[nerd32]

.010 liters of 2.00 molar silver nitrate is added to .010 liters of 2.00 molar sodium phosphate solution at 23 degrees celcius in a caloromiter, a silver phosphate precipitate forms and the temp. of the aqueous mixture increases to 40.1 degrees C.  The specific heat of the aqueous mixture is 4.18J(g*C), density of the mixture is 2.5g/mL . The caloromited itself absorbs a negligble amount of heat

calculate delta H in kCal/mol for the reaction

[nerd32]

Im confused on this problem because the chapter in our book only spends two paragraphs talking about this.  I don't know what to do with the density and all of my example problems deal with grams of a substance, not liters..im lost on this

[opti384]

Start with heat = specific heat x mass x temperature change.

[rabolisk]

Dimensional analysis will help you a lot in this problem. Basically you know that temperature changes as heat is absorbed (relates delta T to delta H*). You also know that a reaction occurs, and there is an enthalpy change associated with reaction. That's the chemistry of this problem.

To solve this, balance the reaction. Relate heat (enthalpy) evolved from the reaction to change in temperature via specific heat. The mathematical part of this problem is dealing with the fact that specific heat is in units of J/C*g, while delta H is to be in units of kCal/mol. A hint is that you will need density...

I am purposely being vague so you can figure it out on your own with a few nudges from me.

*Change in enthalpy is not necessarily the same as heat evolved/absorbed, but in most chemical reactions under constant pressure, they are. This is beyond the scope of gen chem, but this is why the term heat and enthalpy are generally interchangeable in chemistry.

[nerd32]

Alright so I found q= S x M x change in T, which is -3573.9J.  I know need to find Joule/mol, so what do i divide by.  do I use 3 mol of silver nitrate of 1 mol of sodium phosphate(from the balanced equation)?

[rabolisk]

Whenever you calculate delta H for the reaction, the units are always in energy/mol, to signify that this is delta H for the reaction as written. However, there is some confusion as to whether "1 mol" of reaction is simply the reaction with the coefficients as written, or the reaction with the lowest integer coefficients...

I suggest just converting joules to kcals for now. When the question asks to calculate delta H, they may only be asking for the delta H of the reaction that actually occurred. Sorry if the above didn't make much sense.