- Factors that affect the rate of a reaction
- Substance particle size
- Physical state of substances
- Reagent concentration
- Temperature
- Catalysts
- References
The speed of a chemical reaction is the speed at which the transformation of substances called reactants occurs, into other substances called products. The factors that affect speed can be several; nature of reagents, particle size, physical state of substances…
The reactants can be atoms or molecules that collide or collide with each other causing a breakdown of bonds between them. After the break, new bonds are created and products are formed.
If at least one of the reactants is totally consumed in the reaction, fully forming the product, the reaction is said to be complete and run in only one direction.
In some cases the products formed collide again and break their bonds to reorganize and become the reactants again. This is called a reverse reaction.
Both reactions occur at different speeds, however when the speed of the forward reaction equals the speed of the reverse reaction, a kinetic equilibrium is established, which means that the reaction is in equilibrium.
Factors that affect the rate of a reaction
Every chemical reaction is subject to a series of factors, which cause the speed of it to pass quickly or slowly. We find reactions that happen in seconds, such as explosions, and others that take a little longer, such as the oxidation of an iron rod put out in the open.
These factors that affect the rate of a chemical reaction are:
Substance particle size
It is also known as a contact surface. If the substances have a large contact surface, that is, they are very compact, the reaction is slower than when the contact surface is small.
An example is the reaction of Alka seltzer in tablet and Alka seltzer in powder. Alka seltzer is a mixture of acetyl salicylic acid, with sodium bicarbonate, calcium phosphate and citric acid.
If the substances are atomic species, they also show a variation in their reactivity due to the size of the atom and the number of electrons at its last level.
Due to this, sodium (Na) reacts with water in a violent way compared to calcium (Ca). Similarly, iron (Fe) is easily oxidized by the action of water vapor present in the ambient air, compared to lead (Pb) whose reaction is much slower.
The ionic species have a very high reactivity (low reaction rates), when compared to their neutral species. Thus, Mg + 2 is more reactive than Mg.
Physical state of substances
The state of aggregation of the reactants also influences the reaction rate. In the solid state, the particles (atoms) are very close to each other, so the mobility between them is very little, and the collisions are very slow.
In the liquid state the particles have more mobility, which makes the reactions faster compared to the solid state.
In the gaseous state, the reaction has a much higher speed, thanks to the large separation between the reagent particles.
To increase the reaction speed of a substance, it can be dissolved in water, in such a way that the molecules are solubilized and the mobility between them increases.
Reagent concentration
The concentration of a substance refers to the number of particles (atoms, ions or molecules) that are in a given volume.
In a chemical reaction, if there are a lot of particles, the number of collisions between them will be very high, so the speed of the reaction will be high.
The higher the concentration of the reactants, the higher the reaction rate of product formation will be.
Temperature
In a system made up of reagents, all the particles that make it up are in motion, either vibrating, as in solid substances, or moving in the case of liquids and gases.
In both cases, vibrational E and kinetic E are observed respectively. These energies are directly proportional to the temperature at which the system is.
As the temperature of the system increases, the molecular movements of the substances increase.
The collisions between them are becoming stronger, enough to cause the breakdown and formation of bonds, overcoming the obstacle that constitutes the activation energy Ea.
As the temperature of the system increases, the reactivity increases and the reaction speed is lower, therefore faster.
Catalysts
They are chemical substances that influence a chemical reaction, either increasing the reaction rate or slowing it down. Its main characteristic is that it does not participate in the chemical reaction, which means that at the end of the reaction, it can be isolated from the system.
An example is the hydrogenation of an unsaturated organic compound with lithium aluminum hydride as a catalyst:
CH3 - CH = CH - CH3 + H2 CH3 - C2 - CH2 - CH3
In a chemical equation, the catalyst is placed above the arrow that indicates the direction of the reaction.
In a chemical reaction it can happen that both the catalyst and the reactants are not in the same physical state, this type of system is known as "heterogeneous".
These are called contact catalysts. The "homogeneous" catalysts are those that have the same physical state of the reactants and are called transport.
References
- Levine, I. Physicochemistry. vol.2. McGraw-Hill 2004
- Capparelli, Alberto Luis Basic physicochemistry. E-Book.
- Fernández Sánchez Lilia, Corral López Elpidio, et.al (2016). Kinetics of chemical reactions. Recovered: zaloamati.azc.uam.mx.
- Anne Marie Helmenstine, Ph.D. Factors That Affect the Rate of a Chemical Reaction. Recovered: thoughtco.com.