A-level chemistry question

Kahynickel SG

For UltimaOnile:

I request to review a question i set for AS. It relates to the difference in bonding of CO2 and SiO2 and their relation to bond energy, bond polarity.

In part (a) I have given the relavane melting point of CO2 and SiO2 and asked students to draw the structures of CO2 and SiO2.

Then in part (b) using these diagram I asked from the perspective of bond energy and bond polarity the difference in the bonding of these two oxides using the data from the data booklet. I made two headings bond energy and bond polarity. This is what I expect students to state and the relevant problems I am facing with this question.

"Si=O bonds are not stable" is given in the question.

BOND ENERGY: C-O, Si-O, C=O values quoted from the data booklet.

C=O bond is stronger than C-O in CO2 therefore a simple molcular structure is favoured over a giant covalent. while in SiO2, Si-O bonds are stronger because as given Si=O does not exist therefore a giant -covalent structure is likely to occur than a simple molecular structure. [Now should I give the information regarding Si=O or is there any other way of rephrasing the question?]

BOND POLARITY: Si-O bonds are more polar in SiO2 than C=O in CO2 since the difference in the electronegativities is larger in case of Si than C because Si has is less electronegative than C. Hence a three-dimensional network of Si-O bonds in SiO2 is favoured. [ I also like to involve the Si=O bond here is it workable?]

I would also like to involve the parameter of bond length here. Since multiple bonds are stable than single bonds but double bonds are shorter than single bond. Here is what i wish to stae w.r.t this case.

shorter C=O bond is favoured in CO2 as compared to long C-O bonds if CO2 were to be giant covalent because both C and O are small atoms they can easily overlap to form stronger C=O bond. In SiO2 silicon has a lrager atomic radius than C and thus will form weaker Si=O bonds or a lot of longer Si-O yet strong covalent bonds thus a giant covalent stritcure is more favoured over a simple molecular form.

I am looking forward to your suggestions.

UltimaOnline SG

a) By comparing the lengths and strengths of single bonds versus double bonds. state the correlation between the two parameters.

b) By comparing the relative atomic radii of C vs Si, suggest the difference in bond lengths that the C and Si atoms would form with O atoms.

c) Hence, deduce whether single bonds or double bonds, are favoured when C atoms, and Si atoms, bond with O atoms.

Personally, I would prefer the student explain this in terms of atomic orbitals (for which the question can be similarly structured, as in the example above, to guide the student towards this explanation).

Alternatively, if you leave your question as more open-ended essay style rather than structured, your mark scheme could accept a variety of points. For such essay style questions, the Cambridge mark scheme often has "accept any 5 of the following 7 points" that will suffice for the student to gain full marks for the question.

Carbon and silicon are both Group IV elements. Why then, under standard conditions, does carbon dioxide exist as a simple covalent molecule, while silicon dioxide exist as a giant covalent lattice?

To form the pi bond between C and O atoms, involves the more effective sideways or side-on overlap between the (less diffused) unhybridized 2p orbitals of both C and O atoms. To form the pi bond between Si and O atoms, involves the less effective sideways or side-on overlap between the (more diffused) unhybridized 3p orbital of Si and the (less diffused) unhybridized 2p orbital of O.

Consequently, the C=O pi bond is significantly stronger than the (hypothetical) Si=O pi bond, and hence it is more energetically favourable for C atoms to form (relatively strong) double bonds with O, and in contrast for Si atoms to form (a greater number of stronger) single bonds with O.

Conclusion : it is energetically favoured that (under standard conditions), carbon dioxide has a simple covalent molecular structure, while silicon dioxide has a giant covalent lattice structure.

Kahynickel SG

Thank you for your invaluable suggestions.

I would like this question to be structure-one rather than essay-type since CIE exam papers prefers structured question for AS. However essay-type questions cannot be completely ruled out. They are often asked in A2.

Secondly your "hybridization answer approach" will prove quite challenging for students I have included the comparison of atomic radii of C and Si in the question.

Finally, after this part (a) and part (b) I have also introduced part (c) which relates to the use and property of three compounds.

part (c)

The industrial processes use different compounds carrying out different purposes. Choose one use and property on which that particular use depends of each the following material listed below.

