Conducting Reactions Below Room Temperature
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 chemica…
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.
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 CO2 gas 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 N2 to 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 N2 to 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.
Certain chemical reactions must be performed below room temperature for safety?or to obtain the desired product.
A cooling bath allows for a system to be maintained at a certain temperature range for the duration of the reaction. This is achieved by placing the reaction flask into the bath, cooling the reaction without ever having direct contact with the reagents.
The bath?is typically a well-insulated vessel?such as a Dewar flask?containing the cryogenic components necessary to reach the desired temperature. In simple setups like this, temperature is not stable, and the bath must be monitored and adjusted throughout the procedure.
This video will explore the different cooling baths regularly used to carry out reactions below room temperature.
During a chemical reaction?the species involved must collide for new bonds to form. Raising the temperature increases the internal energy of the system and will cause these species to move more quickly, meaning they will collide more often. As a result, reactions proceed faster at higher temperatures.
However, in some cases, it is desirable to carry out reactions at low temperatures, despite the lowering of the rate of reaction. For example, some reactions are too vigorous, and must be cooled to prevent spilling and pressure build up. Highly exothermic reactions could also rapidly boil over and spurt out if not cooled, creating a safety hazard.
Cooling can?be utilized to provide an economic benefit. For example, preventing the boiling off of a solvent or the decomposition of a reagent saves both time and resources.
Cooling is also frequently used to control which product is yielded by a reaction that has competing pathways. In these reactions?the pathway with the lower activation energy is generated at lower temperatures, while the pathway with the higher activation energy is preferred at higher temperatures.?
Now that you understand the importance of running reactions below room temperature, let's take a look at how to prepare various types of cooling baths.
Ice-water baths are easy to set up, and are available in every teaching chemistry laboratory. While ice-water itself has a temperature of 0 ?C, a melting-point depression can be achieved by the addition of certain salts.
This allows ice-water baths to reach a temperature of -40 ?C. The final temperature can be adjusted by increasing or decreasing the concentration of salt additive.
To set up an ice-water bath, begin by weighing the appropriate amounts of ice and salt additive, as outlined in the ice-bath table found in the text protocol.
Next, add the salt to the ice. Pour a small amount of deionized water into the container. Using a stirring rod, mix the bath thoroughly.
Now that the bath has been set up, check with a thermometer to ensure that the desired temperature has been reached. If it has not, add more salt as necessary. When the correct temperature is reached, place the reaction vessel into the ice bath.
Ice-water baths do not retain their temperature long, and need to be adjusted every 20?30 min. To maintain the target temperature, it may be necessary to remove the liquid water and add more ice and salt.
For temperatures down to -78 ?C, dry-ice baths are utilized. Dry-ice is solid carbon dioxide, so efficient heat-transfer from it to a reaction vessel requires a solvent. Because dry-ice sublimes at -78 ?C, a solvent with a freezing point below that must be used if this temperature is to be reached. Solvents with higher freezing points can be utilized to create warmer dry-ice baths. To prepare a dry-ice bath, begin by putting on cryogenic protection gloves and safety goggles. Never let dry-ice touch bare skin.
For a 1?L bath, obtain about 1/3 of a block of dry-ice and break it into smaller pieces into the container.
Next, slowly add the chosen organic solvent to the dry-ice while stirring with a glass rod. There will be a vigorous fizzing as carbon dioxide gas develops.
Continue to slowly add solvent and stir until most of the dry-ice dissolves, forming a homogenous slurry. This ensures that heat transfer to the reaction flask is as uniform as possible.
Using a cold temperature thermometer or thermocouple, ensure that the bath has reached the desired temperature, then place the reaction vessel into the bath.
Monitor the bath in regular intervals, and add chunks of dry-ice when a rise in the bath temperature is noticed.
Finally, when the desired bath temperature is below what dry-ice can provide, liquid nitrogen is utilized. Liquid nitrogen has a melting point of -196 ?C, and solvents are only needed when creating warmer baths.
Due to the extremely low temperatures of liquid nitrogen, a Dewar is the only acceptable vessel.
To prepare a liquid-nitrogen cooling-bath, begin by putting on safety goggles and cryogenic protection gloves. Use care when handling liquid nitrogen, as it can cause frostbite and permanent eye damage.
For a bath with additives, determine the appropriate organic solvent for the desired temperature, as shown in the liquid nitrogen table found in the text. Add the solvent to the Dewar, then slowly add the liquid nitrogen.
Insert a cold-temperature thermometer or thermocouple into the bath to ensure that the desired temperature has been reached. Then, place the reaction vessel into the bath.
For a bath without additives, simply add the appropriate amount of nitrogen to the Dewar to obtain a temperature as low as -196??C.
Monitor the bath in regular intervals to see if additional nitrogen is needed.
Many different types of reactions?across various scientific disciples?utilize cooling baths to operate below room temperature.
Mechanical laboratory processes, much like very exothermic reactions, can also create undesirable heat.
In this example?bulk barium copper tetrasilicate was prepared through both solid state and melt flux synthesis. Then, these layered materials were exfoliated using sonication techniques.
Sonication uses sound waves to agitate particles. However, because it is a high-energy process, it can create excess heat in a sample.
Therefore, an ice-water bath was used to cool the sample during the one-hour sonication process. Preventing this excess heating ensured the integrity and consistency of product yield.
In this example, a dry-ice bath was used to ensure that diiodomethyllithium was synthesized by deprotonation of diiodomethane.
Reagents were added to a round-bottomed flask containing a stir bar. Then, the round-bottomed flask was placed in a Dewar. Dry-ice and acetone were added to the Dewar, and the entire apparatus was covered to minimize exposure to light. Maintaining low system energy was essential for the stability of the product.
Dry-ice and liquid nitrogen baths are frequently used as cold traps to condense samples. In particular, these cold traps can aid the safe transport of air-sensitive compounds while preventing contamination of equipment. In?this example, a liquid nitrogen cold trap was used to condense a volatile and oxidation sensitive sample, for later preparation for mass spectrometrical analysis.
The system was first cleaned and heated, to remove any potential contaminants. The lockable test tube was then submerged in liquid nitrogen, to allow for condensation of the sample through the Schlenk line. The sample was then removed for analysis through mass spectrometry.
You've just watched JoVE's introduction to conducting reactions below room temperature. You should now understand ice-water, dry-ice, and liquid nitrogen cooling baths, and why they are chemically important.
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Q1: 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.
Q2: 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.
Q3: 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.
Q4: 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.
Q5: 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.
Q6: 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.
Q7: 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.