Chemistry82 views·Updated May 12, 2026·4 pages

Understanding Free Energy: Enthalpy and Entropy Explained

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Free energy is the ultimate judge of whether chemical reactions... Show more

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$22.5- free energy Key Facts: - free energy change ($\Delta$G) = represents the overau change in energy during chemical reaction Notes: - t$

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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$22.5- free energy Key Facts: - free energy change ($\Delta$G) = represents the overau change in energy during chemical reaction Notes: - t$

Working Through a Free Energy Calculation

Let's tackle a real example: 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(l) + CO₂(g). 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: (0.135 + 0.07 + 0.214) - (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.

of 4

$22.5- free energy Key Facts: - free energy change ($\Delta$G) = represents the overau change in energy during chemical reaction Notes: - t$

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.

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$22.5- free energy Key Facts: - free energy change ($\Delta$G) = represents the overau change in energy during chemical reaction Notes: - t$

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 May 12, 2026·4 pages

Understanding Free Energy: Enthalpy and Entropy Explained

sheila@study_withsheila

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... Show more

of 4

$22.5- free energy Key Facts: - free energy change ($\Delta$G) = represents the overau change in energy during chemical reaction Notes: - t$

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Understanding Free Energy Change

Key Rule: If ΔG is negative, your reaction is energetically feasible. Positive ΔG means it won't happen spontaneously.

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$22.5- free energy Key Facts: - free energy change ($\Delta$G) = represents the overau change in energy during chemical reaction Notes: - t$

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Working Through a Free Energy Calculation

Let's tackle a real example: 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(l) + CO₂(g). 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: (0.135 + 0.07 + 0.214) - (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.

of 4

$22.5- free energy Key Facts: - free energy change ($\Delta$G) = represents the overau change in energy during chemical reaction Notes: - t$

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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.

Memory Trick: For feasibility, you need TΔS to be larger than ΔH - think of entropy fighting against unfavourable enthalpy changes.

of 4

$22.5- free energy Key Facts: - free energy change ($\Delta$G) = represents the overau change in energy during chemical reaction Notes: - t$

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Limitations You Need to Know

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...

What is the Knowunity AI companion?

Where can I download the Knowunity app?

You can download the app from Google Play Store and Apple App Store.

Is Knowunity really free of charge?

That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.

Can't find what you're looking for? Explore other subjects.

Students love us — and so will you.

4.6/5App Store

4.7/5Google Play

Stefan SiOS user

Samantha KlichAndroid user

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