what is q when converting liquid water to ice?

Question

what is the value of q, in kj, for the process in which 20.2 g of liquid water at 100 C is converted into ice at 0 C?

Answer 1

The solution to this problem requires three steps, each of which calculates heat (q) necessary for the change.

Step 1. heat to convert 20.2 g of liquid water from 100º to liquid water at 0º.

q = mC∆T = (20.2 g)(4.184 J/g/deg)(100 deg) = 8452 J

Step 2. heat to convert 20.2 g of liquid water at 0º to solid water (ice) at 0º. This is a phase change with no change in temperature and we need to use the ∆H_fusion for water which is 334 J/g.

q = m∆H_fusion = (20.2 g)(334 J/g) = 6747 J

Step 3. Sum the heat values from each step.

q = 8452 J + 6747 J = 15,199 J x 1 kJ/1000 J = 15.2 kJ (to 3 significant figures)