Rates of Rxn Exam

EXAMINATION QUESTIONS FOR RATES OF REACTION

Increased surface area (1)

1.

more collisions (1)

2  [9]

(c )

(i)

2H2O2 → 2H2O + O2

(ii)

Speeds up (alters the rate of) a chemical reaction 1 Rema emains un unchanged (o (or no not use used d up up)

1

Remain Remainss unchan unchanged ged (or (or not not used used up or or not in in the overal overalll reactio reaction n equati equation on)) 1 Offe Offers rs alter alterna nativ tivee rea react ctio ion n rou route te (or (or act actss as as an an int inter erme medi diat ate) e)

1

(iii) (iii)

1

[10]

2.

(a )

(&)

(c) (c)

Graph starts at origin

1

Graph se!ed to left and has decreasing gradient to ma"imum

1

Graph after ma"imum decreases in steepness# never touches  x a"is#  x a"is# levels out less than $ mm from x from  x a  a" "is%

1

'inimum energ

1

o start a reaction (or ( or for a reaction to occur )

1

'ole 'olecu cule less gain ain ene energ ( or always some molecules have E > E a)

1

*ue to collisions

1

(d)

*ecreases  E a lo!ered (1)  alternative route (1) So more molecules have energ , - a (1)

1

ma" 2 [10]

3.

(a )

(i)

 . u m & e r o f  m o l ec u le s or (1)  p a r t i c l e  . O  m o le s o r m o ls o r a to m s  . o t f r a c t i o n o r p r o p o r t i o n

(ii) (ii)

( ' o le c u la r) - n e rg 

he total total num& num&er er of of parti particle cless (or (or molec molecule ules) s) in in the the sample sample OR the number of molecules present

( 1 )  . o t v e l o c i t 

(iii (iii))

.o molec molecul ules es have have no ener energ g OR all molecules have some energy Do not allow “if there are no molecules there is no energy”

(&)

(i)

4

he mi minimum en energ required  (1) for a reaction to occur (1) OR to start reaction or for a successful collision

(ii)

/hanges0 /a /atals lst (1) -"planation0 lternative route  (1), !ith a lo!er activation energ  (1) OR a lower activation energy 1! so more molecules can react 1!"more molecules have this energy #f change incorrect $% & 0

$

'llow answers anywhere in b ii!

[9]

4.

(a )

2

fraction or  or no% m o le c u le s or  p a r t i c l e s

h i g h e r a n d d is p l a c e d t o le f t ( 1 ) (1 )

not m o le s

m u s t c r o s s o n l  o n c e # m u s t  . O  (1 ) c r o s s  x a " i s # m u s t g o t h r o u g h o r i g i n

(inetic m o l e c u la r )

c a n o n l s c o r e th i s m a r if   f ir s t m a r  o&tained ( ie s h a p e )

e n e r g  (1 )

(&)

See a&ove

2

(c )

-nerg 3 -a or must have enough energ (to react) (1)

1

(d) (d)

Incr Increa ease se conc concen entr trat atio ion n (or (or press pressur ure) e) (1)

1

(e )

'an (1) more molecules have - , - a 4 enough energ (1)  NOT KE increases increases with T 

2

(f)

5o!ers -a (1) alternative route (1)

2 [10]

5.

(a )

ctiva ivation en energ67 g67 he minimum energ needed for a reaction to occur 4 start (1) 1

(&)

/atal talst st effect07 t07 lternative route (or more molecules have -a) (1) 5o!er activation energ (1) 2

(c) (c)

Incr Increa ease se in moles oles of gas gas07 07 8osition of E  of E mp unchanged (1) 'ore molecules !ith E  !ith  E mp (1) rea under curve increases (1) 'olecules !ith E  !ith E ≥ E a increased (1) emperature emperature decreased07 8osition of E  of E mp moves to the left (1) 'ore molecules !ith E  !ith  E mp (1) rea under curve unchanged (1) 'olecules !ith E  !ith E ≥ -a decreased (1) /atalst introduced07 8osition of E  of E mp unchanged (1) 'olecules !ith E  !ith E mp unchanged (1) rea under curve unchanged (1) 'olecules !ith E  !ith E ≥ E a increased (1) 12 [1(]

6.

