Week 5 and 6 Assignment Question 9 - Deciding Which Equation to Use for Systems or Surroundings [ENDORSED]

[Sam Leistiko 2B]

Here is the homework problem.

Consider the following reaction at 298 K.

4Al(s)+3O2(g)⟶2Al2O3(s)ΔH∘=−3351.4 kJ

Calculate the following quantities. Refer to the standard entropy values as needed.

I correctly calculated the answer using standard molar entropies to find ΔSsys, and I correctly calculated ΔSsurr using ΔSsurr = −ΔHsys / T.

However, I do not understand why I need to use different equations for ΔSsys and ΔSsurr. Why is ΔSsys not calculated using ΔHsys / T? Additionally, when is it proper to use ΔHsys/T, and what is the underlying concept?

Thanks!

[Chem_Mod]

Using standard molar entropies to find ΔS_sys is the total entropy change for the system as it includes changes in 'structures' due to the chemical reaction inside the system: R ---> P

The surroundings only gain or lose heat (due to the reaction in the system), and therefore use: ΔS_surr = −ΔH_sys / T

Great question. I usually comment on this in class.