CHem

M10/4/CHEMI/HP2/ENG/TZ1/XX/M+

MARKSCHEME
May 2010

CHEMISTRY

Higher Level

Paper 2

18 pages

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M10/4/CHEMI/HP2/ENG/TZ1/XX/M+

This markscheme is confidential and for the exclusive use of examiners in this examination session.
It is the property of the International Baccalaureate and must not be reproduced or distributed to any other person without the authorization of IB Cardiff.

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Subject Details:

M10/4/CHEMI/HP2/ENG/TZ1/XX/M+

Chemistry HL Paper 2 Markscheme

Mark Allocation
Candidates are required to answer ALL questions in Section A [40 marks] and TWO questions in Section B
[2  25 marks]. Maximum total = [90 marks].
1.

A markscheme often has more marking points than the total allows. This is intentional. Do not award more than the maximum marks allowed for part of a question.

2.

Each marking point has a separate line and the end is signified by means of a semicolon (;).

3.

An alternative answer or wording is indicated in the markscheme by a slash (/) – either wording can be accepted. 4.

Words in brackets ( ) in the markscheme are not necessary to gain the mark.

5.

Words that are underlined are essential for the mark.

6.

The order of marking points does not have to be as in the markscheme, unless stated otherwise.

7.

If the candidate’s answer has the same “meaning” or can be clearly interpreted as being of equivalent significance, detail and validity as that in the markscheme then award the mark. Where this point is considered to be particularly relevant in a question it is emphasized by writing OWTTE (or words to that effect). 8.

Remember that many candidates are writing in a second language. Effective communication is more important than grammatical accuracy.

9.

Occasionally, a part of a question may require an answer that is required for subsequent marking points.
If an error is made in the first marking point then it should be penalized. However, if the incorrect answer is used correctly in subsequent marking points then follow through marks should be awarded. Indicate this with ECF (error carried forward).

10.

Only consider units at the end of a calculation. Unless directed otherwise in the markscheme, unit errors should only be penalized once in the paper. Indicate this by writing –1(U) at the first point it occurs and U on the cover page.

11.

Significant digits should only be considered in the final answer. Deduct 1 mark in the paper for an error of 2 or more digits unless directed otherwise in the markscheme.
e.g. if the answer is 1.63:
2
reject
1.6
accept
1.63
accept
1.631
accept
1.6314 reject
Indicate the mark deduction by writing –1(SD) at the first point it occurs and SD on the cover sheet.

12.

If a question specifically asks for the name of a substance, do not award a mark for a correct formula, similarly, if the formula is specifically asked for, do not award a mark for a correct name.

13.

If a question asks for an equation for a reaction, a balanced symbol equation is usually expected, do not award a mark for a word equation or an unbalanced equation unless directed otherwise in the markscheme.

14.

Ignore missing or incorrect state symbols in an equation unless directed otherwise in the markscheme.

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M10/4/CHEMI/HP2/ENG/TZ1/XX/M+

SECTION A
1.

(a)

(i)

Copper:
0 to +2 / increases by 2 / +2 / 2+;
Allow zero/nought for 0.
Nitrogen:
+5 to +4 / decreases by 1 / –1 / 1–;
Penalize missing + sign or incorrect notation such as 2+, 2+ or II, once only.

(ii)

nitric acid/HNO3 / NO3–/nitrate;
Allow nitrogen from nitric acid/nitrate but not just nitrogen.

[1]

(i)

0.100  0.0285 ;
2.85 10 3 (mol);
Award [2] for correct final answer.

[2]

(ii)

2.85 10 3 (mol);

[1]

(iii)

(b)

[2]

(63.55  2.85  10 3 ) = 0.181 g ;
Allow 63.5.

[1]

 0.181

100 =  39.7 % ;

 0.456


[1]

(iv)

(v)

(c)

 44.2  39.7

100   10/10.2 %;

44.2


Allow 11.3 % i.e. percentage obtained in (iv) is used to divide instead of 44.2 %.

(i)

1s22s22p63s23p63d9;
Do not allow [Ar]3d9.

(ii)

d orbitals are split;
(3d) electrons move between orbitals and absorb light/energy / complementary colour is transmitted when energy absorbed by d electrons moving;
Accept levels instead of orbitals.

