H2 Chem - Inorganic Chemistry
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Hi UltimaOnline,
I have a couple of questions on inorganic chemistry:
Which of the following is true?
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BF4- is able to act as a ligand.
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Aqueous bromine and aqueous chlorine can be distinguished by adding iron (II) sulphate, followed by aqueous sodium hydroxide to separate volumes of aqueous bromine and aqueous chlorine.
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When AgNO3(aq) is mixed with FeCl2(aq), a grey precipitate is formed, which does not dissolve in NH3(aq).
Answer: 1, 2 and 3
Remarks: For 1, how is BF4- ion able to act as a ligand despite it not having lone pair of electrons? For 2, what happens? I suppose there could be a redox reaction between iron (II) and bromide/chloride ions, but how is this a way to distinguish between the halogens? For 3, what is the chemistry involved in this reaction? Isn’t AgCl of cream colour, which dissolves in ammonia?
2. [DHS Prelim 2012]
S is a transition element. The 3d sub–shell of S in the compound K[S(C2O4)2(NH3)2] contains 3 electrons. How many unpaired electrons does S contain when it is in the elemental state?
A: 2 B: 3 C: 4 D: 5
Answer: C
Remarks: I thought the answer is 6: from the compound, I gather that the charge on S is 3+. Hence, during the ionisation, I would think that 1 4s electron and 2 3d electrons will be taken away. As electronic configuration of S is [Ar]3d54s1 prior to ionisation, wouldn’t there be 6 unpaired electrons in the elemental state of S?
3. http://img.photobucket.com/albums/v700/gohby/Chemistry/trichloro_zpsb0anyptf.jpg
Answer: B
Remarks: I’d like to confirm if my understanding is correct. Firstly, sodium chloride and aqueous bromine will be formed. When I add excess concentrated sodium hydroxide, bromine will disproportionate (assuming that the reaction is carried at temperatures above 15 degrees) to Br- and BrO3-. I am assuming BrO3- to be colourless. As trichloromethane is denser than water it will form a colourless layer below all the ions.
Thank you! :)
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Hmm.. for some reason the formatting goes haywire even though everything looks alright when I clicked on "edit"
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Originally posted by gohby:
Hi UltimaOnline,
I have a couple of questions on inorganic chemistry:
Which of the following is true?
-
BF4- is able to act as a ligand.
-
Aqueous bromine and aqueous chlorine can be distinguished by adding iron (II) sulphate, followed by aqueous sodium hydroxide to separate volumes of aqueous bromine and aqueous chlorine.
-
When AgNO3(aq) is mixed with FeCl2(aq), a grey precipitate is formed, which does not dissolve in NH3(aq).
Answer: 1, 2 and 3
Remarks: For 1, how is BF4- ion able to act as a ligand despite it not having lone pair of electrons? For 2, what happens? I suppose there could be a redox reaction between iron (II) and bromide/chloride ions, but how is this a way to distinguish between the halogens? For 3, what is the chemistry involved in this reaction? Isn’t AgCl of cream colour, which dissolves in ammonia?
2. [DHS Prelim 2012]
S is a transition element. The 3d sub–shell of S in the compound K[S(C2O4)2(NH3)2] contains 3 electrons. How many unpaired electrons does S contain when it is in the elemental state?
A: 2 B: 3 C: 4 D: 5
Answer: C
Remarks: I thought the answer is 6: from the compound, I gather that the charge on S is 3+. Hence, during the ionisation, I would think that 1 4s electron and 2 3d electrons will be taken away. As electronic configuration of S is [Ar]3d54s1 prior to ionisation, wouldn’t there be 6 unpaired electrons in the elemental state of S?
3. http://img.photobucket.com/albums/v700/gohby/Chemistry/trichloro_zpsb0anyptf.jpg
Answer: B
Remarks: I’d like to confirm if my understanding is correct. Firstly, sodium chloride and aqueous bromine will be formed. When I add excess concentrated sodium hydroxide, bromine will disproportionate (assuming that the reaction is carried at temperatures above 15 degrees) to Br- and BrO3-. I am assuming BrO3- to be colourless. As trichloromethane is denser than water it will form a colourless layer below all the ions.
