Edexcel A-Level Chemistry AS 8 Redox Chemistry and Groups 1 2 And 7 Questions

D

(b)(iv)

An answer that makes reference to the following point:

- +4 / 4

Allow (+IV)

Do not award -4

(b)(iii)

An answer that makes reference to the following point:

- the Roman numeral is the charge the chlorine would have if the species were fully ionic / is the oxidation number of the chlorine (in the ions)

OR the oxidation number of chlorine in chlorate( V ) is ( + )5 and in chlorate(VII) is (+)7

Allow is the oxidation number of chlorine in the compound Ignore just is the charge on the chlorine Do not award is the oxidation number of chlorine in the molecule

Allow valency of chlorine in chlorate(V) is (+)5 and in chlorate(VII) is (+)7

Allow chlorine is +5 in chlorate(V) and +7 in chlorate(VII)

Do not award the charge on chlorate(V) is +5 and on chlorate(VII) is +7

An explanation that makes reference to

Ignore explanations in terms of electron loss/gain

the following points:
- calcium/Ca oxidised
and
from 0 (in Ca ) to (+)2 (in $\mathrm{CaCl}_{2}$ )

- chlorine/ $\mathrm{Cl}_{2} / \mathrm{Cl}$ reduced
and
from 0 (in $\mathrm{Cl}_{2}$ ) to -1 (in $\mathrm{CaCl}_{2}$ )

The only correct answer is A (gains electrons and decreases in oxidation number)
B is incorrect because oxidising agents are reduced during a reaction so there is a decrease in oxidation number
C is incorrect because oxidising agents are reduced during a reaction so they gain electrons
D is incorrect because oxidising agents are reduced during a reaction so they gain electrons and there is a decrease in oxidation number

(b)(v)

An answer that makes reference to the following points:

- either (+)5 or (+)6

- because it must be between (+)4 and (+)7

(1)

(1)

Allow must reduce to (+)4 and oxidise to (+)7
Chlorine goes from +5 to +4 and +7 scores M2
Allow TE on answers in (b)(i) and (b)(ii)
throughout

B

species

- balancing

(1)

Example of equation
$$\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+14 \mathrm{H}^{+}+6 \mathrm{e}^{-} \rightarrow 2 \mathrm{Cr}^{3+}+7 \mathrm{H}_{2} \mathrm{O}$$

M2 dependent on M1

Ignore state symbols even if incorrect

(2)

B

Indicative points:

- IP1: $\mathrm{Ca}((\mathrm{s}))+2 \mathrm{H}_{2} \mathrm{O}((\mathrm{l})) \rightarrow \mathrm{Ca}(\mathrm{OH})_{2}((\mathrm{aq}))+\mathrm{H}_{2}((\mathrm{~g}))$

- IP2: effervescence

- IP3: (indicator) turns blue

- IP4:

faster / more vigourous reaction with calcium and

linked to an observation

OR

no (observable) reaction with magnesium

- IP5:

Ca has lower ionisation energy

OR

Ca has more shielding / more shells / larger atoms

- IP6:

darker blue / more alkaline / higher pH with Ca and

linked to greater solubility of $\mathrm{Ca}(\mathrm{OH})_{2}$

Ignore any reference to C a / M g dissolving

Allow multiples

Ignore state symbols

Ignore equations involving magnesium

Allow fizzing/bubbling

Allow turns purple

If stated, initial colour must be green/yellow Ignore forms solution with pH>7

Accept reverse arguments

eg reaction with calcium:

- has faster bubbling

- changes colour faster

- forms (white) ppt faster

- gets hotter (Ignore more exothermic)

- (solid) disappears faster

Ignore (indicator) remains green with Mg Ignore no (white) ppt with Mg

Accept reverse arguments

Allow Ca has less attraction on (outer) $\mathrm{e}^{(-)}$

Allow Ca loses electron(s) more easily Ignore calcium ions/ $\mathrm{Ca}^{2+}$ for calcium/Ca

