Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: highschoolhelp16 on April 15, 2012, 07:38:57 PM

How do you calculate the amount of heat required to convert 100.0g of solid methanol at its normal freezing point to gaseous methanol at its normal boiling point? I have tried everything I can think of, and after 2hrs of ridiculous answers I need help.

So what does "everything you can think of" consist of?

I have tried q=m*s*deltaT, and I have done multiple equations involving the boiling and vaporization point. I tried (100.0g)(0.424ca/gc)(337.2k175.5k)=6,877.28cal, i think that is in the right direction but I am unsure. it think that is the heat required to bring the methanol to its boiling point.

I think you are forgetting to include the enthalpy of fusion (energy needed for solid :rarrow: liquid at the melting temperature) and the enthalpy of vaporisation (energy needed for liquid :rarrow: gas at the boiling temperature) into your calculation. Your calculation of (100.0g)(0.424ca/gc)(337.2k175.5k)=6,877.28cal only gives the energy needed to heat the liquid from 175.5 K to 337.2 K, it doesn't take into account the conversion of solid to liquid or liquid to gas.

so how would i fit 3.16 KJ/mol into the equation?

Well, how many moles of methanol do you have?

0.062 mol nh4no3

Methanol has molecular formula CH_{3}OH.

sorry I was looking at the wrong problem, its 0.16 mol CH3OH

I think you might like to check that again because I got a different answer. Remember you have 100 g of methanol. But anyways, once you have worked out the number of moles, you can find the energy needed for the phase transformation. I assume 3.16 kJ/mol is the enthalpy of fusion?

its 3.12 mol, and yes the Hfusion is 3.16kJ/Mol

Ok, so if you have 3.12 moles and the enthalpy change is 3.16 kJ PER mole, then the energy change needed for solid to liquid transformation is? It is a similar process for finding the energy needed to go from liquid to gas, but you'll be using a different enthalpy value obviously. Then you need to add these energy values onto your calculation of (100.0g)(0.424ca/gc)(337.2k175.5k) but you will need to convert to the same units.

so is the energy change needed 9.86kJ/Mol?

It will just be 9.86 kJ, the 'moles' unit cancel out when you multiply.

so the equation should be (100.0g)(0.424cal/gc)(337.2K175.5K)(9.86kJ)?

or is 9.86kJ the energy required to convert the 100g from solid to boiling point?

No, 9.86 kJ is the energy needed to convert solid to liquid. Now you need to find how much energy is needed to transform the liquid into gas, because the question states that you want gaseous methanol. I have stated in my previous post how to do this.

so the energy needed to convert from liquid to gas should be 110.136kJ?

Yes (I am assuming you used the correct H_{vap} value)

so do I need to put these values into my original equation of (100.0g)(.424cal/gc)(337.2k175.5k)? or was that just completely off track?

Yes you add the two values onto whatever you got from (100.0g)(.424cal/gc)(337.2k175.5k) but you will need to change the units so that they match, either convert kJ into cal or vice versa.

So if I get 3,684.58kJ from that equation, I should add 110.14kJ, and 9.86kJ, to get 3768.58kJ total.

So if I get 3,684.58kJ from that equation
That seems to be an awfully large number...(check again) but if thats what you got, then adding 110.14 kJ and 9.86 kJ onto it is correct.

could the equation be wrong. Im using q=mass*specific heat*delta T

Well initally you got 6,877.28 cal, how many kJ is this?

I got 3684.58 kJ

In thermochemistry 1 calorie is about 4.184 J
http://en.wikipedia.org/wiki/Calorie

so 28.75kJ?

Well going by your equation (100.0g)(.424cal/gc)(337.2k175.5k), this should give 6856.08 cal. Then converting to kJ gives 28.69 kJ. But maybe you did some rounding somewhere.

You're right I made a calculation error. so 28.69kJ is the heat required to convert 100.0g of solid methanol to gaseous methanol?

No, remember you got to add 110.14kJ and 9.86kJ

so 148.69 is the total heat required?

Yes. 148.69 kJ

Thank you so much for your help I really appreciate it :)

:) I'm glad we finally sorted that one out!