Second law, entropy and spontaneity
- In which of the following processes entropy increases?A. A liquid evaporates to vapour.B. Temperature of a crystalline solid lowered from 130 K to 0 K.C. 2NaHCO_3(s) → Na_2CO_3(s) + CO_2(g) + H_2O_(g)D. Cl_2(g) → 2Cl_(g)Choose the correct answer from… Answer: 4. A, C and D Key idea: entropy rises with disorder, i.e. when more gas or more particles are formed. A: liquid to vapour, disorder... 1
- For irreversible expansion of an ideal gas under isothermal condition, the correct option is : Answer: 3. Δ U=0, Δ S_total≠0 Key idea: for an ideal gas, internal energy depends only on temperature. Isothermal means Δ U=0 . An irreversi... 1
- For the reaction, 2Cl(g) → Cl_2(g), the correct option is : Answer: 4. Δ_rH < 0 and Δ_rS < 0 2Cl(g)→ Cl₂(g) forms a Cl-Cl bond. Bond formation releases energy, so Δ_rH<0 . Two particles become one, so... 1
- In which case change in entropy is negative ? Answer: 4. 2H(g) → H ₂ (g) Entropy measures disorder. Evaporation, sublimation and isothermal expansion all increase randomness ( Δ S>0 ). I... 1
- For a sample of perfect gas when its pressure is changed isothermally from p_i to p_f, the entropy change is given by Answer: 1. Δ S = nRln(p_i/p_f) Key idea: for an isothermal ideal gas, Δ S=nRln(V_f/V_i) , and V∝1/p . Δ S=nRlnV_f/V_i=nRlnp_i/p_f The T does... 1
- The correct thermodynamic conditions for the spontaneous reaction at all temperatures is : Answer: 3. Δ H < 0 and Δ S > 0 Key idea: Δ G=Δ H-TΔ S . A reaction is spontaneous at all temperatures when Δ G<0 for every T. This needs Δ H... 1