In each case choose a diffrerent property on which the use depends.

Al2O3

Use Abrasive / catalyst in cracking of hydrocarbons /

Property Inert

SiO2

use Ceramics (porcelain, chaina dish) / stationary phase in TLC, HPLC, GLC /

Property high melting point inert/ can be derivatized on Silanol groups

MgO

use Refractory material in furnace lining

Property absorbs acidic impurites and electrical insulator

UltimaOnline SG

Thank you for your invaluable suggestions.

Secondly your "hybridization answer approach" will prove quite challenging for students I have included the comparison of atomic radii of C and Si in the question.

Finally, after this part (a) and part (b) I have also introduced part (c) which relates to the use and property of three compounds.

part (c)

The industrial processes use different compounds carrying out different purposes. Choose one use and property on which that particular use depends of each the following material listed below.

In each case choose a diffrerent property on which the use depends.

Al2O3

Use Abrasive / catalyst in cracking of hydrocarbons /

Property Inert

SiO2

use Ceramics (porcelain, chaina dish) / stationary phase in TLC, HPLC, GLC /

Property high melting point inert/ can be derivatized on Silanol groups

MgO

use Refractory material in furnace lining

Property absorbs acidic impurites and electrical insulator

Kahynickel, an excellent exam question that you've written for your students, as usual.

Kahynickel SG

As you will be aware of the situation CIE exams are appoaching I am preparing two final mock exams. This question was a part of my second mock. I would also request you look into the parts of two questions.

Question 1 Aluminium is the third most abundant element in the Earth’s crust, occurring combined in many minerals.

Give the electronic configuration of Al³⁺ ions in the space below.

(a)(i) Al³⁺……………………………………………………………………………………………………

(ii) Using your answer from (a) sketch the shapes of the orbitals having lowest energy, highest energy and the next available orbital in Al³⁺. Label in each case the name of the orbitals. [Below i have provided the axes forthe answers] the answers are:

Is 2p 3s

lowest energy highest energy the next available orbital

[is this right]

[How about the approach of this question? Will students get it!!......

Question 2 [It is also a part of the same mock exam]

Nitrogen, which makes up about 80% of the Earth’s atmosphere, is very unreactive.

(a)(i) Explain the lack of reactivity of nitrogen.

It has strong N-N triple bonds which needs high energy to break. bond energy may be quoted.

(ii) State one use of nitrogen which depends upon its lack of reactivity.

It is used to provide inert atmosphere during synthesis of organic compounds.

(b)(i) On the other hand, Fluorine is most reactive member of group VII. Draw the dot and cross diagram for fluorine.

All lone pairs and bond pair shown

F.xF

(ii) Fluorine reacts explosively with hydrogen. This reaction is performed in dark i.e. in the absence of sunlight.

H₂(g) + F₂(g) 2HF(g)

The corresponding reaction of chlorine requires sunlight. By using your answer from (c)(i) and choosing suitable data from the Data Booklet to support your answer explain the difference in the reactivity of fluorine and chlorine.

Data from data booklet: F-F 158 kJmol-1 Cl-Cl 244 kj mol-1

Flourine has samller atomic/ covalent radius than Chlorine but lone pairs in F-F causes repulsion since they are very near to each other (second shell) as comapred Cl-Cl in which lone pairs repulsion are insignificant thus F-F has lower bond energy than C-Cl and reacts violently with hydrogen.

UltimaOnline SG

Question 1 Aluminium is the third most abundant element in the Earth’s crust, occurring combined in many minerals.

Give the electronic configuration of Al³⁺ ions in the space below.

(a)(i) Al³⁺……………………………………………………………………………………………………

Is 2p 3s

lowest energy highest energy the next available orbital

[is this right]

[How about the approach of this question? Will students get it!!......

Question 2 [It is also a part of the same mock exam]

Nitrogen, which makes up about 80% of the Earth’s atmosphere, is very unreactive.

(a)(i) Explain the lack of reactivity of nitrogen.

It has strong N-N triple bonds which needs high energy to break. bond energy may be quoted.

(ii) State one use of nitrogen which depends upon its lack of reactivity.

It is used to provide inert atmosphere during synthesis of organic compounds.

