o level chemistry

ArJoe SG

one of the reasons why chlorine react faster than bromine is because of electronegativity.But in o levels, can you state that chlorine produces more effective collisions since it collide diffuses faster due to lower mr, higher frequency of collisions more than activation energy, and since it is a gas?

UltimaOnline SG

Yes that’s fine. As long as the concept is consistent with the results, it’s acceptable.

ArJoe SG

Hi ultimaonline, thank you for taking your time to reply, really appreciate it. I have a few o level questions to ask. 1) what are the difference between addition and condensation polymerisation? Can i state addition produces polymers that have the same empirical formula as the monomer itself whereas condensation does not? I am very certain about the other 2 differences.,

UltimaOnline SG

Sure no prob.

Q1. Yes, that's correct.

Q2. No, that's not quite correct (you stated two facts correctly, but there's no direct link between them, or the link is indirect and tenuous at best). Different isomers (eg. structural, functional group, geometric, etc) have different thermodynamic stabilities and hence possess different potential enthalpies, and therefore their combustion enthalpies (ie. magnitude of enthalpy difference between the isomers, and the combustion products) are different. A good example at A levels would be terminal alkenes vs internal alkenes (internal alkenes have greater number of stronger C-C sigma bonds with the higher % s orbital character), or cis alkenes vs trans alkenes (cis alkenes have greater steric van der Waals strain or repulsion between bulky alkyl groups on same side of the double bond). At O levels this question shouldn't, and wouldn't, be asked by Cambridge at all. Whoever asks this question at O levels (eg. school prelim paper, Cambridge, etc) should be shot, as the correct answer requires a minimum of A level Chemistry.

Q3. Correct, per unit volume of the fuel, there is a greater number of C and H atoms, and hence greater number of C and H atoms oxidized (exothermically via combustion) to CO2 and H2O, and concordantly, larger amount of heat released during combustion. An additional secondary effect, results from having stronger van der Waals attractions between molecules with larger number of atoms, allowing for closer packing of the molecules, ie. increasing density of fuel, hence resulting in greater number of fuel molecules per unit volume.

SBS n SMRT SG

To answer Q2

Just to add,(though it's A level requirement) I am quite uncomfortable mentioning enthalpy change of combustion (as the definition of enthalpy change of combustion is the energy change when 1 mole of an reactant is completely combusted in excess oxygen [if we mention standard enthalpy change of combustion it's at standard conditions of 298K, 1 atm])

For isomers (since TS mentioned same molecular formulae, I will assume it's referring to structural isomers - rather than geometric isomers), there can be a case when enthalpy change of combustion maybe different:

E.g.

C3H6O2 can refer to either ethylmethanoate (CH3CH2OCOH) or propanoic acid (CH3CH2COOH). When 1 mole of either compound completely combusted in oxygen, there will be 3 mole of CO2 and 3 mole of H2O

Equations will be as follows

CH3CH2OCOH + 7/2O2 --> 3 CO2 + 3 H2O (equation A)

CH3CH2COOH + 7/2O2 --> 3 CO2 + 3 H2O (equation B)

Using Bond energies to derive enthalpy changes of equation A & B

Enthalpy change of equation A is

= bond energy of reactants - bond energy of products

= [6 C-H + 1 C=C + 2 C - O + 1 C = O + 7/2 O = O] - [6 C = O + 6 O - H]

= 6006 - 7200

= -1194KJ/mol

Enthalpy change of equation B is

= bond energy of reactants - bond energy of products

= [5 C-H + 2 C-C + 1 C - O + 1 C = O + 1 O - H + 7/2 O = O] - [6 C = O + 6 O - H]

= 6064 - 7200

= -1136 KJ/mol

Hence, enthalpy change of combustion of ethylmethanoate is higher than enthalpy change of combustion of propanoic acid (theorectical - need to be verified by bomb calorimetric results / experimental readings)

In conclusion, enthalpy change of combustion may be different for structural isomers BUT for geometric isomer do refer to UltimaOnline answer

NOTE: The above is A level Chemistry already, it will be very out of syllabus for O level

P.S: UltimaOnline, if there are errors in my explanation above do point it out, thanks :) Too long never touch Chemistry cause in NS ...

UltimaOnline SG

Yes, good additional points on varying combustion enthalpies of structural isomerism by SBS n SMRT (aren't you in Uni now? which course? if chemistry related, then any 'NS effect' would already have worn off, yah?)

Basic idea is to draw an energy profile diagram with y-axis as energy/enthalpy level. Near the top would be the elements, near the bottom would be the combustion products, and in the middle (with varying energy/enthalpy levels and thermodynamic stabilities) would be the different isomers. By Hess Law, overall (formation + combustion) enthalpy would be the same. But if the isomer is less stable = more energy, its formation enthalpy would be less exothermic but its combustion enthalpy would be more exothermic; if the isomer is more stable = less energy, its formation enthalpy would be more exothermic but its combustion enthalpy would be less exothermic. Examples would be (as I mentioned earlier) internal vs terminal alkenes, and cis vs trans alkenes.

