Temperature dependence of ΔS°, ΔG° and ΔH°

• etotheipi
In summary, standard ΔS°, ΔG°, and ΔH° are measured under standard conditions, which typically include 0.1M for solutions, 1 bar for gases, and 298K. However, values for ΔS°, ΔG°, and ΔH° can be found for all temperatures, as these quantities are not fixed at 298K. This is because ΔG° is dependent on temperature, as shown in the equation ΔG° = ΔH° - TΔS°. While an approximation at some levels is that ΔH° and ΔS° do not change with temperature, when calculating ΔG° for a specific temperature, the values of ΔH° and ΔS°
etotheipi
I was taught that standard ΔS°, ΔG° and ΔH° are the corresponding quantities measured under standard conditions which I was told was 0.1M for solutions, 1 bar for gases, but importantly also at 298K.

On many places on the internet, however, I have read that 298K is not actually a standard condition. Instead, we can find values for ΔS°, ΔG° and ΔH° for all temperatures.

This makes sense, since considering the definition of ΔG°, for instance,

ΔG° = ΔH° - TΔS°

it would be silly for ΔG° to be fixed at 298K considering that we can choose T to be whatever we want. I thought it should be read more like "The standard change Gibbs energy at e.g. 500K is ΔH° (at 500K) subtract 500K x ΔS° (at 500K).

A common approximation at my level is that ΔH° and ΔS° don't change with temperature, so when calculating ΔG° we can still use the values of ΔH° and ΔS° for 298K. However, this ΔG° would correspond to the reaction under standard conditions at the specified temperature.

I was wondering if anyone could check if I am understanding this correctly? Thank you :)

0.1M for solutions is unusual but may be common in some specialized field.
Even if you take Delta H0 and Delta S0 to be independent of temperature, Delta G0 will be temperature dependent because of the T in -T Delta S0, you are right, here.

etotheipi

1. How does temperature affect the values of ΔS°, ΔG° and ΔH°?

The values of ΔS°, ΔG° and ΔH° are all temperature-dependent. As temperature increases, the values of ΔS° and ΔH° also increase, while the value of ΔG° decreases. This is because at higher temperatures, there is more energy available for a reaction to occur, leading to an increase in entropy and enthalpy, and a decrease in free energy.

2. What is the relationship between temperature and the spontaneity of a reaction?

The spontaneity of a reaction is directly related to the temperature dependence of ΔG°. If the value of ΔG° is negative, the reaction is spontaneous at that temperature. However, as temperature increases, the value of ΔG° becomes more positive, and the reaction becomes less spontaneous. At a certain temperature, known as the equilibrium temperature, the value of ΔG° becomes zero and the reaction is at equilibrium.

3. How do changes in temperature affect equilibrium constant (K)?

The equilibrium constant (K) is also temperature-dependent. As temperature increases, the value of K also increases for exothermic reactions and decreases for endothermic reactions. This is because at higher temperatures, the equilibrium shifts in the direction of the reaction that produces more heat, in order to maintain a constant temperature.

4. Can the temperature dependence of ΔS°, ΔG° and ΔH° be predicted?

The temperature dependence of ΔS°, ΔG° and ΔH° can be predicted using thermodynamic equations and principles. However, it is important to note that these values are also affected by other factors such as pressure and concentration, so predictions may not always be accurate.

5. How does temperature dependence affect the rate of a reaction?

The temperature dependence of ΔG° also affects the rate of a reaction. As temperature increases, the rate of a reaction also increases due to the increase in kinetic energy of the reactant molecules. This leads to more frequent and successful collisions, resulting in a faster reaction rate.

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