T525: Hot Packs vs Hydrogen Balloon

This demo is cross-listed as Kinetics demonstration K925 

Introduction

A reusable hot pack made of a supersaturated sodium acetate solution that crystallizes spontaneously once triggered by a metal piece releases heat.  A balloon filled with hydrogen gas also releases heat when lit on fire.  Due to differences in energy density, the crystallization of sodium acetate releases more heat per mole than the hydrogen gas explosion.  However, the hydrogen balloon reaction is more dangerous, because of the difference in reaction rates.  The hydrogen reaction is fast enough to explode, so even though the sodium acetate reaction releases more heat per mole, it is safe to make into hand warmers, while explosive hydrogen is quite dangerous.

This demo teaches students about energy density as well as kinetics, linking thermochemistry concepts to kinetics concepts.

Equations

Exploding balloon:      H2 (g) + ½ O2 (g)  →  H2O (g)                       ∆H˚ = –241.8 kJ/mol

Hot pack:                     NaOOCCH3 (aq) → NaOOCCH3 (xtal)         ∆H˚ ≈ –xtal lattice energy

                                                                                                                   = –763 kJ/mol

Supplies

  • Balloon
  • Tape
  • Yardstick
  • Matches
  • Earplugs
  • Fire extinguisher
  • Goggles
  • Sodium acetate hot packs (three is good – sandwich the thermometer between two, one extra if it fails)
  • Electronic thermometer

 

Prior to Lecture  (prep time ~ ? min.)

  1. Immediately before lecture, inflate desired number of balloons with hydrogen.  Attach ~6ft. string to each.
  2. Tape the ends of the strings to the floor or to the lecture table at such a height that the balloons can be reached with the candle on the yardstick.
  3. Assemble the following:
  • Attach the candle to the yardstick.

To Conduct Demonstration

  1. Light the candle on the end of the yardstick and lower the lights.
  2. Bring the candle under the hydrogen balloon until the tip of the flame touches the balloon.  Be prepared for an explosion.  DO NOT LIGHT THE STRING ON FIRE!
  3. Note the initial temperature of the thermometer
  4. Click the metal button in the hot pack, the sodium acetate will crystallize.  Sandwich the thermometer between the two packs and note the temperature change (this works well with a document reader so students can see the readout change)
  5. Ask students which reaction releases more heat?
  6. Students will often say the hydrogen, but it is the sodium acetate!  Kinetics (an explosion) tells us the rate but the actual energy released is more (per mole) for the sodium acetate crystallization.

Demo Time: 5-10 minutes

Safety and Disposal

Hearing damage can occur with especially large balloons or small rooms.  Earplugs or ear protectors maybe worn.

The sodium acetate hot packs are reusable and should not be disposed of.

 

ACKNOWLEDGMENT:

Doug Gin, Spring 2016

Kristin Boles, Spring 2016