# Relationship between enthalpy entropy and equilibrium constant

### Thermodynamics Notes

To know the relationship between free energy and the equilibrium entropy, and temperature, thereby eliminating ΔH from the equation for ΔG. At equilibrium none of the properties of a system change with time. Note how silicates have a nearly constant heat capacity of ~1 J K-1 g-1 above K. How to integrate the heat capacity divided by T (to determine entropy S): An important relationship between enthalpy change H and heat change Q is revealed by. As a consequence of the second law of thermodynamics, at constant pressure and During the approach to equilibrium, the free enthalpy decreases, reaching a where H is the enthalpy, T is the absolute temperature and S is the entropy.

The difference between Go and G for a reaction is important. There is only one value of Go for a reaction at a given temperature, but there are an infinite number of possible values of G.

The figure below shows the relationship between G for the following reaction and the logarithm to the base e of the reaction quotient for the reaction between N2 and H2 to form NH3. They therefore describe systems in which there is far more reactant than product.

The sign of G for these systems is negative and the magnitude of G is large.

The system is therefore relatively far from equilibrium and the reaction must shift to the right to reach equilibrium. Data on the far right side of this figure describe systems in which there is more product than reactant. The sign of G is now positive and the magnitude of G is moderately large.

## van 't Hoff equation

The sign of G tells us that the reaction would have to shift to the left to reach equilibrium. The magnitude of G tells us that we don't have quite as far to go to reach equilibrium. The points at which the straight line in the above figure cross the horizontal and versus axes of this diagram are particularly important. The straight line crosses the vertical axis when the reaction quotient for the system is equal to 1.

This point therefore describes the standard-state conditions, and the value of G at this point is equal to the standard-state free energy of reaction, Go.

• 19.7: Free Energy and the Equilibrium Constant

Because there is no driving force behind the reaction, the system must be at equilibrium. The relationship between the free energy of reaction at any moment in time G and the standard-state free energy of reaction Go is described by the following equation. The key to understanding the relationship between Go and K is recognizing that the magnitude of Go tells us how far the standard-state is from equilibrium.

A calorie is defined as the amount of energy required to raise the temperature of 1 g of water from Many times it is easiest to solve equations or problems by conducting "dimensional analysis," which just means using the same units throughout an equation, seeing that both sides of an equation contain balanced units, and that the answer is cast in terms of units that you want.

As an example, consider the difference between temperature units of K and heat units of J. Two bodies may have the same temperature, but contain different amounts of heat; likewise, two bodies may contain the same heat, but be at different temperatures. In fact, the heat capacity C describes the amount of heat dQ involved in changing one mole of a substance by a given temperature increment dT: The specific heat is essentially the same number, but is expressed per gram rather than per mole.

Don't forget significant digits. Think about some everyday experiences you have with chemical reactions.

Your ability to melt and refreeze ice shows you that H2O has two phases and that the reaction transforming one to the other is reversible--apparently the crystallization of ice requires removing some heat. Frying an egg is an example of an irreversible reaction.

If you dissolve halite in water you can tell that the NaCl is still present in some form by tasting the water.

### Temperature Dependance of K

Why does the NaCl dissolve? Does it give off heat? Does it require energy? How is it that diamond, a high-pressure form of C, can coexist with the low pressure form, graphite, at Earth's surface? Do diamond and graphite both have the same energy?

The Equilibrium Constant

If you burn graphite and diamond, which gives you more energy?