Question

Predict the sign for the entropy change ΔSsys for each of the following reactions without doing any calculations and explain your reasoning.

a.Mg(s) + Cl2(g) MgCl2(s)

b.HCl(g) + NH3(g) NH4Cl(s)

c.2 O3(g) 3 O2(g)

d.CH3CH3(g) H2C=CH2(g) + H2(g)

Answer 1

• Change in Entropy of system (ΔSsys) tells us about the degree of randomness of the solution.
• When Change in Entropy comes positive, it means randomness increases and when it comes as negative, randomness of solution decreases.
• Gas has highest degree of randomness and is much more than solid,liquid and gas and therefore has highest ΔSsys.
• The order of entropy change is: Solid<liquid<aqueous<<<gas.
• As gas has very high ΔSsys, we neglect entropy of solid, liquid and aqueous while calculating sign of entropy.

a). Mg(s) + Cl₂(g) ------> MgCl₂(s) ΔSsys= 0-2 = negative [as entropy of solid is very low, we neglect it]

b). HCl(g) + NH₃(g) ------> NH₄Cl(s) ΔSsys= 0-2 = negative (moles of product - moles of reactant)

c). 2 O_{3(g) ------>} 3O₂(g)   ΔSsys=3-2 = positive

d) Equation 4 is wrong.

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