Tag Archives: Stopped-Flow

Factors Affecting Reaction Rate in Chemical Kinetics

Chemical kinetics or reaction kinetics is the study of rates of chemical processes. The study of chemical kinetics includes investigations on how different experimental conditions can influence the speed of chemical reactions and produce information about the reaction’s mechanism and transition states. It also includes the construction of mathematical models that can describe the characteristics of a chemical reaction.

Factors affecting reaction rate

Chemical kinetics deals with the experimental determination of reaction rates from which rate laws and rate constants are derived. In order for a reaction to occur, a collision must take place. This collision has to be of proper orientation and have sufficient energy to break the necessary bonds.

There are several factors that influence the rates of chemical reactions. For instance, the particle size distribution of a material, which can be deduced using a particle size analyser, will affect the reactivity of solids participating in chemical reactions. The following are other factors:

Nature of reactants

Reaction rate variation is dependant on which substances are reacting. Fast reactions include acid reactions, the formation of salts and ion exchange. Reactions tend to be slow when covalent bond formation takes place between the molecules and when large molecules are formed.

Physical state

The physical state, i.e. solid, liquid or gas, of a reactant is also an important factor of the rate of change. When reactants are in the same phase, thermal motion brings them into contact. If they are in different phases, the reaction is limited to the interface between reactants and reactions can only occur at their area of contact. Vigorous shaking and stirring may be needed to complete the reaction.


According to the collision theory of chemical reactions, concentration is an important factor because molecules must collide in order to react with each other. When the concentration of the reactants increases, the frequency of the molecules colliding increases, striking each other more frequently. Increasing the amount of one of more reactants causes the collusions to happen more often, increasing the rate of reaction.


Molecules at a higher temperature have more thermal energy and collision frequency is greater at higher temperatures.


Catalysts are substances used to facilitate reactions but remain chemically unchanged afterwards. The rate of reaction is increased when the catalyst provides a different reaction mechanism to occur with lower activation energy.


When the pressure is increased in a gaseous reaction, the number of collisions between reactants will also rise, increasing the rate of reaction.

Instruments that can be used for studying reaction rate


A stopped-flow instrument is a mixing device that is most frequently used to study rapid kinetics. Small volumes of solution are rapidly driven from syringes into a high efficiency mixer to initiate a fast reaction. These reactions are usually recorded by spectroscopic techniques such as UV absorbance, fluorescence or circular dichroism. The most commonly used detection method is fluorescence spectroscopy because of its high sensitivity.

Stopped flow instruments can be equipped with up to four syringes, one for the sample and two or three syringes for double or sequential mixing of reagents.

Rapid kinetics fluorescence

This is another instrument designed specifically for the detection of rapid kinetics. Combined with the Bio-Logic stopped-flow quenched-flow equipment, it makes a very powerful kinetics analysis system, one with high sensitivity and very efficient data acquisition. The Spectrometer can be configured to measure reactions by UV absorbance plus fluorescence or two different fluorescence wavelengths.

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