(a )

the minimum energ6 -nerg required for a reaction to occur6 (or to start a reaction or for successful collisions)

(&) (&)

a"es a"es la&e la&ell lled ed07 07 0 num num&er &er (or fraction or %) of molecules (or particles) "0 energ (or KE); curve starts at origin6 se!ed to right6 approaches " a"is as an asmptote6 (penalise a curve that levels off > 1 % of max pea! hei"ht or a curve that crosses the ener"y axis)

1 1

1 1 1 1

second curve displaced to the left (an# #oes not cross T 1 curve for a secon# time) time ) 1 and pea higher6 1 man fe!er molecules6

1

fe!er molecules have E have  E > E a ; (can score this mar! from suita$ly mar!e# curves)

1

(&)

molecules (or particles or collisions) do not have enough energ6 (or orientation may $e wron")

1

increase the pressure6 (or increase the concentration or re#uce the volume) increases the collision frequenc6 (or more collisions) (#o not allow if st ate# to $e #ue to increase in ener"y implie# $y temperature increase)

1 1

add a catalst6

1

lo!ers activation energ (or - a) ( of & mar!);

1 [1(]



1

 . u m & e r o f  fraction (1 )

2

- n e r g  (1 )

7.

1 c u rv e 0 o r ig i n a n d s h a p e (1 ) 2 c u rv e 0 lo ! e r a n d to r ig h t (1 )

- a

t 20

more molecules (1) have sufficient energ (1)  plus reference to -a or shaded area on graph (1)

5arger mass0

more particles (1) higher curve (1) most pro&a&le energ is same (1) [10]

8.

(a )

(i) (ii)

9

:  . u m & e r o f  m o le c u le s

 p e a  h i g h e r ( 1 ) s t a r ts a t o r ig i n # d i s p l a c e d t o l e f t# c ro s s e s o v e r o n c e (1 ) m a " o f  y & e t ! e e n th e s e l im its

 p e a  h i g h e r ( 1 ) ( / % - % i f n o t h i g h e r ) s ta r ts a t o r i g i n # d i s p l a c e d &  s a m e a m o u n t ( se e lim its ) a n d d o e s n o t c r o ss o v e r (1 )

;

-nerg

(&)

(c )

(i)

collide (1) !ith sufficient energ (or - ≥ -a) (1) (or with correct orientation) orientation )

(ii) (ii)

molecu molecules les (or (or part particle icles) s) have have more more energ energ (or (or move move faste faster) r) (1) more molecules (or collisions) have - ≥ -a (or sufficient energ) (1)

(i)

equili&rium re reached (1) (or rate for!ard reaction < rate &ac!ard)

(ii) (ii)

Reac Reacti tion on is endo endoth ther ermi micc (1) or ∆H +ve or reverse reaction is e"othermic endothermic reaction favoured (1) (or reaction shifts to R  or moves for!ard or more products formed)

9

=

[11]

9.

(a )

(i)

infinit or never (1)

 'oint   Explanation

no ma"imum energ for molecules (1)

 p e a  h i g h e r ( 1 ) m a " a t s am e e n erg  (1 )

 . u m & e r o f  m o le c u le s ! ith a g i v e n energ

-nerg

(ii) (iii (iii))

(&)

no dif differen erence ce (1)

$

 euirement 1  1 

/ollision (1)

 euirement *

>ith sufficient energ or correct orientation (1) 2 [)]

10.

(a )

Same

(&)

(i)

(ii) (ii)

(c )

(d)

1 *ecreases

1

'ore moles on left hand side

1

-quili&rium moves to increase the pressure (Or to oppose the chan"e or to compensate for low pressure)

1

/ost /ost of prod produc ucin ing g high high press pressur uree (1) /ost of plant to resist high pressure (1) /orrect safet factor !ith reason (1)

ma" 2

.o change

1

/atalst has no effect on equili&rium position (Or catalyst affects rate of forwar# an# $ac!war#s reactions eually)

1

.egative

1

Reaction (or euili$rium) moves euili$rium) moves in the e"othermic direction (or to the ri"ht)

1

In order to oppose the change (or to raise the temperature)

1

(e )

Reccled ( or re+use# or ,put $ac! in- ) in- )

1 [1*]

1  . u m & e r + o f  m o le c u le s (1 )

11.

