Why do some chemical reactions need to be performed below room temperature?

Reactions are cooled for several reasons: to prevent vigorous or highly exothermic reactions from boiling over and creating safety hazards, to control which product forms when competing reaction pathways exist, and to preserve reagents or solvents from decomposition. Lower temperatures favor pathways with lower activation energy, allowing chemists to obtain desired products while maintaining safety.

How does temperature affect the rate of a chemical reaction?

Raising temperature increases molecular movement and collision frequency between reactant species, causing reactions to proceed faster. The rule of thumb states that increasing temperature by 10 K approximately doubles the reaction rate. However, despite this acceleration, chemists sometimes deliberately cool reactions to control selectivity, prevent decomposition, or manage exothermic heat release.

What is an ice-water bath and how is it prepared?

An ice-water bath is the simplest cooling method, reaching 0°C naturally or -40°C with salt additives that depress the freezing point. Prepare it by mixing weighed ice and salt with deionized water, then stir thoroughly. Check the temperature with a thermometer and adjust salt concentration as needed. These baths require monitoring every 20-30 minutes and must be refreshed with ice and salt to maintain temperature.

What are dry-ice baths used for and how do you set one up?

Dry-ice baths reach temperatures down to -78°C and are used when ice-water baths are insufficient. Prepare by wearing cryogenic protection gloves and safety goggles, then slowly add an organic solvent to dry-ice pieces while stirring with a glass rod. Continue until a homogenous slurry forms, ensuring uniform heat transfer. Monitor regularly and add dry-ice chunks when temperature rises.

When is liquid nitrogen used as a cooling bath and what precautions are necessary?

Liquid nitrogen cooling baths reach -196°C and are used when lower temperatures than dry-ice provides are required. A Dewar flask is the only acceptable vessel due to extreme cold. Wear safety goggles and cryogenic protection gloves at all times, as liquid nitrogen causes frostbite and permanent eye damage. Add solvent for warmer baths or use nitrogen alone for maximum cooling.

How do cooling baths prevent direct contact between reagents and cooling agents?

Cooling baths work by placing the reaction flask inside an insulated vessel such as a Dewar flask containing cryogenic components. The reagents never touch the cooling bath directly; instead, heat transfers through the flask walls. For effective temperature transfer, solvents are often mixed with cryogenic components, and additives like salts lower the freezing point of the mixture to achieve desired temperatures.

What are practical applications of cooling baths in laboratory chemistry?

Cooling baths are used in diverse applications including sonication processes that generate excess heat, synthesis of air-sensitive compounds requiring low system energy, and cold traps for condensing volatile samples. For example, ice-water baths cool samples during high-energy sonication, while dry-ice and liquid nitrogen baths condense oxidation-sensitive compounds for analysis via mass spectrometry or preparation for growing crystals for x-ray diffraction analysis.

Conducting Reactions Below Room Temperature

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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 opening is generated while the glassware is immersed in the cooling bath, then room temperature air quickly flows inside and carries moisture in.

Low temperature reaction setup diagram with dropping funnel, reaction flask, cooling bath in Dewar.

Figure 1. Example for a cooling bath set-up in a three-neck round-bottom flask with dropping funnel, thermometer under inert atmosphere.

1. Making an Ice-Water Bath

For an ice-water bath with an additive, weigh and add the appropriate amount of ice to the bath vessel of choice. A Dewar is helpful but not necessary. The vessel can be plastic, rubber (e.g. a bucket), or glass (e.g. a large crystallization dish). For this experiment, add 500 g of ice to a 1 L vessel.
Weigh the appropriate amount of additive consulting an ice-bath (Table 1), and add the additive to the ice. For this experiment, weigh and add 165 g (=5x 33 g) of NaCl to the bath vessel.
Add a small amount of deionized water to the bath vessel and stir thoroughly using a stir rod. Add just enough water until all the ice is covered.
Check with a thermometer to ensure that the desired temperature has been reached. Adjust the amount of additive if necessary. The bath will not maintain its temperature for very long and adjustments need to be made in frequent intervals of about every 20–30 min. For this it may be necessary to pipette off some of the water in the bath and add more ice and additive.

Substance	g/100 g H₂O	Final Temperature (°C)
Na₂CO₃	20	-2.0
NH₄NO₃	106	-4.0
NaC₂H₃O₂	85	-4.7
NH₄Cl	30	-5.1
NaNO₃	75	-5.3
Na₂S₂O₃● 5H₂O	110	-8.0
CaCl₂ ● 6H₂O	41	-9.0
KCl	30	-10.9
KI	140	-11.7
NH₄NO₃	60	-13.6
NH₄Cl	25	-15.4
NH₄NO₃	45	-16.8
NH₄SCN	133	-18.0
NaCl	33	-21.3
CaCl₂● 6H₂O	81	-21.5
H₂SO₄ (66.2%)	23	-25
NaBr	66	-28
H₂SO₄ (66.2%)	40	-30
C₂H₅OH (4°)	105	-30
MgCl₂	85	-34
H₂SO₄ (66.2%)	91	-37
CaCl₂● 6H₂O	123	-40.3
CaCl₂● 6H₂O	143	-55

Table 1. Salt/ice cooling mixtures that are obtained by mixing the salts with water or ice at the specified temperatures and in the specified amounts.¹

2. Making a Dry-Ice Bath

Put on cryogenic protection gloves and safety goggles. Always practice this when handling dry ice and never touch it with bare hands as it can rapidly burn skin and cause frostbites.
For a bath vessel with a volume of about 1 L, take about 1/3 of a block of dry ice (usually available in ~2 lb blocks) and break it up into a few smaller pieces.
Add the pieces of dry-ice to the bath vessel.
Slowly add the organic solvent (e.g. acetone) to the dry ice while stirring with a glass rod. There is a vigorous fizzing as a result of CO₂gas development.
Continue to slowly add solvent and stir until a homogenous slurry forms and most of the dry ice dissolves. This is to ensure that the heat transfer to the reaction flask is as uniform as possible.
Insert a cold-temperature thermometer into the bath to ensure that the desired temperature is reached.
Check on the dry-ice-bath in regular intervals and add more chunks of dry ice when a rise in the bath temperature is noticed. The interval time depends on the degree of insulation, but is usually around every 45–60 min.

3. Making a Liquid Nitrogen Bath

Put on cryogenic protection gloves and safety goggles. Always practice this when handling liquid nitrogen as it can rapidly burn skin tissues and eye fluids, causing frostbites or permanent eye damage.
For a bath without additives, add the appropriate amount of N₂to a Dewar to obtain a temperature of -196 °C. Move to step 3.3 if this is the desired temperature.
For a bath with additives, add an organic solvent of choice (consult Table 3 to find the right solvent for the right temperature) to the Dewar first, then slowly add liquid N₂to the solvent.
Insert a cold-temperature thermometer into the bath to ensure that the desired temperature has been reached. In contrast to the other baths, the liquid N₂ bath inside of a Dewar maintains its temperature for hours at a time.
Check on the bath in appropriate intervals (a few hours) to see if more N₂ is needed.