[millie]

I am so stuck. please help me.  The question is from the Chemistry Revision book by Letts on Enthalpy.

I have the answers but have no idea how to get it.

1.) Addition of Zinc powder to 55.0cm3 of aqueous Copper 2 sulphate at 22.8 degrees celcius raised the temperature to 32.3 degrees C. 0.3175g of Copper were obtained.

a.) Calculate the energy released in this reaction?
b.) Calculate the enthalpy change for this reaction per mole of copper formed.

I can do a.) the answer is 2.18 kj
but I cant do b.) the answer is -437kj mol-1



And the second question:-


Combustion of 1.6g ethanol C2H5OH raised the temperature of 150g of water from 22 degrees C to 71 degrees C. Find the enthalpy change of combustion of ethanol?

The answer is :-

-883 kj mol-1


PLEASE HELP ME!!!!
Thanks. Millie

[rpirrie]

use q = m*cp*deltaT to get the energy released to the water for combustion of that amount of ethanol in moles, then "ramp it up" to the value obtained if you had burned a whole mole of ethanol. Key point: you got this energy (assuming perfect transfer ) from less than a mole of ethanol. same thing for the copper. the energy released is from less than a mole. In 1b, they are assuming a cp for aqueous copper sulfate of 4.18 J/g*degrees C, and a density of 1g/cm cubed

[millie]

hi. thank you. i forgot to put -

density of solution = 1 g cm-3 and specific heat capacity of solution = 4.18kj  - for both questions.

could you show me the workings for number 2 please.

I did Q=mc delta T
Q= mx4.18x(71-22)

What is the mass? .

then what do i do?

 sorry. ive not done chemisty in a very long time and i have a gamsat  exam in 10 weeks and am panicking. thanks again

millie

[millie]

any help asap would be much appreciated. have stared at this for 7 hrs. am so fed up. someone please tell me how to get the answers . thank you. from a desperate chemistry beginner.

[Trave11er]

Hi millie!

Well, you probably know that the reaction taking place is: Zn + Cu2+ = Zn2+ + Cu. What you need is to find, how much energy is released when one mol of copper is formed.
You know that the ammount of copper being formed is: n(Cu) = 0,3175g:(63,55 g/mol) = 4.996*10^-3 mol and energy released in this process is 2,18 kJ, the per mole of copper it will be Q = 2,18 kJ:(4.996*10^-3 mol) = 437 kJ mol-1.
You may notice similarity with the answer you seek for, but it is not the same. You see, energy released in reaction is what you observe, but enthalpy is energy released from the point of view of reaction vessel. So when you measure that you say when one mol of copper is formed then 437 kJ of energy is released, but for the reaction vessel it means that it energy decreases by 437 kJ per mol of copper, so enthalpy= -437 kJ mol-1 and in general case H (enthalpy) = -Q.

[Trave11er]

P.S. in the previous post I incorrectly wrote "enthalpy" and "H", it should be "enthalpy change" and "ΔH" instead.

Second question:
Molar mass of ethanol is M(C2H5OH)= 2*12+ 6*1+ 16 = 46 g*mol-1 and its quantity is then
n(C2H5OH)=1.6 g: 46g*mol-1 = 0.0348 mol.
And the ammount of energy released when this of ammout of ethanol burns equals to
Q=mc(T2-T1)=150g * 4.18 J mol-1 °C-1 * (71°C-22°C) = 30723 J = 30.723 kJ.
Thus per one mol of ethanol energy released equals to Q=30.723 kJ:0.0348 mol = 883 kJ mol-1 and enthalpy change is then ΔH=-883 kJ mol-1 respectively.
This is a very cool topic, because pretty same calculations let you now how much coal you should burn on a power plant to supply energy to a city or for example or much energy batteries or accumulators can release depending on which chemicals you use in them and many other applications (as in the first task) so good luck to you with!

[millie]

thank you so so much for you answers . i understand the first one and got most of the way through the 2nd one then got stuck.

i get that 0.0348 moles of ethanol is used.
and i get that delta H is 30.72.

but how do you get 883kj mol-1?

i thought- 1 mole of ethanol takes 30.72 kj
so 0.0348 takes 30.72x0.0348= 1.069

what am i doing wrong? so sorry for being dumb. thanks againl

[sjb]

thank you so so much for you answers . i understand the first one and got most of the way through the 2nd one then got stuck.

i get that 0.0348 moles of ethanol is used.
and i get that delta H is 30.72.

but how do you get 883kj mol-1?

i thought- 1 mole of ethanol takes 30.72 kj
so 0.0348 takes 30.72x0.0348= 1.069

what am i doing wrong? so sorry for being dumb. thanks againl

Not quite. If 0.5 of a car costs £1,500, how much does a full car cost?

[Trave11er]

It means that these 30.723 kJ released when 0.0348 mol of ethanol were burned. But enthalpy change of combustion of ethanol implies enthalpy per one mol of ethanol.
Generally, all the enthalpy changes you will find in textbooks or reference books will be represented per one mol of substance - it is like when you go to the shop you see price of apples in $ per kg (-883 kJ mol-1)  and then you calculate how much 600 g (0.0348 mol) of apples will cost to you (30.723 kJ) - your task was just the opposite: you knew how much you paid for some definite ammount and then were to find the price per kg.
Good luck!

[millie]

ah, i see. thank you so so much for your *delete me*. i really appreciate it.