How to judge spontaneous and non spontaneous reactions

In high school chemistry, only qualitative analysis is required, and no calculations are required.

High School Chemistry Elective 4: Chemical Reactions.

There is a formula mentioned in the principle:

g=h-t·s

where g is Gibbs free energy.

h is the enthalpy change of the reaction.

t is the temperature, and s is the change in reaction entropy.

The change of Gibbs free energy can be used as a criterion for the spontaneity and equilibrium of the constant temperature and constant pressure process, and if g is negative, the reaction has a tendency to proceed spontaneously.

The enthalpy change can be roughly understood as the heat absorption and release of the reaction. If the reaction is exothermic, h is less than zero, and endothermic is greater than zero.

Entropy is a numerical measure of how chaotic a system is. Entropy change can be roughly understood as the physical quantity that expresses the physical state of the products and products of the reaction. The amount of reaction that produces gas, the amount of gas substance.

The response to the increase is an increase in entropy.

React. To sum up, 1If the enthalpy of entropy increases, g is less than zero, and the positive reaction is spontaneous. If the enthalpy of entropy reduction increases, g is greater than zero, and the reverse reaction is spontaneous;

2.From the formula, it can be seen that g is mainly affected by h at low temperature, and g is mainly affected by s at high temperature. So:

If the entropy of the reaction increases and the enthalpy increases, the low temperature reverse reaction is spontaneous, and the high temperature positive reaction is spontaneous.

If the entropy of the reaction decreases by enthalpy, the low temperature positive reaction is spontaneous, and the high temperature reverse reaction is spontaneous.

It is important to note that spontaneous reactions are defined as processes or reactions that can be initiated automatically once initiated under given conditions without external help, rather than reactions that can be carried out without conditions. For example, red phosphorus.

and oxygen do not react at room temperature, but its reaction is spontaneous.

At the same time, the larger the value of g, the more likely it is that the corresponding forward and reverse reactions will be spontaneous.

Judging by the Gibbs free energy change of the reaction, the Gibbs free energy change is less than 0 and can be spontaneous.

Or according to the comparison between the concentration quotient (pressure quotient) and the standard equilibrium constant, if the concentration quotient (pressure quotient) is less than the standard equilibrium constant, it is positive and spontaneous.

For the redox reaction in aqueous solution, it can also be judged by the electromotive force of the galvanic battery, and the electromotive force greater than 0 is spontaneous, otherwise it is not spontaneous.

This is judged by physicochemical only, Gibbs free energy.

Under given conditions, the process or reaction that can be carried out automatically once triggered without the need for external help is called a spontaneous reaction.

Iron rusts in humid air; Iron from a solution of copper sulfate.

Copper is displaced; Zinc from dilute sulfuric acid.

hydrogen is displaced, etc.

Chemical thermodynamics states that reactions with an increase in entropy and a decrease in enthalpy must be spontaneous. A spontaneous response is not necessarily a quick response.

If there is no palladium as a catalyst, the hydrogen-oxygen mixture can be maintained without obvious reaction for a long time at room temperature and pressure. Under the same conditions, the reverse reaction of the spontaneous reaction is non-spontaneous and can only be carried out if an external force is given to do the work.

Note. The reaction always takes place under certain conditions. In order to discuss the spontaneity of the reaction, it is also necessary to specify the conditions for the reaction.

If there is no explanation of the reaction conditions, only talking about "so-and-so reaction is not spontaneous" and "so-and-so reaction spontaneously", in fact, it refers to the reaction carried out under the temperature and normal pressure of Changyou. "Certain conditions" means "a certain temperature and a certain pressure".

With Djibouti. The free energy calculation formula ( g

h-t s) to determine whether the reaction can proceed spontaneously, we can see that t has an effect on the value of g, and in some cases, may determine whether g is greater than 0 or less than 0; where h is a physical quantity related to temperature and pressure.

Therefore, the discussion of whether g is greater than 0 or less than 0 is actually carried out at a certain temperature and pressure.

Free energy. g = h- s t, h is the enthalpy change, s is the entropy change. When the free energy is less than 0, the reaction can be spontaneous; The free energy is greater than 0 and the reaction cannot be spontaneous. It can be seen that the enthalpy decreases and the entropy increases.

It is conducive to the spontaneous infiltration of the reaction.

The following is a specific introduction to the defeat of the rock.

1. The most basic criterion for judging whether the reaction is spontaneous is the potential criterion, that is, the spontaneous reaction is always in the direction of decreasing potential.

2. Isothermal isobaric, and the reaction that does not do non-volumetric work only has chemical potential.

In this term, the chemical potential is the partial Mole Gibbs function.

Therefore, there is a common saying that the withered Gibbs becomes less than 0, and the reaction can be spontaneous.

3. If there is non-volumetric work in a system, such as electrical work.

This involves two potentials, one chemical potential and one electric potential, and if the reaction is spontaneous it must be the sum of the chemical potential and the electric potential is reduced. This will lead to a situation where the chemical potential increases, the electric potential decreases, and the electric potential decreases more than the chemical potential increases, so that the sum of the chemical potential and the electric potential still decreases, and the reaction is still spontaneous, but it will be wrong to judge just by the Gibbs function.

4. To discuss whether a change is spontaneous, we must first determine whether the change involves other non-volume work such as electrical work, and if it does, it cannot be judged by Gibbs function change, because the usual Gibbs function change only represents the chemical bite, and the potential must be added as the sum judgment. The use of the criterion must meet the constraints, and the original criterion is not applicable if the system changes and the conditions are different.

Judgment of the spontaneity of chemical reactions:

1. Spontaneous process: under certain conditions, it can be carried out automatically without external force.

2. Enthalpy judgment: a spontaneous process, the system tends to transform from a state of high energy to a state of low energy. to chemistry.

In terms of reactions, exothermic reactions tend to be spontaneous. However, endothermic reactions are also spontaneous, and there are also non-spontaneous febrile reactions.

3. Entropy change judgment: In a system isolated from the outside world, the spontaneous process will lead to an increase in the entropy of the system.

4. The method of judging the free energy change g h t limb beam s

g 0, the reaction proceeds spontaneously in the forward direction. g 0, the reaction is in equilibrium. g 0, the reaction proceeds spontaneously in reverse.

A reaction in which the entropy of exothermic increases is certainly a spontaneous reaction.

h＜0，△s＞0，△g＜0

A reaction that entropies decrease endothermically, is certainly a non-spontaneous reaction.

h 0, calendar s 0, g 0

An exothermic reaction with reduced entropy, lowering the temperature, favors the spontaneous progress of the reaction. h 0, s 0, g 0 to ensure g, t to be lowered.

A process of increasing entropy by endothermy, raising the temperature, favors the spontaneous occurrence of the reaction. h 0, s 0, to ensure g 0, t should be raised high enough.

To determine whether the reaction can be spontaneous or not, it is based on this formula: g

h t s, g – Gibbs free energy change, h – enthalpy change, s – entropy change.

t – Kelvin temperature.

Determine whether the reaction can proceed spontaneously using the Gibbs free energy criterion. If the mu answer g<0 reaction can be carried out spontaneously; Otherwise, it should not be done spontaneously. Under constant temperature and pressure:

When h<0, s>0, the reaction proceeds spontaneously.

When h>0, s<0, the reaction is not spontaneous;

When h>0 and s>0, it needs to be carried out spontaneously under the condition of higher temperature.

When h<0 and s<0, it needs to be carried out spontaneously under the condition of lower temperature.