# chemistry a level ocr question

so this is the question: A fuel is being developed using a 1:1 molar ratio of octane and ethanol. Write the equation for the complete combustion of this fuel.

Calculate the energy released, in kJ, by the complete combustion of 8.00kg of this fuel

ΔcH (C8H18)= -5470kJ mol-1 ΔcH (C2H5OH) = -1367kJ mol-1

I understand how to calculate the moles of ethanol and octane . it is :

Mr: (C8H18)= 114
Mr: (C5H5OH)= 46

Mass= moles x mr
= (1x114) + (1x46)= 160g
C8H18 (50mol) & C5H5OH (50mol)
so 8.00kg contains 50mol of each

im not sure how to calculate the enthalpy change of combustion tho. I've assembled a hess cycle. I got -341850 (kJ) . the mark scheme shows 341850 (kJ) . can someone explain this?
I calculated (-5470) * 50 ) + ( -1367 * 50) = -341850 (kJ) . however the mark scheme did (50x5470) + (50x1367) = 341850 (kJ) . im not sure how these values became positive. because the arrows of a hess cycle combustion go down . the equation is Equation: C8H18 + C2H5OH + 15 1/2O2 ----> 20 CO2 + 12 H2O.
Original post by astiaf
so this is the question: A fuel is being developed using a 1:1 molar ratio of octane and ethanol. Write the equation for the complete combustion of this fuel.

Calculate the energy released, in kJ, by the complete combustion of 8.00kg of this fuel

ΔcH (C8H18)= -5470kJ mol-1 ΔcH (C2H5OH) = -1367kJ mol-1

I understand how to calculate the moles of ethanol and octane . it is :

Mr: (C8H18)= 114
Mr: (C5H5OH)= 46

Mass= moles x mr
= (1x114) + (1x46)= 160g
C8H18 (50mol) & C5H5OH (50mol)
so 8.00kg contains 50mol of each

im not sure how to calculate the enthalpy change of combustion tho. I've assembled a hess cycle. I got -341850 (kJ) . the mark scheme shows 341850 (kJ) . can someone explain this?

Hi, I remember doing this question in my mock so I just found the paper and I put -341,850 and I got it right, the markscheme says ignore the sign. This is because its just "energy" rather than an enthalpy change - an enthalpy change can be negative or positive depending on whether more energy is required to break the bonds than is released when forming or vice versa. But in this case we're just measure the energy released which is just a quantity (i hope that makes sense)
Original post by lukeb44
Hi, I remember doing this question in my mock so I just found the paper and I put -341,850 and I got it right, the markscheme says ignore the sign. This is because its just "energy" rather than an enthalpy change - an enthalpy change can be negative or positive depending on whether more energy is required to break the bonds than is released when forming or vice versa. But in this case we're just measure the energy released which is just a quantity (i hope that makes sense)

ok thanks I understand