[1]

[1]

[2]

–7–

2.

(a)

to maintain a constant volume / OWTTE;

(b)

(i)

M10/4/CHEMI/HP2/ENG/TZ1/XX/M+

(ii)

(c)

(d)

[H+] order 1, [CH3COCH3] order 1, [I2] order 0;
(rate = )k [H+] [CH3COCH3];
Award [2] for correct rate expression.
Allow expressions including [I2]0. neither were correct / Alex was right about propanone and wrong about iodine /
Hannah was right about propanone and hydrogen ions but wrong about iodine /
OWTTE;

[CH3COCH3] = 0.100 mol dm–3 and [H+] = 0.100 mol dm–3;
4.96  10 6 k  4.96 10 4 ;
(0.100  0.100) mol–1 dm3 s–1;
Ignore calculation of [I2].
No ECF here for incorrect units.

[1]

[2]

[1]

[3]

(i)

Number of particles

Ea (with catalyst)
Ea (no catalyst)

Energy axes correctly labelled x = energy/velocity/speed, y = number/% of molecules/particles/probability; graph showing correct curve for Maxwell-Boltzmann distribution;
If two curves are drawn, first and second marks can still be scored, but not third. Curve(s) must begin at origin and not go up at high energy.

two activation energies shown with Ecat shown lower;
Award the mark for the final point if shown on an enthalpy level diagram.
(ii)

[3]

catalyst provides an alternative pathway of lower energy / OWTTE;
Accept catalyst lowers activation energy (of reaction).

[1]

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3.

M10/4/CHEMI/HP2/ENG/TZ1/XX/M+

(a)
;

[1]

Brackets not required for mark.
Allow correct condensed structural formula.
Continuation bonds from each carbon are required.
Cl atoms can be above or below carbon spine or alternating above and below.
(b)

(c)

plastics are cheap/versatile/a large industry / plastics have many uses / OWTTE; plastics are not biodegradeable / plastics take up large amounts of space in landfill / pollution caused by burning of plastics / OWTTE;
Do not accept plastics cause litter.
Allow plastics don’t decompose quickly / OWTTE.
(i)

Step 1:
CH 2CHCl  H 2  CH 3CH 2Cl ;
Step 2:
CH3CH 2 Cl  OH   CH 3CH 2 OH  Cl ;
Accept NaOH or NaCl etc. instead of OH – and Cl –.
Allow abbreviated formulas C2H3Cl, C2H5Cl,C2H5OH.

(ii)

(i)

[2]

H2SO4/H+/acidified and Cr2O72–/(potassium/sodium) dichromate;
Accept suitable oxidizing agents (e.g. KMnO4 etc.) but only with acid.
Ignore missing or incorrect oxidation states in reagents.
(heat under) reflux;
Second mark can be scored even if reagent is incorrect.

(d)

[2]

[2]

CH 3COOH (aq)  H 2O (l)  CH 3COO  (aq)  H3O + (aq) ;
OR

CH 3COOH (l)  H 2 O (l)  CH3COO  (aq)  H 3O + (aq) ;
OR

CH 3COOH (aq)  CH 3COO  (aq)  H + (aq) ;
Must include  .
Ignore state symbols.
(ii)

Ka =10–4.76 / 1.74 105
[H + ]2
1.74 10 5 =
/ [H + ] = 0.00187 ;
0.200
pH = 2.73;
Award [3] for correct final answer, allow mark for correct conversion of [H+] to pH even if [H+] incorrect.

[1]

[3]

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(e)

M10/4/CHEMI/HP2/ENG/TZ1/XX/M+

(initial) [CH3COOH] = 0.500 mol dm–3 and) eqm [CH3COOH] = 0.200 mol dm–3;
(initial) [CH3COO–] = 0.300 mol dm–3 and) eqm [CH3COO–] = 0.300 mol dm–3;
Allow 0.02 moles and 0.03 moles instead of 0.200 and 0.300 mol dm–3.
[CH 3COOH]
[SALT]
;
= 1.16  10 5 mol dm  3 / pH  pK a  log

[ACID]
[CH 3COO ] pH = 4.94;
Award [3 max] for correct final answer if no working shown.
[H + ]= K a

(f)

[4]

(if acid added) CH 3COO   H   CH 3COOH ;
(if alkali added) CH 3COOH  OH   CH 3COO   H 2 O ;
Explanation marks cannot be awarded without equations.
Accept H +  OH   H 2O as OH – reacts with H+ in the buffer to form water.

