Equilibria Ans

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  • 8/10/2019 Equilibria Ans

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    1. B[1]

    2. (a) (i) Mark the two points independently, subject to the constraint inReject column

    Effect:

    (Equilibrium) shifts to the right (1

    ALLOW: favours forward reaction / increase the amount of product/ increase the yield (of product)

    Reason:

    Eothermic (in forward direction) (1

    NOTE: !ust(equilibrium) shifts in the eothermic directionscores (1 !

    Equilibrium shifts to left will score (0) for (a)(i)

    (ii) "irst mark:"ctivation energy for the reaction is too high / (if cooled) moleculeswould not have enough energy to react / few(er) molecules have therequired Ea/more molecules have energy #Eaat higher temperatures#R

    not (technologically) feasible to cool the gases before they enter theconverter/costly to cool the gases (1

    $econd mark:(cooling the gases would ma$e) the rate (too) slow /rate is faster if thetemperature is high (so the gases are not cooled) (1 !

    Cooling the gases decreases the yield (of products) /an

    incorrect Le Chatelier argument

    (iii) Mark the two points independently, subject to theconstraint in Reject column

    Effect:

    (Equilibrium) shifts to the right

    ALLOW: favours forward reaction / increase the amount of product /increase the yield of product (1

    Reason:

    %hifts / moves in the direction of fewer (moles of gas) molecules

    ALLOWshifts in direction of fewer moles (of gas molecules) (1

    INO!Eeffect on the rate !

    Equilibrium shifts to left will score (0) for (a)(iii)

    "#### fe$er atoms

    1

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    (b) (i) (%n :&! / !& (1

    (%n '

    :&' / '& (1

    NOTE%

    (%n :ust !)&*(%n '

    :ust ' scores (1 !

    (ii) *+,& +-&& 'e,. * & 2-!*

    ACCE&Tmultiples !

    (iii) "g . "g&& e(,)/ "g , e(,). "g&

    ACCE&TmultiplesINO!Estate symbols even if incorrect 0

    "Ag ' e

    Ag'

    (iv) +"g & *+,& 1-& +"g&& * & !-!* (2

    (1for multiplication of the silver half2equation by three or cq multiplefrom (b)(ii)

    (1for rest of equation correctNOTE: Equation must be completely correct for the second mar$3

    INO!Estate symbols even if incorrect !

    if any e

    are left in the final e*uation+ second mar, cannot -e

    scored[13]

    '. 4[1]

    +. B[1]

    . 4[1]

    2

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    -. 4[1]

    . "[1]

    /. (a) (i) p4*)(*-p4-

    -4*p4-)5(

    +

    !+p

    .u

    uu=

    6artial pressure symbol can be shown in various ways eg ppp4* (4*)p etc

    ALLOWp in upper or lower case round brac$etsINO!Eunits 0

    0

    1tate sym-ols gi2en as (l)

    ' in -ottom line

    (ii) 6 4-+*- 7 138 (atm) (16 4* 7 138 (atm) (1

    0 mar$ for recognition that pressures are equal

    INO!Eunits !

    (iii) 3p7 ((!!3!)/(138)!)

    7 938!' (1

    atm,0(1stand alone mark but must match e0pression used in (a(iii

    #R

    83!' 0916a,0/ 8!3' $6a,0(2

    ALLOW;E from (a)(i) if inverted and/or (a)(ii) !

    Ans$ers to other than 4 significant figures

    (b) (i) 4-+*-: +3!4* : +3! (1for both values

    4-+4*!-: 1

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    (ii)

    !3'+

    +!=310/>

    !

    !

    (iii) 4orrect answer with or without wor$ing scores 2 marks

    03F 09,'7 930!'ED- !

    (1

    D-& 7 031< 09,+

    p- 7 !3=1/!3= (1

    no ;E from an incorrect D-& !

    5

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    (iv) p- 7 13= / 13FF (1

    p- 7 p3a/ D-& 73a(when acid is half neutraliHed) (1 !

    >'8 3a

    (v) %igmoid curve starting between p- ! and 1 (!3=) ending between

    p- 0! and 01 inclusive (1

    with steep rise (may be vertical or gently sloping) of between + ,F units between p- < and 0!3 %loping section should not etendover more than 'cm+3 (1

    Ihen 0!3' cm+ a*- added3 (1"AA*I tolerance for grid

    Ceverse curves lose first mar$ +

    (vi) "irst mark

    ;hymolphthalein more suitable as it changes(from colourless to blue) in steep region of titration(p- =3+ to 093

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    13. "[1]

    11. (a) (i) !g(*+)!. !g* & 1*!& *!

    4orrect formulae (1Balancing (1)44#5multiples or equation divided by ! %econd mar$ on correctspecies only Lgnore state symbols even if incorrect)44#5

    !*1Etra oygen molecules on both sides in a balanced equation !