Thank you! :)
Hi Gohby,
Q1. BF4- cannot act as a ligand, but it can function as a source of a F- ligand. (Qn is phrased ambiguously, so qn's fault). If molarities are less than standard molarities, only Cl2, but not Br2, is strong enough an oxidizing agent to oxidize Fe2+ to Fe3+, which you can confirm by the colour of the Fe(OH)3(s). The grey ppt isn't AgCl (which is white, btw), but Ag(s).
Q2. Ignoring the dative bonds from the ligands, since the 3d subshell of the metal ion contains 3 electrons, and it's OS is +3, thus the metal ion's 4s orbital is vacant. Hence reducing the metal ion to it's elemental atomic form, we add 3 electrons. 2 of these electrons will first go into 4s (since we're working backwards ; when ionizing from atom to cation, always remove 4s electrons before adding to 3d), and the 3rd will go into the 3d subshell. This gives a total of 4 electrons (all unpaired) in the 3d subshell.
Q3. You're correct. But just to add, the actual mechanism for all 3 halogens (chlorine bromine iodine), is the halogen first disproportionates into the halide ion and the halonate(I) ion. The halonate(I) ion then further disproportionates into the halide ion and the halonate(V) ion. For chlorine, the disproportionation of chlorate(I) ion occurs only upon heating at 70 deg C. For bromine and iodine, this occurs readily at room temperature (they require much lower Ea for the disproportionation redox reaction). Bromate(I) ion can only be isolated at 0 deg C, while iodate(I) ion can never be isolated at any temperature. Fluorine behaves very differently with NaOH(aq) because it is the king of electronegativity. Although the reaction of fluorine with NaOH(aq) is not in the H2 syllabus (and thus H2 student's needn't know or memorize what happens), but Cambridge may still come up with "suggest suggest" type of challenging questions for this beyond-syllabus reaction.
Welcome :) -
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Hi UltimaOnline,
Many thanks once again for your cogent explanation. :)
For 1, why isn’t Br2 able to oxidise Fe2+ to Fe3+ since the Ecell is +0.30V?
For 2, I don’t understand why wouldn’t the electronic configuration of the element be [Ar]3dâ�µ4s¹ instead of [Ar]3dâ�´4s² since a half-filled 3d subshell would afford extra stability. If it were [Ar]3dâ�µ4s¹ then there would be 6 unpaired electrons.
5: What is the trend of the pH of aqueous Group II chloride going down the group? Are they all neutral?
6: Which of the following gives the best description of the reactions of Group II metals and their compounds?
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All Group II oxides undergo neutralisation with hot acids to give a salt and water.
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Beryllium hydroxide is amphoteric due to the high charge density of the Be²+ ion.
Remarks: The answer is 1 (I agree), but what’s wrong with choice 2? Is it because it does not give the best description of the Group II reactions (since it only talks about beryllium as opposed to Group II hydroxides in general), or is it because choice 2 should be “Beryllium hydroxide is amphoteric because it reacts with acids and bases” → but that begets the question as to why does it even react with acids and bases in the first place.
7: X is a mixture of two compounds. When X is treated with an excess of dilute hydrochloric acid, a colour gas is evolved and some, but not all of the mixture dissolves. Which one of the following mixtures could be X?
A Ba(NO3)2 and Ca(OH)2
B Ag2SO4 and CaCO3
C CaCO3 and MgSO4
D Ca(OH)2 and MgCO3
Remarks: Since not all of the mixture dissolves and silver chloride is insoluble, B is the only option - but what is the colour gas that is evolved? They aren’t referring to carbon dioxide right?
8: ACJC Prelim 2012 P1 Q17
Remarks: I agree with the answer B, but why is D not a possible answer? (I reckon the position of elements down the group would be X -> Z -> Y)
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Originally posted by gohby:
Hi UltimaOnline,
Many thanks once again for your cogent explanation. :)
For 1, why isn’t Br2 able to oxidise Fe2+ to Fe3+ since the Ecell is +0.30V?
