All you need to know about reaction kinetics?
The study of the kinetics of a chemical reaction is the study of the speed of this reaction. In this article, we will review the distinction between slow and fast reactions, then the different methods of monitoring reactions, and finally, we will focus on the catalysis of reactions.
At the end of this article, you must, therefore, master:
- defining a fast/slow response
- methods for monitoring a slow response
- the definition of a half-reaction time and a kinetic speed
- the definition of a catalyst and the classic examples
Fast reaction vs. slow reaction
A reaction is rapid if it cannot be followed with the naked eye or with a conventional measuring device. As soon as the reagents are mixed, the final state is reached instantly. For example, this is the case of precipitation reactions, acid/base reactions, and explosive reactions.
On the contrary, a reaction is slow if it can be followed with the naked eye or with a usual measuring device. It can last from a few minutes to a few years. This is, for example, the case of redox reactions, the formation of rust, the aging of wine.
Fast reaction:
A chemical reaction is considered slow when it takes a while to observe the evolution of the chemical system with the naked eye or to use measuring devices. Product training is therefore carried out on a time scale ranging from a few seconds to several hours or even several days or months.
Note: when the duration begins to be counted in days, we speak of infinitely slow or extremely slow reactions. Some examples of slow reactions:
- esterification reactions
- hydrolysis reactions
- certain redox reactions (such as corrosion)
An example of a well-known slow reaction is the dissolution of drugs in water. Rust formation is an extremely slow reaction.
Fast chemical reactions
Unlike a slow reaction, a chemical reaction is considered to be rapid if the formation of products is instantly perceived by human senses as soon as the reactants are brought into contact with each other. The reaction is then too short for its evolution to be followed with the naked eye or using measuring devices. This, therefore, means that it takes place on a time scale, which is less than a second. Some examples of quick reactions:
- precipitation reactions
- acid-base reactions
- explosive combustion reactions
- Certain redox reactions.
An example of a known rapid reaction is the precipitation reaction. Take, for example, the case of soda (NaOH) in a solution of Copper Sulfate (CuSO4).
The mixing of the reactants instantly leads to the formation of a precipitate of copper hydroxide (Cu (OH) 2) according to the following reaction equation: Cu 2+ + 2 OH - → Cu (OH) 2
Methods for monitoring a slow response
In the case of a slow reaction, we can set up monitoring methods which allow us to obtain a relationship of the type
ξ= f ( t )
We try to write the progress of the reaction as a function of time.
Qualitative methods are available such as thin-layer chromatography (TLC), which makes it possible to ensure the formation of the products but does not make it possible to follow the reaction rate. There are also quantitative methods that make it possible to determine the concentration of a species and therefore, to go back to the reaction speed.
- chemical methods such as dosing or titration
- physical methods
- spectrophotometry: measures the absorbance of a solution then with Beer Lambert's law we go back to a concentration, so we use it when the color of the solution varies during the reaction
- conductimetry: we measure the capacity of the solution to conduct current, we use it when we are in the presence of ions
- manometry: we measure the pressure, we use it when we are in the presence of gas, and then we use the ideal gas equation pV = nRT to go back to the concentration
Monitoring a chemical reaction over time
What is kinetics?
The chemical kinetics or reaction kinetics is to study the evolution of a chemical reaction in time. In order to monitor this evolution over time, we will, for example, be interested in modifying the concentrations of the reagents or the concentrations of the products as the reaction progresses.
How to track a chemical reaction over time?
The objective here is to know the evolution of the chemical system at each instant of the reaction. To do this monitoring, it is possible to plot the evolution curve of the concentrations of the chemical species present in this chemical system:
- The evolution curve of the concentration of a reagent will be decreasing since the concentration of this reagent decreases as the reaction progresses
- The evolution curve of the concentration of a product will be increasing since the concentration of this product increases as the reaction progresses
In order to draw these curves, we will use measuring devices such as a conductumeter by measuring the conductivity or a spectrophotometer by measuring the absorbance.
Reaction time and half-reaction time
What is the reaction time?