Accept reverse arguments

Ignore less (white) ppt with Ca

Allow hydroxide solubility increases down group Allow $\mathrm{Mg}(\mathrm{OH})_{2}$ insoluble

Ignore explanations of solubility

An answer that makes reference to the following points:

- observation

(1)

- product

(1)

Solid dissolves / forms a colourless solution Allow solid disappears

Allow gets warmer

Allow steam given off

Allow sizzling sound Ignore bubbles/effervescence/fizzing

Strontium hydroxide / Sr(OH)2 If name and formula given both must be correct Do not award hydrogen / oxygen

Marks are independent

An answer that makes reference to the following points: (Sodium hydroxide) - no change / no reaction / colourless solution (1) (Sulfuric acid) - white and precipitate (1) TE for both marks on 1(c) for incorrect group 2 / group 1 cation Allow (white) precipitate does not form Ignore nothing / no observation Ignore just colourless / remains clear Ignore solid dissolves Do not award white solution Do not award effervescence / gas evolved Allow ppt(e)/solid/crystals for precipitate Ignore name or formula of precipitate Do not award effervescence / gas evolved

An answer that makes reference to the following points:

- the temperature of decomposition of carbonates / thermal stability increases (down the group)

- the size of the cation increases (but has the same charge)

- (so) polarises the anion / carbonate (ion) less

(1)

Ignore references to rate Ignore "decomposition decreases down the group"

Allow amount of heat energy needed to decompose increases down the group

Accept reference to the metal ion size/radius Allow charge density of (cat)ion decreases Ignore "ion size decreases"

Do not award atomic radius

Orange-red flame colour. Accept brick-red, red, orange, yellow-red or yellow-orange;
ignore just yellow.

An answer that makes reference to the following points:

(formula)

- $\mathrm{SO}_{4}{ }^{2-}$

(1)

(positive test result)

- white precipitate / white solid

(1)

Mark independently

Ignore name

Allow ionic equations that include $\mathrm{SO}_{4}{ }^{2-}$

Do not award other ionic equations

Allow white ppt(e)

Do not award if other chemical tests are mentioned If the precipitate is given as AgCl do not award this mark

A description that makes reference to the following points:

- add the acid (quickly) until just short of the rough end-point

- add the acid drop-by-drop

- with swirling

and

until the indicator colour (just) changes

Accept within $1-4 \mathrm{~cm}^{3}$ (any value in this range) before the rough titre

Allow to a value in the range $19.4-22.4 \mathrm{~cm}^{3}$

Allow 'until close to the rough value' Ignore 'carbonate added'

Do not award 'until the rough value is reached'

Standalone mark (award even if M1 not given)

Allow 'dropwise'

Allow 'a few drops at a time'

Ignore 'add very slowly'

Clip with

(b)

An answer that makes reference to one of the following points

- All the acid/reactant/solid/solution/substance weighed out should be added / transferred (to the flask) (1)

Or

none of the acid/reactant/solid/solution/substance weighed out/solution should be left behind (in the beaker)

Or

the solution remaining in the beaker will contain some dissolved ethanedioic acid/ (if washings not added) the solution concentration will be lower

Or

to ensure the amount of acid in the solution is known accurately

Allow"so that all of the solution is transferred"

Ignore just "transfer losses" just "to ensure accuracy"

An explanation that makes reference to the following points:

- elements become less electronegative (down the group)

- the outer electron(s)/shell is further away from the (positive) nucleus (so the bonding electrons are less strongly attracted)

Allow decreases Allow descriptions using names of elements

Allow increased repulsion (from inner electrons) Allow more shielding (by inner electrons) Allow more (inner) shells of electrons Allow effective nuclear charge is lower Ignore other comments about nuclear charge Ignore size/radius of atom increases

Allow reverse argument Marks are independent

This question assesses the student's ability to show a coherent and logically structured answer with linkages and fully sustained reasoning.

Marks are awarded for indicative content and for how the answer is structured and shows lines of reasoning.

The following table shows how the marks should be awarded for indicative content.

