Heat to change states?

Question

Compounds like CCl2F2 are known as chlorofluorocarbons, or CFCs. These compounds were once widely used as refrigerants but are now being replaced by compounds that are believed to be less harmful to the environment. The heat of vapoization of CCl2F2 is 289J/g.

What mass of this substance must evaporate in order to freeze 100g of water initially at 18 degrees C?

Heat of fusion of water is 334 J/g and the specific heat of water is 4.18 J/g-K

Answer 1

First off, we need to figure out exactly how many joules does it take to freeze the water. This is a two step process where we:

1) Take all of the water to that temperature
2) Actually freezing it all off at that temperature

For the first part, we use Q=sm(del T) where I use the s as the specific heat of water (4.18 J/g-C)

Q= 4.18(100g)(-18)
Q= -7524 J

Thus we need to remove about 7.5 kJ from the water to bring it to zero degrees celsius. Next we use the heat of fusion to figure out how much energy we need to remove to freeze all of that water.

Q= -Hfus (m)
Q= -334(100)
Q=-33400 J (The negative just lets me know that the water is gonna lose heat to the CFCs)

Adding it all up gives me 40924 J to remove from the system.
Finally, figuring out how much CFCs you need to boil off, you use the heat of vaporization formula and rearrange for the mass.

Q=Hvap (m)
Q/Hvap = m

40924(J) / 289(J/g) = 141.61 g

So we need about 141.61 g of CFCs to vaporize so that 100g of 18 degree celsius water can freeze.

Hope this is helpful to you.

Answer 2

nobody cares go spoof jkjk