EdExcel a Level Chemistry Unit 4 Mark Scheme Results Paper 1 June 2007

Mark Scheme (Result s) Summer 2007 GCE

GCE Chemistry (6244) Paper 01

Edexcel Limited Registered in England and Wales No 4496750 Registered Office: One90 High Holborn, London W

General Guidance on Marking

Examiners should look f or qualit ies t o reward rat her t han f ault s t o penalise. This does NOT mean giving credit f or incorrect or inadequat e answers, but it does mean allowing candidat es t o be rewarded f or answers showing correct applicat ion of principles and knowledge. Examiners should t heref ore read caref ully and consider every response: even if it is not what is expect ed it may be wort hy of credit .

Crossed out work should be marked UNLESS the candidate has replaced it with an alternative response.

Using t he mark scheme The mark scheme gives you: an idea of t he t ypes of response expect ed how individual marks are t o be awarded t he t ot al mark f or each quest ion examples of responses t hat should NOT receive credit . Candidat es must make t heir meaning clear t o t he examiner t o gain t he mark. Make sure t hat t he answer makes sense. Do not give credit f or correct words/ phrases which are put t oget her in a meaningless manner. Answers must be in t he correct cont ext . 1 2 3 4 5

/ means t hat t he responses are alt ernat ives and eit her answer should receive f ull credit . ( ) means t hat a phrase/ word is not essent ial f or t he award of t he mark, but helps t he examiner t o get t he sense of t he expect ed answer. [ ] words inside square bracket s are inst ruct ions or guidance f or examiners. Phrases/ words in bold indicat e t hat t he meaning of t he phrase or t he act ual word is essential t o t he answer. ecf / TE/ cq (error carried f orward) means t hat a wrong answer given in an earlier part of a quest ion is used correct ly in answer t o a lat er part of t he same quest ion.

1.

(a)

✁

✆

602

elect ron af f init y (of O)

elect ron af f init y (of O)

( 150) ( 2186) ( 249) ( 657) LE

nd

✂

✂

✂

✂

✂

✂

✂

✄

☎

✄

(1)

Correct answer only wit h no working (1 max)

(LE =) -3844 (kJ mol ) (1)

-1

(LE =) -(+657)-(+249)-(+2186)-(+150)+(-602) (1)

OR

Hf

OR 1 and 2

st

H 3 1 plus 2

nd

(1)

H 2 (Ent halpy of ) at omisat ion (of Mg)

st

(1)

H 1 (Ent halpy of ) f ormat ion (of MgO)

EXPECTED ANSWER Mg2+(g) (+) O2- (g)

✁

(iii)

(ii)

(i)

6244/ 01

✁ ✂

1

Recognisable abbreviat ion such as “ EA” f or elect ron af f init y.

ACCEPT

any incorrect sign in algebraic expression (0)

Doubling elect ron af f init y and/ or at omisat ion values scores (0)

REJECT if st at e symbols missing If 2e included in box

(2)

(3)

(1)

MARK

(b)

(iii)

“ Charge densit y”

charge (on ions)

2

Theoret ical value based on purely/ 100 % ionic model

covalent charact er (st rengt hens lat t ice) (1)

Mark each aspect independent ly

Smaller OR more endot hermic OR Less negat ive OR Lower

Less (exot hermic) (1)

(1)

Dist ance bet ween ions OR Sum of (ionic) radii OR Type of cryst al st ruct ure OR Madelung const ant

Radius/ size (of ions) (1)

(ii)

“ Mg ion” “ I ion” OR “ iodine ion”

(i)

ACCEPT (Mg ) polarises (I ion) “ dist ort ion” if clearly linked t o t he iodide ion 2+

EXPECTED ANSWER The elect rons around t he iodide ion are drawn t owards t he magnesium ion

Any implicat ion of magnesium iodide being totally covalent

Higher/ great er

“ atomic radius”

Wrong polarisat ion e. g. “ magnesium ion is polarised” “ I polarises Mg2+”

REJECT Any ref erence t o at oms or molecules e. g. “ Mg polarises…. . ” “ iodine/ I/ I2 is polarised”

(2)

(2)

(1)

MARK

(c)

(i)

