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The tricky thing involved in the 11 question is the balancing of the redox reaction equation. You're given this reaction:http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s13_qp_11.pdf
Can anyone please explain question 11 and 12?
Your cycle is correct, but you don't need to make any cycles here. Do this question with bond energies. Find the energy absorbed for bond-breaking, and the energy released when bonds were formed. Then simply subtract the energy released from energy absorbed to get the enthalpy change.View attachment 59660
Is my enthalpy cycle for Q12 correct? View attachment 59661
If it is, how do I calculate the enthalpy change of combustion now?
thanks a lotThere are 4 C--H bonds and 2 O=O bonds broken. So calculate the energy absorbed for this:
4 * [C--H] + 2 [O=O] = 4 * 410 + 2 * 496 = 2632
There are 2 C=O bonds and 4 O--H bonds formed. So calculate the energy released:
2 * 740 + 4 * 460 = 3320
Now find the enthaply:
energy absorbed - energy released
= 2632 - 3320
= - 688 kJ/mol
So the answer is D.
Actually this addition of Bond energy value has been made for 2016 examinations. Previously in the old data booklet, there was no discrimination between these two values of C=O. As this question from one of the previous years, so i used the previous data booklet value, because had i used the new one for 2016 syllabus, the answer would have been wrong. But we're to use the new value in all the questions relating to C=O of CO2, in our examinations of 2016 onwards.thanks a lot
but wait...why didn't we use the value of 'C=O in CO2' which is 805 as per the data booklet?
oooh right! that clears it up! thank youActually this addition of Bond energy value has been made for 2016 examinations. Previously in the old data booklet, there was no discrimination between these two values of C=O. As this question from one of the previous years, so i used the previous data booklet value, because had i used the new one for 2016 syllabus, the answer would have been wrong. But we're to use the new value in all the questions relating to C=O of CO2, in our examinations of 2016 onwards.
Is there a specific reason why it started to curve at 10 and ended at 20 ?
There are two moles of C. So you multiply its enthalpy change by 2.I'm having great trouble in enthalpy cycles...is this one correct now? and also why did we multiply the enthalpy change of combustion of hydrogen and carbon by 2?
I did the working wrong because I multiplied the combustion of ethene by 2 instead of carbon and hydrogen
View attachment 59664
Here , thought its too much to write...I'm having great trouble in enthalpy cycles...is this one correct now? and also why did we multiply the enthalpy change of combustion of hydrogen and carbon by 2?
I did the working wrong because I multiplied the combustion of ethene by 2 instead of carbon and hydrogen
View attachment 59664
No the graph drawn here is not correct.Is there a specific reason why it started to curve at 10 and ended at 20 ?
There are two moles of C. So you multiply its enthalpy change by 2.
There are two moles of H2. So you multiply its enthalpy change by 2.
There is only one mole of C2H4. So you leave it as it is (or multiply 1 if you wish)
Your cycle is correct. Some may be picky about the box you have, where you only have 1 mole of CO2 and H2O, but in reality you should have 2 each. But that doesn't affect anything, it's just a small technicality.
thank you so much...but if mine is correct why did Mohammed Elatta draw it in another way?Here , thought its too much to write...
Can you please show me a sketch ?No the graph drawn here is not correct.
Yes the pH starts at 11.3 and ends at 1.6.
However this titration will have two end points. This is because they told us the compound undergoes two acid base reactions. We even write their equations.
So it will gradually decrease as usual and then at the 10cm3 mark there will be a steep part to show one end point, then it will decrease gradually again, then at 20cm3 mark it will have a steep decrease again to show the second end point. Then gradual decrease again.
Have a read through this page for better understanding this topic:
http://www.chemguide.co.uk/physical/acidbaseeqia/phcurves.html
You need to find the moles of CaCO3 first. 1200 million tonnes is 1200x10^12. Let's just ignore the 10^12 as the answer will also be in million tonnes. So moles of carbonate = 1200/100.1=11.988View attachment 59681
CaCO3 >> CaO + CO2
total mass 1200 million tonns 1 mol of carbonate will decompose into 1 mol of CO2...
what next?
oh! that was simple, (maybe I just had a dizzy mind then ) Thank you!..You need to find the moles of CaCO3 first. 1200 million tonnes is 1200x10^12. Let's just ignore the 10^12 as the answer will also be in million tonnes. So moles of carbonate = 1200/100.1=11.988
CO2 will have an equal number of moles too so to find the mass of co2 = moles×molar mass = 11.988×44=527. So B is the answer
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