For 2, I don’t understand why wouldn’t the electronic configuration of the element be [Ar]3d�4s¹ instead of [Ar]3d�4s² since a half-filled 3d subshell would afford extra stability. If it were [Ar]3d�4s¹ then there would be 6 unpaired electrons.
5: What is the trend of the pH of aqueous Group II chloride going down the group? Are they all neutral?
6: Which of the following gives the best description of the reactions of Group II metals and their compounds?
-
All Group II oxides undergo neutralisation with hot acids to give a salt and water.
-
Beryllium hydroxide is amphoteric due to the high charge density of the Be²+ ion.
Remarks: The answer is 1 (I agree), but what’s wrong with choice 2? Is it because it does not give the best description of the Group II reactions (since it only talks about beryllium as opposed to Group II hydroxides in general), or is it because choice 2 should be “Beryllium hydroxide is amphoteric because it reacts with acids and bases� → but that begets the question as to why does it even react with acids and bases in the first place.
7: X is a mixture of two compounds. When X is treated with an excess of dilute hydrochloric acid, a colour gas is evolved and some, but not all of the mixture dissolves. Which one of the following mixtures could be X?
A Ba(NO3)2 and Ca(OH)2
B Ag2SO4 and CaCO3
C CaCO3 and MgSO4
D Ca(OH)2 and MgCO3
Remarks: Since not all of the mixture dissolves and silver chloride is insoluble, B is the only option - but what is the colour gas that is evolved? They aren’t referring to carbon dioxide right?
8: ACJC Prelim 2012 P1 Q17
Remarks: I agree with the answer B, but why is D not a possible answer? (I reckon the position of elements down the group would be X -> Z -> Y)
Hi Gohby,
Q1. Under standard molarities, yes (and H2 students must memorize this) both Cl2 and Br2 can oxidize Fe2+ to Fe3+. But since the answer given stated this option was correct (of course, the student can't see the mark scheme, but based on option elimination : A is 1+2+3, B is 1+2, C is 2+3, D is 1 only), therefore it implies the molarities are less than 1M for the halogens (ie. a diluted solution such that only Cl2, but not Br2, is able to oxidize Fe2+ to Fe3+). Even under standard molarities, there may be an observable difference between the amount of Fe(OH)2 vs Fe(OH)3 ppts generated using Cl2 vs Br2. But you're right, this question is problematic.
Q2. Usually for such qns, you'll not be expected to state the electron configuration as 4s1 3d5 unless the element is specifically stated as Cr. Because in most other such MCQs, the element may not be either Cr or Cu (where a tweak is needed). However in this particular MCQ, your argument that the qn implies it is Cr, and therefore having electron configuration 4s1 3d5 (instead of 4s2 3d4) is fair, and as such Cambridge (in a P2 or P3 qn) would accept either answer. However, since this MCQ did not have an option "6 unpaired electrons", therefore the DHS teacher didn't want the student to tweak the electron configuration. But you're right, this question is problematic.
Q5. They tend towards neutral down the Group. Near the top of the group, they are acidic, due to high cationic charge densities, their hexaaquated complex ions have weakened (less polarized) O-H bonds and tend to dissociate protons, making the solution acidic. The counter ion Cl- has low anionic charge density, and so its effect on pH is negligible.
Q6. Replace the word "hydroxide" with "oxide", and you're good to go. The hydroxide is still significantly more basic than acidic.
Q7. Replace the typo word "color" with "colorless", and you're good to go.
Q8. As you say, D is only *possible*, it is not *confirmed*. And therefore, D is wrong. D implies "confirmed", not "possible".
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Thanks UltimaOnline, much appreciated!
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Originally posted by gohby:
Thanks UltimaOnline, much appreciated!
Totally welcome, Gohby, feel free to keep your questions coming! :) -
Hi UltimaOnline,
I have some questions on transition elements:
Q1
Copper (I) sulphate is a white power which reacts with water to give a blue solution and a pink-colored solid. By reference to the E data from the Data Booklet, identify the products formed, describe the reaction undergone.
