Thermodynamics

Question 1

Anhydrous magnesium chloride, MgCl2, can absorb water to form the hydrated salt MgCl2.4H2O:
MgCl2 (s) + 4H2O (l) → MgCl2.4H2O (s)
Suggest one reason why the enthalpy change for this reaction cannot be determined directly by calorimetry

Answer 1

Not possible to prevent some dissolving

Question 2

A reaction is exothermic and has a negative entropy change.
Explain which statement is correct and why
A. The reaction is always feasible
B. The reaction is feasible above a certain temperature
C. The reaction is feasible below a certain temperature
D. The reaction is never feasible

Answer 2

• ΔG = ΔH - TΔS
• Remember, for reaction to be feasible ΔG must be 0 or negative
• ‘Exothermic’ tells us ΔH is negative and ‘negative entropy change’ tells us S is negative, making -TΔS positive
• Therefore, T cannot be too large or else it would make -TΔS more positive than ΔH is negative, making ΔG positive which makes it unfeasible.
• So, C is the correct option