Heat of Combustion of Ethanol (1 Viewer)

[mazza_728]

Hey guys,
im having a lil trouble with this stuff and my teacher doesnt help because he's never taught the chemistry course and i really doubt if he knows anything!
Can someone please explain how you answer this question:
The heat of combustion of ethanol is 1360KJ/mol. What mass of ethanol needs to be burnt to raise the temperature of 350g water through 77 degrees if 50% of the heat released by the ethanol is lost to the surroundings? the specific heat capacity of wate is 4.2J/K/g.
And if someone is really bored and they have the conquering chemistry textbook could u answer 30-34 otherwise hopefully ill understand it after this question. thanksxoxo

 

[Frigid]

the heat of combustion is given by the equation:

delta-H = m x C x delta-T

where delta-H is the energy in joules;
m is the mass in grams;
C is the specific heat of the substance (water is 4.2 J/K/g);
delta-T is the temperature change.

now, taking the heat of combustion value of 1360kJ/mol and multiplying that value by 50%:

1360 x 50% = 680 kJ/mol = 680000 J/mol

We substitute m=350, C=4.2, delta-T=77 into our original equation:

delta-H = 350 x 4.2 x 77 = 113190 J for 'x' grams of ethanol.

the molar mass of ethanol is 46 (12 x 2 + 1 x 6 + 12 x 1).

by similar ratios, the energy per 'x' grams of ethanol is equal to the energy per mole of ethanol.
113190 / 'x' = 680000 / 46

therefore, solving for x,
x = 7.66 (3 sig fig)

the mass of ethanol that needs to be burnt is about 7.66 grams.

and dun be a lazy ass, DO THE REST YOURSELF!

 

[abdooooo!!!]

i believe the answer should be negative... i heard they hammer you for it if you have a positive answer for heat of combustion even though it suppose to be minus enthalpy of change... and if you don't write the special symbol for heat of combustion another mark gone

just joking

but it just might be true... read the marking guidelines and the answers on that 2001-2002 past paper book...

 

[mazza_728]

Thanks guys i finally understand - ftr11 u really helped! it just clicked.. thankyou xoxo

 

[The Bograt]

In the combustion of ethanol your answer should always be negative, this shows that the reaction was exothermic.

 

[crazylilmonkee]

omg im so glad this wasnt in the exam!! or was it.. i never got taught it.. had to use damn excel, shitty book!!!

 

[Ragerunner]

Wasn't there one with burning ethanol in air and how much CO2 released?

I couldn't get that one

gayyyyyyyyy stupid maths questions.

 

[crazylilmonkee]

oh shit there was too!!
i had like 72 L or something
hahaha that was so messed up

 

[Frigid]

Originally posted by The Bograt
In the combustion of ethanol your answer should always be negative, this shows that the reaction was exothermic.

Bullshit! the enthalpy of an exothermic reaction should be negative, but the heat of combustion of ethanol is a positive value!

 

[iambored]

delta is the triangle thing /_\

for an exothermic reaction:

delta T = negative

delta H = positive


delta T corresponds to the substance itself. as the substance is giving out heat, it is negative, as the SUBSTANCE IN ITSELF is LOSING HEAT

delta H corresponds to the heat being given out (as opposed to the substance giving out the heat). therefore, as heat is being given out, it is a positive value

 

[Phatphil]

so delta h is not enthalpy?!??!

 

[abdooooo!!!]

it is

 

[The Bograt]

Yeah the change in heat (delta H) is a negative value. I'm sure of it

 

[spice girl]

just to clear up, 'enthalpy' is heat lost or gained in a process kept at constant pressure (which is the case in nearly all of class lab experiments). So in HSC levels, 'enthalpy' and 'heat' may be used in the same way.

some things are not constant pressure. for example, igniting petrol in a closed container.

 

[The Bograt]

Thanks for clearing that up