[AlphaScent]

Hello all,

I have a question from a quiz I am correcting and I believe the key may be wrong.  Please set me straight if I am messing up thermodynamics right now.

Question:

Two solutions originally at 24.6 deg C are mixed in a calorimeter.  When 100.0 mL of 0.1M silver nitrate (aq) is mixed with excess NaCl (aq), the temperature of the solution rises to 28.3 deg C.  Determine Delta H rxn for the reaction as written below:

AgNO3(aq) + NaCl(aq)  AgCl (aq) + NaNO3 (aq)

assume d = 1.015 g/ml for both solutions and specific heat of each solution is 4.195 J/g°C.

Since the solutions are both the same temperature does the ΔH rxn not only come from the limiting reactant silver nitrate?

using MW AgNO3= 169.87 g/mol; you have 1.69 g of silver nitrate reacting in the reaction.

so plugging into q = mcΔT; you get -26.37 J (exothermic makes it negative).  Divide by moles of silver nitrate, 0.01 gives, after conversion, -2.64 kJ/mol for ΔH rxn.

My teacher used 100 mL of solution and multiplied by density to get 101.5 g and plugged that into q = mcΔT.  But the reaction depends on the heat from the reaction not water.  Does he use the mass of the water as well because of the heat capacities?  I have read that the m in the equation if the reactants + water + calorimeter....

I am a little confused as to if I have assumed wrong. 

Much appreciated!

Cheers!

[AlphaScent]

Im wrong.

You are given the heat capacity of the solution!!!  So you use the density and 101.5 g.

I feel foolish.

Cheers!

[mjc123]

Unless you've missed something out, you're not told the volume of NaCl solution, only that it is "excess". So you don't know the total volume of solution, so there is no way to work out an answer.