The study of the rate of a chemical reaction and the conditions affecting its change is dealt with by one of the areas of physical chemistry - chemical kinetics. She also considers the mechanisms of these reactions and their thermodynamic validity. These studies are important not only for scientific purposes, but also for controlling the interaction of components in reactors in the production of various substances.
The concept of speed in chemistry
The reaction rate is usually called a certain change in the concentrations of the reacted compounds (Δ) per unit time (Δt). The mathematical formula for the rate of a chemical reaction is as follows:
ᴠ = ± ΔC / Δt.
The reaction rate is measured in mol / l ∙ s if it occurs in the entire volume (i.e., the reaction is homogeneous) and in mol / m 2 ∙ s if the interaction occurs on the surface separating the phases (i.e., the reaction is heterogeneous). The “-” sign in the formula is related to the change in the concentration values of the starting reacting substances, and the “+” sign - to the changing concentration values of the products of the same reaction.
Examples of reactions at different rates
Chemical interactions can occur at different speeds. So, the growth rate of stalactites, that is, the formation of calcium carbonate, is only 0.5 mm per 100 years. Some biochemical reactions, such as photosynthesis and protein synthesis, are slow. Corrosion of metals proceeds at a rather low rate.
At an average rate, reactions requiring one to several hours can be characterized. An example is cooking, accompanied by decomposition and conversion of the compounds contained in the products. The synthesis of individual polymers requires heating the reaction mixture for a certain time.
An example of chemical reactions, the rate of which is quite high, can be neutralization reactions, the interaction of sodium bicarbonate with a solution of acetic acid, accompanied by the release of carbon dioxide. We can also mention the interaction of barium nitrate with sodium sulfate, in which precipitation of insoluble barium sulfate is observed.
A large number of reactions can proceed with lightning speed and are accompanied by an explosion. A classic example is the interaction of potassium with water.
Factors affecting the rate of a chemical reaction
It is worth noting that the same substances can react with each other at different speeds. So, for example, a mixture of gaseous oxygen and hydrogen may not show signs of interaction for a rather long time, however, when the container is shaken or shocked, the reaction becomes explosive. Therefore, certain factors are identified by chemical kinetics that have the ability to influence the rate of a chemical reaction. These include:
- the nature of the interacting substances;
- reagent concentration;
- temperature change;
- the presence of a catalyst;
- pressure change (for gaseous substances);
- contact area of substances (if we are talking about heterogeneous reactions).
The effect of the nature of matter
Such a significant difference in the rates of chemical reactions is explained by different values of the activation energy (E a ). By it is meant a certain excess amount of energy in comparison with its average value, which is necessary for a molecule in a collision in order for a reaction to occur. It is measured in kJ / mol and the values are usually in the range of 50-250.
It is generally accepted that if E a = 150 kJ / mol for any reaction, then when n. at. it practically does not leak. This energy is spent on overcoming repulsion between the molecules of substances and on weakening the bonds in the starting materials. In other words, activation energy characterizes the strength of chemical bonds in substances. By the value of the activation energy, you can tentatively estimate the rate of a chemical reaction:
- E a <40, the interaction of substances occurs rather quickly, since almost all collisions of particles lead to their reaction;
- 40 <E a <120, an average reaction is assumed, since only half of the molecular collisions will be effective (for example, the reaction of zinc with hydrochloric acid);
- E a > 120, only a very small fraction of particle collisions will lead to a reaction, and its speed will be low.
Concentration effect
The dependence of the reaction rate on concentration is most likely characterized by the law of mass action (ZDM), which states:
The rate of a chemical reaction is directly proportional to the product of the concentrations of the reacted substances, the values of which are taken in degrees corresponding to stoichiometric coefficients.
This law is suitable for elementary one-stage reactions, or for any stage of the interaction of substances characterized by a complex mechanism.