Mark each aspect independently

3

Is dissolved t o f orm an inf init ely dilut e solut ion OR Is dissolved in a large/ excess/ inf init e amount of wat er

Just “ hydrat ed” Just “ complet ely hydrat ed”

“ Added t o wat er” / ” react s wit h wat er” inst ead of “ dissolved”

IGNORE “ st andard condit ions”

REJECT Any implicat ion of an endot hermic process e. g. energy required

“ …. 1 mol of gaseous atoms”

ACCEPT Energy or heat

is dissolved such t hat f urt her dilut ion causes no f urt her heat change (1)

EXPECTED ANSWER Ent halpy change when 1 mol of gaseous ions (1)

(2)

MARK

(ii)

H LE ✁

✁

✂

✄

✁

H HYD ) (1)

( 2874)

✆

✞

☎ ✟

☞

✠

✟

( 2374) ( 1360)

✝

☞

☛

☞

✡

✠ ✡

✠

✟

✄

✁

(

H LE ✁

✁

✂

H HYD ) (1) st at ed in words or

so Hsoln less exot hermic/ more endot hermic/ more posit ive f or BaSO4 so less soluble OR so Hsoln more exot hermic/ more negat ive/ less endot hermic f or MgSO4 so MgSO4 more soluble (1)

symbols

H SOLN

(but ) lat t ice energies change less (1)

OR (bot h) lat t ice energies and hydrat ion ent halpies decrease f rom MgSO4 t o BaSO4 (or down group) (1)

Ent halpy of solut ion of MgSO4 less endot hermic/ more exot hermic/ more negat ive t han f or BaSO4, so MgSO4 more soluble t han BaSO4 (or reverse argument ) (1)

1014(kJ mol ) (1)

1

( 1920) ✆

H SOLN BaSO 4

954(kJ mol ) (1)

✆

1

H SOLN MgSO4

Expression quot ed or correct ly used in at least one of t he calculat ions below

EITHER H SOLN (

EXPECTED ANSWER

4

“ The hydrat ion energies decrease f ast er…. . ”

Answer only wit h no working (1)

Answer only wit h no working (1)

ACCEPT

✌

✌

(4)

MARK

(Total 17 marks)

Just “ solubilit y/ Hsolut ion depends on a balance bet ween lat t ice and hydrat ion energies”

(-)500 and (-)560 st at ed wit hout f urt her explanat ion

Just “ solubilit y/ Hsoln depends on a balance bet ween lat t ice and hydrat ion energies”

REJECT

✌

✌

2.

(a)

(1)

EXPECTED ANSWER

(1)

5

ACCEPT

Any compressed f ormulae e. g. CH3CONH2 OR

C2H5Br OR CH3CH2Br

REJECT Any compressed f ormulae e. g.

(2)

MARK

(b)

Step E: LiAlH4 (1)

Step D: P2O5 OR P4O10 (1)

Step C: PCl 5 OR SOCl 2 OR PCl 3 (1)

N. B. only award t he acid/ H2SO4 mark if a correct (or a near-miss) oxidising agent given

Step B: K2Cr 2O7 (1) and H2SO4 (1)

Step A: NH3 (1) Ignore st at e or dilut ion or solvent f or ammonia

EXPECTED ANSWER Accept names or formulae, but ignore correct or incorrect conditions

6

C2H5OH and Na OR H2 and Ni / Pt / Pd (cat alyst ) OR Lit hal

OR alkaline KMnO4 (1) then acidif y (accept any acid) (1)

Cr 2O72- and H+ (2) OR CrO42- and H+ (2) OR Acidif ied dichromat e ions (2) OR Acidif ied K2Cr 2O7 (2) OR acidif ied dichromat e((VI)) (1) OR KMnO4 (1) and H2 SO4 (1)

“ Hydrochloric acid” / ” HCl” inst ead of “ H2SO4”

ACCEPT

NaBH4 (0)

Incorrect oxidat ion number f or manganat e(VII)

Hydrochloric acid/ HCl wit h KMnO4 (0)

CrO42- alone (0)

Incorrect oxidat ion number f or dichromat e(VI)

REJECT

(6)

MARK

(d)

(c)

(CH2) 6

N

H

O C

H N

C

O (CH2) 4

(1)

C

O

✄

✞

✁

☎

✟

✆

CH3CH2NH2 + H2O + NaCl

CH3CH2 NH2 H2O

(1)

✆

OR CH3CH2NH3Cl + NaOH

OH

✁

CH3CH2 NH3

CH 3CH 2 NH 3

(1)

CH 3CH 2 NH 3 Cl as product

HCl as a react ant and

ACCEPT

7

CH3CH2NH3+ + NaOH CH3CH2NH2 + H2O + Na+ (1)

(charges are not required).

