Conducting Reactions Below Room Temperature

Overview

Source: Laboratory of Dr. Dana Lashley - College of William and Mary

Demonstration by: Matt Smith

When new bonds are formed in the course of a chemical reaction, it requires that the involved species (atoms or molecules) come in very close proximity and collide into one another. The collisions between these species are more frequent and effective the higher the speed at which these molecules are moving. A widely used rule of thumb, which has its roots in the Arrhenius equation¹, states that raising the temperature by 10 K will approximately double the rate of a reaction, and raising the temperature by 20 K will quadruple the rate:

(1) Equation 1

Equation (1) is often found in its logarithmic form:

(2) Equation 2

where k is the rate of the chemical reaction, A is the frequency factor (relating to frequency of molecular collisions), E_a is the activation energy required for the reaction, R is the ideal gas constant, and T is the temperature at which the reaction is taking place.

A higher temperature therefore means a reaction is completed much faster. Nonetheless, in some cases it is desirable to carry out reactions at low temperatures, in spite of the lowering effect on the rate of the reaction. A few scenarios in this regard are elaborated upon further below.

When it is useful to run a reaction below room temperature, chemists use cooling baths to maintain a certain temperature or temperature range. Reactions are cooled down to a desired temperature by placing the reaction flask inside an appropriate cooling bath. The reagents in the reaction never come in direct contact with the chemicals in the cooling bath. The cooling bath may consist of a single cryogenic (cooling) component (such as ice, dry ice, or liquid nitrogen) or may be a mixture of the cryogenic component with a certain solvent and/or an additive salt. The purpose of the solvent is to effectively transfer temperature of the cooling agent to the reaction flask, and the purpose of the additive is to lower (or depress) the freezing point of the mixture. (Note that it is possible for a substance to be both a solvent and an additive.)

Procedure

Cooling Bath Setup

For a general set-up, prepare the cooling bath of choice as described below and immerse the reaction flask into the bath (Figure 1). Don't fill the bath vessel all the way, but leave enough room to allow for immersion of the reaction flask.

Note: if the reaction is moisture sensitive, be very careful when adding reagents to the flask or any other part of the apparatus (e.g. a dropping funnel). If an o

Application and Summary

When is it useful to run a reaction at a low temperature?

In order to answer this question let us investigate four different applications:

Application 1. Sometimes reactions are too vigorous and exothermic and the reaction mixture must be cooled in order to prevent spilling and pressure build-up due to gas development. A highly exothermic reaction can also become a safety hazard as the reaction mixture can rapidly boil over (many organic solven

References

Gordon, A. J., Ford, R. A. The Chemist's Companion - A Handbook of Practical Data, Techniques, and References. Chapter 11 (1973) ISBN: 978-0-471-31590-2.
Rondeau, R. E. Slush baths. J. Chem. Eng. Data. 11, 124 (1966)