Question

chem:1220

ΔS is positive for the reaction ________.
A) 2NO (g) + O2 (g) → 2NO2 (g)
B) 2N2 (g) + 3H2 (g) → 2NH3 (g)
C) C3H8 (g) + 5 O2 (g) → 3CO2 (g) + 4 H2O (g)
D) Mg (s) + Cl2 (g) → MgCl2 (s)
E) C2H4 (g) + H2 (g) → C2H6 (g)

Answer 1

In order to have a positive dS (of the reaction) we must create CHAOS in the reaction

chaos is created when:

- moles of solid turn to liquid or gas (more cahtoic system)

- moles of liquid turn to gas ( similar, more chaos)

- som moles of gas react to form MORE moles of gas

then, knowing this:

A)

2+1 = 3 mol of gas form 2 mol of gas; therefore, there is a decrease in gas, therefore, chaos, i.e. negative dS

b)

2+3 = 5 mol of gas form 3 mol of gas,  therefore, there is a decrease in gas, therefore, chaos, i.e. negative dS

c)

1+5 = 6 mol of gas form 3+4 = 7 mol of gas, therefore, there is an increase in gases / chaos, it must have a POSITIVE dS value

d)

1 mol of gas (Cl2) forms a solid MgCl2, there is an ordering in the system, therefore negative dS

e)

1+1 = 2 mol of gas form 1 mol of gas, therefore, there is a decrease in gas, therefore, chaos, i.e. negative dS

The best answer is then C

choose C) C3H8 (g) + 5 O2 (g) → 3CO2 (g) + 4 H2O (g)