Number of indicative marking

points seen in answer

Number of marks awarded for

indicative marking points

6

5-4

3-2

1

0

\end{tabular}

The following table shows how the marks should be awarded for structure and lines of reasoning

\end{tabular}

Guidance on how the mark scheme should be applied.

The mark for indicative content should be added to the mark for lines of reasoning. For example, a response with five indicative marking points that is partially structured with some linkages and lines of reasoning scores 4 marks (3 marks for indicative content and 1 mark for partial structure and some linkages and lines of reasoning).

If there were no linkages between the points, then the same indicative marking points would yield an overall score of 3 marks (3 marks for indicative content and no marks for linkages).

In general it would be expected that 5 or 6 indicative points would get 2 reasoning marks, and 3 or 4 indicative points would get 1 mark for reasoning, and 0, 1 or 2 indicative points would score zero marks for reasoning.

If there is any incorrect chemistry, deduct mark(s) from the reasoning. If no reasoning mark(s) awarded do not deduct mark(s).

Comment: Look for the indicative marking points first, then consider the mark for the structure of the answer and sustained line of reasoning.

Indicative content:

IP1 all are disproportionation reactions (because chlorine is both oxidised and reduced)

IP2 chlorine changes/is reduced from 0 to -1 (in $\mathrm{Cl}^{-}$ in all reactions)
$$\text { IP3 } \mathrm{Cl}_{2}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{HCl}+\mathrm{HOCl}$$

IP4 $\mathrm{Cl}_{2}+2 \mathrm{NaOH} \rightarrow \mathrm{NaClO}+\mathrm{NaCl}+\mathrm{H}_{2} \mathrm{O}$$$\text { IP5 } 3 \mathrm{Cl}_{2}+6 \mathrm{NaOH} \rightarrow \mathrm{NaClO}_{3}+5 \mathrm{NaCl}+3 \mathrm{H}_{2} \mathrm{O}$$

IP6 chlorine oxidised to +1,+1 and +5 shown in the appropriate reaction

Allow for a description, including the name, of disproportionation if a reaction type is not given for all three equations Allow disproportional/disproportion reaction

Allow $\mathrm{Cl}_{2}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{H}^{+}+\mathrm{Cl}^{-}+\mathrm{HOCl}$

Allow single arrow instead of equilibrium sign Allow HClO or ClOH

Allow $\mathrm{Cl}_{2}+2 \mathrm{OH}^{-} \rightarrow \mathrm{ClO}^{-}+\mathrm{Cl}^{-}+\mathrm{H}_{2} \mathrm{O}$$$\text { Allow } 3 \mathrm{Cl}_{2}+6 \mathrm{OH}^{-} \rightarrow \mathrm{ClO}_{3}^{-}+5 \mathrm{Cl}^{-}+3 \mathrm{H}_{2} \mathrm{O}$$

Allow oxidation numbers shown next to a combination of correct names and formulae even if the equations are not balanced

Ignore state symbols even if incorrect

An answer that makes reference to four of the following points:

Any two from:

(Similarities)

- both produce hydrogen halides

- formation of hydrogen halides is protonation

(1)

- both give off misty/steamy/white fumes

(1)

Any two from:

(Differences)

- only the reaction of sodium bromide is redox / bromide ions are stronger reducing agents (than chloride ions)

(1)

- (red-) brown fumes / gas / liquid produced with sodium bromide

(1)

- sulfur dioxide (colourless choking gas) / bromine given off with sodium bromide

(1)

Both similarities and differences can be shown in equations or with observations

Accept NaX+H_2 SO_4 HX+NaHSO_4

Allow 2 NaX+H_2 SO_4 2 HX+Na_2 SO_4

Do not award white smoke

Do not award rotten egg smell / references to yellow solid

Accept 2 HBr+H_2 SO_4 Br_2+SO_2+2 H_2 O

Apply list principle

An answer that makes reference to the following points:

- (dark) grey / black

and solid

Ignore shiny / metallic