Reaction Enthalpies - #7.53

[Mara Lockhart 3J]

To calculate the change in temperature of solution, we have to plug into the equation q=mCsdeltaT. For the q, I am confused as to where -20,000 came from? The question gave a hint of "Assume the density and molar heat capacity of the hydrochloric acid solution are the same as those of water and that all the heat is used to raise the temperature of the solution." I looked up heat capacities, but did not find the number 20,000 anywhere. Where does this number come from?

[Neil DSilva 1L]

I'm going to assume you're looking at the solutions manual. just comes from the previous step, where the enthalpy per mole of zinc consumed is determined and then multiplied by the number of moles of zinc. Zinc releases and this is equivalent to when converted to joules.

[Astred 1D]

Hi,

So the goal in this problem is to find the final temperature, So you need to find "q". Because the reaction take place at constant pressure, we know that "q=deltaH" You use standard formation enthalpies given in the back of the book, and the equation that finds deltaH (standard formation of products-standard formation of reactants). You then get a DeltaH, but remember this value is per moles of zinc, and you want your value in just kj, so simply multiply by the moles per 8.5g of zinc. You then get the value you were looking for.

Hope this helps.

[Mara Lockhart 3J]

Thank you both! I had gotten stuck, but this helped a lot.