# Gibbs Free Energy

How can enthalpy and entropy be used to explain reaction spontaneity?

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Gibbs free energy is a useful formula we can use to determine whether chemical species which we mix together are going to react with one another in the given conditions. It is important to remember a chemical reaction is trying to achieve greater stability – the following formula can tell us whether a reaction would achieve a more stable configuration:

• is the gibbs free energy (kJ),
• If this is negative the reaction is spontaneous in the given conditions
• If this is zero the reaction is at equilibrium
• If this is positive the reaction is not spontaneous in the given conditions
• Is the change in enthalpy (kJ) – this means the change in the total heat content of the system
• If this value is positive the system is endothermic
• If this value is positive the system is exothermic
• is the temperature of the system in kelvin (K) (be careful a common mistake is to use this in degrees)
• When going from degrees celsius to kelvin, add 273.15
• When going from kelvin to degrees celsius, subtract 273.15
• Is the change in entropy (kJ/K). Entropy is the measure of disorder of a system. So if your room is really messy and chaotic, then it has high entropy. Disorder is more probable then order hence a system tends to become more disorder over time. However, it is important to keep in mind a system can become more ordered.

Entropy is increasing if:

• From (l) → (g)
• From (s) → (l)
• From (s) → (g)
• Fewer moles → more moles

This table summarises the conditions’ effect on Gibbs free energy, however, it is recommended to derive these by looking at the equation to minimise the need to memorise.

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Master.

Question 1.

IS the following reaction spontaneous or non spontaneous if the reaction vessel is at 273.1 K, the delta H is 273, delta S is 180 kJ/K?

Question 2.

Reactants are added to a reaction vessel at 244 degrees celsius, the H of products is 88 kJ/mol and the H of reactants is 164 kJ/mol. The delta S of this reaction is 273 kJ/K.

Is the reaction spontaneous or non spontaneous?

Question 3.

Delta S is 3400 J/K and Delta H is 270 kJ.

What temperature, if any, is required for this reaction to be spontaneous?

Question 1.

Delta G = -48885 kJ

Therefore the reaction is spontaneous.

Question 2.

244 -273 = -29K

164 – 88 = 76kJ

Delta G = 7993 kJ

Therefore the reaction is non spontaneous

Question 3.

3400/1000 = 3.4kJ

Delta G is negative ( Therefore, delta G < 0)

0 < 270 – T(3.4)

270 < T(3.4)

79.4 < T

T  > 79.4

Learning Outcomes.

• analyse the differences between entropy and enthalpy
• use modelling to illustrate entropy changes in reactions
• predict entropy changes from balanced chemical reactions to classify as increasing or decreasing
entropy
• explain reaction spontaneity using terminology, including:
• Gibbs free energy
• Enthalpy
• Entropy
• solve problems using standard references and 𝛥𝐺𝑜 = 𝛥𝐻 𝑜 − 𝑇𝛥𝑆𝑜
(Gibbs free energy formula) to
classify reactions as spontaneous or non-spontaneous
• predict the effect of temperature changes on spontaneity