kclo4 - 5-6-2005 at 00:32
How do you calculate how much heat is evolved during a reaction and what determines if it is exothermic or endothermic
guy - 5-6-2005 at 01:26
You have to determine by experiment or look at a table. Exothermic means heat is released, hot (product has less energy and negative enthalpy).
Endothermic means heat is absorbed, cold (product has higher energy positive enthalpy)
neutrino - 5-6-2005 at 04:06
If you’re talking about an aqueous reaction, you measure the temperature change of the solution and find the amount of heat gained/lost.
axehandle - 5-6-2005 at 04:45
Google for "bomb calorimeter".
E.g. http://thermal.sdsu.edu/courses/me395/thermo/bomb_html/Bomb_...
[Edited on 2005-6-5 by axehandle]
carbocation - 7-6-2005 at 01:05
if you want to calculate the enthalpy for a reaction in the liquid phase, you use q=mcΔt where q is the heat energy for the reaction, m is the
mass of the reaction mixture, and delta T is obviously the temperature change. If the value of q ( Kj mol-1 units) is negative, then the reaction is
exothermic and vice versa. Experimentally, you measure the heat energy change and use the above calculation, ideally plotting it on a graph over about
5 mins at 30 sec intervals and extrapolating backwards to find the theoretical ΔT value at the moment which the reaction commences.
Of course, for theoretical calculations at a basic level, you can use ΔHf and ΔHc values. For basic level calculations of formation of ionic
compounds, you use the relevant ΔH values and use born-haber cycles. Theoretical calculations are usually a bit different to the experimental
value.
carbocation - 7-6-2005 at 01:10
Sorry, I didn't properly explain myself then. The first bit (q=mcΔt) is for experimentally determining enthalpy values. I think what you
were asking is how to calculate what the energy change is going to be before you do the experiment? the easiest thing to do would be to look in
a data table for the enthalpys of formation and combustion. Basically, (and you'll have to consult a basic chem textbook for more information) it
doesn't matter what route a reaction takes, as the overall energy change will be the same. So you can use the enthalpy values to work out how
much heat will be generated by what products will be made. What reaction did you want to calculate the change for?
kclo4 - 9-6-2005 at 09:23
Well the reaction I want to calculate is 6NH4ClO4 + 16B = 8B2O3 + 6NH4Cl but the more I learn about calculating the temperature change harder it seems
since this is obviously a out of solution reaction. There would seem to be many other factors it almost seems to be impossible for me to calculate the
reaction or be able to add in all the factors (having not even had a single chemistry class in my life). But if someone has an answer I would rather
have the formula then answer to the reaction so I do not have to ask every time I figure the heat of a propellant
[Edited on 9-6-2005 by kclo4]
The_Davster - 9-6-2005 at 20:13
This should work (theoreticaly)
Net energy change = 8 x (molar enthalpy of formation of B2O3) + 6 x(molar enthalpy of formation of NH4Cl) - 6 x (molar enthalpy of formation of
ammonium perchlorate)
This is some times known as the formation reaction method or Hess' shortcut. The equation is
(delta)H(net)= (sum of)nH(products) -(sum of)nH(products)