The duration of a chemical reaction corresponds to the final time t required for the limiting reagent to be completely consumed.
At this final time t, the progress of the reaction noted x is maximum: x = x final
What is the half-reaction time?
- When a reaction evolves slowly enough, it is not clear when it ends. The half-reaction time then makes it possible to characterize the evolution of the system. Thus, the half-reaction time corresponds to the time which elapses between the start of a reaction (t = 0) and the instant (noted t 1/2) when the advancement has a value equal to half of final advancement (x final): At t = t 1/2 [x = frac {x_ {final}} {2}]
- The half-reaction time, therefore, gives an idea of the duration of a chemical reaction. Depending on the precision chosen, it is considered that the reaction is finished when the time which has elapsed since the start of the reaction is equivalent to five to ten times the value of the half-reaction time.
Note: Because of its name, it is common to think that the duration of the reaction is equal to twice the time of half-reaction. However, this is an error because the speed at which a chemical reaction takes place is not constant and, moreover, tends to decrease when the concentration of the reactants decreases.
Half reaction time, total duration and reaction speed
We define the half-reaction time of a reaction and we note it as the duration at the end of which the advancement is equal to half of the advancement in the final state.
A 
, so we have: 
The total duration of the reaction can be estimated by 
We define a speed of reaction, as the derivative of advancement over time divided by the volume of the medium.
v =1Vd ξdt
In this formula:
- is the speed in
- is the volume in
- Then we have who is the advancement in
- is finally the time in
The speed is maximum at it then decreases to cancel when the final state is reached.
Kinetic factors
A kinetic factor is a quantity or a parameter that influences the speed of the reaction and, therefore, its duration. Here are two examples of kinetic factors.
Temperature
By increasing the temperature of the reaction medium, thermal agitation is favored, and therefore the rupture of existing bonds or the formation of new bonds. Temperature is, therefore, a kinetic factor.
Example: to store food, the temperature is lowered to block certain reactions.
The initial concentration of reagents
Under all equal conditions, the increase in the initial concentration of the reactants makes it possible to reduce the reaction time and, therefore, to increase the reaction speed.
Catalysis
To catalyze, a reaction means to accelerate it. There are three types of catalysis:
- heterogeneous the reagent and the catalyst are not in the same phase, for example, liquid / solid
- enzymatic, the catalyst is an enzyme
- homogeneous, the reagent and the catalyst are in the same phase, for example, liquid
Example we are interested in the disproportionation of hydrogen peroxide H2O2
We put into play 2 redox couples:
- H2O2/ $ H_20 $
- O2/H2O2
The equation is therefore
2H2O2 --> 2H2O+O2
Homogeneous catalysis
Let's catalyze the reaction by introducing ions Fe3+/Fe2+.
The reaction is broken down into 2 steps
Step 1 2Fe3+ + H2O2 --> 2Fe2+ + O2 + 2H+
Step 2 2Fe3+ + H2O2 + 2H+ --> 2Fe3+ + 2H2O
Balance sheet equation 2H2O2 --> O2 + 2H2O, we find the basic equation
These 2 reactions are faster than the basic reaction. So we really accelerated the reaction.
Note that the ions added for catalysis do not appear in the balanced equation.
Heterogeneous catalysis
Catalysis is said to be heterogeneous when the reactions and the catalysts are in 2 different phases. Let's catalyze the reaction by introducing a platinum wire. The reactions are liquid and the catalyst solid.
Oxygen bubbles are observed on the surface of the wire. The transformation takes place on the surface of the catalyst, which provides the energy necessary to break the first bonds. The larger the surface, the faster the reaction is.
Properties:
A catalyst is a chemical species that has a purely kinetic role. It indeed allows the increase of the reaction speed, but it does not modify the final state of the system. It participates in the transformation but is regenerated at the end of the reaction
, and therefore it does not appear in the balanced equation. A catalyst is selective; it will promote a particular reaction. That concludes this recap on the kinetics of chemical reactions does not hesitate to consult our other files or to practice on our annals.
Author: Vicki Lezama