Theromodynamics and Thermochemistry Test - 8

Enthalpy is defined as a measure of the total energy of a thermodynamic system including the internal energy. It is a state function and an extensive property of the system.
Specific, enthalpy, heat capacity and compressibility are intensive properties as these properties do not depend on the quantity of matter.

In thermodynamics, a state function is a property of a system that depends only on the current state of the system, not on the way in which the system acquired that state. For example, internal energy, enthalpy and entropy are state functions because they describe quantitatively an equilibrium state of a thermodynamic system, irrespective of how the system arrived in that state.

According to the first law of thermodynamics, energy can be changed from one form to another, but it cannot be created or destroyed.

As volume is constant, so work done in this process is zero. Hence, heat supplied is equal to change in internal energy.

For an exothermic reaction the total internal energy of the system decreases, hence ΔU is negative.

The products formed are more stable than the reactants, hence,ΔH is negative.

Therefore, the observations (a) and (d) are not correct.

NH₃(g) + 7/4O₂(g) → NO₂(g) + 3/2H₂O(g) 

ΔH^o(reaction) = ΣH_f^o(products) - ΣH_f^o(reactants) 

ΔH^o = [ΔH_f^o(NO₂(g)) + 3/2 x ΔH_f^o(H₂O(g))] - [ΔH_f^o(NH₃(g)) + 7/4 x ΔH_f^o(O₂(g))] 

ΔH^o = [+34 + (3/2 x -242)] - [-46 + (7/4 x 0)] 

ΔH^o = [+34 + (-363)] - [-46 + 0] = -329 + 46 = -283 kJ

Yes, this is the correct answer.

Entropy of a pure substance in the solid state is less than the entropy of the substance in the gaseous state.