Posted: Thu Jan 30, 2020 11:44 am
I had a hard time matching the data given to a heating curve because b) and c) look pretty similar to me. Does anyone have tips for interpreting these curves? Will we have problems like this on the test?
Posted: Thu Jan 30, 2020 1:25 pm
In general, the lower the heat capacity of a substance, the steeper the line (and the greater the slope). This is because it takes less heat energy to raise the temperature of the substance, so temperature would increase at a faster rate relative to those with higher heat capacities. For instance, in the problem, the heat capacity for the liquid is 2x greater than the heat capacity for the solid and gas states, so you can expect the slope for the solid and gas states to be 2x steeper, which is why (c) is the right answer. Furthermore, the enthalpy of vaporization is 2x greater than enthalpy of fusion; because it takes more heat energy to change the vaporize the substance, the length of the line marking a phase change from liquid --> gas will be *longer* than the line marking a phase change from solid --> liquid. This is also seen in (c).
In contrast, in (b), the slopes for the heat capacities of solid and gas are smaller/flatter than that of liquid. This is incorrect because you would expect the slopes for solid/gas to be steeper since they have lower heat capacities.
It's also good to become familiarized with the heating curve for water because it helps a lot when calculating enthalpy for phase changes. I think it's important to understand that temperature stays constant at the actual phase change (fusion, vaporization, etc) because energy is being put in to break/separate the intermolecular forces. A heating curve usually shows q (heat) vs T (temperature), and the slope = heat capacity while the length of lines = enthalpy of phase change.