(b)(i) On the other hand, Fluorine is most reactive member of group VII. Draw the dot and cross diagram for fluorine.

All lone pairs and bond pair shown

F.xF

(ii) Fluorine reacts explosively with hydrogen. This reaction is performed in dark i.e. in the absence of sunlight.

H₂(g) + F₂(g) 2HF(g)

Data from data booklet: F-F 158 kJmol-1 Cl-Cl 244 kj mol-1

Another well-written question.

Yes that's correct, 3s is the next available orbital. Yes, the phrasing of the question is fine, students should have no problems understand what's required.

(Regarding relative energy levels of orbitals, it is the 4s versus 3d orbitals that students have difficulty with. Despite being slightly further away from the nucleus, the 4s orbitals have slightly lower energy compared to the 3d orbitals. The reasons for this are beyond an 'A' level discussion).

Only suggestion to add to the very last part of your question :

You might further split your last question "By using your answer from (c)(i) and choosing suitable data from the Data Booklet to support your answer explain the difference in the reactivity of fluorine and chlorine" into two sub-parts, eg. along the lines of :

a) Using information in the Data Booklet, compare the atomic radii of fluorine and chlorine, and compare the molecular bond dissociation enthalpies of F2 and Cl2. Identify the apparent contradiction and suggest an explanation for it.

b) Using information in the Data Booklet, calculate the enthalpies of the reactions between (i) hydrogen and fluorine, and (ii) hydrogen and chlorine. Hence explain the difference in the reactivity of fluorine and chlorine, with hydrogen gas.

Kahynickel SG

I would like to modify your part (a) by the words:

By considering the atomic radii of flourine and chlorine and bond energy of F2 and Cl2 explain the difference in the reactivity of F2 and Cl2 towards hydrogen.

As for the part (b) my students have already done this part as part of their practice problems. [This particular question has appeared in CIE A2 Nov/2011/42 Q.1-an A2 question-paper based on an AS content]

Regards.

UltimaOnline SG

Looks good.

Kahynickel SG

Another painful mind exercise for you if you like.

Question

Elements of group I and II are prepared by the electrolysis of their molten salts. Sodium is manufactured from molten sodium chloride and magnesium from magnesium chloride.

(a) Give the balanced equations, including state symbols, for the reaction occurring at each electrode during the electrolysis of molten sodium chloride.

(i) Release of 1 mole of product in its standard state at anode.

2Cl-(g) Cl2(g) + 2e-

(ii) Gain of all electrons from anode to form product at cathode.

2Na+(l) + 2e- 2Na(s)

(b) Give an overall reaction for the process.

2Cl-(g) + 2Na+(l) 2Na(s) + Cl2(g)

(c) In a certain electrolysis cell for the manufacturing of sodium, 6500 dm³ of the product was collected at anode. Using your answer from (b) calculate the amount, in tonnes, of the product formed at cathode.

moles of Cl2 =6500/24= 270.8 or 271

Cl2: Na from (b)

1: 2

moles of Na= 271.2 x 2 = 542

mass of Na= 542x23 = 12466g = 12.5 tonnes

d) A white powder contains the three oxides MgO, Al₂O₃ and SiO₂. Three samples were tested each containing only one of these oxides. Read the following description of tests then answer the questions that follow.

Adding aqueous HCl to a sample of white power produces a colorless solution. To this resultant solution dilute sodium hydroxide was added dropwise, a white precipitate was seen which dissolves in excess of the reagent. A second sample of white powder produces a white precipitate on adding little water and then in excess. Adding aqueous HCl to this second sample of white powder also gave no reaction. Adding aqueous HCl to the third sample of white power produces a colorless solution which gave a white precipitate on further adding NaOH_(aq) dropwise until in excess.

(i) Which two oxides might be present in the first sample of white powder?

MgO and Al2O3

(ii) Which of these samples first, second or third was MgO? third

(iii) Using your answer from (d)(i) give the exact formulae of the two species present in colorless solution in the first sample stating the pH associated with each species and the total pH of the colorless solution as a result of both these species. Mg(aq)2+ and Al(aq)3+ 6.5, 3 , 2.95

Formula: [Mg(OH2)6]2+ pH 6.5 Total pH 2.95-2.98 (must be less than 3.0)

Formula: [Al (OH2)6]3+ pH 3.0

(iv) Identify the oxide which was actually present in the first sample. Also give two reactions resulting in the formation of colorless solution and the addition of dilute NaOH.

