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