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Suppose a reaction had a positve entropy change and a negative enthalpy change.

According to Le Chatelier's principle, the reaction should be less favorable at higher temperatures, as heat is released. The same is applicable when looking at a Van't Hoff plot:

enter image description here

However, the entropy change is also positive, and as

$\Delta G=\Delta H-T\Delta S$

one would expect the gibbs energy to decrease at higher temperatures, therefore increasing the reaction's equilibrium constant:

$K=e^{\frac{-G^{0}}{RT}}$

So what is going on here, why do those two statements contradict each other?

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6 Answers 6

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  • Using your first approach, van't Hoff :

$$ \begin{eqnarray} && \ln \frac{k_2}{k_1} = -\frac{∆H^\circ}{R} (\frac{1}{T_2} - \frac{1}{T_1}) \end{eqnarray} $$

For an exothermic reaction $\Delta H <0$

Now , Let $T_2>T_1$ therefore $K_2<K_1$ i.e. equilibrium constant decreases.

  • Using your second approach,

$$ \begin{eqnarray} \Delta G^\circ = \Delta H^\circ - T \Delta S^\circ \end{eqnarray} $$ $$ \begin{eqnarray} \Delta G = \Delta H - T \Delta S \end{eqnarray} $$ $$ \begin{eqnarray} \Delta G = \Delta G^\circ + RT \ln(Q) \end{eqnarray} $$ At equilibrium $ \Delta G = 0 $ and $Q= K_{eq}$ and hence $$ \begin{eqnarray} && K=e^{\frac{-\Delta G^\circ}{RT}} =e^{\frac{-(\Delta H^\circ - T \Delta S^\circ)}{RT}} \end{eqnarray} $$

For Temperature $T_1$ and Equilibrium Constant $K_1$ $$ \begin{eqnarray} K_1=e^{\frac{-(\Delta H^\circ - T_1 \Delta S^\circ)}{RT_1}} \end{eqnarray} $$ Similarly, For Temperature $T_1$ and Equilibrium Constant $K_1$ $$ \begin{eqnarray} K_2=e^{\frac{-(\Delta H^\circ - T_2 \Delta S^\circ)}{RT_2}} \end{eqnarray} $$

Dividing the above two equations, you get $$ \begin{eqnarray} && \frac{k_2}{k_1} = e^{-\frac{∆H^\circ}{R} (\frac{1}{T_2} - \frac{1}{T_1})} \end{eqnarray} $$

The Van't Hoff Equation !!! And it would give the same result as before.

I didn't conclude Dependence of $K_{eq}$ and T from $\Delta G^\circ$ because if a reaction is exothermic, it only tells that $\Delta H^\circ <0$ and doesn't tell anything about $\Delta G^\circ$ ..

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    $\begingroup$ This answer does not answer the question. The van't Hoff equation describes the change of an equilibrium constant with temperature and is not in question. The "reaction" questioned with different signs for enthalpy and entropy will not reach equilibrium as long as those conditions persist. $\endgroup$
    – jimchmst
    Commented Feb 28 at 10:09
  • $\begingroup$ A reaction with a negative enthalpy change must lose heat with a decrease in entropy for equilibrium there is an equal reverse reaction that absorbs heat with an increase in entropy. A reaction that both releases heat [exothermic] and increases entropy cannot reach equilibrium. It must be either a composite reaction or a steady state with continuous energy input . $\endgroup$
    – jimchmst
    Commented Mar 2 at 19:17
  • $\begingroup$ @jimchmst I think heat has correctly pointed it out jim in the comment to your answer ,,,I'll give an example ,,,Take Delta H∘ to be -7kJ ,,,, Delta S∘ to be + 1.314 J ,,, T = 1000 K ,,R = 8.314 J mol⁻¹K⁻¹ ,, and Keq to be e (2.71828) ,,,,As in this example Delta G is zero ,,,,Delta H∘ is negative and Delta S∘ is positive,,, and the reaction can reach equilibrium $\endgroup$
    – Amit
    Commented Mar 3 at 5:27
  • $\begingroup$ The question that must be answered is whether those conditions can exist in reality. the primary consideration is that at equilibrium DeltaG EQUALS ZERO =deltaH -Tdelta S delta H equals TDelta S your example Delta H= -7 ; TDelta S = Some plus value THEY DO NOT HAVE the SAME SIGN THEY are NOT EQUAL. Also the standard values do not apply to equilibtium They are the energy changes from a defined standard state to equilibrium. $\endgroup$
    – jimchmst
    Commented Mar 3 at 23:32
  • $\begingroup$ Equilibrium means reaction rates are the same in each direction that is only possible if the energy and entropy changes are opposing at equilibrium. $\endgroup$
    – jimchmst
    Commented Mar 3 at 23:44
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I think it all comes down to how much the change in temperature affects the numerator and the denominator of the exponent in the equation for equilibrium constant. Afterall temperature is "hidden" in ΔG and directly present in RT expression. Increase of temperature causes decrease of ΔG and because for given example it is negative (negative ΔH and positive ΔS) so when decreasing it gets more negative and the minus sign makes the numerator larger, but on the other hand increase of the temperature maked the denominator greather as well.

