Calculating Mass of Octane Needed to Heat Aluminum Block

[chemisttree]

Yes, you do. You made a super small error... you know your stuff but now you are pulling your hair out because of some trivial thing.

Your answer was 6930 kJ is required to raise the block by 70C. It isn't the heat released in the aluminum block engine. Its the energy absorbed by the block to raise the temperature by 70C. How much energy had to have been produced by the octane so that only 20% of it is equal to 6930 kJ?

Once you have that energy, you need to somehow convert it into mass of octane. Is there something you know about octane that tells you how much energy is produced per unit mass (or amount) that is burned?

 

[Bystander]

Yes, you do. You made a super small error... you know your (snip)

Amajor conceptual error --- reread the problem statement --- carefully. Ask yourself if there's a glass in your mother's kitchen into which you can pour an entire gallon of milk.

 

[ghostanime2001]

if what you say is true then i know 20% of energy is absorbed by the engine. THat 20% of energy released by octane is 6930. 100% of energy released by octane by burning would be 6930/0.2 = 34650 is this the total energy released by octane ? i assume so, and 20 percent of that is absorbed by octane which is 6930. Now how many moles is 34650 kJ ? uhhh i know 1 mole of combusion of octane is 5517 kJ/mol so 34650/5517 = 6.281 mol x 114g/mol molar mass of octane = 715.986 g

BUT THATS STILL WRONG... WHAT THE HECK... WHY DO SOME PROBLEMS ALWAYS SCREW ME UP AARRGGGGGGG

 

[Borek]

Even if it is wrong (numerically) you did this part correctly now.

Edit: it is not wrong, 715.986 rounds down to 720. Check signifficant digits throughout the problem.

 

[ghostanime2001]

................ I still don't know why i divided 6930 by 0.2

 

[ghostanime2001]

i did it only because i had no other choice to go. but i did it without understanding what I am doing.

 

[Borek]

I still don't know why i divided 6930 by 0.2

It follows from the % definition. There were X kJ, of which 20% were used to heat the block. Amount of heat used to heat the block was 6930 kJ. That means that

20% = 6930kJ / XkJ * 100%

Solve for X.

 

[chemisttree]

You found your original logical error. You should have divided instead of multiplying. That is minor from my POV. You could have caught it if you had used units in your calculations. Why did you divide 6930 (kJ absorbed by engine) by 20%? Look at the units if you did it the other way.

If the answer is wrong now it is either due to bad assumptions (like heat released per mole of octane burned) or arithmetic.

Try it on your own for awhile.

Edit. BTW, regarding whether to multiply or divide... think of it like this. I have a number (6930) that I am about to apply a factor to. I want the result to be bigger (arrived at logically). Do I multiply by a number less than one (20% or 0.2 in this case) or divide by a number less than one?

 

[ghostanime2001]

I don't understand

 

[ghostanime2001]

ive done all i tried but still don't understand.

 

[ghostanime2001]

All i know is that there's a freakin aluminum block engine. Heat is produced inside the engine from burning octane. 20% of that heat is used to raise the temperature of the aluminum engine block from 15 to 85 celcius. what mass of octane was used to raise the temperature of the engine block from 15 to 85 ?
q=6930 is the energy that raised the engine block from 15 to 85 so I DONT GET IT ..... T_TTTTTTTTTTTTTTTTTTTTTTT

 

[chemisttree]

Yes you do... Where did you get the value for the heat of combustion of octane?

DON'T GET SLOPPY WITH YOUR UNITS! Slow down.

 

[ghostanime2001]

The heat of combustion of octane is -5517 kJ from using Hess Law' and using heats of formation of CO2 and H2O in my first post.

 

[ghostanime2001]

is this question asking How much mass of octane provides 6390 KJ of energy that is 20% of the total energy produced from its total combusion inside the combustion chamber of the engine. So, how much mass of octane provides 6390 kJ of energy of which is used to heat the block from 15 to 85 celcius ?? what mass of octane delivers its 20% of energy from its total energy produced in the engine to the heating of the engine block ? In other words how many grams of octane provides 6390 kJ of energy that directly goes to heating up the aluminum engine block from 15 to 85

 

[chemisttree]

My bad, I thought that number was for energy per liter.

Carefully read post #34.

 

[ghostanime2001]

so what is it ? do i have the right idea now from post#44

 

[chemisttree]

You had it right in post #33. Borek caught it in #34.


720 g = 0.72 kg

 

[ghostanime2001]

okay so i got the right answer but that doesn't prove i understand the quetsion any better

 

[ghostanime2001]

it says what mass of octane can raise the temperature of the aluminum engine block from 15 to 85 not what the total mass of octane is present inside the combusion chamber of the engine block.

 

[Borek]

it says what mass of octane can raise the temperature of the aluminum engine block from 15 to 85 not what the total mass of octane is present inside the combusion chamber of the engine block.

Original question doesn't refer to the mass inside, but to the amount of octane that have to be burnt.

Think how car engine works - you put gasoline mixed with air in. It burns inside. Some of the energy is converted to work that you can get from the rotating crankshaft, some is lost in exhasut gases, some heats the engine block.

That's the same kind of situation - you are told how much temperature of the block rose, you are told what part of the heat was used to heat the block, you are asked how much gasoline (octane) was burnt. You are not told it was inside.

 

[ghostanime2001]

this question is still foggy to me

 

[lewdtenant]

So, you have an engine block, and octane is used to heat it from 15 to 85 C. You have a certain amount of fuel that is combusted, but only 20% of the heat from that combustion goes towards heating up the aluminum block. The rest of it is waste heat. Regardless of how much octane you have, you can assume (for this problem) that 20% of the heat goes towards heating the block and 80% is lost as waste heat. So, you need to find how much octane is needed to supply the right amount of heat to raise the temperature 70 C. But you want to determine the total amount of octane needed, including the excess waste heat.

Maybe a helpful analolgy: (Please realize that even though this analogy deals with fuels, it is totally unrelated to the above problem). The space shuttle is attached to a rocket to launch it into outerspace. The rocket is propelled by liquid hydrogen that is in a fuel tank. The shuttle and rocket have a combined weight, and engineers calculate how much hydrogen they will need to launch it. But they also have to account for the weight of the fuel tank and the weight of the fuel itself. So, they have to add additional hydrogen to launch this extra weight.

The engine block problem is the same concept. You want to know how much extra octane is needed to heat the block to 85 C.