Rate of a Chemical Reaction - Chemical Kinetics

Chemical kinetics is the area of chemistry in which reaction rates are studied. Kinetics is largely an experimental science.

Reaction rate is called the speed at which a chemical reaction occurs.

Some chemical reactions are complete within a fraction of a second (explosions) while others take years (corrosion of metals) or even centuries (formation of mineral’s in Earth).

The basic concepts covered in this post are the following:

Factors that affect the rate of chemical reactions
Definition of the rate of reaction

 

What are the factors that affect the rate of chemical reactions?

Four factors allows us to change the rate at which a reaction occurs:

 

Physical state and nature of the reactants

Reactants in the same physical state tend to react faster since there is a greater chance for collision. Most of the reactions we consider are homogeneous involving either all gases or  all liquids

Reactants in different physical states tend to react slower since there is a smaller chance for collision

Gases and liquids react faster than solids because of the increase in surface area.

Large and complex molecules tend to react slower than small molecules since the reaction site may be hindered and therefore statistically there is smaller chance for collision at it.

 

Reaction temperature

Reaction rates generally increase with temperature (Fig. I.1). Increasing temperature increases the kinetic energies of molecules. The higher the temperature, the higher the kinetic energies of the molecules and the greater the number of collisions.

 

 Reactant Concentrations

The higher the concentration of reactants, the greater the chance of collision and the greater the reaction rate. For gaseous reactants, the pressure is directly related to the concentration. The greater the pressure the greater the reaction rate.

 

Catalysts

Catalysts increase reaction rates and are (theoretically) recoverable at the end of the reaction. Catalysts accomplish this by reducing the energy required for the reaction (activation energy)

Speed of a chemical reaction— reaction rate— is the change in the concentration of reactants or products per unit of time. The units for reaction rate are usually expressed as Molarity per second (M/s) —that is, the change in concentration measured in Molarity divided by a time interval measured in seconds.

 

Let us consider the general reaction:

 

A  +   2B   →   C  + 3D

The average rate of reaction can be expressed as:

Average rate of disappearance of A: - Δ[Α] / Δt

Average rate of disappearance of B: -(1/2) * Δ[B] / Δt

Average rate of appearance of C: + Δ[C] / Δt

Average rate of appearance of D: + (1/3) * Δ[D] / Δt

The first two expressions for the reactants are negative, because their concentrations will decrease with time

Where Δ[Α] Δ[B] Δ[C] and Δ[D] is the concentration change of A,B,C and D (Molarity) with time (s).

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Relevant Posts - Relevant Videos

Chemical Kinetics (a video series)

Rates of Chemical Reactions and the Collision Model

How to determine the order of a reaction by the initial rates method?

 

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References

1. David W. Oxtoby, H.P. Gillis, Alan Campion, “Principles of Modern Chemistry”, Sixth Edition, Thomson Brooks/Cole, 2008
2. Steven S. Zumdahl, “Chemical Principles”  6th Edition, Houghton Mifflin Company, 2009
3. Ralph H. Petrucci, “General Chemistry”, 3rd Edition, Macmillan Publishing Co., 1982
4. Chemical Kinetics - Reaction Rates -Concentration Changes

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Key Terms

reaction rate, chemical kinetics, factors affecting reaction rates, rate of a chemical reaction, nature of reactants, , reactant temperature, , reactant concentrations, , catalysts, ,

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