    (ii) %tand alone mar$s

    g!&/ agnesium ionsmaller or fewer electron shells/ greatercharge density (1#R

    agnesium ionhas same charge (as calcium ion) but is smaller (1

    4auses more polarisation /distortion of nitrate / anion (electron clouds)/ M* (bond)(1#R

    g* produced has stronger lattice (1#R

    production of g* is more eothermic (1

    #Rreverse argument based on 4a!&

    [email protected] / calcium / atoms / molecules

    (b) !a*+. !a*!& *!)44#5multiples or equation divided by ! Lgnore state symbols even ifincorrect)44#5

    Etra oygen molecules on both sides in a balanced equation 0

    (c) o asN3double bond would be shorter (than single bond) / shorter than dative(covalent) bond3)44#5

    %tructure has double and single bonds (between and *)Oouble and single bonds have different lengths 0

    Implication that the single co2alent and dati2e co2alent -onds

    ha2e different lengths

    7

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    (d) (i) ar$ independently Poes dar$er (brown) (1"AA*IPoes brownerLgnore comments on miture first becoming paler if volume increases

    Equilibrium moves in the endothermic direction (1

    #R

    Equilibrium moves left as forward reaction is eothermic (1

    Qor second mar$ )44#5Equilibrium moves left to counteract addition of heat / increase intemperature(1

    #R

    Ceaction removes added heat by moving left (1 !

    ro$n (gas e2ol2ed)

    (ii) Equilibrium moves right ("AA*I forwards) (so *!concentrationdecreases) (1#R

    Ceaction reduces pressure (1

    "s fewer moles / molecules("AA*I particles) (of gas) on C-% (1

    %tand alone mar$s !

    (e) "t ;!more molecules/collisions have energy greater than (or equal to)E"(1

    ;his can be shown on the diagram by indicating areas to right of vertical line

    Energy must be at leastE"for successful collision / for reaction (1

    #R%o more collisions have sufficient energy to react (1

    Lgnore references to the average energy and speed of the molecules ![12]

    12. B[1]

    1'. 4[1]

    1+. B[1]

    8

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    1. "[1]

    1-. B[1]

    1. "[1]

    1/. 4[1]

    1. 4[1]

    23. (a) (i) (p- 7) ,log D-&

    *C(p- 7) ,log D-+*

    &

    *C

    )ccept

    Oefinition in words(Qor eample: Lt is minus / negative log(arithm) of the hydrogen ionconcentration)

    Base 09 does not have to be there but reRect ln 0

    Just"concentration of hydrogen ions

    B curly -rac,ets

    Dlog >'

    (ii) (p- 7 2log 939099) 7 !(399) 0

    If any units gi2en

    9

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    (b) (i) D-+*& 7 E4**D4-

    4**-ED4-,

    +

    +a3

    *C

    D-+*&

    !

    73aD4-+4**- (1ALLOW

    D-" for D4-+4**- and D", for D4-+4**

    , in rearranged epression

    )ccept

    D-& for D-+*&

    D-+*& 7 S 03F' 09,F

    *C

    D-+*& 7 130=(' 09,1(mol dm,+) (1

    p- 7 +3+= / +31ignore sf ecept one sf (1

    ;hird mar$ ;E from D-& only if p- less than

    &.6. 7#RRE78 )&$5ER, 5%89 #R 5%89#8 5#R;%&

    G0 / >

    '0 8 salt0

    10

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    (ii) "irst mark:

    (Oilution/addition of water) shifts the equilibrium

    4-+4**- 4-+4**,& -&/

    4-+4**- & -!* 4-+4**,

    & -+*&

    to the riht*Cthe above stated in words such as: degree of dissociation increases/

    proportion of dissociation increases/ more dissociation (as theethanoic acid is diluted) (1

    $econd mark:

    so the D-& is greater than epected/ so the decrease in D-& is less thanepected / so that the decrease in D-& is less than that forhydrochloric acid (1

    Each mark is a stand alone mark.

    ALTE!NATIHE !O?TE%

    "irst mark:

    D-& 7 S3a D-" *C (3a D-")T

    *C

    p- 7 Tp3a2 TlogD-" (1

    $econd mark:

    use of mathematical epression given (e3g3D-&

    affected by factor of0/S09 on dilution *C substitution of numerical values into theequation) (1

    INO!E: any comments or calculations relating to -4l(aq) !

    !eFect Fust a reference to a #: increase in p> for

    C>4COO>(a*) compared $ith a 7# increase in p> for

    >Cl(a*)

    (c) (i) 8hese marks are stand alone.

    aintains an almostconstant p- / resists change(s) in p- (1

    for smalladdition of -&or *-,ions (3B3 both ions needed) / forsmalladditions of acid or al$ali / for smalladditions of acid or

    base (1

    INO!Eany references to named buffer mitures !

    "resists small change(s) in p>

    O!

    "p> does not change

    (ii) citric acid 0

    11

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    (iii) "irst mark:

    (buffer contains) reservoir of -" and",

    *C(buffer contains) large concentrations of D-" andD",

    *Cbothequations:

    -" ", & -& anda" a&& ",

    $econd mark:

    ("ddition of al$ali/base)

    -" & *-,. ",& -!**Cdescription/equations to show that -&reacts with *-,(to form -!*)

    and more acid dissociates (to replace -&) (1

    8hird

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