Question

The molar heat of combustion of ethanol is 1360kJ/mol. What will be the final temperature if 250g of water at 20 Celsius, is heated by the combustion of 2.0g of ethanol if there is a 30% heat loss to the surroundings? (The specific heat capacity of water is 4.2 J K^-1 g^-1).

Answer 1

number of moles of ethanol used = 2/(2x12+6+16) = 0.0357 moles

therefore find the heat released by burning this much, 0.0357x1360 = 48.571 KJ (<--- note this is in kilojolues)

now 30% of this is lost , actual heat used is 70% of the above amount, 0.70x48.571= 34KJ

using the equation enthalpy change = mc(change in temperature)

where the enthalpy change is 34 000 J (notice you must change from KJ to J)

therefore 34 000 = 250x4.2x(X-20)

X-20= 32.38

X=52.38 degress celcius

the final temperature is therefore 52 degress celcius

Answer 2

Thankyou so much, much appreciated. But you made a little mistake with the molar mass of ethanol calculation, its supposed to be (12.01*2+1.008*6+16.00) but I understood what you meant. Thankyou very much sir!