Textbook Question
Without doing any calculations, determine the sign of ΔSsys for each chemical reaction. b. CH2=CH2( g) + H2( g) → CH3CH3( g)
Ch.18 - Free Energy and Thermodynamics
Chapter 18, Problem 37b
Without doing any calculations, determine the signs of ΔSsys and ΔS surr for each chemical reaction. In addition, predict under what temperatures (all temperatures, low temperatures, or high temperatures), if any, the reaction is spontaneous: b. N2(g) + O2(g) → 2 NO(g), ΔH°rxn = +182.6 kJ; d. 4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2O(g), ΔH°rxn = -906 kJ.
Verified step by step guidance1
insert step 1> Identify the type of reaction and the change in the number of moles of gas for each reaction.
insert step 2> For reaction b, N2(g) + O2(g) → 2 NO(g), note that the number of moles of gas remains the same (2 moles of reactants to 2 moles of products), suggesting a small or negligible change in entropy of the system (ΔSsys).
insert step 3> For reaction d, 4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2O(g), observe that the number of moles of gas increases from 9 moles of reactants to 10 moles of products, indicating an increase in entropy of the system (ΔSsys > 0).
insert step 4> Consider the sign of ΔH°rxn for each reaction: for reaction b, ΔH°rxn is positive, indicating an endothermic reaction, which means ΔSsurr is negative; for reaction d, ΔH°rxn is negative, indicating an exothermic reaction, which means ΔSsurr is positive.
insert step 5> Predict the spontaneity based on ΔG = ΔH - TΔS: for reaction b, since ΔH is positive and ΔSsys is small, the reaction is non-spontaneous at all temperatures; for reaction d, since ΔH is negative and ΔSsys is positive, the reaction is spontaneous at all temperatures.
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Entropy (ΔS)
Entropy, denoted as ΔS, is a measure of the disorder or randomness in a system. In chemical reactions, an increase in the number of gas molecules typically leads to a positive change in entropy, while a decrease results in a negative change. Understanding the signs of ΔSsys (system entropy) and ΔSsurr (surroundings entropy) is crucial for predicting the spontaneity of reactions.
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Entropy in Thermodynamics
Enthalpy (ΔH)
Enthalpy, represented as ΔH, is the heat content of a system at constant pressure. It indicates whether a reaction is exothermic (releases heat, ΔH < 0) or endothermic (absorbs heat, ΔH > 0). The relationship between ΔH and ΔS is essential for determining the spontaneity of a reaction using the Gibbs free energy equation, ΔG = ΔH - TΔS.
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Gibbs Free Energy and Spontaneity
Gibbs free energy (ΔG) combines enthalpy and entropy to predict the spontaneity of a reaction. A reaction is spontaneous when ΔG is negative, which can occur under specific temperature conditions depending on the signs of ΔH and ΔS. For reactions with positive ΔH and positive ΔS, spontaneity is favored at high temperatures, while for negative ΔH and positive ΔS, spontaneity is favored at all temperatures.
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Related Practice
Textbook Question
Without doing any calculations, determine the sign of ΔSsys for each chemical reaction. a. Mg(s) + Cl2(g) → MgCl2(s) b. 2 H2S(g) + 3 O2(g) → 2 H2O(g) + 2 SO2(g) c. 2 O3(g) → 3 O2(g) d. HCl(g) + NH3(g) → NH4Cl(s)
Textbook Question
Without doing any calculations, determine the signs of ΔSsys and ΔS surr for each chemical reaction. In addition, predict under what temperatures (all temperatures, low temperatures, or high temperatures), if any, the reaction is spontaneous. a. C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g) ΔH°rxn = -2044 kJ
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Textbook Question
Without doing any calculations, determine the signs of ΔSsys and ΔSsurr for each chemical reaction. In addition, predict under what temperatures (all temperatures, low temperatures, or high temperatures), if any, the reaction is spontaneous. c. 2 N2(g) + O2(g) → 2 N2O(g) ΔH°rxn = +163.2 kJ
Textbook Question
Calculate ΔSsurr at the indicated temperature for each reaction. d. ΔH°rxn = +114 kJ; 77 K