(a )

(ii)

(&)

(c)

/

(i)

*

- n e r g  (1 )

/ (1)

=

 euirement 1  1 

collisions (1)

 euirement *

!ith sufficient energ (1) 2

.reater effect  

emperature (1)

 Explanation

?or small ∆# more molecules (1) have energ ≥ -a (1) = [+]

12.

(a )

removal4loss of electrons

1

(&)

no change

1

equal num&er of gaseous moles on either side

1

 &oth sides affected equall

1

increases

1

equili&rium moves to lo!er the temperature4oppose the change

1

endothermic reaction favoured 4for!ard reaction is endothermic

1

(c )

(i)

+2

1

+$

1  @ 

+

 @ 

(ii)

.O=  + 9H  + =e  A .O +2H2O

(iii)

g A g  + e

(iv)

.O=  + 9H  + =g A .O + 2H 2O + = g

+

 @ 

1

 @ 

+

1 +

1 [1*]

13.

(a )

12 (8a)

1

 pp < mole fraction B total pressure ! mo  mole fraction < 1241C9

1

< C%11$

1 (allow /1*)

(&)

DE (8a)

1

(pSO = ) 2 (c )

(pSO 2 ) 2

F p <  

×

(pO 2 )

1

(0f K  p wron" allow conseuential units only) (penalise suare $rac!ets in expression $ut then mar! on) DE 2 2 < 29 × 12 < C%DD

1 1 (2llow /34) (2llow full mar!s in calculation conseuential on their values in (a) an# ($))

 @1

(d)

8a

1

2

1 (5ust $e correct to score any mar!s in this section)

-"othermic

1

Reduce  to shift equili&rium to the right or for!ard reaction favoured & lo!  or F  p increases for lo!  or lo!  favours e"othermic reaction

1

(e )

Increase

1

 .one

1 [1,]

N"#$

(a )

If F p  has   lose mar in (a) &ut allo! full mars in (d) If F  p !rong4upside do!n etc# allo! ma" 2 in (d) for su&stitution of num&ers (1) and consequential units (1)

(&)

'ar fo for mo moles of of SO SO 2/l2 can &e scored in part (c) (ii) if not gained in (&) 1%J$ get (2) If moles of SO 2/l2 < 1# this is a /hemical -rror# hence a 2 mar penalt •

(c) (c)

(d)

If total moles given in (&) < 1%J$# this scores 2 in (&)6 &ut if the no moles of SO 2/l2 < 1 in (c)(ii)# lose &oth mars in (c)(ii) for pp of SO 2/l2 < (141%J$) B 12$# i%e% the 2 mar   penalt is in (c)(ii)%

•

If total moles given in (&) < 2%$# score Kero in (&)# &ut can gain full mars i n (c)(ii) consequentiall# i%e% the 2 mar penalt is in (&)%

•

If moles of SO 2/l2 < 1 and total in (&) does not equal 2%$# still lose &oth in (&) &ut can get all 9 conseq in (c)(ii) for 14" etc and C%J$4" etc

(i) (i)

llo llo! ! L Lotal tal pre press ssu ure < sum sum of par parti tial al press ressu uresM resM for for (1) or p < " B ptot

(ii) (ii)

?irs ?irstt mar mar  is is for for mole ole fra fract ctio ion% n% If either num&er in either mole fraction is not consequential on (&)# then lose &oth mars for that partial p%

If p/l 2 is not equal to pSO 2 or an num&er used in F  p is not conseq on (c)(ii)# allo! units onl SIG ?IGS6 must &e = sig figs in (&) &ut then allo! 2 sig figs in (c) and (d)6 (ignore e"tra figs) &ut penalise incorrect rounding

14.

