Predict the sign of ∆Ssys for the following processes.

Predict the sign of ∆Ssys for the following processes.

a) N₂(g) → 2 N(g)

b) K(l) → K(s)

c) 3 O₂(g) → 2 O₃(g)

d) CaCO₃(s) → CaO(s) + CO₂(g)

e) Compressing 5.0 L of Cl₂(g) to 1.0 L (constant temperature)

f) Heating steam from 100 °C to 200 °C

Solution:

The change in entropy, ΔS, represents the degree of disorder in a system. When looking at an equation there are two things you have to look for to predict the sign of ∆S.

1. Phases (gas,liquid, solid) going from reactants to products. Gases are more disordered than liquids, and liquids are more disordered than solids.

   For example, going from a solid to a gas results in an increase in entropy (ΔS > 0).

2. Coefficients in the equation. Coefficients represent moles and if we go from less to more moles, ∆S will increase as well.

   Now, lets look at the examples.

   a) N2(g) → 2 N(g) The phase is the same (gas), but the number of moles increases from 1 to 2 → ΔS is positive

   b) K(ℓ) → K(s), going from liquid to solid ∆S is negative

   c) 3 O2(g) → 2 O3(g) Phase is the same (gas) but we go from 3 to 2 moles, ∆S is negative

   d) CaCO3(s) → CaO(s) + CO2(g) We go from solid to solid and gas, ∆S is positive

   e) compressing 5.0 L of Cl2(g) to 1.0 L (constant temperature), Compressing means particles are not as free to travel, decreasing disorder. ∆S is negative

   f) heating steam from 100 °C to 200 °C. Increasing temperature increases disroder, ∆S is positive

   
   
   
   

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