The influence of reactant concentration on the reaction

Experimental facts show that increasing the concentration of reactants at a constant temperature increases the reaction rate, but it takes longer to complete the reaction
.

It can be seen from Figure 3-1 that the reaction occurs after the reactants are mixed in a certain proportion, and the reaction rate decreases with the continuation of time

3.
2.
1 The influence of reactant concentration on reaction rate

1.
Rate equation

Experiments show that for the reaction

aA+bB=gG+hH

The relationship between the instantaneous rate r of the reaction and the concentration of the reactant is

r=k[A] ^m [B] ⁿ

This is the rate equation of the reaction, or the law of mass action
.

k is a rate constant, and k does not change due to the change of reactant concentration at a constant temperature; k, m and n can be measured by experiments

[Example 3-1] The measured initial concentration of reactants and reaction rate data for the reaction aA+bB=gG+hH are shown below.
Try to give the reaction rate equation based on the experimental data
.

The experimental data in the solution comparison table can be found:

When the initial concentration of reactant B does not change, the initial concentration of A doubles, and the reaction rate also doubles, that is, r∝[A] ¹ ;

When the initial concentration of reactant A does not change, the initial concentration of B increases by 1 time, and the reaction rate does not change, that is, r∝[B] ⁰
.

Therefore, the reaction rate equation is

r _B =k _B [A]

2.

Reaction order

If the reaction rate equation is

r=k[A] ^m [B] ⁿ

Then (m+n) is called the reaction order of the reaction, or the reaction is an (m+n)-order reaction
.

The reaction stages may be provided only a certain kind of the reactants, such as the reaction of reactant A is an m-order reaction, the reaction product B is an n-order reaction

The reaction order can be an integer
.

For example, the chemical equation for the reaction of H ₂ and I ₂ and the rate equation of the reaction are

H ₂ +I ₂ =2HI r=k[H ₂ ][I ₂ ]

The reaction order is 2, in other words, both H ₂ and I ₂ are first-order reactions
.

The reaction order can be a fraction
.

For example, the chemical equation of the reaction between Cl ₂ and H ₂ and the rate equation of the reaction are

H ₂ +Cl ₂ =2HCI r=k[H ₂ ][Cl ₂ ] ^1/2

The reaction order is 3/2, or it is a first-order reaction for H _{2 and} a 1 /2- order reaction for Cl ₂
.

There is no definite reaction order for some reactions
.

For example, the chemical equation and rate equation of the reaction between Br ₂ and H ₂ are

The number of reaction stages can also be zero
.

For example, the chemical equation for the reaction of Na and H ₂ O and the rate equation for the reaction are

2Na+2H ₂ O=2NaOH+H ₂            r=k

The rate of the reaction has nothing to do with the concentration of the reactants, it is a zero-order reaction
.

3.

Rate constant k

If the rate equation of the reaction aA+bB=gG+hH is r=k[A] ^m [B] ⁿ , when the concentration of various reactants is 1mol·dm ^-3 , r=k
.

The rate constant k is the reaction rate when the concentration of various reactants is 1 mol·dm ³ , or is called the specific rate constant
.

Under the same concentration and reaction order conditions, the size of the rate constant can be used to compare the rate of chemical reactions
.

The unit of the rate constant is related to the reaction order
.
The international unit of reaction rate is mol·dm ^-3
.
s ^-1 , if the concentration unit is mol·dm ^-3 , for the n-order reaction, the unit of the rate constant k is

It can be seen that the unit of the rate constant k of the n-order reaction is mol ^(1-n) .
dm ^-3(1-n) .
s ^-1 or (mol·dm ^-3 )(1-n).
s ^-1
.

The unit of the rate constant of the zero-order reaction is mol·dm ^-3 .
s ^-1 , which is consistent with the unit of the reaction rate; the unit of the rate constant of the first-order reaction is the reciprocal of time, that is, s1; the unit of the rate constant of the second-order reaction Is mol ^-1 .
dm ³ ·s ^-1
.

When the change in the concentration of different substances is used to express the reaction rate, the value of the rate constant in the rate equation is often different, which is related to the stoichiometric number of the substance in the reaction formula
.
For reaction

aA+bB=gG+hH

The general formula of the rate equation is

r _i =k _i [A] _m [B] _n

No matter what kind of substance concentration change is used to express the reaction rate, the [A] _m [B] _n term is the same, so the relationship between the reaction rate constants is consistent with the relationship between the reaction rates
.
Because the relationship between the reaction rates is

Therefore, the relationship between the rate constants is