Reaction Heats

1. Dec 7, 2008

ghostanime2001

1. The problem statement, all variables and given/known data

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 is required to raise the temperature of this block from 15 celcius to 85 degrees celcius? c(Al) = 0.9 kJ/(kg . degrees celcius)

2. Relevant equations

$$\Delta$$H=mc$$\Delta$$T

3. The attempt at a solution
$$\Delta$$H=(110kg)(0.9)(85-15)=(110kg)(0.9)(70)=6930 kJ x 0.2 % = 1386 kJ

Im clueless as to find the mass of octane after this step :(

2. Dec 7, 2008

Staff: Mentor

You will need specific enthalpy for the reaction of octane burning to solve the question.

3. Dec 7, 2008

ghostanime2001

The heat of reaction for the combustion of octane is -5517 kJ

4. Dec 7, 2008

Joskoplas

Nope. It's -5517 kJ/mol ... that "per mole" part is important. Presumably if you knew how much heat was produced, you could figure how how many moles of octane -- and if you knew that, from the molecular weight, how many grams of octane.

5. Dec 7, 2008

ghostanime2001

okay but none of them give me my answer: THe answer to this question is 0.72 kg

But anyways here's what ive tried to do

because only 20% of the heat produced is used to heat the block:

5517 kJ/mol x (1/1386 kJ) THe kJ part cancels out which gives me: 3.9805 mol and multiply that with 114 g/mol which gives me 453.77 g which is definently not the answer !

6. Dec 7, 2008

Staff: Mentor

Apart from the fact that it is either 0.2 or 20%, but not 0.2%, are you sure have calculated here total amount of heat that have to be produced from the octane burning?

7. Dec 7, 2008

ghostanime2001

The entire Question goes like this:
Aviation gasoline is almost pure octane $$C_{8}H_{18}$$. Octane burns according to the equation:

$$\underbrace{C_{8}H_{18(l)}}_{a} + \underbrace{O_{2(g)}}_{b} \rightarrow \underbrace{CO_{2(g)}}_{c} + \underbrace{H_{2}O_{(l)}}_{d}$$

$$\Delta$$$$H^{\circ}_{f}$$ = -209 kJ for $$C_{8}H_{18}$$,

$$\Delta$$$$H^{\circ}_{f}$$ = -286 kJ for $$H_{2}O$$,

$$\Delta$$$$H^{\circ}_{f}$$ = -394 kJ for $$CO_{2}$$,

i) Balance the equation
$$C_{8}H_{18}$$ + $$\frac{25}{2}O_{2}$$ $$\rightarrow$$ $$8CO_{2}$$ +$$9H_{2}O$$

ii)What is the heat of formation of reactant (b)
0 kJ because its an element

iii) Calculate the heat produced when 1.00 L of $$C_{8}H_{18}$$ is burned. The density of $$C_{8}H_{18}$$ is 703 g/L.

(-1) x 1. $$8C_{(s)} + + 9H_{2(g)} \rightarrow C_{8}H_{18(l)}$$ $$\Delta$$$$H^{\circ}_{f} = -209 kJ$$

(9) x 2. $$H_{2(g)} + \frac{1}{2}O_{2} \rightarrow H_{2}O_{(l)}$$ $$\Delta$$$$H^{\circ}_{f} = -286 kJ$$

(8) x 3. $$C_{(l)} + O_{2(g)} \rightarrow CO_{2(g)}$$ $$\Delta$$$$H^{\circ}_{f} = -394 kJ$$

1. $$C_{8}H_{18(l)} \rightarrow 8C_{(s)} + 9H_{2(g)}$$ $$\Delta$$$$H^{\circ}_{f} = 209 kJ$$

2. $$9H_{2(g)}) + \frac{9}{2}O_{2} \rightarrow 9H_{2}O_{(l)}$$ $$\Delta$$$$H^{\circ}_{f} = -2574 kJ$$

3. $$8C_{(l)} + 8O_{2(g)} \rightarrow 8CO_{2(g)}$$ $$\Delta$$$$H^{\circ}_{f} = -3152 kJ$$
===========================================
$$C_{8}H_{18(l)} + \frac{25}{2}O_{2} \rightarrow 8CO_{2(g)} + 9H_{2}O_{(l)}$$ $$\Delta$$$$H^{\circ}_{c} = -5517 kJ$$

iv)Calculate the heat produced when 1.00 L of $$C_{8}H_{18(l)}$$ is burned. The density of $$C_{8}H_{18(l)}$$ is 703 g/L.
$$D = \frac{m}{V}$$

$$703g/L = \frac{m}{1 L}$$

$$m = 703 g$$

$$\frac{703g}{114g/mol} = 6.17 mol$$

$$\frac{5.52 x 10^{3} kJ}{mol}$$ x 6.17 mol = 3.4x$$10^{4}$$ kJ

v)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 is required to raise the temperature of this block from 15$$\circ$$C to 85$$\circ$$C degrees celcius? $$c_{Al}$$ = 0.9 kJ/(kg $$\circ$$C)

m=110 kg
c=0.9
$$\Delta$$T=70 (85-15)
$$\Delta$$H=(110)(70)(0.9)=6930 kJ (Total heat released in the aluminum block engine)

Last edited: Dec 7, 2008
8. Dec 7, 2008

ghostanime2001

I dont understand what to do after part v)

9. Dec 21, 2008

ghostanime2001

10. Dec 21, 2008

ghostanime2001

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 (the answer is 0.72 kg) But i dont understand how X_X

11. Dec 21, 2008

ghostanime2001

12. Dec 22, 2008

Ivan Seeking

Staff Emeritus
Everyone here donates their time. You will have to be patient.

13. Dec 22, 2008

chemisttree

Do it in pieces. How much energy will you need to raise the block by 1 degree C? Then multiply that by delta T, then account for the 20% efficiency.

You have the skills you need to answer this. Trust what you know and relax.

14. Dec 22, 2008

Staff: Mentor

And that's where the problems started...

ghostanime2001: as I already hinted at, amount of energy needed to heat the block if 20% is used for heating is NOT 6930 kJ x 0.2.

15. Dec 22, 2008

ghostanime2001

Then what do i do....... ill say it again ive done ALL i can.... i have no more ideas.

16. Dec 22, 2008

chemisttree

Borek, humor me.

Ghost, so tell me, how much energy do you need to raise the engine block by one degree C?

17. Dec 22, 2008

ghostanime2001

deltaH=(110)(70)(1) = 99 kJ

Last edited: Dec 22, 2008
18. Dec 22, 2008

chemisttree

$$\Delta$$H=mc$$\Delta$$T

$$\Delta$$T = 1 (if you want the energy per degree C)... you need 'c'

19. Dec 22, 2008

ghostanime2001

so i am right. u said the energy per degree ive given you the energy per degree man.. what else do u want yo WHy in the world would i need C man ????????? i dont need the heat capacity of octane dawg

Last edited: Dec 22, 2008
20. Dec 22, 2008

chemisttree

Uhhhh, I guess. I just thought that you were trying to do too much at once and when you made a mistake it looked too complicated for you to figger out.... dawg.

You should have written:

$$\Delta$$H=(110 kg)(0.9 kJ/kg degrees C)(1 C)= 99 kJ (heat required to raise block by 1 C)