According to collision theory, the rate of a chemical reaction is directly proportional to the number of effective collisions between reactant particles.
In the laboratory, factors can be modified to vary the number of effective collisions and in turn the reaction speed.
The following table gives a general description of how some of these factors influence reaction speed.
Click on one of the factors to see details.
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Factor |
Increased reaction speed |
Reduced reaction speed |
|---|---|---|
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High concentration |
Low concentration |
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Few chemical bonds to break |
Large number of chemical bonds to break |
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Weak bonds |
Strong bonds |
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The physical state of the reactants
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High kinetic energy |
Low kinetic energy |
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Low attraction between particles |
Strong attraction between particles |
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Large contact surface |
Small contact surface |
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High temperature |
Low temperature |
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Presence of a catalyst |
Presence of an inhibitor |
Usually, an increase in the concentration of a reactant incurs an increase in reaction rate.
An increase in the number of reagent particles in a given volume generally results in a higher number of collisions, increasing the likelihood of effective collisions.
The effect of a change in reactant concentration on the rate of a reaction is not always directly proportional. For example, doubling the reactant concentration does not necessarily double the reaction rate. The effect of an increase in reagent concentration depends on the type of chemical reaction and the nature of the reactants.
To know precisely the effect of an increase in reactant concentration on reaction rate, we need to use a mathematical relationship called the law of reaction rates.
The number of bonds to be broken, the strength of the bonds, the force of attraction between the particles and the kinetic energy of the particles are all factors that influence the energy of activation. By the same token, they influence the rate of reaction.
The lower the activation energy, the faster the chemical reaction.
The higher the activation energy, the slower the chemical reaction.
When the activation energy |(E_a)| is low, more particles possess the energy required to make effective collisions. The greater the number of effective collisions between reactants, the faster the reaction.
When the activation energy |(E_a)| is high, fewer particles have the energy required to make effective collisions. The fewer the number of effective collisions between reactants, the slower the reaction.
The number of chemical bonds to be broken as well as the strength of the bonds vary according to the nature of the reactants involved in a chemical reaction.
The Number of Chemical Bonds to Break
When comparing similar molecules, the more chemical bonds to be broken in the reactants, the higher the activation energy. Thus, the reaction rate is slower.
Under similar reaction conditions, the combustion of propane |(\text{C}_3\text{H}_8)| is slower than that of methane |(\text{CH}_4).| In fact, the number of bonds to be broken is greater in a molecule of |\text{C}_3\text{H}_8| than in a molecule of |\text{CH}_4.|
Slower reaction
It takes at least |4\ 006\ \text{kJ}| of energy to break the 10 chemical bonds in each molecule of one mole of |\text{C}_3\text{H}_8.|
Faster reaction
It takes |1\ 656\ \text{kJ}| of energy to break the 4 chemical bonds present in each molecule of one mole of |\text{CH}_4.|
The Strength of Chemical Bonds to Break
The lower the bonding energy of the reactants, the lower the reaction activation energy and the faster the reaction rate.
The bond energy of sodium bromide (NaBr) is higher than that of sodium iodide (NaI).
When these two compounds react with chloromethane |(\text{CH}_3\text{Cl}),| the reaction with |\text{NaI}| is faster than the reaction with |\text{NaBr}.|
Slower reaction
||\text{CH}_3\text{Cl}+\text{NaBr}\rightarrow \text{CH}_3\text{Br}+\text{NaCl}||
Faster reaction
||\text{CH}_3\text{Cl}+\text{NaI}\rightarrow \text{CH}_3\text{I}+\text{NaCl}||
Steric hindrance, or steric effects, is a phenomenon where the arrangement of atoms in a compound makes it difficult to access certain atoms. These atoms therefore make fewer collisions, reducing the number of effective collisions and slowing down the reaction.
As a result, even if the reactants have few or weak bonds, a reaction may be slower than expected due to steric hindrance.
The force of attraction between particles of a substance as well as the particle kinetic energy vary according to the physical state of that substance.
The Force of Attraction Between Particles
The stronger the attractive forces between reactant particles, the greater the amount of energy needed to break these forces, the higher the reaction activation energy and the slower the reaction rate.
According to the particle model, attractive forces are greater between particles of a solid substance than between particles of the same substance in the liquid or gaseous state. Furthermore, when an ionic compound is dissociated in solution, the attractive forces between ions are significantly reduced.
Particle Kinetic Energy
Particles with high kinetic energy are more likely to reach an energy level greater than or equal to the activation energy |(E_a)| of the reaction.
According to the particle model, the physical state of a substance influences the kinetic energy of its particles.
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In the solid state, the kinetic energy of particles is very low. In fact, solid particles only vibrate in place.
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In the liquid state, the kinetic energy of particles is low. In fact, while remaining very close together, particles in a liquid can slide relative to one another.
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In the gaseous state, the kinetic energy of particles is very high. In fact, particles in a gas can move at high reaction rates in all directions at random.
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A homogeneous reaction is a chemical reaction where all reactants are in the same physical state.
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A heterogeneous reaction is a chemical reaction where not all reactants are in the same physical state.
Homogeneous reactions are generally faster than heterogeneous reactions.
For example, the combustion of propane, which is a homogeneous reaction in which all substances are gaseous, is much faster than the combustion of wood, which is a heterogeneous reaction.
When one of the reactants is solid, only the particles on the surface of the solid can come into contact with the particles of the other reactants. Fragmenting a solid reactant increases its surface area in contact with the other reactants, thereby exposing its particles to more collisions. By allowing a greater number of collisions, the number of effective collisions also increases, thus boosting the reaction rate.
According to collision theory, the reaction rate of a chemical reaction depends, among other things, on the kinetic energy of the reactant particles.
The temperature is an index of the degree of particle agitation. Increasing the temperature of the reaction medium therefore increases the kinetic energy of the reactant particles, which tends to increase the number of effective collisions and the reaction rate.
The Maxwell-Boltzmann distribution curve shows the kinetic energy of reactant particles as the temperature of the reaction medium is varied.
We can see that as the temperature of the reaction medium rises, a greater number of reactant particles have a kinetic energy equal to or greater than the activation energy |(E_a)| of the reaction.
The refrigerator lowers the temperature of food to slow down the fermentation and oxidation reactions of certain foods.
Source : Andrey_Popov, Shutterstock.com
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A catalyst is a substance that increases the reaction rate of a chemical reaction by decreasing its activation energy, without being consumed in the process.
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An inhibitor has the opposite effect of a catalyst, which means that it decreases the reaction rate of a chemical reaction by increasing its activation energy, without being consumed in the process.
According to collision theory, the reaction rate is directly proportional to the number of effective collisions.
By lowering the activation energy of a reaction |(E_{a\ cat}),| the addition of a catalyst allows a greater number of reactant particles to reach the energy level required to make effective collisions. As a result, the reaction is faster.
By increasing the activation energy of a reaction |(E_{a\ inh}),| the addition of an inhibitor allows fewer reactant particles to reach the energy level required for efficient collisions. As a result, the reaction is slower.
Consult this concept sheet to learn more about the effect of catalysts on reaction rate and the types of catalyst.