Al2O3

• Balanced equations for the two reactions occurring when the oxide you have chosen in (iii) produced:

a) Colorless solution Al2O3 + 6HCl 2AlCl3 + 3H2O

b) Reaction with NaOH(aq) Al(OH)3(s) + OH-(aq) AL(OH)4-(aq)

(v) By considering your reactions a and b what “word” is used for this type of oxide.

amphoteric

(vi) Identify the oxide present in the second sample and explain why no reaction took place here.

SiO2. Since it is an acidic oxide and does not react with HCl and is insoluble in water forming a precipiatate.

(vii) Give the formula of the white precipitate formed in the third sample. Also state the pH of this solution.

Mg(OH)2 (s). pH would be 9

Kahynickel SG

Just another mind and academic enjoyment if you like more.

Question

Elements of group I and II are prepared by the electrolysis of their molten salts. Sodium is manufactured from molten sodium chloride and magnesium from magnesium chloride.

(a) Give the balanced equations, including state symbols, for the reaction occurring at each electrode during the electrolysis of molten sodium chloride.

(i) Release of 1 mole of product in its standard state at anode.

(ii) Gain of all electrons from anode to form product at cathode.

(b) Give an overall reaction for the process.

(d) A white powder contains the three oxides MgO, Al₂O₃ and SiO₂. Three samples were tested each containing only one of these oxides. Read the following description of tests then answer the questions that follow.

(i) Which two oxides might be present in the first sample of white powder?

(ii) Which of these samples first, second or third was MgO?

Formula…………………………… pH…………… Total pH……………

Formula…………………………... pH……………

(iv) Identify the oxide which was actually present in the first sample. Also give two reactions resulting in the formation of colorless solution and the addition of dilute NaOH.

………………………………………………………………………………………………………………………………….…….

• Balanced equations for the two reactions occurring when the oxide you have chosen in (iii) produced:

a) Colorless solution………………………………………………………………………………………………………..

b) Reaction with NaOH(aq)………………………………………………………………………………………….……..

(v) By considering your reactions a and b what “word” is used for this type of oxide.

………………………………………………………………………………………………………………………………….……

(vi) Identify the oxide present in the second sample and explain why no reaction took place here.

……………………………………………………………………………………………………………………………………..…

(vii) Give the formula of the white precipitate formed in the third sample. Also state the pH of this solution.

………………………………………………………………………………………………………………………………………..

Kahynickel SG

I apologize for posting it again. Actually the server was down and for unknown reasons this site was not available so I did not post answers on the second occasion.

UltimaOnline SG

Another well-written question by Kahynickel. Your students are certainly getting good practice for their 'A' level exams!

I've a suggestion for a bonus related question that could be added at the end of your list of questions :

Would a reaction occur if NaOH(aq) was added to SiO2 at room temperature? If yes, write an equation for the reaction. If no, explain why.

Answer :

SiO2 is chemically inert to both aqueous acids and aqueous alkalis, hence no reaction occurs with NaOH(aq) at room temperature.

Being a giant covalent acidic oxide, it will react only with fused (ie. pure molten) NaOH(l) at high temperatures, to generate the sodium silicate salt Na2SiO3 and water.

Due to SiO2 having a giant covalent lattice structure, this particular acid-base reaction has a very high activation energy, in order to break the multitude of strong covalent bonds in the giant covalent lattice, required for the reaction to proceed.

Kahynickel SG

In the oxides test quetions above if I include P4O10 it will make students to think a bit deeper than in case of 3 oxides given (MgO, Al2O3 and SiO2). However,only 3 oxides will be tested positive in the three samples.

UltimaOnline SG

Indeed, that will make for a more challenging, interesting question.

Not to mention that P4O10, even though an acidic oxide, because it is the anhydride of phosphoric(V) acid, will therefore still 'dissolve' in aqueous acidic solutions (eg. HCl(aq) ), in the sense of hydrolysis of the P4O10 to generate phosphoric(V) acid. Which means this 'dissolving' would be a hydrolysis* reaction, not a Bronsted-Lowry acid-base reaction, and the resulting solution would have two different acids.

