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# Enthalpy help Watch

1. The standard enthalpy change of combustion of ethanol is -1371 kjmol-1. Calculate the mass of ethanol needed to heat 500cm3 of water by 25c

can someone help??
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3. First I'd calculate the mass of water you have. Density = mass/volume, so mass = density * volume.

The density of water is 1000kgm-3, so we convert the volume of water, 500cm3, into m3, which is 500 x 10-6 or 5 x 10-4 m3. Multiplied together, this gives a mass of about 0.5kg.

From here, you can calculate the energy needed to heat the water by 25 degrees using where Q is the energy, m is the mass of water, c is the specific heat capacity of water, and is the change in temperature.

Now, you know that the energy provided when combusting 1 mole of ethanol is 1371 kJ. We only need 52.25 kJ, which by dividing means we need about 0.038 moles of ethanol.

Finally, you can use rearranged to where is the relative molecular mass of ethanol, about 46.

of ethanol

I think this is right anyway. Hope it helps!
4. Have a look at 'chemistry student teacher' on youtube. He has a couple of good videos on enthalpy that actually goes through similar calculations have a watch ans itll help im sure!!!
5. (Original post by tomlfc)
The standard enthalpy change of combustion of ethanol is -1371 kjmol-1. Calculate the mass of ethanol needed to heat 500cm3 of water by 25c

can someone help??
First of all I'd find out what one mole of ethanol weighs. C2H5OH = 46g.

After that you would associate this mass with the energy created when it burns as the enthalpy change of combustion is the energy change when mole of a substance burns in oxygen.
So 46g ---> -1371kj

Now I would do an Eh=CmDeltaT calculation to find out the energy need to increase the temperature of 500cm^3 of water by 25 degrees celsius. Note the mass of water has to be in litres when doing the calculation.
Eh=4.18x0.5x25 = -52.25kj

Then you would calculate the associated mass using the enthalpy of combustion.

-1371kj ---> 46g
-1kj ---> (46/1371)
-52.25kj---> (46/1381)x52.25
-52.25kj---> 1.75g

1.75g is needed to heat the 500cm^3 of water by 25 degrees celsius.

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