If you want to determine the speed of a chemical reaction, the equation of which can be arbitrarily written as:
α + bB = ϲ, then,
in accordance with the above stated wording of the law, the speed can be found by the equation:
V = k · [A] a · [B] b , where
a and b are stoichiometric coefficients,
[A] and [B] are the concentrations of the starting compounds,
k is the rate constant of the reaction in question.
The meaning of the rate coefficient of a chemical reaction is that its value will be equal to the speed if the concentrations of the compounds are equal to units. It should be noted that for the correct calculation according to this formula, the aggregate state of the reagents should be taken into account. The concentration of the solid is taken equal to unity and is not included in the equation, since during the reaction it remains constant. Thus, in the calculation by ZDM include the concentration of only liquid and gaseous substances. So, for the reaction of obtaining silicon dioxide from simple substances described by the equation
Si (tv) + Ο 2 (t) = SiΟ 2 (tv) ,
speed will be determined by the formula:
V = k · [Ο 2 ].
Typical task
How would the rate of the chemical reaction of nitrogen monoxide with oxygen change if the concentrations of the starting compounds were doubled?
Solution: The reaction equation corresponds to this process:
2ΝΟ + Ο 2 = 2ΝΟ 2 .
We write the expressions for the initial (ᴠ 1 ) and final (ᴠ 2 ) reaction rates:
ᴠ 1 = k · [ΝΟ] 2 · [Ο 2 ] and
ᴠ 2 = k · (2 · [ΝΟ]) 2 · 2 · [Ο 2 ] = k · 4 [ΝΟ] 2 · 2 [Ο 2 ].
Next, you should separate the left and right parts:
ᴠ 1 / ᴠ 2 = (k · 4 [ΝΟ] 2 · 2 [Ο 2 ]) / (k · [ΝΟ] 2 · [Ο 2 ]).
The concentrations and rate constants are reduced, and remains:
ᴠ 2 / ᴠ 1 = 4 · 2/1 = 8.
Answer: increased by 8 times.
Temperature effect
The dependence of the rate of a chemical reaction on temperature was determined experimentally by the Dutch scientist J. H. Van Hoff. He found that the rate of many reactions increases 2–4 times with an increase in temperature for every 10 degrees. For this rule, there is a mathematical expression that looks like:
ᴠ 2 = ᴠ 1 · γ (Τ2-Τ1) / 10 , where
ᴠ 1 and ᴠ 2 are the corresponding velocities at temperatures Τ 1 and Τ 2 ;
γ is the temperature coefficient, equal to 2–4.
At the same time, this rule does not explain the mechanism of the influence of temperature on the value of the rate of a particular reaction and does not describe the totality of laws. It is logical to conclude that with increasing temperature, the random movement of particles increases and this provokes a greater number of their collisions. However, this does not particularly affect the efficiency of the collision of molecules, since it depends mainly on the activation energy. Also, a considerable role in the efficiency of particle collisions has their spatial correspondence to each other.
The temperature dependence of the rate of a chemical reaction, taking into account the nature of the reagents, obeys the Arrhenius equation:
k = A 0 · e -Ea / RΤ , where
And o is the multiplier;
E a - activation energy.
An example of a problem on the Van Goff law
How should the temperature be changed so that the rate of a chemical reaction, in which the temperature coefficient is numerically equal to 3, grows 27 times?
Decision. We use the formula
ᴠ 2 = ᴠ 1 · γ (Τ2-Τ1) / 10 .
From the condition ᴠ 2 / ᴠ 1 = 27, and γ = 3. You need to find ΔΤ = Τ 2 –Τ 1 .
Transforming the original formula, we get:
V 2 / V 1 = γ ΔΤ / 10 .
We substitute the values: 27 = 3 ΔΤ / 10 .
From this it is clear that ΔΤ / 10 = 3 and ΔΤ = 30.
Answer: the temperature should be increased by 30 degrees.
Catalyst Effect
In physical chemistry, the rate of chemical reactions is also actively being studied by a section called catalysis. He is interested in how and why relatively small amounts of certain substances significantly increase the rate of interaction of others. Such substances that can accelerate the reaction, but themselves are not consumed in it, are called catalysts.