[2]

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M10/4/CHEMI/HP2/ENG/TZ1/XX/M+

SECTION B
4.

(a)

(b)

(c)

average mass of isotopes of an element compared to (1/12 g of) 12C / average mass of an atom relative to C–12 having a mass of exactly 12 / OWTTE;
Allow element instead of atom.
Must refer to average mass and C–12.

[1]

Diagram of mass spectrometer containing in the correct sequence: vaporization/vaporized sample; ionization/electron gun; acceleration/oppositely charged plates; deflection/magnetic field; detection; Award [3] for 5 correct labels, [2] for 3–4 correct labels, [1] for 2 correct labels.
Award [1] for correct order for at least 4 correct labels.
Award [1] for diagram which must at least show ionization (e.g. electron beam), acceleration (e.g. charged plates) and deflection (e.g. magnetic field) even if these are incorrectly labelled.

[5]

63x + 65(1–x) = 63.55;
(or some other mathematical expression).
63

Cu = 72.5 % and 65Cu = 27.5 %;
Allow 63Cu =0. 725 and 65Cu =0. 275.
Award [2] for correct final answer .

(d)

60

Co/ 131I/125I;
Must contain correct mass numbers.
Allow other formats such as cobalt-60, Co-60 etc.
Award no marks if a correct radioisotope is given with an incorrect radioisotope.
Allow any other radioisotope if you can verify its use.

(e)

Sc has no d electrons as an ion / Cu has d electrons;
Cu compounds are coloured / Sc compounds are colourless;
Cu has more than one oxidation state / Sc has only one oxidation state;
Cu compounds can act as catalysts / Sc cannot act as catalysts;

(f)

2Na (s)  2H 2 O (l)  2NaOH (aq)  H 2 (g) / Na (s)  H 2 O (l)  NaOH (aq)  1 H 2 (g)
2
Award [1] for correct balanced equation.
Award [1] for correct state symbols for sodium, water, sodium hydroxide and hydrogen. Second mark is not dependent on equation being correctly balanced.

(g)

(Rb more reactive because) electron lost further from nucleus so less tightly held;
Rb electron is in 5th energy level and (Na less reactive) as electron lost in 3rd energy level / OWTTE;
Allow [1 max] for electron arrangements of Na (e.g. 2,8,1) and Rb if second mark is not scored.

[2]

[1]

[3 max]
[2]

[2]

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(h)

(i)

M10/4/CHEMI/HP2/ENG/TZ1/XX/M+

AlCl3 covalent and Al2O3 ionic;
AlCl3 (simple) molecular and Al2O3 (giant ionic) lattice;
OR

AlCl3 is covalent and simple molecular/small molecules held together by dipole–dipole attractions;
Al2O3 is ionic and Al3+ and O2– ions are held together in a (giant) lattice;
[2 max]
(ii)

AlCl3 is acidic and Na2O is basic and P4O10 is acidic;

AlCl3  3H 2 O  Al(OH)3  3HCl / AlCl3  6H 2 O   Al(H 2 O)6   3Cl – and
3

 Al(H 2O)6 

3

  Al(H 2 O)5OH   H  ;
2

Accept suitable alternative hydrolysis expressions or reactions with a base.
Na 2 O  H 2O  2NaOH ;
P4 O10 + 6H 2 O  4H 3 PO 4 ;
Accept suitable reactions with an acid or base.
(iii) AlCl3 does not conduct in the solid/molten state;
Al2O3 conducts when molten (but not when solid);
Al2O3 contains mobile ions when molten and AlCl3 has neither mobile ions nor delocalized electrons / OWTTE;
Reference to mobile ions or electrons must be made.

[4]

[3]

– 12 –
5.

