# Equilibria Ans

• View
216

0

Embed Size (px)

### Text of 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-+*&

!

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

Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents
Documents