tbr test 3 physical sciences q 32

[2010premed]

---

Members do not see this ad.

Adding NH4NO3(s) to water lowers its temperature. How can this be explained?

A. The solvation energy is greater than the lattice energy.
B. The solvation energy is less than the lattice energy.

Answer: B

Howcome looking at it in the following way is wrong?

since delta H is positive, then products minus reactants is positive. Energy of products (solvation energy) is larger than energy of reactiavts (lattice energy)

 

[PiBond]

Adding NH4NO3(s) to water lowers its temperature. How can this be explained?

A. The solvation energy is greater than the lattice energy.
B. The solvation energy is less than the lattice energy.

Answer: B

Howcome looking at it in the following way is wrong?

since delta H is positive, then products minus reactants is positive. Energy of products (solvation energy) is larger than energy of reactiavts (lattice energy)

you're thinking of it in terms of heat of formation (Hess's law). the question is testing whether you know which bonds are stronger, the ones broken or formed based on the fact that the reaction is endothermic.

since the water got cooler, the reaction is endothermic. Given that delta H is positive and that we know that our delta H = energy of bonds broken (from lattice) - energy of bonds formed (solvation shell), we can see that the the solvation shell bonds formed must be of lower energy than our lattice energy in order for delta H to be positive.