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Predicting Reaction Feasibility: Understanding ΔG, ΔH, and ΔS

This article explores the criteria for determining if a chemical reaction is feasible using Gibbs free energy (ΔG). It explains the role of enthalpy (ΔH) and entropy (ΔS) in the equation ΔG = ΔH - TΔS, where a negative ΔG indicates a spontaneous reaction. Specific examples, like the reaction of sodium bicarbonate with hydrochloric acid and the formation of CO2 from graphite, illustrate how to calculate ΔG at standard temperature. Understanding these principles helps predict the feasibility and spontaneity of various reactions.

joel-wood
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Predicting Reaction Feasibility: Understanding ΔG, ΔH, and ΔS

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  1. Reaction Feasibility Will a specific reaction occur ?

  2. How can we predict if a reaction is feasible ? NaHCO3 + HCl → NaCl + H2O + CO2 ΔH is +ve (endothermic) Entropy increases We know this reaction is feasible

  3. How can we predict if a reaction is feasible ? ΔG = ΔH - T ΔS ΔG = free energy ( kJmol-1) ΔH = enthalpy change ( kJmol-1) T = temperature (K) ΔS = entropy change ( kJK-1mol-1) ΔS must be converted from JK-1mol-1 (/1000)

  4. How can we predict if a reaction is feasible ? ΔG = ΔH - T ΔS A reaction is feasible if ΔG is negative ΔG < 0 A reaction becomes feasible when ΔG = 0

  5. Calculate ΔG for the formation of CO2 from graphite at 298K, givenΔHf = -393.5kJmol-1& ΔS is +3.3 JK-1mol-1 ΔG = ΔH - T ΔS (-393.5) – (298 x 3.3/1000) ΔG = -394.5 kJmol-1 Feasible reaction ΔG = -ve

  6. Is the decomposition of limestone spontaneous at standard temperature?ΔS = +165 JK-1mol-1 ΔH = +178 kJmol-1 ΔG = ΔH - T ΔS (+178) – (298 x 165/1000) ΔG = +128.83 kJmol-1 Not feasible ΔG = + ve

  7. Feasible  Spontaneous Thermodynamics tells us nothing about the speed a reaction happens It only tells us that it can happen C + O2 → CO2 ΔG = -394.5 kJmol-1 EA is large so reaction very slow at 298K

  8. Calculate ΔG for the rusting of Fe at 298K, givenΔHf = -825 kJmol-1& ΔS is -272 JK-1mol-1 2Fe(s) + 3/2O2(g) → Fe2O3 (s) ΔG = ΔH - T ΔS (-825) – (298 x (-272/1000) ΔG = -744 kJmol-1 Feasible reaction ΔG = -ve

  9. At what temperature does this reaction cease to be feasible?ΔHf = -825 kJmol-1 & ΔS is -272 JK-1mol-1 ΔG = ΔH - T ΔS Reaction becomes feasible when ΔG = 0 ΔH = T ΔS ΔH = T ΔS = 3033K T = -825/-0.272

  10. Free energy & Equilibrium ΔG = ΔH - T ΔS ΔG = 0 for a system at equilibrium Changes of state are systems at equilibria ΔS =ΔH T ΔH = T ΔS ΔH = T ΔS

  11. Calculate the entropy change when ice melts given thatΔHfusion (melting) = + 6.0 kJmol-1 ΔG = ΔH - T ΔS ΔG = 0 for a change of state ΔH = T ΔS ΔS =ΔH T ΔS = + 6/273 = 0.0219 kJK-1mol-1 = 0.0219 x 1000 JK-1mol-1 = 21.9 JK-1mol-1

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