Frozen Board: An endothermic reaction

A beaker is placed in a small puddle of water on a balsa wood board. Two white powders, barium hydroxide octahydrate and ammonium chloride, are introduced into the beaker and stirred, forming a slurry. After two minutes, frost is observed on the outside of the beaker and the beaker is picked up. The board is attached to it because the water between the beaker and the board has frozen. A great example of an endothermic reaction. 

Optional:  In a large lecture hall, most students cannot see frost forming on the outside of the beaker.  In order for this demonstration to have some educational value, it is best if care is taken to frame the demonstration.  If the instructor would like to go into some depth with this demonstration the following procedures should be implemented.  A digital temperature probe can be used to measure the initial temperature of each of the two solids, room temperature, +20°C.  After the reaction occurs and the board is lifted, it is imperative to for the student volunteer to report that the outside of the beaker is "very cold" and the temperature of the slurry/mixture is displayed.  Most students are surprised that the temperature of the mixture is about -20°C.

Curriculum Notes 

This demo is usually performed when thermochemistry or thermodynamics are being discussed. It is an endothermic reaction. The big idea for most calorimetry themed demonstrations is energy is conserved.  Energy cannot be created or destroyed, but it can be exchanged.

qlostqgain = 0   or   qreleased + qgain = 0

This demonstration also illustrates how entropy can act to drive a reaction to spontaneity. 

The equation representing this endothermic reaction shows that it is entropy driven: Ba(OH)2*8 H2O(s) + 2 NH4Cl(s) --> BaCl2*2 H2O(s) + 2 NH3(aq) + 8 H2O(l) This is a neutralization reaction with the hydroxide ion acting as the base and the ammonium ion acting as the acid. 

When presenting this demonstration as a component of calorimetry/thermochemistry.  It is important for the students to identify what gains heat and what looses heat.  Students have difficulty with the idea that the bulk material they can see is NOT the chemical reaction. A chemical reaction has no mass, has no specific heat, and does not change temperature.  A chemical reaction consists of bonds breaking and bonds forming and this is potential energy.  In this demonstration, the chemical reaction took in heat from the surroundings.  The chemical reaction gained heat.  The bulk chemicals and the beaker "lost heat" or had heat transferred out.  When heat is transferred out, the surrounding decrease in temperature.  The mixture released heat to the chemical reaction.

The thermochemical date for barium hydroxide, ammonium chloride, barium chloride, ammonia and water are given in the table below.


Substance

∆H˚f (kJ/mol)

S˚298(J/mol·K)

∆G˚f (kJ/mol)

Ba(OH)2·8H2O(s)

-3342

427

-2793

NH4Cl(s)

-341.4

94.6

-203

BaCl2·2H2O(s)

-1460.1

203

-1296.5

NH3(aq)

-80.29

111

-26.6

H2O(l)

-285.83

75.291

-237.2

The enthalpy for the reaction is calculated using the following equation:
∆H˚= ∑∆H˚f (products) - ∑∆H˚f(reactants) 
∆H˚ = 63.5 kJ

The entropy for the reaction is calculated using the following equation:
∆rS˚= ∑∆S˚The entropy for the reaction is calculated using the following equation:

∆rS˚= ∑∆S˚f (products) - ∑S˚f(reactants) 
∆rS˚ = 368 J/K = .368kJ/K

Calculating the free energy from the above values at 298K:

∆G˚ = ∆H˚ - T∆S˚ = 63.5-298(.368) = -46.1 kJ(products) - ∑S˚f(reactants) 
∆rS˚ = 368 J/K = .368kJ/K

Calculating the free energy from the above values at 298K:
∆G˚ = ∆H˚ - T∆S˚ = 63.5-298(.368) = -46.1 kJ

https://lecturedemos.chem.umass.edu/thermodynamicsI7_1.html

Lead Time 
One day of lead time is required for this project.
Discussion 

The equation representing this endothermic reaction shows that it is entropy driven: Ba(OH)2*8 H2O(s) + 2 NH4Cl(s) --> BaCl2*2 H2O(s) + 2 NH3(aq) + 8 H2O(l) This is a neutralization reaction with the hydroxide ion acting as the base and the ammonium ion acting as the acid. The two relatively low entropy crystalline solid reactants react to form many small molecules in the high entropy liquid and aqueous states. The positive enthalpy change in this reaction (63.6 kJ/mol) is more than offset by the entropy change near room temperature (368 J/mol*K) to yield a strongly negative free energy change (-47.7 kJ/mol). Thus the reaction is spontaneous and endothermic.

From the University of Massachusetts - Amherst  Department of Chemistry Web Site

https://lecturedemos.chem.umass.edu/thermodynamicsI7_1.html

"For a reaction to be spontaneous at constant temperature and pressure, the change in free energy (∆G) must be negative. 