Remarks: I understand that disproportionation took place given the products formed as hinted in the question. However, if Cu+ is disproportionated, why is it stated in the question that Copper (I) sulphate “reacts with water”?
Q2: Chromium is a metal commonly used to make stainless steel and for the chromium-plating of steel articles.
Remarks: for Q2b(i) Can I confirm if the answer for Q is [Cr(H2O)4Cl2]+? Do we write down any excess water molecules which are not attached to the transition element in the complex?
As for Q2c, would the proposed response be correct?
Ligand exchange took place as the sulphate ions that were initially bound to the chromate ion were replaced by the oxalate ion. This is because the ligand strength of the oxalate ion is stronger than that of the sulphate ion, as the size of the d-orbital splitting in the complex is larger.
Thank you!
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Originally posted by gohby:
Hi UltimaOnline,
I have some questions on transition elements:
Q1
Copper (I) sulphate is a white power which reacts with water to give a blue solution and a pink-colored solid. By reference to the E data from the Data Booklet, identify the products formed, describe the reaction undergone.
Remarks: I understand that disproportionation took place given the products formed as hinted in the question. However, if Cu+ is disproportionated, why is it stated in the question that Copper (I) sulphate “reacts with water�?
Q2: Chromium is a metal commonly used to make stainless steel and for the chromium-plating of steel articles.
Remarks: for Q2b(i) Can I confirm if the answer for Q is [Cr(H2O)4Cl2]+? Do we write down any excess water molecules which are not attached to the transition element in the complex?
As for Q2c, would the proposed response be correct?
Ligand exchange took place as the sulphate ions that were initially bound to the chromate ion were replaced by the oxalate ion. This is because the ligand strength of the oxalate ion is stronger than that of the sulphate ion, as the size of the d-orbital splitting in the complex is larger.
Thank you!
Hi Gohby, no prob :)
Q1. It's just the phrasing. Of course, water isn't oxidized or reduced, but it's presence is required for the disproportionation of Cu+ to occur.
Q2bi) Since 0 moles of AgCl is precipitated, all 3 moles of Cl- must be ligands instead of counter ions. Hence Q is the neutral coordination complex [Cr(Cl)3(H2O3)].3H2O and yes for such questions (in which the overall formula is given as CrCl3.6H2O you need to include and balance the water molecules, and you can regard the H2O outside the coordination complex as 'water of crystallization'.
Q2c) Not quite. Since the exact colour change isn't specified, you cannot be sure if the new ligands have a stronger or weaker ligand field strength, and/or that the new magnitude of d-d orbital splitting is larger than before the ligand substitution. You can only state with certainty that ligand substitution / exchange has occurred, either because the new ligands have a stronger ligand strength, ie. Kstab or Kf larger, or because the molarity of the new ligands are significantly larger than that of the old ligands, or both. In answering the question, you can only confidently state that the magnitude of d-d splitting has changed upon ligand substitution, resulting in light energy in a different region or wavelength of the electromagnetic spectrum is absorbed, and therefore the observed colour (which are the colours not absorbed) will concordantly be different. -
Hi UltimaOnline,
I have 2 questions on Inorganic Chemistry from RI Prelim 2011 P1
Q17: Why is D wrong? Since the acid strength of HX increases down the group, wouldn’t HAt(aq) have a lower pH than HCl(aq) of the same concentration?
Q18: Remarks: I reckon that B is wrong because iron (III) iodide is soluble so there wouldn’t be a brown ppt. However, I do not understand why A & C are wrong and D is correct.
Thank you very much for your help! :)
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Originally posted by gohby:
Hi UltimaOnline,
I have 2 questions on Inorganic Chemistry from RI Prelim 2011 P1
Q17: Why is D wrong? Since the acid strength of HX increases down the group, wouldn’t HAt(aq) have a lower pH than HCl(aq) of the same concentration?
Q18: Remarks: I reckon that B is wrong because iron (III) iodide is soluble so there wouldn’t be a brown ppt. However, I do not understand why A & C are wrong and D is correct.
