Explosive Science - with Chris Bishop

The lecture begins with an introduction to the fascination with explosions and the science behind them. The speaker emphasizes the importance of safety and the dangers of replicating experiments at home due to the rapid and unforgiving nature of explosives. The lecture's first demonstration aims to recreate the explosive power of the Krakatoa volcanic eruption in 1883, illustrating the concept of a physical explosion caused by a sudden release of pressure.

Exploring the historical use of gunpowder as an explosive, the speaker explains its composition of charcoal, sulfur, and potassium nitrate. A demonstration of gunpowder ignition highlights its rapid burning but lack of explosive force when unconfined. The importance of confinement for achieving an explosion with low explosives is discussed, using a cardboard tube to contain the gunpowder and demonstrate the effect.

The lecture introduces flash powder, a different type of explosive used in pyrotechnics for its bright flash. The speaker compares the combustion of flash powder in the open versus its confinement in a metal cylinder, emphasizing how confinement leads to an explosion. The demonstrations involve audience participation, showcasing the dramatic difference in the outcome based on the confinement of the explosive.

The speaker discusses the importance of oxygen in the combustion process, using lycopodium powder as an example. The demonstration shows the slow combustion of the powder when unconfined and how mixing it with air or liquid oxygen greatly increases the rate of combustion. This leads to the discovery of nitrocellulose (gun cotton) by a Swiss chemist, which revolutionized the field of explosives due to its high oxygen content and explosive properties.

Nitrocellulose is highlighted as a significant advancement in explosives due to its composition of oxygen and nitrogen directly within its molecules. The speaker demonstrates the pyrotechnic effects of nitrocellulose and explains its low smoke production and high energy release. The lecture also touches on the concept of activation energy and its role in initiating explosions, using a volunteer to illustrate the energy release from a reaction between hydrogen and oxygen.

The lecture delves into the concept of activation energy and how different energy sources, such as heat or light, can initiate chemical reactions. The speaker uses a ball on a hill analogy to explain how activation energy breaks molecular bonds, allowing for new bonds and energy release. Demonstrations with hydrogen and chlorine gas, as well as magnesium silicide with hydrochloric acid, illustrate the varying activation energies required for different reactions.

The speaker discusses the critical role of nitrogen in explosives, using the example of nitrogen triiodide to demonstrate an explosive with a very low activation energy. The lecture then transitions to a real-life application of explosives, showcasing the deployment of a car airbag to protect passengers in collisions. The airbag's explosive, sodium azide, is highlighted for its rapid release of nitrogen gas upon activation.

The lecture introduces nitroglycerin, a highly powerful but dangerous explosive due to its sensitivity. The speaker explains how the Swedish chemist Alfred Nobel mitigated this sensitivity by combining nitroglycerin with an absorbent material to create dynamite. The invention of the detonator by Nobel is also discussed, which allows for the controlled detonation of dynamite and other high explosives.

The speaker clarifies the difference between deflagration (burning) and detonation (explosive shock wave). Using a trail of nitrocellulose powder, the lecture demonstrates deflagration, where heat transfer causes a slow combustion process. In contrast, detonation is a rapid process involving a shock wave that travels faster than the speed of sound, which is illustrated using a plastic tube and piston.

The lecture explains the mechanism of a shock wave in high explosives, using a clear plastic tube and cotton wool to demonstrate how a shock wave can initiate an explosion. The speaker also introduces shock tubing, a tool used in the explosives industry, to visually demonstrate the travel of a detonation wave. The lecture concludes with a dramatic demonstration of a shock wave traveling through a long section of shock tubing.

To conclude the lecture, the speaker performs a final demonstration involving liquid nitrogen, showcasing a physical explosion on a larger scale. The demonstration involves adding liquid nitrogen to a sealed container, causing the nitrogen to turn into a gas and expand, leading to a dramatic increase in pressure and a powerful explosion. The lecture ends with a thank you to the department of chemistry, sponsors, and the assistant, Chris Braxton.