∆G = ∆H - T∆S

In an endothermic reaction, the ∆H is positive corresponding to the absorbed heat. Thus this positive ∆H must be offset by a sufficient increase in entropy (∆S). Two solids creating a different compound and an aqueous solution of ammonia represents a large increase in entropy."

The enthalpy for the reaction is calculated using the following equation:
∆rH˚= ∑∆H˚f (products) - ∑∆H˚f(reactants) 
∆rH˚ = 63.5 kJ

The entropy for the reaction is calculated using the following equation:
∆rS˚= ∑∆S˚f (products) - ∑S˚f(reactants) 
∆rS˚ = 368 J/K = .368kJ/K

Calculating the free energy from the above values at 298K:
∆G˚ = ∆H˚ - T∆S˚ = 63.5-298(.368) = -46.1 kJ

Materials 
  • 32 g barium hydroxide octahydrate
  • 11 g ammonium chloride
  • 250 ml beaker
  • stir rod
  • deionized water in wash bottle
  • 1 m "2x4" balsa wood board

Optional (enhancement)  for a large lecture hall: digital thermometer, computer interface, and display of the temperature

Procedure 

Mechanics of presenting the demonstration  Option A:

Set the board on a table in front of the class, broad (4") side facing up. Squirt about 2-3 mL of DI water onto the center of the board to make a puddle. Set the beaker in the center of the puddle. Add the barium hydroxide and the ammonium chloride to the beaker. Stir vigorously while holding the beaker firmly in place. After two minutes, lift up the beaker. It should be frozen to the board. If it is not, replace it in the puddle of water and continue stirring for another minute or so and try again.

This demonstration works well if performed by a student volunteer, but ask them discretely if they have any respiratory disorders such as asthma before you allow them to do the demo, because the ammonia vapors could cause an adverse reaction to someone with a respiratory disorder. Allow about 10 minutes for this demo.

Option B (enhanced): Set the board on a table in front of the class, broad (4") side facing up. Squirt about 2-3 mL of DI water onto the center of the board to make a puddle. Set the beaker in the center of the puddle. Using a digital thermometer, measure the initial temperature of the barium hydroxide and the ammonium chloride before mixing.  Use a digital display.  

Making this demonstration interactive - active learning. 

Give students the reactants and ask the students to complete and balance the equation.  Provide a hint that the barium hydroxide is a base and the ammonium ion can serve as an acid. Add the barium hydroxide and the ammonium chloride to the beaker. Stir vigorously while holding the beaker firmly in place. After two minutes, lift up the beaker. It should be frozen to the board. If it is not, replace it in the puddle of water and continue stirring for another minute or so and try again.  The person doing the mixing should report to the class that the beaker is "very cold". Measure the final temperature of the mixture.  Ask students to identify what gained heat and what released heat?  Is qrxn (heat of the reaction) positive or negative?  Is this an exothermic or an endothermic reaction?  Is the change in enthalpy for this reaction positive or negative?  Ask students to draw an enthalpy diagram for this reaction and compare it to the enthalpy diagram for the reaction HCl with NaOH.

There are several clicker questions that can accompany this demonstration.  There are a set of PowerPoint slides to accompany this demonstration that will help the instructor structure an interactive session.

This demonstration works well if performed by a student volunteer, but ask them discretely if they have any respiratory disorders such as asthma before you allow them to do the demo, because the ammonia vapors could cause an adverse reaction to someone with a respiratory disorder. Allow about 10 minutes for this demonstration and the accompanying interactive class activities.

Safety Precautions 

Barium hydroxide is toxic and corrosive and ammonium hydroxide is an irritant. The small quantity of ammonia gas gas evolved during the demonstration could act as an irritant. Avoid beathing dust or fumes from demo. If it any chemicals get on your skin, wash thoroughly and flush with water for 15 minutes. If any chemicals get in your eyes, flush with water for 15 minutes and seek medical attention.

Footnotes 

A good video of this demonstration is available on-line

https://www.youtube.com/watch?v=IZaGmUGBdC0

UMass Amherst Chemistry URL https://lecturedemos.chem.umass.edu/thermodynamicsI7_1.html

References

1. B.Z. Shakhashiri; Chemical Demonstrations: A Handbook for Teachers of Chemistry; Wisconsin; 1983; Volume 1;p 10-12
2. L. Summerlin, J. Ealy; Chemical Demonstrations: A Sourcebook for Teachers. ACS publications. Washington D.C; 1983; p. 44

Prep. Notes 

When you retrieve this demo the beaker will be frozen to the board and generating ammonia fumes, so set it on a cart in front of the fume hood until it thaws.

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group