Chemical reaction rate

The rate of a chemical reaction is a value that shows the change in the mass of one of the elements (product or reagent) of the reaction per unit of time, also, the rate is expressed in concentrations, fractions and other units measurements.

Factors

There are various factors that affect the rate of a chemical reaction. Reactions always proceed at different speeds, the rate of formation and breaking of the bond is influenced by physical properties: pressure and temperature. From a chemical point of view, the geometry of the molecules is an important factor, because not all molecules can at least reach each other! Also, the speed of the chemical reaction depends on the concentration of the substances being mixed and the interaction between miscible substances. Different radiation, for example, infrared, can also affect the flow reactions.

If you put a piece of copper (Cu) in a glass with a solution of ferric chloride (FeCl₃), the molecules on the surface of the piece of copper will interact first of all. Around a piece of copper are formed CuCl molecules that will not react further with anything and the reaction will stop if you shake the mixture, then the CuCl molecules will be distributed over the solution and the reaction will continue until the solution runs out ferric chloride or copper. Therefore, it is always important to consider exactly how the reaction proceeds.

FeCl₃ + Cu => FeCl₂ + CuCl

Chemical kinetics studies the rate of chemical reaction and reaction mechanisms. The rate of a chemical reaction is an indicator of how fast it occurs formation of products and absorption of reagents.

Reaction mechanism

The reaction mechanism is a detailed representation of all stages of the reaction, taking into account the products that are not present in the resulting mixture. The reaction can take place in several stages, for example, it has been experimentally established that the following reaction proceeds in two stages:

2ICl_(g) + H_2(g) → 2HCl_(g) + I_2(g)
1. ICl_(g) + H_2(g) → HCl_(g) + HI_(g)
2. ICl_(g) + HI_(g) → HCl_(g) + I_2(g)
Here, in parentheses, the aggregate states of substances are indicated, as you already know, the aggregate state of the substance it also affects the reaction rate, the aggregate states of substances denote: g - gaseous (gas); s - solid (solid); l - liquid (liquid)

Note that in the first reaction, the HI product appears, which is not present in either the reagents or in the resulting substance is 2HCl + I₂. The occurrence of intermediates can have a strong impact at the rate of a chemical reaction.

As a measure of the absorption of reagents and the occurrence of products, their concentration, concentration is used it can be expressed in various units of measurement, for example:

• Molarity - mol/l
• Molality - mol/kg
• The mass of the substance per one liter of solution
• Molar fraction

In any case, the concentration is indicated by square brackets: [C], where C is the substance whose concentration is measured.

To determine the rate of concentration change, we can measure the concentration of substances with a certain interval, for example, every second. We can present the obtained data in the form of a table or graph and based on from the data obtained, make an equation.

The rate of change in the concentration of the substance in the reaction is:
-d[A]/dt

For a theoretical reaction
aA + bB → cC + dD
the rate of change in the concentration of each reagent and each product will be:
-(1/a)(d[A]/dt) = -(1/b)(d[B]/dt) = (1/c)(d[C]/dt) = (1/d)(d[D]/dt)

It is impossible to predict the rate of concentration change based only on the chemical reaction equation, the change in the amount of reacting substances is always determined experimentally.

Definition

In order to determine the rate of conversion of reagents into a chemical reaction, it is necessary to conduct a series of experiments in which only one parameter will change, for example, temperature. We will conduct experiments at temperatures of 10, 50 and 100 degrees and will be able to make an assumption about how temperature affects the reaction rate.

Consider the following example of the effect of the concentration of substance A on the reaction rate:

A → B

We will conduct an experiment under certain conditions (fixed temperature and pressure, absence of sunlight) for a concentration of [A] = 1 mol/L. Let's determine the time it takes for substance A to completely turn into substance B. Then, under the same conditions, we will conduct an experiment for [A] = 2 mol/l and for [A] = 3 mol/L. If the reaction rate increases two and three times, respectively, then the speed is directly proportional concentrations A, i.e. V ∝ [A] (V is velocity). If , when doubling the concentration , we get a quadruple increase in speed, and tripling - an increase of 9 times, then V ∝ [A]².

Reaction order

In this transformation, the order of the reaction is determined by the degree to which [A] is located, i.e. for V ∝ [A], the order reactions - 1, for V∝ [A]² reaction order - 2 and so on. The order of the reaction is not necessary integer, for V ∝ [A]^0.63 the reaction order is 0.63.

In general, the reaction order will be defined as the sum of the degrees of each reagent. It is important to understand, that a change in the concentration of one of the reaction components indicates a change in the concentration of the others participants of the reaction.

For reaction
aA + bB + cC → dD
the expression
is valid -d[A]/dt • 1/a = -d[B]/dt • 1/b = -d[C]/dt • 1/c = +d[D]/dt • 1/d

Equilibrium constant

The relationship between V and [A] can usually be expressed in terms of the constant k, i.e.

V = k • [A]ⁿ
where n is the reaction order

V = k • [A]ⁿ = -d[A]/dt ⇒ k • dt = - d[A]/[A]ⁿ
Integrating, we get:
∫(1/x)dx = ln(x) + C
for convenience of calculations, instead of C, we write ln[A]₀, then
ln[A] = -kt + ln[A]₀

We have obtained the equation of the straight line y=mx+b, where y=ln[A] is the concentration of A at time t, m=-k is the coefficient of the relationship between V and [A], x=t is time, b=ln[A]₀ is the initial concentration A.

Transformation period

For a first-order reaction, we can get the time during which half of the initial substance was converted, that is, the time for [A] = 1/2[A]₀:

[A] = 1/2[A]₀
kt = ln[A]₀ - ln[A] = ln[A]₀/ln[A] = ln2 ⇒ t _½ = ln(2)/k = 0.693/k
thus, for a reaction of the first kind, the conversion time of the product does not depend on the initial concentration

Similarly, another conversion period can be calculated, a third of the product or one fourth of the product or which will be necessary.

For a reaction of the second order or higher, it is necessary to use more complex mathematical models, consider an example for a reaction of the second kind:

C → D
V ∝ [C]² → -d[C]/dt = k[C]²
V = k[C]² → 1/[C] = kt + 1/[C]₀
t_½ = 1/k[C]

Temperature

An increase in temperature indicates an increase in the internal energy of the molecules of the system, as a result, the amount of the number of molecules capable of producing some reaction increases. Also, the increase in internal energy provokes an increase in the speed of movement of molecules, which also contributes to an increase in the reaction rate. But, in the case of enzymes (molecules in living organisms), the reaction occurs under comfortable conditions, therefore, in such in reactions, the temperature must be in a certain range.

Molecular weight

Theories about the reaction rate were based on the assumption that for the interaction of two particles it is necessary so that they would collide. The number of molecules that collide simultaneously is called molecular weight. Depending on the number of colliding molecules, there are: monomolecular (for one), bimolecular (for two molecules) and trimolecular reactions. Reactions involving more than three molecules at the same time currently unknown to science.

Steric factor

What determines whether the molecules collide with each other or not? Firstly, on their number: the higher the concentration, the more often the molecules are found. Secondly, the fact that the molecules collided does not mean that they will react, since their geometry may not allow them to react with each other, in connection with which it was introduced a coefficient called the steric factor, which shows the probability that when when two molecules collide, a chemical reaction will occur.