4.15 (7th edition)

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4.15 (7th edition)

Postby Noh_Jasmine_1J » Tue Feb 05, 2019 3:00 pm

for 4.15 in the 7th edition book, the balanced chemical equation is 2HCl(aq)+Zn(s)--->H2(g)+ZnCl2(aq) and it tells you that a piece of zinc metal of mass 8.5 g is dropped into an apparatus containing 800.0ml of 0.5M HCl(aq). If the initial temp of the HCl acid solution is 25C, what is the final temp of the solution?

I know that you're supposed to calculate the enthalpy of the reaction first, but in the solutions manual it does the enthalpy of formation of reactants-enthalpy of formation of products, but I thought it was the other way around

Jonas Talandis
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Re: 4.15 (7th edition)

Postby Jonas Talandis » Tue Feb 05, 2019 9:38 pm

The reason the book has the enthalpy of formation of the reactants-products is because the signs of these values flip. Considering that most enthalpies of formation are given as negative values, we know that that is the energy required to break the initial bonds. For this reason, the sign of the values associated with the reactants change to positive values because we aren't forming them, we're breaking them and that requires a positive amount of energy to do. For the products, the values remain negative because these are actually forming and release energy. Therefore instead of products-reactants, because of the sign flip, it's reactants-products.

Phan Tran 1K
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Re: 4.15 (7th edition)

Postby Phan Tran 1K » Thu Feb 07, 2019 6:26 pm

For reaction enthalpies of combustion, its products-reactants, while it is the other way around for reaction enthalpies of formation. I just kinda memorize this.

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