SBS n SMRT SG

I am currently still serving NS (2 years is quite a long time) ... goign to NUS Pharmacy next year

UltimaOnline SG

Eh I last time NS 2.5 years leh... 2 years considered short liao... Lol.

Just as commando unit always get all the glory, other units like guardsman and armour, nearly as siong as commando, but get no glory. Similarly, pharmacists and nurses may work as hard as doctors, but get much less prestige and pay. C'est la vie.

I am currently still serving NS (2 years is quite a long time) ... goign to NUS Pharmacy next year

TIB783G SG

For chem, This years question include:

'How will not recycling metals will affect future generations

My answer is: 'It would be costly to dig up metals which will affect the industry which will in turn affect the economy.

And also:

'How does the incomplete combustion of carbon greatly affect the human life (or something like that)

My answer is: Ít greatly affects the transmission of oxygen which inturn can lead to death

How close am I getting it correct for both questions?

UltimaOnline SG

For chem, This years question include:

'How will not recycling metals will affect future generations

My answer is: 'It would be costly to dig up metals which will affect the industry which will in turn affect the economy.

And also:

'How does the incomplete combustion of carbon greatly affect the human life (or something like that)

My answer is: Ít greatly affects the transmission of oxygen which inturn can lead to death

How close am I getting it correct for both questions?

I'm an A level expert, not an O level expert, but to be frank, it doesn't look like you'll get any marks for both questions. Or at most 1 mark (for both questions combined), if Cambridge is lenient. It's actually also costly to recycle metarials, but we need to do so, because non-renewable resources are rapidly being depleted from the earth, leaving insufficient resources for future generations. Incomplete combustion results in the production of carbon monoxide, which is directly toxic to humans, as well as soot, which contributes to haze pollution, etc.

ArJoe SG

Omg… the isomer question came out for this year o level pure chem paper…. and it is of a level standard????

These are some questions from this year paper… please help me see whether my answer is good enough.

What are the similarities and differences between diamond and graphite in terms of structure and bonding(4 marks)?
1)diamond and graphite have giant molecular structures with strong covalent bonds within the atoms
2)diamond have a tetrahedral structure with strong covalent bonds whereas geaphite has a hexagonal structure with weak van der waals forces between the layer of hexagonal layers
3)diamond cannot conduct electricity, due to its tetrahedral strucutre, each carbon atom is covalently bonded to 4 other carbon atoms, therefore there are no delocalised electrions.
graphite is covalently bonded to 3 other carbon atoms, therefore it has delocalised electrons.

why are the burning of alchols an exothermic reaction(3 marks)?
Heat energy is absorbed by reactants to break the covalent bonds between the reactant molecules and particles. Heat energy is released by the products in forming covalent bonds between the co2 and h2o molecules. There is a net release of heat energy of the ptoducts as the heat energy released by products is greater than the heat energy absorbed by the reactants, enthalpy change is negative

ArJoe SG

For chem, This years question include:

'How will not recycling metals will affect future generations

My answer is: 'It would be costly to dig up metals which will affect the industry which will in turn affect the economy.

And also:

'How does the incomplete combustion of carbon greatly affect the human life (or something like that)

My answer is: Ít greatly affects the transmission of oxygen which inturn can lead to death

How close am I getting it correct for both questions?

i think these are combined science questions.. i dont remember these questions being in the pure chem paper

UltimaOnline SG

Omg… the isomer question came out for this year o level pure chem paper…. and it is of a level standard????

These are some questions from this year paper… please help me see whether my answer is good enough.

Yes, your answers are very good. You should gain full marks (or close to full marks) for these questions. Are you an O level student or an O level tutor, ArJoe?

ArJoe SG

Yes, your answers are very good. You should gain full marks (or close to full marks) for these questions. Are you an O level student or an O level tutor, ArJoe?

I am just a student, do i sound like a tutor? Haha

UltimaOnline SG

I am just a student, do i sound like a tutor? Haha

Well if you're an excellent O level student (as you sound to be), you then have the capability to moonlight (ie. earn some cash on the side) as an O level tutor, perhaps when you're in a JC or Uni. Feel free to continue posting your Chemistry questions here if/when you go on to study Chemistry in a JC.

TIB783G SG

Are my answers totally off the track? For the first part I wrote since it will be costly to dig up metals, It also affects the industry which means that if every company spends money digging up new metals and if we continue doing that, It's supply will be affected, which in turn will affect the economy .

UltimaOnline SG

Unfortunately, that's not the most important point, so nope you don't get the mark.