(e) (e)

If effe effect ct !ro !rong ng## no no mar mars s for for e"pl e"plan anat ation ion%% If effect missing# e%g% ans!er states Lequm shifts to rightM# mar on% In the e"planation# the !ord LendothermicM (or its equivalent) is essential%

(a) (a)

n equi equili li&r &riu ium m opp oppos oses es chan change ge (1)

(&)

(i)

1

 Effect on yiel# of hy#ro"en6 hy#ro"en6 decreases  decreases (1)  Note 7/E/ if not #ecrease #ecrease $ut mar! on if no answer   Explanation6 pressure  Explanation6 pressure lo!ered (or increase opposed) (1)  & favouring fe!er moles moles (of gas) (1)

(ii)

 Effect on yiel# of hy#ro"en hy#ro"en00 increase (1) 7E if wron" as a$ove  Explanation0  Explanation0 pressure 4 concentration 4 reactants 4 steam reduced (1)  & shifting to right (1) or steam remove# or forwar# reaction favoure# 

D

(c)

 eason 10 10 cost of high temperature 4 energ (1)  eason *0 *0 cost of plant (to resist high ) too high (1)  OR plant could not contain high 

2 [9]

15.

(a) (a)

rate rate for for!a !ard rd rea react ctio ion n < rate rate &ac &ac!a !ard rd rea react ctio ion n (1) concentration remains constant (1) -O. /%ual2 'llow /.he same if clear that means constant

2

(&) (&)

fe!e fe!err mole moless (of (of gas) gas) on R%H% R%H%S S (1) (! %&'#!$#) (methanol favoured) & reducing applied pressure (1) Or removing constraint

2

(c)

8o!er 8o!er 4 energ energ requir required ed to to provi provide de high high pressur pressuree 4 pump pumping ing (1) Strong pressure vessel 4 or equipment (1) 3igh maintenance costs 'ny two

2

(d)

-ffect0 decre creases (1) -"planation0 reaction e"othermic (1) sstem tries to lo!er  or remove constraint or oppose the change or endothermic reaction favoured =

(e )

to spe speed up re reaction ion (1) or otherwise to slow or ta4es too long or to give more molecules %

%'

1 [10]

16.

(a )

mar la&elled X on curve A !here curve C Noins A6

1

(&)

equili&rium opposes a change6 ( of & mar!)

1

(c)



1

more ammonia is produced (or ield increases)6 fe!er moles (of gas) on right ( or 9 mol goes to 2 mol)6

1 1

(d)

equili&rium moves to oppose increase in pressure (or oppose change)6

1

C

1

amount of ammonia (or ield or equili&rium) unchanged6 reaction is faster6

1 1 [9]

17.

(a) (a)

(i) (i)

ll ll (re (reag agen ents ts)) (re (reag agen ents ts)) are are in the the sam samee pha phase se4s 4sta tate te4a 4are re gase gasess (1)

(ii) (ii)

he for!ar for!ard d and and &ac!a &ac!ard rd reation reationss are are occurr occurring ing (1) at the same4equal rate (1) or concentrations of reactants (and products) are constant (1) and reaction is continuous (1) -ote5 “concentrations of reactants an6 pro6ucts are the same” is incorrect

=

(&) (&)

(i) (i)

(/on (/onccentr entrat atio ion n of hdr hdrog ogen en 4pro 4produ duccts) ts) inc increas reased ed (1) -7 if a pro6uct state6 this must be 3 *

-quili&rium moves to right 4 for!ard reaction favoured (1) to remove added !ater 4 sstem reacts to oppose change (1) 8ar4 $% if effect wrong Do not allow “rate” answers

(ii) (ii)

(conce (concentr ntrati ation on of hdrog hdrogen en 4 prod product ucts) s) increa increased sed (1) -quili&rium moves to right 4 for!ard reaction favoured (1) Reaction e"othermic 4 gives out heat 4 moves to oppose change (1)  2llow max (1) for exothermic if other answers incorrect 

D

(c )

.one (1) Rates of &oth for!ard and &ac!ard reactions increased 4 changed (1)  & same amount (1) 'llow: 'ctivation energy of forwar6 an6 bac4war6 reactions lowere6 by the same amount 1! $% if effect wrong

= [1*]

18.