✁

Return of smell:

CH 3CH 2 NH 2

H

Rest of molecule and cont inuat ion bonds (1) 2nd mark is CQ on a polyamide being drawn

pept ide link

H

N

Loss of smell:

(ii)

(i)

EXPECTED ANSWER (subst it ut ed) ami de OR (N-subst it ut ed) amide OR secondary amide OR 2 amide

✂ ✠

✄

✝ ✄

CONH

REJECT Polyamide ami ne

(Total 13 marks)

(2)

(2)

(1)

MARK

3.

(a)

Middle of vert ical pH range bet ween 4 and 6 (1)

Vert ical range: at least t hree pH unit s in t he range 3 t o 8 e. g. pH range 3 t o 6 OR 3 t o 7 OR 3 t o 8 OR 4 t o 7 OR 4 t o 8 OR 5 t o 8 (1) (do not need t o st art / f inish on whole numbers)

Vert ical at 25 cm 3 (1)

St art ing pH ~ 1 and f inishing pH bet ween 9 and 11 (1)

Do not worry about general shape of t he curve, t he scoring point s are:

EXPECTED ANSWER

8

ACCEPT

pH range 3 t o 5

REJECT

(4)

MARK

(c)

(b)

9

No sharp/ clear/ precise end point OR very small range over which indicat or changes colour

f or a sharp colour change of indicat or (1)

[ If say ammonia is a st rong base or et hanoic acid is a st rong acid, or bot h, (0 out of 2) ]

Too small a vert ical (region) OR no vert ical (region) OR no point of inf lexion OR no sudden change in pH OR no st raight sect ion

More t han one indicat or f or ext ended vert ical regions

ACCEPT

pH change around equivalence point t oo small OR pH changes over t oo big a volume (1)

EXPECTED ANSWER Bromocresol green Indicat or(s) CQ on graph [ check t able on quest ion paper]

No suit able indicat or OR No “ easy” colour change

REJECT

(Total 7 marks)

(2)

(1)

MARK

4.

(b) (c)

(a)

C

O

C

H

H

H

C

H H

EXPECTED ANSWER

0.50

2.50

0.485

0.485 3.75

2.50 0.485

0.485

[ HCOOC2 H 5 ][ H 2O] [ HCOOH ][C2 H 5OH ]

✁

✁

No, (as) equal numbers of moles on bot h sides OR volumes cancel OR mol dm -3 cancel OR unit s cancel OR crossing out unit s t o show t hey cancel

3.33 only scores

answer K c 0. 647 mol -1dm 3

If [ H 2 O] omit t ed in (ii), t hen

(2) but t his will give K c 1. 33 mol -1dm 3 wit h V

10

Mark CQ on K c expression in (ii)

“ equal powers/ moles on bot h sides” OR “ powers cancel”

omit t ed f rom calculat ion (1)

✄

IGNORE sig. f igs.

(2.50) 2 0.50 3.75

ACCEPT

(2) if it is st at ed t hat V cancels eit her here or in (iv)

Kc

✄

= 3. 33 (1) stand alone mark

(1) Must have clearly divided moles of each component by 0. 485 f or 1st mark e. g. [ HCOOC2H5] = [ H2O] = 5. 16 (mol dm -3) and [ HCOOH] = 1. 03 (mol dm -3) and [ C2H5OH] = 7. 73 (mol dm -3)

Kc

Kc

✄

(iv)

(iii)

(ii)

Moles: HCOOC2H5 : 2. 50 and moles H2O : 2. 50 (1) for both

est er (i) Moles: C2H5OH: 3. 75 (1)