(Technically, hydrolysis may be considered a Lewis acid-base reaction, with P4O10 as the Lewis acid and water as the Lewis base).

Good one, Kahynickel!

Kahynickel SG

After teasing questions this is starter that I have prepared for my mock.

John Dalton’s atomic theory, published in 1808, contained four predictions about atoms

Prediction 1: No atom can be split into simpler parts.

Prediction 2: All the atoms of a particular element have the same mass.

Prediction 3: All the atoms of one element are different in mass from all the atoms of other elements.

The fourth prediction is missing.

(a) Define the term isotopes.

………………………………………………………………………………………………………………………………………

……………………………………………………………………………………………………………………………………….

Few atoms are shown below.

₅B¹²₆C¹² ₇N¹⁴ ₆C¹⁴

All three predictions were proved to be wrong later on when the scientific world progressed with the help of modern electronic equipment and theories.

(b) Suggest by using the above atoms why prediction 2 and 3 were proved to be wrong.

Prediction 2:

………………………………………………………………………………………………………………………………………

……………………………………………………………………………………………………………………………………….

Prediction 3:

…………………………………………………………………………………………………………………………………….

………………………………………………………………………………………………………………………………………..

c) Give the missing fourth prediction John Dalton made regarding atoms which is still valid.

………………………………………………………………………………………………………………...............................

d) (i) Define the term relative isotopic mass.

……………………………………………………………………………………………………………………………………….

(ii) A mass spectrum of a sample of indium showed two peaks at m/z = 113 and m/z = 115. The relative atomic mass of this sample of indium is 114.5

Calculate the relative abundances of the two isotopes.

There have been questions in CIE where strutcural formulae are aksed. I have come with a different question in which observations are asked. This type of questions are rare. Enjoy it.

a) Complete the following reaction table by using the instructions given below.

If a reaction occurs with one or more observations state that observation.

If a reaction occurs without any observation then place a tick in the appropriate column.

If no reaction occurs write “no reaction”.

Reagents and conditions

CH₂=CHCH₂OH

HOOC-CHO

HCl / ZnCl₂

White ppt

No reaction

NaOH

No reaction

dissolves

H⁺ + Cr₂O₇^2- +

heat

Orange to green

LiAlH₄

No reaction

Tick

Red P + limited Br₂ + heat under reflux

White ppt

✓

b) Give the reagent and conditions required in the last column of this reaction table.

(c) Both A and B give the same organic product when reacted with a reagent not given in the chart. Identify this reagent and the conditions needed to carry out this reaction.

………………………………………………………………………………………………………………………………………

Separate samples A and B were mixed together. Other than those listed in the chart suggest reagent and conditions which gives a positive test of alcohol group in this sample. Also give the structural formula of the organic product formed.

Reagent and conditions…………………………………………………………………………………………………………....

Observation………………………………………………………………………………………………………………………….

Structure of the organic product

(d) Under the reagents and conditions given in (c) suggest the structural formula of another organic product formed during this reaction.

Kahynickel SG

My apologies for ruptrued table. Following is the corrected version.

CH2=CHCH2OH HOOC-CHO

HCl / ZnCl₂

NaOH

H⁺ + Cr₂O₇^2- + heat

LiAlH₄

Red P + limited Br₂ + heat under reflux

✓ ✓

UltimaOnline SG

Nice teaser qn, Kahynickel!

Kahynickel SG

Any suitable carboxylic acid with sweat-smelling liquid (ester).

Since this is the only possible identification test for an alcohol in the presence of a COOH.

then in the very next part I expected B- a COOH- to react with A-an alcohol.

UltimaOnline SG

Any suitable carboxylic acid with sweat-smelling liquid (ester).

Since this is the only possible identification test for an alcohol in the presence of a COOH.

then in the very next part I expected B- a COOH- to react with A-an alcohol.

Cool.

Kahynickel SG

UltimaOnline with your support and guidance I have prepared two mocks. I would like to send a copy of each of these to you so that it may serve as a practice test for students there.

Would you like to give your e-mail address for this purpose to my mailbox if you intend to.

Regards.