(a )

(i)

(ii)

(&)

(i)

(ii)

(c )

Temperature

change decrease (1)

 Explanation

e"othermic reaction (1)

 'ressure chan"e chan"e

decrease (1)

 Explanation

fe!er moles of gas on l%h%s (1) 9

Temperature

to increase reaction rate (1)

 'ressure

to increase reaction rate (1)

 eason 1

large surface area (1)

 eason *

lo!er cost in e"pensive 8t (1) 9

(i)

enthalp of formation (1)

(ii) (ii)

stan standa dard rd cond condit itio ion ns (1) 1 &ar pressure and stated (fi"ed) temperature (1) reactants and products in standard states (1)

(iii (iii))

8oll 8ollut utan ants ts (aci (acid d rai rain) n) @ .O .OB  produced & com&ustion engines engines

(iv) (iv)

deco decomp mpos ositi ition on is e"ot e"othe herm rmic ic (1) 5o!  reduces effect of heat evolved (1)

(d) (d)

J

.O produ roduce ced d in Stag tage = (1) can &e reccled to Stage 2 (1)

2 [1)]

19.

(a )

(i)

 ates6 Rates are equal# for!ard and &ac!ard (1) 7oncentrations6 /oncentrations are constant (1) ; of < mar4

(ii)

 Euili$rium yiel#6 *ecreases (1) if wrong allow ma= 1 for a correct moles statement

 Explanation6 'ore moles 4 molecules of product (or 2 → 9) (1) Reaction 4 equili&rium moves to left 4 reduce constraint (1) -O. “volume” answers 'llow one for “Reaction favours fewer molecules”

(iii) (iii)

-nthal -nthalp p of of react reaction ion is is positi positive ve 4 endo endother thermic mic (1)

(iv) (iv)

oth oth for!a for!ard rd and and &ac!a &ac!ard rd rates rates chang changed ed 4 incre increase ased d (1)  & equal amount (same proportion) proportion) (1) allow one for “%a of forwar6 an6 bac4war6 reactions re6uce6 by an eual amount”

E

(&)

(i)

he reaction is e" e"othermic (1) High temperature gives a lo! equili&rium ield (1) Rate of reaction higher at higher temperature (1) 'n “euilibrium statement” nee6e6 eg low temp favours the reaction Do not allow answers base6 on cost of higher  temperature etc

(ii) (ii)

High Higher er press pressur uree giv gives es a high higher er iel ield d (1) 9 moles of gaseous reactant form 2 moles of gaseous product (1) Higher pressure generation or equipment is e"pensive to produce (1) %uilibrium statement reuire6 $ost factor  -7 -O. a safety answer 

D [1>]

20.

(a )

(i)

enthalp change !hen 1 mol of a su&stance (or compound) (5 mar) is (completel) &urned in o"gen (or reacted in e"cess o"gen) at 2E F and 1CC 8a (or under standard conditions)

1 1 1

(ii) (ii)

heat heat prod produce uced d < mass mass of !ater !ater B Sp Sp heat heat capaci capacit t x  x∆T (or mc∆T) < 1$CB9%1EBD9 (note if mass < 2%12 lose first 2 mars then conseq) < 9C1CC P or < 9C%1 P (allo! =%79C%2 must have correct units) moles methanol < mass4' r  < 2%124=2 (1) < C%CDD=

1

∆ 8 

1 1  @1

< @ 9C%14C%CDD= < @ DC$ P (mol )

(allo! @DC2 to @DCE or ans!er in P) (note allow conse mar!in" after all mista!es $ut note use of */1* " loses * mar!s

(&)

(i)

equili&rium shifts to left at high pressure  &ecause position of equili&rium moves to favour favour fe!er moles (of gas)