It has been proved that catalysts change the mechanism of the chemical interaction itself, contribute to the appearance of new transition states, which are characterized by lower heights of the energy barrier. That is, they help to reduce the activation energy, and hence increase the number of effective particle stresses. The catalyst cannot cause a reaction that is energetically impossible.
So hydrogen peroxide is able to decompose with the formation of oxygen and water:
H 2 Ο 2 = H 2 Ο + Ο 2 .
But this reaction is very slow and in our first-aid kits it exists unchanged for quite a long time. Opening only very old bottles with peroxide, you can notice a small clap caused by the pressure of oxygen on the walls of the vessel. Adding just a few grains of magnesium oxide will provoke active gas evolution.
The same decomposition reaction of peroxide, but already under the action of catalase, occurs during the treatment of wounds. In living organisms there are many different substances that increase the rate of biochemical reactions. They are called enzymes.
Inhibitors have the opposite effect on reactions. However, this is not always bad. Inhibitors are used to protect metal products from corrosion, to extend the shelf life of food, for example, to prevent the oxidation of fats.
Area of contact of substances
In the event that the interaction occurs between compounds having different aggregate states, or between substances that are not able to form a homogeneous medium (immiscible liquids), then this factor also affects the rate of a chemical reaction significantly. This is due to the fact that heterogeneous reactions are carried out directly at the interface between the interacting substances. Obviously, the broader this boundary, the more particles are able to collide, and the faster the reaction.
For example, burning wood much faster in the form of small chips than in the form of a log. For the same purpose, many solids are triturated into a fine powder before being added to the solution. So, powdered chalk (calcium carbonate) acts faster with hydrochloric acid than a piece of the same mass. However, in addition to increasing the area, this technique also leads to chaotic rupture of the crystal lattice of the substance, which means it increases the reactivity of the particles.
Mathematically, the speed of a heterogeneous chemical reaction is found as a change in the amount of a substance (Δν) that occurs per unit time (Δt) per unit surface
(S): V = Δν / (S · Δt).
Pressure effect
A change in pressure in the system affects only when gases are involved in the reaction. The increase in pressure is accompanied by an increase in the molecules of the substance per unit volume, that is, its concentration increases proportionally. Conversely, lowering the pressure leads to an equivalent decrease in the concentration of the reagent. In this case, the formula corresponding to ZDM is suitable for calculating the rate of a chemical reaction.
Task. As the reaction rate described by the equation increases
2ΝΟ + Ο 2 = 2ΝΟ 2 ,
if the volume of a closed system is reduced by three times (T = const)?
Decision. As volume decreases, pressure increases proportionally. We write the expressions for the initial (V 1 ) and final (V 2 ) reaction rates:
V 1 = k · [NΟ] 2 · [Ο 2 ] and
V 2 = k · (3 · [NΟ]) 2 · 3 · [Ο 2 ] = k · 9 [ΝΟ] 2 · 3 [Ο 2 ].
To find how many times the new speed is greater than the initial one, you need to separate the left and right parts of the expressions:
V 1 / V 2 = (k · 9 [ΝΟ] 2 · 3 [Ο 2 ]) / (k · [ΝΟ] 2 · [Ο 2 ]).
The concentrations and rate constants are reduced, and remains:
V 2 / V 1 = 9 · 3/1 = 27.
Answer: speed increased by 27 times.
Summing up, it should be noted that many factors influence the rate of interaction of substances, and more precisely, the quantity and quality of collisions of their particles. First of all, it is the activation energy and the geometry of the molecules, which are almost impossible to correct. As for the remaining conditions, then to increase the reaction rate should:
- increase the temperature of the reaction medium;
- increase the concentration of the starting compounds;
- increase the pressure in the system or reduce its volume, when it comes to gases;
- to bring dissimilar substances to one aggregate state (for example, having dissolved in water) or to increase the area of their contact.