Equilibria Ans

8/10/2019 Equilibria Ans

1/142

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

8/10/2019 Equilibria Ans

2/142

(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

8/10/2019 Equilibria Ans

3/142

-. 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

8/10/2019 Equilibria Ans

4/142

(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

8/10/2019 Equilibria Ans

6/142

(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

8/10/2019 Equilibria Ans

7/142

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

8/10/2019 Equilibria Ans

8/142

(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

8/10/2019 Equilibria Ans

9/142

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

8/10/2019 Equilibria Ans

10/142

(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

8/10/2019 Equilibria Ans

11/142

(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

8/10/2019 Equilibria Ans

12/142

(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