7 posts • Page 1 of 1
For phase change questions, when do you know to start off with q=mcdeltaT to find the heat and when to use the Hvap etc.? For example, why do you use q=mcdeltaT first in question 4.1 where as in 4C.13 you use H=6.01 kJ/mol first?
You use q=mCdeltaT when you want to find the energy it takes to raise the temperature of something from one point to another. However when the thing is undergoing phase changing we then use Hvap etc. It wouldn't make sense to use q=mCdeltaTto find the energy in phase change because the temperature is the same (delta T=0) during phase changes.
There are important differences between the two situations. One has a temperature change while the other one has a constant temperature. Therefore, the equations need to take these differences into consideration, resulting in different equations.
These calculations frequently have to be divided into two parts because the phase change will occur at a constant temp (so you use things like Hvap) while the second part might deal with an increase or decrease in temperature.
ayushibanerjee06 wrote:For phase change questions, when do you know to start off with q=mcdeltaT to find the heat and when to use the Hvap etc.? For example, why do you use q=mcdeltaT first in question 4.1 where as in 4C.13 you use H=6.01 kJ/mol first?
You start off with q=cmdeltaT when the current state of the substance is not in the horizontal phase transition. For example, if there's some ice at -5 C you would start off with q=cmdeltaT solving for the q from that -5 C to 0 C, where you would then use q = ndeltaHfusion to solve for that ice transitioning into liquid water. After, use q=cmdeltaT for the chnaging temperature of water. Repeat the steps until the final phase and temperature is reached. It helps to draw a diagram to not get lost.
I find it helpful to remember the heating curve graph that Lavelle showed in class. If the reaction is going through a phase change, its temperature stays constant, so it wouldn't make sense to use the q = mC(delta T) since delta T would be 0. Instead, you use the hopefully given delta H value and multiply it by the number of moles of substance to find the change in enthalpy during a phase change. When the temperature is increasing or decreasing, then you can find q, since delta T would not = 0.
For water the most important temperatures you have to remember is 0 and 100 degrees Celsius. When doing calculations and you pass either one of these numbers you have to take into account of a phase change with either Hvap or Hfus. For example if you're going from -20 degrees C to 106 degrees C you'd have to calculate the amount of energy required to get water to 0 degrees using MCdeltaT then add Hfus then add the energy required to heat the water up to 100 degrees then add Hvap and so on. MCdeltaT is used only when raising temperatures while Hvap or Hfus is when you encounter a phase change.
Who is online
Users browsing this forum: No registered users and 3 guests