Now it would make sense to consider how strong is the influence of temperature change on the ΔG value. In the reasoning below I will neglect influence of the temperature on ΔH and ΔS.

We have the equation:

ΔG = ΔHTΔS

I would say that this influence of temperature is determined by value of ΔS.

Just out of curiosity I took a look in thermodynamic tables and I noticed that for examples given there ΔH values (in J/mol) are three to four orders of magnitude greater than ΔS values (in J/(mol K)).

Let's think of an extreme example:

Let's say we have a reaction that has ΔH = −100 (kJ/mol) and ΔS = 100 (J/(mol K)). Then for two temperatures T1 = 300 K and T2 = 600 K we have:

ΔG1 = −130000 (J/mol) and ΔG2 = −160000 (J/mol)

Looks like for doubling the denominator increase of numerator would be less than two times. Equlibrium constants values that we would got would be:

K1 = 4.32 * 10^(22) and K2 = 8.51 * 10^(13)

So equilibrium constant got smaller with increase of the temperature.

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  • $\begingroup$ G=-RTlnKeq =H-TS; H is negative and T positive; increasing T with no change in H and S makes G more negative and Keq larger. The combination of negative deltaH and positive delta S cannot reach equilibrium. $\endgroup$
    – jimchmst
    Commented Feb 24 at 1:35
  • $\begingroup$ When I do test calculations Keq gets smaller with increase of T for positive S and negative H (when totally neglecting their temperature dependence). It works like that for small and big temperature changes and different H and S combinations. Looks to me that it can even be seen from the plot on the top of this question: when T is increased then (1/T) gets smaller and with (1/T) getting smaller lnKeq decreases. So in other words with increase of T we have decrease of lnKeq. If lnKeq decrease this means that Keq decrease. $\endgroup$
    – Fractal
    Commented Feb 24 at 7:48
  • $\begingroup$ G = H-TS; G = -RTlnKeq if S were positive increasing T makes G more negative causing an increase in Keq. This situation would cause a positive feedback loop with eventual catastrophe. $\endgroup$
    – jimchmst
    Commented Feb 25 at 0:48
  • $\begingroup$ I would like to avoid going in circles. You can take a calculator and see for yourself that this is true: when using -RTlnKeq = ΔH∘ - TΔS∘ for negative ΔH∘ and positive ΔS∘ increase of T leads to decrease of Keq. Looking at your answer for this question I've noticed something that needs clarification. I mean that you, essentially, wrote that at equilibirium 0 = ΔH∘ - TΔS∘. And that is not true (except maybe for some specific examples). In general: ΔG = ΔG∘ + RTlnQ At equilibrium ΔG = 0 and Q = Keq and we get ΔG∘ = -RTlnKeq If you think some other theory applies - please give me the reference. $\endgroup$
    – Fractal
    Commented Feb 25 at 13:31
  • $\begingroup$ At equilibrium DELTA G = ZERO not DELTAG[0] This means that to reach equilibrium deltaH and delta S must have the same sign. If deltaH is negative and delta S is positive the reaction will be increasing in only one direction with increasing T, a positive feedback. A metaphor is putting more fuel on the fire a definite positive entropy event. A negative deltaH combined with a positive delta S makes delta G more negative with increasing T therefore making Keq larger. Thanks for reading my answer but please read what I said. $\endgroup$
    – jimchmst
    Commented Feb 26 at 20:29
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In this answer I will try to resolve your apparent contradiction first by showing arithmetically why only 1 outcome is true and in the next part try to convince your intuition why this is the case.

