To practice problem-solving strategy 11.1 energy efficiency problems. suppose that you have left a 200-ml cup of coffee sitting until it has cooled to 30∘c , which you find totally unacceptable. your microwave oven draws 1100 w of electrical power when it is running. if it takes 45 s for this microwave oven to raise the temperature of the coffee to 60∘c , what is the efficiency of heating with this oven?

efficiency of heating with this oven is 51 %

Explanation:

to raise the temp of 200 ml of coffee from 30°C to 60°C the energy input to microwave oven is:

1100 J/s x 45 = 49,500 J  

AT 100% efficiency

For 1°C the energy required to raise the temperature of 1 ml = 4.2 J

So for 30 C°, 1°C the energy required to raise the temperature of 200 ml =

Q = (4.2) (200)(30) = 25,200 J

efficiency = 25,200/49,500 = 0.51 = 51%  

"standard molar enthalpy:

h2o ( liquid water ) : - 285.8 kj/mole

h2o ( water vapor ) : - 241.8 kj / mole.

δhrxn = - 241. 8 - ( - 285.8 )= -241.8 + 285.8 = 44.0

answer: 44.0 kj"

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