Free energy is the ultimate judge of whether chemical reactions...
Chemistry82 views·Updated 12 Jul 2026·4 pagesUnderstanding Free Energy: Enthalpy and Entropy Explained
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1
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Understanding Free Energy Change
Free energy change (ΔG) is your go-to measurement for predicting whether a chemical reaction will actually happen. Think of it as the energy bookkeeper that considers everything going on during a reaction.
The clever bit is that ΔG takes into account two crucial types of energy: enthalpy change (ΔH) - the heat energy exchanged with surroundings, and the entropy change (ΔS) - how much disorder changes within the system itself.
You'll use the Gibbs equation to calculate this: ΔG = ΔH - TΔS. Remember that temperature must be in Kelvin, and watch out for units - entropy is often given in J K⁻¹ mol⁻¹, so divide by 1000 to match enthalpy's kJ mol⁻¹.
Key Rule: If ΔG is negative, your reaction is energetically feasible. Positive ΔG means it won't happen spontaneously.
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of 4
Working Through a Free Energy Calculation
Let's tackle a real example: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. You're given ΔH° = +91 kJ mol⁻¹ and various entropy values that need converting from J to kJ.
First, calculate ΔS° using products minus reactants: - (2 × 0.102) = +0.215 kJ K⁻¹ mol⁻¹. Notice how the gas formation increases entropy significantly.
Now plug everything into the Gibbs equation at 298 K: ΔG = +91 - (298 × 0.215) = +26.93 kJ mol⁻¹. Since this is positive, the reaction isn't feasible at room temperature.
Pro Tip: Gas formation usually creates large positive entropy changes, which can make unfavourable reactions feasible at higher temperatures.
3
of 4
Finding the Minimum Temperature for Feasibility
When ΔG equals zero, you've found the minimum temperature where your reaction becomes feasible. Set the Gibbs equation to zero: ΔG = ΔH - TΔS = 0, which rearranges to T = ΔH/ΔS.
Using our previous example: T = 91/0.215 = 423 K (150°C). Above this temperature, the reaction becomes thermodynamically feasible because TΔS finally outweighs ΔH.
The pattern is clear: exothermic reactions (negative ΔH) with positive entropy changes are always feasible. Endothermic reactions need high temperatures when ΔS is positive, but they're never feasible if ΔS is negative.
Memory Trick: For feasibility, you need TΔS to be larger than ΔH - think of entropy fighting against unfavourable enthalpy changes.
4
of 4
Limitations You Need to Know
Here's the crucial limitation that might catch you out in exams: ΔG only tells you about thermodynamic feasibility, not reaction rate. A reaction might have a lovely negative ΔG but still appear not to happen at all.
The missing piece is activation energy (Ea). If this energy barrier is sky-high, your reaction crawls along so slowly it seems like nothing's happening, even though ΔG says it should work perfectly.
This is why understanding both thermodynamics and kinetics matters - ΔG predicts the destination, but kinetics determines how long the journey takes.
Exam Alert: Always remember that negative ΔG means thermodynamically feasible, but you might need a catalyst to make it happen at a reasonable rate.
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Chemistry82 views·Updated 12 Jul 2026·4 pagesUnderstanding Free Energy: Enthalpy and Entropy Explained
Free energy is the ultimate judge of whether chemical reactions can actually happen - it's like having a crystal ball that tells you if your reaction will work or not. Understanding how to calculate and interpret free energy changes will...
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Understanding Free Energy Change
Free energy change (ΔG) is your go-to measurement for predicting whether a chemical reaction will actually happen. Think of it as the energy bookkeeper that considers everything going on during a reaction.
The clever bit is that ΔG takes into account two crucial types of energy: enthalpy change (ΔH) - the heat energy exchanged with surroundings, and the entropy change (ΔS) - how much disorder changes within the system itself.
You'll use the Gibbs equation to calculate this: ΔG = ΔH - TΔS. Remember that temperature must be in Kelvin, and watch out for units - entropy is often given in J K⁻¹ mol⁻¹, so divide by 1000 to match enthalpy's kJ mol⁻¹.
Key Rule: If ΔG is negative, your reaction is energetically feasible. Positive ΔG means it won't happen spontaneously.
2
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Working Through a Free Energy Calculation
Let's tackle a real example: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. You're given ΔH° = +91 kJ mol⁻¹ and various entropy values that need converting from J to kJ.
First, calculate ΔS° using products minus reactants: - (2 × 0.102) = +0.215 kJ K⁻¹ mol⁻¹. Notice how the gas formation increases entropy significantly.
Now plug everything into the Gibbs equation at 298 K: ΔG = +91 - (298 × 0.215) = +26.93 kJ mol⁻¹. Since this is positive, the reaction isn't feasible at room temperature.
Pro Tip: Gas formation usually creates large positive entropy changes, which can make unfavourable reactions feasible at higher temperatures.
3
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Finding the Minimum Temperature for Feasibility
When ΔG equals zero, you've found the minimum temperature where your reaction becomes feasible. Set the Gibbs equation to zero: ΔG = ΔH - TΔS = 0, which rearranges to T = ΔH/ΔS.
Using our previous example: T = 91/0.215 = 423 K (150°C). Above this temperature, the reaction becomes thermodynamically feasible because TΔS finally outweighs ΔH.
The pattern is clear: exothermic reactions (negative ΔH) with positive entropy changes are always feasible. Endothermic reactions need high temperatures when ΔS is positive, but they're never feasible if ΔS is negative.
Memory Trick: For feasibility, you need TΔS to be larger than ΔH - think of entropy fighting against unfavourable enthalpy changes.
4
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Limitations You Need to Know
Here's the crucial limitation that might catch you out in exams: ΔG only tells you about thermodynamic feasibility, not reaction rate. A reaction might have a lovely negative ΔG but still appear not to happen at all.
The missing piece is activation energy (Ea). If this energy barrier is sky-high, your reaction crawls along so slowly it seems like nothing's happening, even though ΔG says it should work perfectly.
This is why understanding both thermodynamics and kinetics matters - ΔG predicts the destination, but kinetics determines how long the journey takes.
Exam Alert: Always remember that negative ΔG means thermodynamically feasible, but you might need a catalyst to make it happen at a reasonable rate.
We thought you’d never ask...
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
You can download the app from Google Play Store and Apple App Store.
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