Consider 1 kg of oil, (about one quart) which releases 2.80 x 10Joules of energy

Question

Consider 1 kg of oil, (about one quart) which releases 2.80 x 10Joules of energy when burned. Compare that energy to the energy of heating water to the boiling point, then evaporating the water, followed by heating the water. The reason water extinguishes flames is because it is exceptionally good at absorbing heat (a) It takes 4.186 Joules required to raise 1 g of water by 1 degree C. How many Joules does it take to raise the temperature of 1kg water from 20C to 1000? (b) It takes 2260 Joules to evaporate 1 g of boiling water at 100C and convert it to steam. How many Joules does it take to evaporate 1kg of boiling water and convert it to steam? (c) It takes 1.84 Joules to raise 1 g of steam by 1 degree C. How many Joules does it take to raise the temperature of 1kg of steam from 100C to 300C? (d) Compare the sum of your answers, (a) + (b) + (c) to energy released from burning 1kg of oil. Which is larger, and by what factor?

Explanation / Answer

a)We know

Q=mc delta T

4.186=1*c*1

c=4.186 J/g 0C

Now

Q=mc Delta T

Q=1000*4.186*(100-20)=334.88k J

b)

Q=mc delta T

2260=1*c*100

c=22.6J/g 0C

Now

Q=mc Delta T

Q=1000*4.186*(100-20)=334.88k J

b)Q=mHv

where Hv is the heat of vaporization

For 1g it takes 2260J at 100C

so for 1kg=1000 g takes =2260*1000=2260 kJ

c)Q=mc delta T

1.84 J=1*c*1

c=1.84 J/g 0C

Q=1000*1.84*(300-100)=368 kJ

d)Sum of energy=334.88k J+2260 kJ+368 kJ=2962.88kJ

Energy required to burn 1kg oil=2.8*10^7=28000 kJ

Thus energy required to burn 1kg oil is 9.4 times of this.

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