Arithmetic proof

As alluded to already in other responses, there is no contradiction between the Van't Hoff equation and the equation relating $K$ and $\Delta G$. Given:

\begin{align} K &= \exp\left({-\Delta G^° \over RT} \right). \label{5-7}\tag 1\\ \end{align}

Assuming that $\Delta H^\circ$ and $\Delta S^\circ$ are invariant with temperature, we can differentiate the expression with respect to T to obtain:

\begin{align} \frac{\mathrm{d}K}{\mathrm{d}T} &= \frac{\Delta H^\circ\exp\left( \frac{\Delta S^°}{R}-\frac{\Delta H^°}{RT} \right)}{RT^2} \label{four}\tag 2\\ \end{align}

Since $RT^2 >0$ and $\exp(\frac{\Delta S^°}{R}-\frac{\Delta H^°}{RT}) >0$, the sign of $\frac{\mathrm{d}K}{\mathrm{d}T}$ is only dependent on $\Delta H^\circ$. For exothermic reactions, increasing T will lead to decreasing $K$. Similarly, for endothermic reactions, increasing T will lead to increasing $K$. In other words, Le Chatelier's principle and the Van't Hoff equation are correct.

Intuition

This result might contradict our intuition. For a reaction with $\Delta H^\circ < 0$ and $\Delta S^\circ > 0$, we would expect, given the maths, that $K$ decreases with increasing temperature. However, as you mentioned, the negative $\Delta S^\circ$ term would mean $\Delta G^\circ$ decreases with increasing temperature. This gives rise to an apparent contradiction: at increasing temperatures, $G^\circ$ of products decreases relative to reactants, meaning (at standard state) products are more stable than reactants, yet the position of equilibrium is shifting toward the reactants.

The key to this conundrum lies in the $RT$ term and more specifically what temperature actually means. Notice that in equation (1), changing $T$ does not only change $\Delta G^\circ$ but also $RT$.

Let us consider just enthalpy first

For the sake of discussion let us consider the following: a closed box filled with inert gas with some molecules of A(g) which can isomerise to B(g):

$$\ce{A(g) <=> B(g)}$$

This system has constant total energy $E_{total}$. We consider each molecule of A or B as a smaller subsystem with energy $e_A$ and $e_B$ respectively. The energy of the rest of the system with respect to that one molecule is therefore $E_{total} - e$.

Consider that the isomerisation reaction is exothermic such that when $\ce{A(g) -> B(g)}$, heat is released such that $e$ decreases and $E_{total} - e$ increases. Consider also, for now, that the reaction involves no change in entropy ($\Delta S^\circ = 0$).

Hopefully it is intuitive that at equilibrium at some temperature $T_1$, more molecules of B than A exist, therefore $K > 1$. You can think of it this way: the B isomer contains stronger intramolecular bonds than A is therefore more stable than A. This is also why the reaction is exothermic.

Here is another more useful way to appreciate why $K > 1$. We consider the system as a microcanonical ensemble, such that every microstate occurs with equal probability. Let us rewrite the equation for $K$. Since $\Delta S^\circ = 0$:

\begin{align} K &= \exp\left({-\Delta H^° \over RT} \right) \end{align}

\begin{align} K &= \frac{\exp\left({-H^°_B \over RT} \right)} {\exp\left({-H^°_A \over RT} \right)} \end{align}

\begin{align} K \propto \frac{\exp\left({-H^°_B \over k_BT} \right)} {\exp\left({-H^°_A \over k_BT} \right)} \tag{3} \end{align}

Now, by considering the equation for Boltzmann's distribution,

\begin{align} K \propto \frac{\text{Probability(molecule exists as B isomer)}} {\text{Probability(molecule exists as A isomer)}} \end{align}

\begin{align} K \propto \frac{\Omega(E_{total} - e_B)} {\Omega(E_{total} - e_A)} \end{align}

Where $\Omega(E)$ is the number of microstates accessible to the macrostate with energy $E$.

From the re-expression we can appreciate that the equilibrium constant depends on the probability that the molecule exists as either A or B, which depends on the number of microstates available. Here is the key: because $e_A > e_B$, then $(E_{total} - e_A) < (E_{total} - e_B)$, such that $\Omega(E_{total} - e_A) < \Omega(E_{total} - e_B)$. Therefore, by virtue of the fact that the reaction is exothermic, $K>1$.

