Thread: Enthalpy Changes (Aluminum block question)

1. An aluminum engine block has a mass of 110 kg. If only 20% of the heat produced in the engine is available to heat the block, what mass of octane(C8H18) is required to raise the temperature of the block from 15 degrees celcius to 85 degrees celcius? Specific heat capacity of aluminum = 0.9 kJ/(kg C)

The enthalpy of combustion of octane is -5517 kJ

2.

3. That looks a whole lot like a homework problem. What have you done to try to solve the problem and whare are you getting stuck?

4. Yeah, where exactly is it you have a problem. Do you understand the question?

5. This is the entire question:
Aviation gasoline is almost pure octane, C8H18, Octane burns according to the equation:

C8H18 + O2 --> CO2 + H2O

heats of formation:
C8H18 = -209 kJ
CO2 = -394 kJ
H2O = -286 kJ

a) Calculate the heat of combustion of C8H18
(dont worry about how i used Hess' Law theres too much to type down so im just giving you the correct answer assuming i did this part of the question correctly which i did )

b) An aluminum engine block has a mass of 110 kg. If only 20% of the heat produced in the engine is available to heat the block, what mass of octane(C8H18) is required to raise the temperature of the block from 15 degrees celcius to 85 degrees celcius? Specific heat capacity of aluminum = 0.9 kJ/(kg C)

This is how I *think* im supposed to do it.
m = 110 kg
c=0.9
deltaT=70 (85-15)
deltaH=(110)(70)(0.9)=6930 kJ total energy released inside the aluminum block engine. The question says 20% of that energy produced is used to heat the block. So 6930 x 0.2 = 1386 kJ is used to heat the block right? the rest of the energy 5544 kJ is used to what ? heat the octane in it from 15 to 85 ? what do i do after this step ? This is where i get confused. THis is my question.

6. Hmm, I'm going to assume you have all the equations and stuff right here 'cos it's been a while since I've done anything like this. The question seems to be asking what mass of octane you need to heat the block. You have worked out what amount of energy is needed to heat the block from x to y. The system is only 20% efficent, so the question isn't what happens to the extra heat, but how much extra energy is required.

7. I really do not understand this question and explaining this to me over and over will not help just tell me what i should do.

8. Originally Posted by ghostanime2001
The question says 20% of that energy produced is used to heat the block. So 6930 x 0.2 = 1386 kJ is used to heat the block right? the rest of the energy 5544 kJ is used to what ? heat the octane in it from 15 to 85 ? what do i do after this step ? This is where i get confused. THis is my question.
No, 6930 kJ is the amount used to heat the block. To find the amount burned you have to multiply that by 5, not divide by 5. You don't need to know what happens to the other 80% of the heat produced. It's probably going into the cooling system or the exhaust mostly.

9. If you're going to divide it, then you have to divide it by the percentage(.20)

10. Originally Posted by ghostanime2001
This is the entire question:
Aviation gasoline is almost pure octane, C8H18, Octane burns according to the equation:

C8H18 + O2 --> CO2 + H2O

heats of formation:
C8H18 = -209 kJ
CO2 = -394 kJ
H2O = -286 kJ

a) Calculate the heat of combustion of C8H18
(dont worry about how i used Hess' Law theres too much to type down so im just giving you the correct answer assuming i did this part of the question correctly which i did )

b) An aluminum engine block has a mass of 110 kg. If only 20% of the heat produced in the engine is available to heat the block, what mass of octane(C8H18) is required to raise the temperature of the block from 15 degrees celcius to 85 degrees celcius? Specific heat capacity of aluminum = 0.9 kJ/(kg C)

This is how I *think* im supposed to do it.
m = 110 kg
c=0.9
deltaT=70 (85-15)
deltaH=(110)(70)(0.9)=6930 kJ total energy released inside the aluminum block engine. The question says 20% of that energy produced is used to heat the block. So 6930 x 0.2 = 1386 kJ is used to heat the block right? the rest of the energy 5544 kJ is used to what ? heat the octane in it from 15 to 85 ? what do i do after this step ? This is where i get confused. THis is my question.
Can you explain how you got -5.52 x 10^3 kJ as the answer? Wouldn't the heat of combustion simply be the enthalpy of products - the enthalpy of the reactants? Which is something like -450?

11. nvm, I just realized the equation isn't balanced.

12. a) C8H18 + 12.5 O2 --> 8CO2 + 9H2O

deltaH = -394 * 8 - 286*9 -(-209) = -5517 kJ

b) heat released by heating octan = - (heat aluminium absorbed)
q (Al) = m*C*deltaT = 110*0.9* (85-15) = 6930kJ
Hence 20% q (octan) = - 6930kJ
---> 100% q (octan) = - 6930*5 = 34650kJ
From a) : 1mol of octan releases 5517kJ
34650kJ need 34650/5517 = 6.28 mol
Mass of octan needed = n*M = 6.28 * 114.224 = 717.33 g

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