A-level Chemistry Mark scheme Unit 02 - Chemistry in Action June 2016

AQA Qualifications

A-LEVEL Chemistry

CHEM2 Chemistry in Action Mark scheme

2420 June 2016

Version: 1.0 Final

Mark schemes are prepared by the Lead Assessment Writer and considered, together with the relevant questions, by a panel of subject teachers. This mark scheme includes any amendments made at the standardisation events which all associates participate in and is the scheme which was used by them in this examination. The standardisation process ensures that the mark scheme covers the students' responses to questions and that every associate understands and applies it in the same correct way. As preparation for standardisation each associate analyses a number of students' scripts: alternative answers not already covered by the mark scheme are discussed and legislated for. If, after the standardisation process, associates encounter unusual answers which have not been raised they are required to refer these to the Lead Assessment Writer. It must be stressed that a mark scheme is a working document, in many cases further developed and expanded on the basis of students' reactions to a particular paper. Assumptions about future mark schemes on the basis of one year's document should be avoided; whilst the guiding principles of assessment remain constant, details will change, depending on the content of a particular examination paper. Further copies of this Mark Scheme are available from .uk

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MARK SCHEME ? A-LEVEL CHEMISTRY ? CHEM2 ? JUNE 2016

Question

Answers

more (electron) shells / (outer) electrons further from the nucleus / larger

1a)

atoms / more shielding

so weaker attraction of nucleus/protons for (outer) electrons

White solid / white ash

1b) Bright light / white light

Mg + H2O MgO + H2

1c) Total

BaSO4 is insoluble but Ba(NO3)2 is soluble OR BaSO4 precipitates but Ba(NO3)2product(s) of second reaction is soluble/remains in solution OR BaSO4 is insoluble but no reaction occurs in second case

Ba2+(aq) + SO42-(aq) BaSO4(s)

Mark 1

Additional Comments/Guidance

If `molecules' mentioned CE = 0 It = Ba Mark independently ALLOW energy levels for shells Both ideas must be comparative

1

NOT hold/pull/bonded for `attraction'

idea of nucleus or protons must be clear

ALLOW M2 if electrons implied from mention in M1

ALLOW converse if it is clear that answer refers to Ca

1

ALLOW `white smoke/powder'

IGNORE `product'

NOT ppt

IGNORE fumes

IGNORE tube/glass goes black

1

ALLOW glow/flame for light

1

IGNORE state symbols

1

NOT just `no observation' in second case

Comparison of solubilities must be implied

NOT Barium is soluble/insoluble Correct state symbols required 1

7

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MARK SCHEME ? A-LEVEL CHEMISTRY ? CHEM2 ? JUNE 2016

Question Answers 2a)

The enthalpy / heat energy change when 1 mol (of a substance)

is burned/reacts completely in oxygen

with all reactants and products in their standard states OR With all reactants and products in their normal states at 298K/given temp & 100kPa

2b) H = Hc(reactants) - Hc (products) OR correctly and fully balanced cycle

Mark Additional Comments/Guidance If enthalpy of formation definition given CE=O

1

NOT just `energy'

ALLOW alternatives for substance e.g.

molecule/compound/element

1

ALLOW reacts in excess oxygen

1

ALLOW `everything' for `reactants and products'

Penalise incorrect conditions if given ALLOW `normal states under standard conditions' Correct answer scores 3

1

H = [3(-394) + 4(-286)] ? (-2010) OR H = -2326 + 2010

H = -316 (kJ mol-1)

+316 scores 1 mark only

1

M2 also scores M1

1

IGNORE units

Check for AE in working ? can award M3 as ecf

(error carried forward) from M2 if M2 not given due

to AE

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MARK SCHEME ? A-LEVEL CHEMISTRY ? CHEM2 ? JUNE 2016

2c) H/-1893 = B(reactants) - B(products) OR

Correct answer gains 3 marks 1

H/-1893 = Bonds broken - Bonds formed

OR

H/-1893 = 2B(C-C) + 7B(C-H) + B(C-O) + B(O-H) + 4? B(O=O) ? 6B(C=O) ? 8B(O-H)

-1893= 2B(C-C) + 7(412) + 360 + 463 + 4?(496) ? 6(805) ? 8(463) OR -1893= 2B(C-C) +5939 ? 8534 OR -1893= 2B(C-C) -2595 OR 2B(C-C) = 702 B(C-C) = (+)351(kJ mol-1)

Total

1

M2 also scores M1

May see no 463 in bonds broken and 7x463 in

made (gives 5476 ? 8071)

1

If NOT 351 check for AE. This would lose M2,

but could gain M1 and M3

(+)234 scores 1 (due to 3(C-C))

NOT M3 from incorrect M2 unless incorrect M2 is due to AE IGNORE Units If no other mark awarded then ALLOW 1 if 5939 or 5476 or 8534 or 8071 seen 9

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