1 1

(ii) (ii)

at high high temp tempera eratur turee react reaction ion ield ield is is lo! lo! (or (or at at lo! lo! T ield is high) at lo! temperature reaction is slo! (or at high T reaction is fast) therefore use a &alance (or compromise) &et!een ra rate an and ield

1 1 1

1

(c)

Q

 < Σ∆ 8 c ∆ 8  < ∆ 8 c

Q

(reactants) @ Σ∆ 8 c (products) (or correct ccle)

Q

9

1

9

(/H=OH) < ∆ 8 c (/O) + 2 B ∆ 8 c (H2) @ ∆ 8  < (@2E=) + (2 B @2ED) @ (@1) (mar for previous equation or t his)

1

 @1

< @JD9 (P mol ) ( units not essential $ut lose mar! if units wron") (note : 43 scores 1<=)

1 [1(]

21.

(a )

Homogeneous6

ll reactants in the same phase or state (1)

*namic6 -quili&rium0 -quation6  K c6

(&)

/onditions0

/ontinuous or on7going (1) /oncentrations of reactants and products constant or rates of for!ard and &ac!ard reactions equal (1) 2.H=

 .2 + =H2 ('ust &e decomposition) (1) =

2

.2H2 4.H= (1) $

decomposition favoured & high temp  (1) since the reaction endothermic or logical statement !ith application of 5e /hateliers  principle (1) decomposition favoured & lo! pressure  (1) 2 mole gas giving 9 moles gas or more gas moles on right (1) 9

(c )

In practise

lo! pressure means lo! production  (1) lo! pressure means lo! rate  (1) high temperature means high rate  (1) high temperature e"pensive  (1)

/atalst

equili&rium ield unaffected  (1) rates of for!ard and &ac!ards reactions increased & an equal amount  (1) more hdrogen produced in a given ti me  (1) 'a" D [1(]

22.

(a)

I&%!#$# *& "#+#!"-!#

;ield is increased (A// * ! 2 () ! !-%"$) (1) Reaction endothermic (1) -quili&rium moves to the right OR for!ard# OR -quili&rium moves to oppose change  OR to a&sor& heat  (1) #f “?iel6 statement” incorrect allow ma= one if reaction state6 to be en6othermic

I&%!#$# *& !#$$-!#

;ield is decreased (A// * !  2 () ! !-%"$) (1) Increase in moles of gas ! 2 moles increased to 9 moles ! more moles on right (1) -quili&rium moves to the left OR &ac!ards# OR -quili&rium moves to oppose change  OR to reduce pressure  (1) #f “?iel6 statement” incorrect allow ma= one if number of moles change is correct

D

(&)

E-*/*!*-+ *#/ naffected ! equili&rium unchanged (1) Rate or speed increased (1) ?or!ard and &ac!ards reactions equall or & the same amount (1) A+-&"  !#& !-%# 'ore hdrogen produced (1)

9 [10]

23.

(a )

(&) (&)

(c) (c)

minimu imum energ (1) required &efore a reaction can occur or go or start (1)

2

spee speeds ds up (cha (chang nges es)) reac reacti tion on rate rate (1) !ithout &eing (chemicall) changed (-$# -) (1)

2

prov provid ides es alte altern rnat ativ ivee reac reactio tion n rout routee (1) !ith a lo!er activation energ (1) in b! an6 c! rewar6 > mar4s for > points wherever foun6

2

(d)

(i)

s t a r ts 2 B p la t e a u 0 7 a llo ! s ta r ts & e t ! e e n to p o f a " i s a n Rn R o f c o n c e n tr a tio n ( 1 ) fa lls to K e r o ! h e n T c u r v e l e v e ls (1 )

          n           o                i              t

>

          a           r              t           n           e           c           n           o                /

c u r v e s ta r t s a t o r ig i n # is l e f t o f o r ig i n a l ( 1 ) r e a c h e s s a m e p l a te a u ( 1 )

 im e c a n to u c h a"is

(ii)

(iii (iii))

fe!er e!er coll collis isio ion ns (1) > used up (1) or reactants or reagents or fewer particles

D [1*]