Does anyone know why the answer for question #14 in Chpt 9 of the study guide is not = C x mass x delta T? It states to use C=4.18, m=10g and delta T = +20 deg C, but I came up with something around 800J and the answer is 4180J. Is it just a misprint?

-- Anonymous, February 06, 2000

Answers

No there is not anything wrong with the book answer.(delta = d)

You forgot to take into consideration the enthalpy involved when there is a change in phase. Going from ice to water is known as fusion involving a change in enthalpy. Which is represted by dHfus=6020 J(at standard pressure and volume). To find the heat of the phase chage the equation is q=#of mols*dHfus. You add this to the q for the temperature change which you already calculated to be 800J.

In other problems if you:

go from water to ice you use the negetive dHfus=-6020J

go from water to gas (vaporiztion) dHvap=4070J

go from gas to water you use the negetive dHvap=-4070J

Then calculate q= # of mols * dH; then add that q to the temperature change q.

I hope this has been helpful. Feel free to ask for a clear explanation if I confused you anywhere. Also if you see page 771 of your book it explains heat of fusion in more detail.

-- Anonymous, February 06, 2000