When is g spontaneous

Post by George Ghaly 2L » Fri Feb 22, am Yes a negative value resulting from the Gibbs free energy equation means that the reactants will make products spontaneously.

Jump to. Who is online Users browsing this forum: No registered users and 2 guests. Gibbs free energy measures the useful work obtainable from a thermodynamic system at a constant temperature and pressure. Just as in mechanics, where potential energy is defined as capacity to do work, similarly different potentials have different meanings.

The Gibbs free energy is the maximum amount of non-expansion work that can be extracted from a closed system. Gibbs free energy equation : The Gibbs free energy equation is dependent on pressure.

As such, it is a convenient criterion of spontaneity for processes with constant pressure and temperature. Therefore, Gibbs free energy is most useful for thermochemical processes at constant temperature and pressure. Gibbs energy is the maximum useful work that a system can do on its surroundings when the process occurring within the system is reversible at constant temperature and pressure.

As in mechanics, where potential energy is defined as capacity to do work, different potentials have different meanings.

Energy that is not extracted as work is exchanged with the surroundings as heat. Work Diagram : The reversible condition implies wmax and qmin.

The impossibility of extracting all of the internal energy as work is essentially a statement of the Second Law. This is most commonly electrical work moving electric charge through a potential difference , but other forms of work are also possible. For instance, examples of useful, non-expansion work in biological organisms include muscle contraction and the transmission of nerve impulses. Privacy Policy. Skip to main content.

Search for:. Learning Objectives Calculate the change in standard free energy for a particular reaction. If we could find some way to harness the tendency of this reaction to come to equilibrium, we could get the reaction to do work.

The free energy of a reaction at any moment in time is therefore said to be a measure of the energy available to do work. When a reaction leaves the standard state because of a change in the ratio of the concentrations of the products to the reactants, we have to describe the system in terms of non-standard-state free energies of reaction. The difference between G o and G for a reaction is important. There is only one value of G o 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 N 2 and H 2 to form NH 3. Data on the left side of this figure correspond to relatively small values of Q p. 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. 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, G o.

The point at which the straight line crosses the horizontal axis describes a system for which G is equal to zero. 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 G o is described by the following equation.

We can therefore solve this equation for the relationship between G o and K. This equation allows us to calculate the equilibrium constant for any reaction from the standard-state free energy of reaction, or vice versa. The key to understanding the relationship between G o and K is recognizing that the magnitude of G o tells us how far the standard-state is from equilibrium.

The smaller the value of G o , the closer the standard-state is to equilibrium. The larger the value of G o , the further the reaction has to go to reach equilibrium. The relationship between G o and the equilibrium constant for a chemical reaction is illustrated by the data in the table below. Use the value of G o obtained in Practice Problem 7 to calculate the equilibrium constant for the following reaction at 25C:.

Click here to check your answer to Practice Problem 9. Click here to see a solution to Practice Problem 9. One could write an equation showing these gases undergoing a chemical reaction to form nitrogen monoxide. Fortunately, this reaction is nonspontaneous at normal temperatures and pressures.

However, nitrogen monoxide is capable of being produced at very high temperatures, and this reaction has been observed to occur as a result of lightning strikes. One must be careful not to confuse the term spontaneous with the notion that a reaction occurs rapidly.

A spontaneous reaction is one in which product formation is favored, even if the reaction is extremely slow. You do not have to worry about a piece of paper on your desk suddenly bursting into flames, although its combustion is a spontaneous reaction. What is missing is the required activation energy to get the reaction started. If the paper were to be heated to a high enough temperature, it would begin to burn, at which point the reaction would proceed spontaneously until completion.

In a reversible reaction, one reaction direction may be favored over the other. Carbonic acid is present in carbonated beverages. It decomposes spontaneously to carbon dioxide and water according to the following reaction. The forward reaction is spontaneous because the products of the forward reaction are favored at equilibrium.

In the reverse reaction, carbon dioxide and water are the reactants, and carbonic acid is the product.