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Given the standard enthalpy of formation of $\ce{AgNO3(aq)} = -99 \text{ kJ/mol}$ and the standard enthalpy of formation of $\ce{Ag+(aq)} = +105 \text{ kJ/mol}$, what's the standard heat of formation of $\ce{NO3^{-}(aq)}$?

How can I know this if the standard enthalpy of reaction is not known? This is self test 10.1 in Atkins, PC.

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  • $\begingroup$ We've edited your question to make the chemistry look prettier. You can check out the faq for more information on how to do this yourself. I also added the homework tag. Even though this is not a homework question, you will want a homework style answer - one in which you are shown how to go about solving the problem, but you are not given the answer. $\endgroup$ – Ben Norris Apr 24 '13 at 11:01
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Silver nitrate, $\ce{AgNO3}$ is a strong electrolyte. Based on the information given, I expect you are meant to assume that it dissociates completely when dissolved in water:

$$\ce{AgNO3(aq)-> Ag+(aq) + NO3- (aq)}$$

Thus, we can make the assumption that $$\ce{AgNO3(aq)} \equiv \ce{Ag+(aq) + NO3- (aq)}$$

Then, we can state:

$$\Delta_\mathrm{f} H (\ce{AgNO3(aq)}) = \Delta_\mathrm{f} H (\ce{Ag+(aq)})+\Delta_\mathrm{f} H (\ce{NO3- (aq)})$$

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