(a)

(i)

(ii)

M10/4/CHEMI/HP2/ENG/TZ1/XX/M+

correct substitution of values and numbers of bonds broken /
(1 945)  (3  436) /2253; correct substitution of values and numbers of bonds made / (6  391) /2346; ëH = (sum of energies of bonds broken) – (sum of energies of bonds formed)
= (2253–2346) −93 (kJ);
Ignore units.
Award [3] for correct final answer.
Award [2 max] for +93 or 93.

[3]

entropy of products = 2  192  384 ; entropy of reactants = 193  (3 131)  586 ; ëS Ö (= sum of entropies of products) – (sum of entropies of reactants) /
(384–586) = –202 (J K–1 mol–1);
Award [3] for correct final answer.
Award [2 max] for +202 or 202.
Ignore units. negative as more ordered/less disordered / four moles become two moles / fewer molecules of gas;

[4]

(iii) (ëG Ö = ëH Ö  TëS Ö =  93  298(  0.202)) =  32.8 (kJ mol1 ) ;
(iv) reaction becomes less spontaneous; ëG becomes more positive/less negative / TëS becomes larger;
(b)

(c)

[1]

[2]

macroscopic properties remain constant / concentrations remain constant / no change to copper solution seen; rate of reverse/backwards reaction = rate of forward reaction;

[2]

(Kc=)

[NH 3 ] 2
[N 2 ][H 2 ]3

;

[1]

Do not award mark if [ ] missing or round brackets used.

(d)

(i)

(ii)
(e)

[H2] = 0.11 / 0.11 (mol dm–3);
[N2] = 0.17 / 0.17 (mol dm–3);
Kc = 16;
Ignore units.
Allow ECF from incorrect equilibrium expression and incorrect concentrations for third mark. decrease; heat is a product/reaction is exothermic so equilibrium moves to left / OWTTE;

yield increases / equilibrium moves to the right / more ammonia;
4 gas molecules → 2 / decrease in volume / fewer gas molecules on right hand side;

[3]

[2]

[2]

– 13 –
(f)

M10/4/CHEMI/HP2/ENG/TZ1/XX/M+

high pressure expensive / greater cost of operating at high pressure / reinforced pipes etc. needed;
Do not accept “high pressure is dangerous” without further explanation. lower temperature – greater yield, but lowers rate;
Do not award a mark just for the word “compromise”.

(g)

Kc unaffected; position of equilibrium unaffected; rate of forward and reverse reactions are increased (equally);

[2]

[3]

– 14 –
6.

(a)

M10/4/CHEMI/HP2/ENG/TZ1/XX/M+

(i) direction of electron flow
V

salt bridge
Mg (s)

Ag (s)

Mg2+ (aq)

Ag+ (aq)

correctly labelled electrodes and solutions; labelled salt bridge; voltmeter; Allow bulb or ammeter. direction of electron flow;
(ii)

[4]

Oxidation:
Mg (s)  Mg 2+ (aq)  2e  ;
Reduction:
Ag + (aq)  e   Ag (s);

[2]

Ignore state symbols.
Award [1 max] if equations not labelled reduction or oxidation or labelled the wrong way round.
Allow e instead of e–.
Penalize equilibrium sign or reversible arrows once only in parts (a) (ii) and (d) (ii).

(iii) +0.80 – (–2.37) = 3.17 V correct data; answer with unit;
Award [1] for –3.17 V or correct working of wrong values.
(b)

(i)

Cd/Cd (s) ;
Do not allow Cd2+.

(ii)

5Cd (s)  2MnO 4  (aq)  16H + (aq)  5Cd 2+ (aq)  2Mn 2+ (aq)  8H 2 O (l) correct reactants and products; correct balancing of this equation;
Ignore state symbols.