Now, there are 2 ways to appreciate that increasing the temperature of the system causes $K$ to decrease.

The simplest way is to appreciate that from equation (3), increasing T will cause both the numerator and denominator to approach 1, meaning $K$ decreases from $K > 0$ to $K=1$. But how does this work intuitively?

Let us consider the thermodynamic definition of temperature. For our system,

$$T = \frac{dE}{dS}$$

It is easier to consider the definition of temperature as: $$\frac{1}{T} = \frac{dS}{dE}$$

Therefore, higher temperatures mean that the same increase in energy results in a smaller increase in entropy (in other words, a smaller increase in the number of possible microstates). In other words, a small decrease in energy results in lower entropic cost. Applying this to our system, we see that at higher and higher temperatures, $E_{total}$ increases, such that $(E_{total} - e_B) \approx (E_{total} - e_A) \approx E_{total}$. In other words, as temperature increases, the relative entropic cost of the molecule existing as A or B becomes negligible, to the extent that

$$\text{Probability(molecule exists as B isomer)} \approx \text{Probability(molecule exists as A isomer)}$$

such that $K=1$.

The above is generalizable to endothermic reactions. In the case where $\Delta H^\circ > 0$, at equilibrium at some temperature T more A than B molecules will be present such that $K<1$. As temperature increases, relative entropic cost of converting between A and B decreases such that $K=1$

Now let us consider entropy as well

Without considering entropy, the apparent contradiction does not exist. Therefore, let us consider that the reaction

$$\ce{A(g) -> B(g)}$$

Involves a positive entropy change. For the sake of simplicity, consider that this entropy change does not affect the entropy of the rest of the system. For example, the A isomer forms a closed, rigid ring while the B isomer forms a long linear molecule. (It is not always the case that $\Delta S^\circ$ of a reaction does not affect entropy of the system. Consider kosmotropic ions that reorganise solvent molecules into an ordered clathrate cage.)

How does this affect what have concluded so far for enthalpy? The answer: It doesn't change anything. As we increase temperature, the relative energies of A and B are still defined by the $\Delta H^\circ$. Therefore, increasing temperature means the relative entropic cost of converting between A and B diminishes, such that $K$ decreases.

The difference here is that $\Delta S^\circ$ is not affected by temperature in the same way. The positive entropy change that accompanies $\ce{A(g) -> B(g)}$, and therefore the increased number of possible microstates accessible to B isomers relative to A isomers remains the same at higher temperatures. Therefore, as $T\rightarrow\infty$, $K$ will no longer approach $K=1$, but will instead approach some value above 1, proportional to the magnitude of entropy increase.

Indeed, this is the reason why:

$$\lim_{T \to \infty} K = lim_{T \to \infty} \exp(\frac{T\Delta S^\circ - \Delta H^\circ}{RT}) = \exp(\frac{\Delta S^\circ}{R})$$

The limit of $K$ as $T \rightarrow \infty$ is only dependent on $\Delta S^\circ$ and not on $\Delta H^\circ$.

Conclusion

In other words, we should, in a way, consider enthalpy and entropy as separately contributing to the equilibrium constant. Instead of saying: exothermicity means K decreases with increasing temperature and endothermicity means K increases with increasing temperature, we should see it as: at finite temperatures, exothermicity pushes the equilibrium constant toward $K>1$ and endothermicity pushes the equilibrium constant toward $K<1$, but with increasing temperature the influence of $\Delta H^\circ$ on $K$ diminishes. On the other hand, the influence of $\Delta S^\circ$ on the equilibrium constant does not wane with increasing temperature.

The contradiction is resolved by considering that the equilibrium constant is not solely dependent on the relative free energies of the reactants and products, but also on the entropy of the rest of the system, which is dependent on the temperature.

Of course, the above is highly simplified compared to real-life chemical reactions. Many assumptions were taken, but the goal was to help you appreciate, intuitively, why there is in fact no contradiction.

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The equilibrium constant and standard free energy of a reaction describe the free energy change from the Standard states of Reactants AND Products to equilibrium. Explicit in this definition is that the reaction can reach equilibrium.

At equilibrium delta G = Zero = delta H -Tdelta S. Therefore: delta H = Tdelta S. A system to reach equilibrium must have the same sign for delta H and delta S. When the signs are different the reaction must be composite or possibly a steady state but not equilibrium. Therefore, the concept of equilibrium constant does not apply.