Thank you very much for your help! :)
Q17. Although acidity strength increases down the group, but in aqueous solvents, all HX acids (other than HF) are considered 'strong', meaning they dissociate completely, which means there will be negligible difference in pH between HAt(aq) vs HCl(aq).
Q18. [CuCl4]2- is a favourite complex ion for Cambridge, so all A level Chem students should already be familiar with forming this complex ion in excess Cl-(aq). Cr3+(aq) is acidic enough to protonate CO3 -2, decomposing it into CO2(g). Acidic conditions, you get Cr2O7 2-, alkaline conditions, you get CrO4 2-. Not the other way round. -
Hi UltimaOnline,
Q1: "Fe3+ is more stable in acidic than in alkaline medium."
Remarks: How to I assess the veracity of this statement?
Q2: Referring to Q6 of posted on 15 Oct 2:51PM,
Which of the following gives the best description of the reactions of Group II metals and their compounds?
"Beryllium hydroxide is amphoteric due to the high charge density of the Be²+ ion."
I asked what is wrong with the statement as the answer. You mentioned "Q6. Replace the word "hydroxide" with "oxide", and you're good to go. The hydroxide is still significantly more basic than acidic."
I don't quite get it - both beryllium oxide and beryllium hydroxide are amphoteric - so what wrong with that description? Is it because it is not the best description?
https://en.wikipedia.org/wiki/Beryllium_oxide
https://en.wikipedia.org/wiki/Beryllium_hydroxide
Thank you! :) -
Originally posted by gohby:
Hi UltimaOnline,
Q1: "Fe3+ is more stable in acidic than in alkaline medium."
Remarks: How to I assess the veracity of this statement?
Q2: Referring to Q6 of posted on 15 Oct 2:51PM,
Which of the following gives the best description of the reactions of Group II metals and their compounds?
"Beryllium hydroxide is amphoteric due to the high charge density of the Be²+ ion."
I asked what is wrong with the statement as the answer. You mentioned "Q6. Replace the word "hydroxide" with "oxide", and you're good to go. The hydroxide is still significantly more basic than acidic."
I don't quite get it - both beryllium oxide and beryllium hydroxide are amphoteric - so what wrong with that description? Is it because it is not the best description?
https://en.wikipedia.org/wiki/Beryllium_oxide
https://en.wikipedia.org/wiki/Beryllium_hydroxide
Thank you! :)
Look at the Data Booklet reduction potentials. Is it easier (ie. reduction potential more positive) to reduce Fe3+(aq) or Fe(OH)3(s)? Thusly, determine the veracity of your abovementioned statement.
Fair enough, and my bad. Let me rephrase it this way :
For the oxide and the hydroxide of any metal to behave as a Bronsted-Lowry base, is obvious, as both the oxide and hydroxide anions have a propensity to accept protons to lower their anionic charge densities to stabilize themselves. Both the oxide and the hydroxide of Be are equally acidic, because the high cationic charge density of Be enables both the oxide and hydroxide to accept 2 *additional* moles of OH-(aq) (the oxide will first undergo hydrolysis to generate 2 OH-(aq), which are already present in the hydroxide ; subsequently for every mole of Be2+, 4 moles of OH-(aq) ligands complex with the Be2+ Lewis acid to generate the ionic tetrahydroxoberyllate(II) coordination complex), thus behaving as a (Lewis, not Bronsted-Lowry) acid.
The reason why I earlier suggested replacing the word "hydroxide" with "oxide" in your statement about amphotericity, is simply because the A Level H2 Chemistry syllabus focuses on the acid-base-amphoteric nature of the oxides, rather than hydroxides. So yes, you're right that both the oxide and the hydroxide of Be are equally amphoteric, but for A levels, it's the oxide that's tested rather than hydroxide, so if it were a P2 or P3 qn, I would still recommend you to write "oxide" rather than "hydroxide". Since we're left with P1, yes indeed, you (all students taking the A levels) should be aware that both the oxide and the hydroxide of Be are equally amphoteric.
