using q=n(deltaH) vs q=mc(deltaT)

[Sophia_Kiessling_2L]

I am confused about when to use q=n(deltaH) vs q=mc(deltaT). I am doing problem 4C.11 in the 7th edition textbook and it asks how much heat is needed to convert 80 g ice at 0 degrees C into liquid water at 20 degrees C, and I don't understand why q=n(deltaH) is used for the ice calculation and why q=mc(deltaT) is used for water.

[Christine Chen 1H]

Hey! The reason we use q=n(deltaH) for ice is because it is changing phases from a solid to liquid, but the temperature of ice is not changing since 0 degrees Celsius is the freezing point. q=mc(deltaT) is used to calculate the heat required for a temperature change, which in this case, water changes from its 0 degrees to its new temperature of 20 degrees Celsius.

[David S]

The total heat involved in the process = heat needed to melt the ice into liquid (q=n∆H melting) + heat needed to raise temperature of liquid from 0°C to 20°C (q=nC∆T)

[yuetao4k]

Use q=mcdeltat when dealing with the slope part of the phase change diagram. Use the other equation when the line is flat (no phase change occurring).

[Nghi Nguyen 2L]

q=nC(delta T) is used for temperature changes
q=n(delta H) is used for phase changes