A0924* Turn over<br />
PMT (c) A student suggested that the structure of the nitrate ion, NO3 , is Scientists have found that the bonds between nitrogen and oxygen in the nitrate ion are all the same length. Is the students suggestion supported by this evidence? Explain your answer. (1)(d) Nitrogen dioxide gas can dimerize to dinitrogen tetroxide, N2O4, a very pale yellow gas, as shown in the equation below. 2NO2(g) (cid:85) N2O4(g) H = 58 kJ mol1 (i) What would you see when an equilibrium mixture of these gases is warmed gently? Explain your answer. (2)10 *N35692A01024*<br />
PMT (ii) Explain why an equilibrium mixture of these gases eventually becomes paler in colour when the pressure on it is increased. (2)(e) Two Maxwell-Boltzmann distributions showing the energy of particles in a gas at different temperatures, T1 and T2, are shown below. The activation energy for the reaction is labelled EA. Number of molecules with Energy, E T1 T2 T2 > T1 Energy, E EA Use the distributions to explain why gases react faster when the temperature is increased. (2)(Total for Question 16 = 12 marks) *N35692A01124* 11 Turn over<br />

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Reject Acceptable Answers 2Mg(NO3)22MgO + 4NO2 + O2 Correct formulae (1) Balancing (1) ALLOW multiples or equation divided by 2 Second mark on correct species only Ignore state symbols even if incorrect ALLOW N2O4 Extra oxygen molecules on both sides in a balanced equation Acceptable Answers Reject Stand alone marks Mg2+ / Magnesium ion smaller or fewer electron shells / greater charge density (1) OR Magnesium ion has same charge (as calcium ion) but is smaller (1) Causes more polarisation /distortion of nitrate / anion (electron clouds) / NO (bond)(1) OR MgO produced has stronger lattice (1) OR production of MgO is more exothermic (1) OR reverse argument based on Ca2+ Acceptable Answers 2NaNO32NaNO2 + O2 ALLOW multiples or equation divided by 2 Ignore state symbols even if incorrect ALLOW Extra oxygen molecules on both sides in a balanced equation Magnesium / calcium / atoms / molecules Reject Acceptable Answers Reject No as. double bond would be shorter (than single bond) / shorter than dative (covalent) bond. ALLOW Structure has double and single bonds (between N and O) Double and single bonds have different lengths Implication that the single covalent and dative covalent bonds have different lengths Mark Mark Mark Mark Section B Question Number 16(a)(i) Question Number 16(a)(ii) Question Number 16(b) Question Number 16(c)<br />
Reject Brown (gas evolved) Mark Acceptable Answers Mark independently Goes darker (brown) (1) ALLOW Goes browner Ignore comments on mixture first becoming paler if volume increases Equilibrium moves in the endothermic direction (1) OR Equilibrium moves left as forward reaction is exothermic (1) For second mark ALLOW Equilibrium moves left to counteract addition of heat / increase in temperature(1) OR Reaction removes added heat by moving left (1) Mark Mark Reject Reject Acceptable Answers Equilibrium moves right (ALLOW forwards) (so NO2 concentration decreases) (1) OR Reaction reduces pressure (1) As fewer moles / molecules(ALLOW particles) (of gas) on RHS (1) Stand alone marks Acceptable Answers At T2 more molecules/collisions have energy greater than (or equal to) EA (1) This can be shown on the diagram by indicating areas to right of vertical line Energy must be at least EA for successful collision / for reaction (1) OR So more collisions have sufficient energy to react(1) Ignore references to the average energy and speed of the molecules Question Number 16(d)(i) Question Number 16(d)(ii) Question Number 16(e)<br />

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