2813_1_Jan_02MS

2813_1_Jan_02MS - RECOGNISING ACHIEVEMENT Subject How Far...

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Unformatted text preview: RECOGNISING ACHIEVEMENT Subject: How Far, How Fast? Code: 2813/01 Session: January Year: 2002 Final Mark Scheme MAXIMUM MARK m page 1 1 (a) (b) (C) Abbreviations, annotations and conventions used in the Mark Scheme alternative and acceptable answers for the same marking point separates marking points answers which are not worthy of credit words which are not essential to gain credit (underlining) key words which must be used to gain credit error carried forward alternative wording or reverse ar-ument ora (i) the enthalpy change when 1 mole of compound/substance is formed \/ from its elements under standard conditions (of temperature and pressure) / [2] (ii) temperature 0f298K (or 25 0C) J pressure of 1 atmos (or 100 kPa or 101 kPa) \/ [2] (i) a reaction that gives out heat/energy to its surrounds or in which the reactants react with a decrease in internal enthalpy/energy. [NOT temperature rise] \/ [1] (ii) e.g. combustion/burning of fuels (or stated fuel, e.g. alkanes) 0r respiration or metabolism or (unbalanced) equation representing this. [NOT just ‘burning’ on its own] \/ [1] (i) AH = 4(-242) — 2(+51) - 9 (V for x2 and x4) : ~968 - 102 - 9 (/ for the correct signs) = -1079 kJ mol'1 (/ for the answer) ecf (see separate list of alternatives) [3] (ii) Because the products are gases (if products are identified, both must be correct) [NOT low activation energy] / page 3 2 (a) (b) (C) ecf ecf (d) at a high temperature (accept any stated temperature above 0°C) J [1] photosynthesis requires (only) light. or ‘energy from the sun’ [NOT heat, or heat from the sun] J [1] (i) 6(O-H) + 6(C=O) / = 6 x 464 + 6 x 750 = 7284 (Id moi") « [2] (ii) 3(O=O) + 4(C-H) + 2(C-C) + 2(C-O) + 2(O-H) + C=O \/ = 3x498+4x4l3+2x347+2x358+2x464+750 .1 = 6234 (kl mol ) / (see separate list of alternatives allow [1] if only CC is omitted) [2] (iii) AH : 7284 - 6234 = + 1050 kJ moi" ecf(i.e. (i)-(ii)) / [1] ecf diagram / [to include: C3H603 + 302 as product and AH 0r ‘+1050’, drawn to be consistent with answer to part (iii) above] [1] Total: [8] page 4 (2!) Lu 0)) (C) (d) (i) (ii) distribution curve (T1): starts at( 0,0) and goes to a maximum right hand side tails off to x-axis exponentially [it can reach the axis, but not cross it] second curve (T2): starts at (0,0) and has its maximum at a lower ordinate value and to the right of the T1 maximum the (minimum) energy that molecules/particles need to have in order to react or energy required for effective collisions or minimum energy needed for a reaction to occur or energy needed to break bonds [NOT the energy needed to start a reaction] \/ at higher temperature more molecules have E > Ea [NOT just ‘more molecules have higher energy’] '. greater chance of reacting on collision (or more successful collisions) [NOTjust ‘more collisions’] ‘. faster reaction or increased rate (or accept the converse arguments at a lower temperature) (0 (ii) IIHEII small est Em largest Eact \/\/ (ezlher C<D<B<A or C<D<A<B, i.e.0ne in wrong place: [1]) no bonds broken in B :> low Em the others go in order of bond energies (or wtte — e. g. A has the greatest bond energy) page 5 [2] [2] [1] fu/c [3] (all correct: [2] [2] f V [2] Total: [12] (a) (b) (C) (d) (e) a catalyst speeds up a reaction (without being used up). / it offers a different route / of lower activation energy / [3] heterogeneous / [1] needs to happen in a closed system no change in macroscopic properties forward and backward reactions continue to proceed but at the same rate as each other [NOT same extent] any two/\/ [2] (i) (When a system in dynamic equilibrium is subjected to a change in conditions... ) the (position of) equilibrium [NOT reaction] will shift (or be restored) \/ in the direction that minimises the effect of the change or opposes the change [NOT negates or cancels the change] / [2] (ii) pressure equilibrium shifts to the left \/ because 9 moles of gas on LHS and 10 moles of gas on RHS or less particles on left hand side of equation Vu/c temperature equilibrium shifts to the left hand side J because reaction is exothermic or AH is negative /u/c [4] To speed up reaction. I or To obtain a reasonable yield at reasonable rate. [1] Total: [13] page 6 (b) (i) ammonia is acting as a base/alkali/proton acceptor [NOT ammonia reacts with/absorbs protons] J [1] (11) Mr 1‘01“ (NH4)2SO4 = 132.1 \/ 2Xl7 ——> l32.l (markf0r2x17) J 100 ——> 132.1 x 100/34 = 388-390 g /ecf [3] (iii) fertiliser J [1] Gas/C02 is evolved/given off or reaction fizzes. J [this mark 15 NEGATED if a change of colour is mentioned] MgCO3 + 2HNO3 —-—) Mg(NO3)2 + H20 + C02 correct formulae of reagents \/ equation balanced J [3] Total: [8] CFCs affect the ozone layer J C-Cl bond breaks with UV or energy from sunlight J giving Cl radicals or C1- or Cl atoms (the C1 can be read into an equation, but ‘radical’/‘atom’ has to be in words) J homogeneous J catalysis \/ word explanation of how Cl acts as a homogeneous catalysis (e.g. it is regenerated) \/ mention of chain reaction J hence one Cl breaks down many 03 J Cl+O3—>C10+02 \/ ClO+O—————>Cl+02 or C10+O3——>C1+202 / [ignore 03 ———> O; + O] 10 points: any 8 score Q of w C (at least one sensible sentence): \/ [9] Total: [9] page 7 ...
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