Originally posted by ArJoe: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.,
2) why do isomers(with the same molecular formula) have different enthalpies change of combustion(heat energy generated from combustion)? can i state because they have different physical properties(melting and boiling point), therefore different amount of heat energy is absorbed and produced during bond breaking of reactants and bond making of products, therefore enthalpy change is different?3) why do alcohols with increasing carbon atoms produce more heat energy during combustion? Because larger volume of co2 and h2o are produced, therefore more heat energy is released during bond making....?
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 ...
I am currently still serving NS (2 years is quite a long time) ... goign to NUS Pharmacy next year
Originally posted by SBS n SMRT:I am currently still serving NS (2 years is quite a long time) ... goign to NUS Pharmacy next year
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?
Originally posted by TIB783G: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: �t greatly affects the transmission of oxygen which inturn can lead to death
How close am I getting it correct for both questions?
Originally posted by TIB783G: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: �t greatly affects the transmission of oxygen which inturn can lead to death
How close am I getting it correct for both questions?
Originally posted by ArJoe: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
Originally posted by UltimaOnline:
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?
Originally posted by ArJoe:
I am just a student, do i sound like a tutor? Haha
Originally posted by UltimaOnline:
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.
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 .
Originally posted by TIB783G: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 .