Chemistry

Chemical Balance (continued)


Influence of pressure

The pressure of a gas is associated with the volume of this gas. Increasing pressure benefits the lower volume reaction and decreasing pressure benefits the higher volume reaction.

Gas volume increase = EXPANSION
Gas volume decrease = CONTRACTION

Be the chemical reaction:

The volume occupied by conductors and products follows the stoichiometric ratio. In this case, the product has lower volume. With increasing pressure, the balance will shift to the right direction, because it has less volume.

Pressure increase = EQ for smaller volume side
Pressure Decrease = EQ for higher volume side

For these changes, there is no change in the KC value.

Influence of temperature

It is the only change that can modify the value of KC.


Increasing temperature benefits the endothermic reaction.
Decreasing temperature benefits the exothermic reaction.

The presence of a catalyst (chemical that accelerates the chemical reaction) does not change the KC value. The catalyst promotes a decrease in the moment of equilibrium. Changes in both direct reaction and reverse reaction.

Chemical Equilibrium Displacement Summary

External disturbance

Balance shift

Kc or Kp change

Adding a participant

In the opposite direction to the participant

No

Withdrawal from a Participant

In the sense of the participant

No

Increased total pressure

Toward the smallest volume

No

Total pressure decrease

Toward the highest volume

No

Temperature increase

In the endothermic sense

Yes

Temperature decrease

In the exothermic sense

Yes

Catalyst Presence

No

No

Chemical Balance - 2

Acidity constant (Ka) and basicity constant (Kb)

When an acid comes into contact with water, we say that an ionization has occurred. For the bases, we use the term dissociation. Here's what happens when hydrochloric acid is added in water:

Its equilibrium constant is given by:

Note that the liquid substance water should not participate in the equilibrium constant because its concentration is constant. So if the water concentration is multiplied by KC, we get a new constant, the acidity constant, Ka.

The acidity constant indicates the strength of the acid. The lower the Ka value, the weaker the acid. Less ionized is this acid. This constant is used for weak acids. Strong acids have no Ka, as they dissociate completely and have no balance (direct and inverse reaction).

The acidity constant is linked to the degree of ionization of an acid.

The higher the Ka, the greater the degree of ionization, hence strong acid.
The lower the Ka, the lower the degree of ionization, thus weak acid.

Look at the table of acids with different Ka values ​​and their acid strength:

Acids

Ka (25 ° C)

ACID FORCE

HClO4

10+10

Very strong

HCl

10+7

Very strong

H2ONLY4

10+3

Strong

H2ONLY3

1,5.10-2

Strong

H3POWDER4

7,6.10-3

Weak

HNO2

4,3.10-4

Weak

Hf

3,5.10-4

Weak

CH3COOH

1,8.10-5

Weak

H2CO3

4,3.10-7

Weak

H2s

1,3.10-7

Weak

HCN

4,9.10-10

Very weak

We can also use to demonstrate the value of the acidity constant, the ionization constant, Ki.

THE basicity constant is given by Kb, and indicates the strength of the base and its degree of dissociation. It is similar to Ka, but now it refers to the bases.