By: Mai Pham Phuong

The reaction rate is defined as the speed at which a chemical reaction proceeds (how fast or slow it takes place). This is expressed in terms of either the concentration of a product that is formed per unit of time or the concentration of a reactant that is consumed per unit of time. In order for a reaction to take place, its particles must collide with enough energy and in the correct orientation. Chemical reactions can occur at various speeds, depending on various factors, which are listed below.

1. Nature of the Reactants
In a chemical reaction, bonds in the reactants are broken and new bonds are formed in the products. Therefore, the type, strength, and number of bonds in the reactants can greatly influence the rate of the reaction. Furthermore, the state at which the reactants are in and the size of its particles can also affect the reaction rate.

a) State of Reactants:
In reactions where the reactants are of the same states, the reaction occurs faster when the reactants are in gaseous states than compared to when the reactants are in liquid states. Reactions with reactants in solid states proceed slowest. The reason for this is that gaseous particles have the most freedom of movement, leading to more possible collisions which would cause the reaction to occur. Another explanation is that when the reactants are gases or liquids, they can easily interact and collide but when the reactants are solids, the reaction can only take place on the surface of the solids, causing the reaction to proceed slowly.

b) Aqueous Ions:
Aqueous ions tend to react faster than reactants in other states of matter. This is due to the fact that the ions are already separated, allowing the positive and negative charges of the ions to attract each other easily without having to break any bonds in creating the products. An example is a reaction between lead (II) nitrate and potassium iodide. When both compounds are in solid states, the ionic bonds in the reactants are strong and the ions in each compound are hard to separate from each other, causing the reaction to proceed very slowly. When these compounds are in aqueous solutions, the ions of each compound are dissociated and the lead (II) ions just need to contact the iodide ions for the reaction to take place, resulting in a very fast reaction.

c) Bond Type:
Reactions involving simple ions proceed faster than reactions involving molecules. Since molecules are covalently bonded, the bonds need to be broken down and then reformed, slowing the reaction rate. On the other hand, like the reasoning for reaction with aqueous ions, in simple ions the positive and negative charges of the ions attract each other easily without having to break any bonds to create the products. Therefore, reactions of covalently bonded molecules are usually slow, unless they are highly exothermic.

d) Bond Strength:
Reactions where weaker bonds are broken occur faster than reactions where stronger bonds are broken. This is another reason why reactions with covalently bonded molecules tend to proceed slowly since they are considerable strong. Consider the two reactions below:
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The first reaction is faster than the second reaction because it contains single carbon-carbon bonds, which are weaker than the double carbon-carbon bonds in the second reaction.

e) Number of Bonds:
Reactions involving the breaking of fewer bonds per reactant proceed faster than those involving the breaking of a larger number of bonds per reactant. Consider the two reactions below:

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The first reaction occurs faster than the second reactions as there are more bonds to be broken per molecule of kerosene (C13H28) than per molecule of methane

2. Temperature
Temperature is a measure of the kinetic energy of a system. As the temperature of the reaction is increased, the energy of the reactants increase and at the same time, they move faster. As particles move faster, more collisions will take place. Since the reactant particles need to collide with sufficient energy in order for the reaction to take place, increasing the frequency of collisions and energy of the particles would result in more successful collisions that can overcome the activation energy of the reaction. Therefore, reaction rate increases as the temperature increases. Generally, a 10°C temperature increase doubles the reaction rate. Below is a graph depicting how temperature affects the energy of the reactant molecules.

3. Concentration of Reactants
The concentration of a substance is the amount of that substance per amount of defined space. It is usually expressed in terms of mass unit per volume unit. Usually, an increase in the concentration of reactants results in an increased reaction rate. Increasing the concentration increases the amount of substance in a system, which results in a more collisions and more successful collisions. For example: In a system, every 1 in 1000 particles have sufficient energy to exceed the activation energy. If there are 10,000 particles, 10 of them would react. If there are 50,000 particles, 50 of them would react, which shows that the reaction rate has increased by 5 times. In an aqueous reaction, adding more solute can increase the concentration of the reactant while adding more solvent can decrease the concentration of the reactant.

4. Surface Area of Reactants
As stated above, when the reactants are solids, the reaction can only take place on the surface of the solids. In a solid, the surface area is the exposed matter. Hence, increasing the surface area of the solid would increase the number of collisions that takes place per unit of time (as there is more surface for the particles to interact), which causes an increase in the reaction rate. For the same amount of mass, smaller particles have a greater surface area than compared to larger particles. Increasing the surface area of a solid can be done by crushing it into a powder.

5. Pressure of Gaseous Reactants
Increasing the pressure of a gas, which can be done by forcing its particles into a smaller volume, is the same as increasing its concentration. This causes the particles to be more compressed, leading to the increased frequency of collisions and successful collisions. Solids and liquids are not affected by changes in pressure.

6. Presence of Catalysts and Inhibitors

a) Presence of Catalysts:
Catalysts are substances that alters the reaction rate without being chemically changed at the end of the reaction. As explained before, reactions take place only when the particles that collide have enough energy to overcome the activation energy. The addition of a catalyst to the reaction provides an alternative pathway for the reaction to take place, one with a lower activation energy. As a result of this, more particles will have energy that exceeds the activation energy, leading to more successful collisions. Hence, catalysts increase reaction rates. Addition of a catalyst does not change the energy of the reactants or the products, nor the ΔH of the reaction; catalysts increase both the rates of the forward and the reverse reactions. There are 2 types of catalysts:

  • Homogeneous Catalysts: These catalysts are in the same phase as the reactants.
  • Heterogeneous Catalysts: These catalysts are in a different phase from the reactants and they are usually solids that are involved in a reaction where the reactants are either liquids or gases. Heterogeneous catalysts work through adsorption. The reactants are adsorbed (stick onto the surface) onto the catalyst at a location called an active site. Here, the reactants become more reactive through their interaction with the catalyst, leading to their reaction with each other to form the products. Products are desorbed (broken away) from the surface of the catalyst to allow other reactant molecules to access the active site. Good catalysts adsorb the reactant molecules strongly enough to the active site so that they can react but not to the point where the product molecules are unable to be desorbed.

b) Presence of Inhibitors:
Inhibitors are substances that slow the rate of a reaction down.

Below is a graph depicting how a catalyst works. The green pathway is the alternate pathway that is used when a catalyst is present in a reaction.


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Further Reading: has various topics related to reaction rates.