H

O

✂

“ concent rat ions cancel ”

1st mark if 485 used as V in expression

Obviously round bracket s “ ( )”

REJECT

(1)

(2)

(1)

(2)

(1)

(1)

MARK

(d)

(ii)

(i)

“ no ef f ect ” can be st at ed or implied IGNORE any ref erences t o act ivat ion energy

no ef f ect as cat alyst s do not af f ect (t he value of ) K OR no ef f ect as cat alyst s do not af f ect t he posit ion of equilibrium OR no ef f ect as cat alyst s do not af f ect t he yield OR No ef f ect as cat alyst s increase t he rat e of t he f orward and backward react ions equally/ t o t he same ext ent OR no ef f ect as cat alyst s only increase t he rat e OR no ef f ect as cat alyst s only alt er t he rat e

yield of HCOOC2H5 decreases (1)

11

If st at e exot hermic in f orward direct ion, 1 mark only (out of 4) f or CQ “ increase in Kc”

Kc decreases (1)

numerat or in quot ient (has t o) decrease OR denominat or in quot ient (has t o) increase OR f ract ion (has t o) decrease (1)

ACCEPT Exot hermic in backward direct ion (or words t o t hat ef f ect )

EXPECTED ANSWER (as react ion) endot hermic (1)

Just “ cat alyst s increase rat e”

REJECT

(Total 13 marks)

(1)

(4)

MARK

5.

3H 2 O

6H 2 O

2 H 3 PO4

4 H 3 PO 4

IGNORE “ across group” if used inst ead of “ across period”

met allic charact er (of t he element s) decreases (1)

basic (oxides) t o acidic (oxides) (1) both words needed IGNORE ref erences t o Al 2O3 IGNORE ref erences t o amphot eric charact er of Al 2O3

☎

(c)

✄

covalent

✆

(ii)

P2 O5

OR

2 Na ✄

2OH

4NaOH

12

met al t o non-met al

‘ molecular covalent ’ ‘ simple covalent ’

Giant ionic OR elect rovalent Mult iples

Mult iples e. g. 2Na2O + 2H2O

..

.. 2 Na OH OR ✂

P4 O10

ionic

(ii)

2NaOH ✂

(i)

Na2O + H2O

(i)

ACCEPT

✁

(b)

(a)

EXPECTED ANSWER IGNORE st at e symbols t hroughout t his quest ion.

✁

The elements change f rom basic t o acidic

‘ convalent ’ OR ‘ giant covalent ’ OR dat ive covalent

REJECT

(2)

(1)

(1)

(1)

(1)

MARK

(d)

H 2O

☎

☎

✞

✝

✆

✆ ✟

✠

✝

✡

✡

✟

✡

✡

Na2PbO2 + H2O (1)

✠

OR PbO + 2NaOH

✄

Na 2 Pb(OH ) 4

☎

”

13

OR Na 4 Pb(OH ) 6

f ormat ion of Pb(OH ) 6 4

” inst ead of “

H 2 CO3 ☎

H 2O

Pb(OH ) 4 2

H 2 O (1)

“

CO32-

H 2 CO3 ☎

OR PbO 2 NaOH

H 2O

✆

PbO 2OH

✆

Pb( NO3 ) 2

✞

OR PbO 2 HNO3

H 2O

H 2O

HCO3OR CO 2 H 2 O

OR CO 2

☎

Pb 2

HCO3-

HCO3

☎

PbO 2 H

✂

H3O+ +

H ✁

OR CO2 + 2H2O

CO 2

ACCEPT 2H++ CO32CO2 + H2O ☎

(ii)

(i)

EXPECTED ANSWER

2H +

+

H+ +

H 2 CO3

Any equat ions wit h PbO2

PbO wit h ot her acids

JUST CO 2 H 2 O

REJECT

☎

☛

✄

(2)

(1)

MARK

(e)

[ N. B. Marking cannot allow point s t aken f rom both t he EITHER and OR argument s t oget her]

(so) more easily lose elect rons OR have lower ionisat ion energies OR more easily f orm posit ive ions (in compounds) (1) [ allow 2nd mark even if no specif ic ref erence has been made t o at oms]