[2]

[1]

[2]

– 15 –
(c)

M10/4/CHEMI/HP2/ENG/TZ1/XX/M+

Accept suitable diagram with the following indicated:
Pt electrode;
[H+(aq)] = 1 mol dm–3/0.5 mol dm–3 H2SO4;

H2 gas; at 1 atm / 1.01105 Pa ;
Do not award mark for pressure if no hydrogen gas given.
298 K / 25  C ;
(d)

(i)

(ii)

[4 max]

sodium chloride crystals consist of ions in a (rigid) lattice / ions cannot move
(to electrodes) / OWTTE; when melted ions free to move / ions move when potential difference/voltage applied; [2]

positive sodium ions/Na+ move to the negative electrode/cathode and negative chloride ions/Cl– move to the positive electrode/anode; electrons are released to positive electrode/anode by negative ions and accepted from negative electrode/cathode by positive ions / reduction occurs at the negative electrode/cathode and oxidation occurs at the positive electrode/anode
/ Na+ ions are reduced and Cl– ions are oxidized;
(Positive electrode/anode):
2Cl  Cl2  2e  / Cl  1 Cl 2  e ;
2
(Negative electrode/cathode)
2Na +  2e  2Na / Na +  e  Na ;

[4]

Award [1 max] if equations not labelled or labelled wrong way round.
Allow e instead of e–.
Penalize equilibrium sign or reversible arrows once only in parts (a) (ii) and (d) (ii).

(iii) Products: oxygen at positive electrode and hydrogen at negative electrode; moles of Mg = 0.5 / mole ratio of O2:H2 is 1:2;
Can be implied by calculation.
 1 12.16

mass oxygen   
 32.00 =  8.00 g ;
 2 24.31

 12.16

mass hydrogen = 
 2.02 = 1.01 g ;
 24.31

Do not apply SD rule here.

[4]

– 16 –
7.

(a)

(b)

M10/4/CHEMI/HP2/ENG/TZ1/XX/M+

A: l-bromobutane;
B: 2-bromobutane;
C: 2-bromo-2-methylpropane;
D: 1-bromo-2-methylpropane;
Penalize incorrect punctuation, e.g. commas for hyphens, only once.
Accept 2-bromomethylpropane and 1-bromomethylpropane for C and D respectively.

(i)
(ii)

C/2-bromo-2-methylpropane; unimolecular nucleophilic substitution;

RBr → R+ + Br–;
Allow use of 2-bromo-2-methylpropane instead of RBr.

[4]

[2]
[1]

(iii) A/1-bromobutane/D/1-bromo-2-methylpropane;

curly arrow going from lone pair/negative charge on O in OH– to C;
Do not allow curly arrow originating on H in OH–. curly arrow showing Br leaving;
Accept curly arrow either going from bond between C and Br to Br in 1bromobutane or in the transition state. representation of transition state showing negative charge, square brackets and partial bonds;
Do not penalize if HO and Br are not at 180o to each other.
Do not award fourth mark if OH----C bond is represented.

[4]

– 17 –
(c)

(d)
(e)

M10/4/CHEMI/HP2/ENG/TZ1/XX/M+

(b) (i) no change as [OH–] does not appear in rate equation/in the rate determining step; (b) (iii) rate doubles as the rate is proportional to [OH–] / OH– appears in the ratedetermining/slow step / first order with respect to OH–;
Award [1] if correctly predicts no rate change for SN1 and doubling of rate for SN2 of without suitable explanation. rate of 1-bromobutane is faster;
C–Br bond is weaker/breaks more easily than C–Cl bond;

[2]

[2]

2-bromobutane/B;
(plane-) polarized light shone through; enantiomers rotate plane of plane-polarized light to left or right/opposite directions
(by same amount);
Accept “turn” instead of “rotate” but not “bend/reflect”. physical properties identical (apart from effect on plane-polarized light); chemical properties are identical (except with other chiral compounds);
Do not accept “similar” in place of “identical”.

[5]

– 18 –
(f)

(i)

M10/4/CHEMI/HP2/ENG/TZ1/XX/M+

elimination;

[1]

(ii)

curly arrow going from lone pair/negative charge on O in OH– to H on β-C;
Do not allow curly arrow originating on H in OH–.
Allow C2H5O– instead of OH–. curly arrow going from CH bond to form C=C bond; curly arrow showing Br leaving;
Accept the following for first 3 marks.

curly arrow showing Br leaving; representation of carbocation; curly arrow going from lone pair/negative charge on O in OH– to H on C adjacent to C+ and curly arrow going from CH bond to form C=C bond; two products formed: but-1-ene / but-2-ene/(cis) but-2-ene/(trans) but-2-ene;
Award [1] for two correct answers.

[4 max]