Відео

Have you figured this out? I'm also getting to this stage of having everything multiplied together, but don't know what to put in the denominator

Whoa.

It really was but I figured it out. I’ll try my best to explain it using an example in my homework. It says “Suppose a small sample of X at 20 °C is put into an evacuated flask and heated at a constant rate until 20.0 kJ/mol of heat has been added to the sample. Melting point: 40 °C, enthalpy of fusion: 14.00 kJ/mol. Boiling point: 80 °C, enthalpy of vaporization: 31.00 kJ/mol. (Though we’re not going to use this as the sample is only heated until 20 kJ/mol). Heat capacity at solid phase: 47 J/k•mol. Heat capacity at liquid phase: 34 J/k•mol.” So the first point would be the original temp of the sample. So 20 °C and the x axis is at 0 because no heat was added yet. Next we have to take that point to the melting point (40 °C) but to figure out where to put the point on the x axis, you do the change in temp (T2-T1) x the heat capacity of that phase. Since the original temp is 20 °C that’s T1 and the melting point is T2. So 40 - 20= 20. And since the melting point is 40 °C and the sample is currently at 20 °C, it’s currently in the solid phase. So the heat capacity we use is 47 J/K•mol. So 20K (20K is the same as 20 °C for these calculations since °C and K are on the same temperature scale) so 20K x 47 J/K•mol=940J/mol (K cancels) and we turn it into kJ by dividing by 1000 which gives us .94 which means .94 kJ/mol of heat has been added. So you plot the point at (.94,40). Now we have the flat, horizontal portion of the heating curve since the temperature doesn’t go up during the phase change. To get to the end of the phase change you basically just have to add the .94 to the 14 (heat of fusion) which gives you 14.94. That’s the next plot line (14.94, 40). Since the phase change is done now the temp rises again. So now you have to go from that point to the boiling point. To get the x plot point you do the new change in temp x the heat capacity for the liquid phase since at this point the sample has melted. So 80-40=40 x 34 J/K•mol which gives you 1360 J/mol which again needs to be converted to kJ so it would be 1.36 kJ/mol. To go to the next point you add the 1.36 to the 14.94 to give you 16.3. So the next point is (16.3,80). Now you just do flat horizontal line until the end of the heat added which was 20 for me. And that’s the heating curve

I really hope that’s clear 😅

Wow. How did I miss that? My answer wasn't even one of the choices!! Good catch! Thanks.

This makes WAY more sense than what the website says

Hi, IceKing. Thanks for the questions. One of the clever things about problems like this is as follows: You're being asked to "sketch", which a different skill than "calculate". That's why I spend so much time in the video talking about the qualitatives. Since we never label the x-axis (heat put into the system), the "length" of the fusion and vaporization are just depicted qualitatively. That is, we don't need to know the length. Prove it to yourself by labeling the x-axis, and calculating the exact lengths, and then you will see in the end that you never needed those.

@@PatrickCoppockactually for the topic you do need to calculate the slope for each and the specific points. so i’m still confused:(

s8n

Are you asking for a remake? Send me a snapshot of the one you want done, and I'll do it. (Not sure how to send a snapshot...)