Enthalpy AP Chemistry

corabelle

Hello! My first time on here but having massive troubles with my AP Chemistry homework and need help. I have a question that gives four different reactions and I am asked to calculate the (delta)H for each of them. I am given no more information. I am not sure how to go about the problem...

I have the idea that for each reaction to add together the enthalpy values for the products and subtract from that the sum enthalpys for the reactants....but I do not know where to obtain or calculate these values.

I am online now and will be until this is finished - my MSN is forest_angel@hotmail.com if you'd rather do that than post on here - but I could REALLY use some help on this and I'd appreciate a response asap if anyone has ANY idea or ANY advice.

Thanks again!!
-Cora

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Monique

Staff Emeritus
Gold Member
Hi Corabella, I am afraid I cannot be of too much value on Enthalphy. Did you try looking up formulas and try to apply them? For instance this one:

dH = dE + w = dE + PdV = dE + dnRT

Basically: the difference in heat equals the diffence in energy plus the pressure of the system times the volume.

But your idea sounded as if it might work, did you find the correct answer yet? Maybe you can teach me a few things :P

lavalamp

If you've been set this question, then don't you have a databook to get the values from?

The chances are that you are expected to use Haber cycles to calculate the answers, or if you are at a slightly higher level of Chemistry, then Born-Haber cycles. Either way, you'll need some values. If you don't have some then I'd go and bother my teacher about it.

daniel3777

your book should have an appendix with selected thermodynamic data in it. if i understand the question right, i think you may need the enthalpy of formation of the chemicals.

ShawnD

The short method of doing enthalpy of reactions is energy of formation for the products minus the energy of formation for the reactants.
dH = [product1 + product2] - [reactant1 + reactant2]

Of course you can always have more than 2 products and reactants. You can have as many as you want.

Another way is to have several reactions where you know the energy change in each reaction then you sort of combine the terms. It's a lot like solving for simultaneous equations in math. It's hard to give examples for this method. Post a problem and I could show you how it works.

GCT

Homework Helper
Almost every standard chemistry book has a list of selected reactions with the corresponding change in enthalpy (a list of standard change in enthalpy...look up the chapter on calorimetry).

The topic of enthalpy can be found in the chapter on calorimetry. Although the concept has a broader relevance to chemistry than just calorimetry. Enthalpy has to do with heat. If heat is added to a closed system than the change in heat is directly related to the change in energy of the closed system-constant volume system- (the temperature change). If heat is added to a constant pressure system than the heat energy added to the system is equal to the change in enthalpy of the system which equals the energy change in molecules plus the any work that the molecules performed in expanding against the atmosphere, PV. Thus we have the special term enthalpy rather than just the change in energy of the system. Since most reactions are performed in a constant pressure system, enthalpy applies broadly. The enthalpy of a reaction is related to whether a reaction will proceed; along with entropy. Every reaction has a series of steps; bond breaking, ion formation, hydration, forming ionic bonds, forming covalent bonds etc...Some processes are associated with energy released through bond forming others are associated with energy cost. Enthalpy indicates whether the reaction is favorable in a certain direction in terms of energetics. It can be found whether a reaction is favorable if one were to add and subtract the enthalpies of each step in the process of the reaction. You are currently working with standard change in enthalpy. The standard change in enthalpy has already been found for you. So try applying the latter idea to the equation delta H product (moles of product) - delta H reactant (moles reactant). You should not even need any equations.

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