A system with a negative deltaH and a positive entropy change might be described as a system with a positive feedback.

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  • $\begingroup$ This is incorrect. For most chemical reactions we have to consider $\Delta_\mathrm{r} G = \Delta_\mathrm{r} H^\circ - T\Delta_\mathrm{r} S^\circ+ RT\ln Q$. The $RT\ln Q$ term denotes the entropy of mixing. Whatever the combination of $\Delta H$ and $\Delta S$ signs, equilibrium can always be achieved because the $ \Delta_\mathrm{r} G^\circ$ term can always be balanced by the $RT\ln Q$ term. In other words, the instantaneous change in free energy converting from products to reactants is balanced by the need to maximise entropy of mixing. $\endgroup$
    – Heat
    Commented Feb 28 at 14:58
  • $\begingroup$ You would exclude the $RT\ln Q$ term, as in your answer, only in reactions such as solid to liquid phase changes. In such a reaction, then it is indeed the case that at equilibrium $\Delta_\mathrm{r} H^\circ = T\Delta_\mathrm{r} S^\circ$. Solid to liquid phase changes are always endothermic and positive entropy change. $\endgroup$
    – Heat
    Commented Feb 28 at 15:02
  • $\begingroup$ If your answer were true, you would run into all sorts of problems. Consider, for example, the dissociation of any weak acid which has opposite signs for $\Delta H^\circ$ and $\Delta S^\circ$. (see some examples here: en.wikipedia.org/wiki/Acid_dissociation_constant#Thermodynamics). If it was true that such reactions had no equilibrium, it would be impossible to set up buffer solutions with these acids since adding equal amounts of acid and conjugate base would, according to your "positive feedback" cause one species to completely convert into the other. $\endgroup$
    – Heat
    Commented Feb 28 at 15:08
  • $\begingroup$ Thanks for reading! Now think things thru. First Standard free energies are an artifact so we can make tables of energies. At equilibrium delta G =zero, not deltaG[0] =zero. For a reaction to proceed to equilibrium heat must be added or removed; at constant T and P Delta H =Q delta S = Q/T Both have the same sign. Standard free energies are from equilibrium to a defined standard state . True equilibria are from actual conditions. and can be approached from either direction to equilibrium. In one direction the reaction is driven by removal of energy, the other by increase of entropy. $\endgroup$
    – jimchmst
    Commented Feb 28 at 17:31
  • $\begingroup$ For an exothermic reaction the obligatory increase in entropy is the entropy of the universe; for the endothermic reaction the increase in entropy is in the system. In either case there is a net increase in entropy. An interesting aspect is that equilibrium must be approachable from either direction. This means that the path to equilibrium is spontaneous from both directions. This is only possible if H and S have the same sign. $\endgroup$
    – jimchmst
    Commented Feb 28 at 17:46
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van't Hoff equation is the formal form of Le Chatelier's principle, as in they are the same. A main feature of both these concepts is that they are local properties of temparature and are applicable only in small temparature ranges when enthalpy is independent of temperature.
From van't Hoff equation:

$$ \begin{eqnarray} && \frac{\textrm{d}}{\textrm{d}T}\ln{K}=\frac{1}{K}\frac{\textrm{d}K}{\textrm{d}T}=\frac{\Delta H^{\circ}}{RT^{2}} \\\\ && \frac{\textrm{d}K}{\textrm{d}T}=\left(\frac{K}{RT^{2}}\right)\Delta H^{\circ} \end{eqnarray} $$ As you can see, the sign of the derivative depends on enthaply change. The caveat here is that, enthalpy itself is temparature dependent and ultimately the sign on the derivative depends on the temparature. Hence, Le Chatelier's principle is local.
Temparature dependence of enthalpy is given by Kirchhoff's Law: $\Delta H=C_{\textrm{p}}\Delta T$

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At equilibrium Delta G = Zero; Delta H - TDelta S = Zero; so Delta H =TDelta S. This means if a reaction can reach equilibrium the change in enthalpy and the change in entropy have the same sign and are in opposition to each other. This means that the reaction in one direction is energy driven and in the other entropy driven. My interpretation when the energy and entropy signs are opposite is that the reactions are probably inadequately described and are composite or steady state rather than capable of attaining equilibrium.

The process of a reaction with a negative DeltaH and a Positive Delta S means that the reaction CANNOT reach equilibrium and the concept of equilibrium constant is meaningless.

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