OR atoms become larger (1)

EITHER oxides acidic at t he t op of t he group (1) amphot eric at bot t om of group (1) [ amphot eric must be st at ed f or t he 2nd mark]

Increases as

EXPECTED ANSWER

14

atoms have more shielding/ shells/ energy levels OR “ (out er) elect rons f urt her f rom nucleus” [ no need t o ref er t o at oms in this case]

Oxides more basic down group 1 (out of 2) OR oxides less acidic down group 1 (out of 2)

ACCEPT

OR “ t he element s become larger”

(2)

MARK

(Total 11 marks)

Just “ more elect rons”

REJECT Just “ increases” on it s own (0) OR “ increases” f ollowed by incorrect j ust if icat ion (0)

6.

(a)

(i)

Reduct ion OR nucleophilic addit ion (1) IGNORE het erolyt ic

EXPECTED ANSWER CH 3 CH 2 CH 2 OH (1)

15

Redox “ Nucleophilic reduct ion”

H H H

H C C C O H

H H H

OR

H H H

H C C C OH

H H H

OR f ull st ruct ural f ormula e. g.

C 2 H 5 CH 2 OH

ACCEPT C 3 H 7 OH

REJECT

(2)

MARK

(ii)

OH

oxidat ion OR redox (1)

CH3CH2C

O

(1)

EXPECTED ANSWER

OH

O H

16

“ oxidisat ion”

H H

O

OH

O

H C C C

H H

OR

H H

H C C C

H H

C2H5 inst ead of CH3CH2 OR f ull st ruct ural f ormula e. g.

OR CH3CH2COOH OR CH3CH2CO2H OR C2H5COOH OR C2H5CO2H

C2H5C

ACCEPT O

C 2H 5C

OR

O-

O

CH3CH2C O-

O

REJECT

(2)

MARK

(b)

N (1)

C 2 H 5 MgBr

Dry et hoxyet hane OR dry et her IGNORE ref erences t o I2 OR heat

IGNORE charges

Mg C 2 H 5 Br

nucleophilic addit ion (1) both words needed IGNORE “ het erolyt ic”

CH 3 CH 2 CH (OH )C

✂

(ii)

(i)

(iii)

EXPECTED ANSWER

✁

H

17

H H H

H C C C C N

H H O

OR

H H H

H C C C CN

H H OH

OR f ull st ruct ural f ormula e. g.

OR CH 3CH 2CH (OH )CN

ACCEPT C2 H 5CH (OH )CN

C2H5BrMg C2H5MgI

REJECT

(1)

(1)

(2)

MARK

(c)

(i)

IGNORE punct uat ion

Name: 2-met hylbut an-2-ol OR 2-hydroxy-2-met hylbut ane (1)

EXPECTED ANSWER With propanone: Structure : (CH3) 2C(OH)CH2CH3 (1)

18

CQ name provided t hat it is a t ert iary alcohol

CQ st ruct ure provided it is a t ert iary alcohol

OR

C2H5 in lieu of CH2CH3 OR t he f ull st ruct ural f ormula e. g.

ACCEPT

C 5 H 11OH

REJECT

(2)

MARK

(ii)

C

O

(group in but anal) is planar (1)

Mark each aspect independently unless reference made to carbocations etc

at t acked (wit h equal probabilit y) f rom t wo direct ions (1)

H

Name: hexan-3-ol (1)

CH 3 CH 2 CH 2 CH (OH )CH 2 CH 3 (1)

EXPECTED ANSWER With butanal: Structure :

19

hexane-3-ol CQ name provided t hat it is a secondary alcohol

CQ st ruct ure provided it is a secondary alcohol

H H H O H H H

H C C C C C C H

H H H H H H

OR

H H H OH H H

H C C C C C C H

H H H H H H

ACCEPT t he f ull st ruct ural f ormula e. g.

(2)

(2)

MARK

Ref erences t o carbocat ions OR carbonium ions OR Planar int ermediat es OR SN1 mechanisms scores (0 out of 2) (Total 14 marks)

But anal/ t he molecule/ it is planar OR But anal/ t he molecule/ it is linear

C 6 H 13 OH

REJECT