Okay, stupid question coming, but how exactly do I calculate heat of formation?

Question

jfraser · Answer

most of the time, you won't have to calculate a heat of formation. (DeltaH) Usually, we use the heats of formation to calculate the enthalpy of a reaction, by taking the sum of all the DH's of the products minus the sum of all the DH's of the reactants.

jfraser · Answer

almost every textbook will have a table of formation values in the appendix

anonymous · Answer

Well, the problem is asking us to calculate the heat of reaction for the dissolution of an ionic compound in water in a styrofoam cup calorimeter. It gives me grams of compound, mL of water, and temperature change...

jfraser · Answer

that's not a heat of formation, specifically, but give me a second and I'll work through the #s

jfraser · Answer

the first law of thermo says that whatever energy is lost by one part of a system must be gained by its surroundings. knowing the mass of water and the change in temp of the water allows us to calculate how much heat was absorbed by the water, which will be equal to the heat released by the dissociation.$$Q_{abs} = m_{H_2O}*C_P* \Delta T_{H_2O}$$Since you know the mass and formula of the ionic compound, you can find the moles that were dissolved
$$mol_{cpd} = \frac{g_{cpd}}{MM_{cpd}}$$ the heat of dissocation will be the ratio of$$\frac{kJ}{mol}$$
Divide Q by moles and there you have it.

anonymous · Answer

Okay... I'll try it. I'll let you know if it works.

anonymous · Answer

We don't know the formula of the compound...

jfraser · Answer

in that case, the best you can do is kJ/gram, instead of kJ/mole. Unless you're given more info...