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Gas Laws Save Lives: The Chemistry Behind Airbags

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emmanuel caburian

on 7 March 2011

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Transcript of Gas Laws Save Lives: The Chemistry Behind Airbags

Airbags Jeremys 1st point Jeremys 1st point Jeremys 1st point point 1
point 2
point 3
point 4
point 5 Basic Information
air bags are used to protect a driver and a passenger from a crash
every car has an air bag with an airbag system with different sensors
an airbag fills up with a mixture of potassium and sodium azide which creates nitrogen
an airbag is activated when the requisite threshold is reached
in addition, an airbag distributes the bag to a wider range in front of the body, reducing the peak acceleration that could happen Impacts On Society
1987 to 2005, a total of 19,659 lives were saved
general public uses it
rate of car related deaths decreasing steadily
Help the economy Enviroment
a lot fewer advantages to the environment than thought
almost all airbags are made from nylon 6.6
the nylon 6.6 is made in chemical plants by combining specific chemicals
after an airbag is used, it is disposed of by burning it or they throw it away to dumps where they decompose very slowly
when recycled, they are used to make pellets for reuse in the industry An overview of how does airbag work
1. When a car hits something, it starts to decelerate (lose speed) very rapidly.
2.An accelerometer (electronic chip that measures acceleration or force) detects the change of speed.
3.If the deceleration is great enough, the accelerometer triggers the airbag circuit. Normal braking doesn't generate enough force to do this.
4.The airbag circuit passes an electric current through a heating element (a bit like one of the wires in a toaster).
5. The heating element ignites a chemical explosive. Older airbags used sodium azide as their explosive; newer ones use different chemicals.
6.As the explosive burns, it generates a massive amount of harmless gas (typically either nitrogen or argon) that floods into a nylon bag packed behind the steering wheel.
7.As the bag expands, it blows the plastic cover off the steering wheel and inflates in front of the driver. The bag is coated with a chalky substance such as talcum powder to help it unwrap smoothly.
8.The driver (moving forward because of the impact) pushes against the bag. This makes the bag deflate as the gas it contains escapes through small holes around its edges. By the time the car stops, the bag should have completely deflated. Gas-Generator Reaction Reactants Products

Initial Reaction Triggered by Sensor. NaN3 Na N2 (g)

Second Reaction . Na KNO3 K2O Na2O N2 (g)

Final Reaction. K2O Na2O SiO2 alkaline silicate glass)

Table 1

This table summarizes the species involved in the chemical reactions in the gas generator of an airbag. This initial reaction forms sodium and hot nitrogen gas which inflates the

2 NaN3 —> 2 Na + 3 N2 The sodium byproduct of the first reaction and the potassium nitrate
generate additional nitrogen in the secondary reaction.

10 Na + 2 KNO3 —> K2O + 5 Na2O + N2 And finally the previous two reactions leave potassium oxide and sodium
oxide to react with the third component of the mixture, silicon dioxide,
forming alkaline silicate "glass".

K2O + Na2O + SiO2 —> alkaline silicate The Macroscopic Picture of Gas Behavior: Ideal-Gas Laws

Calculation of the Amount of Gas Needed

Nitrogen is an inert gas whose behavior can be approximated as an ideal gas at the temperature and pressure of the inflating airbag. Thus, the ideal-gas law provides a good approximation of the relationship between the pressure and volume of the airbag, and the amount of N2 it contains. (The ideal-gas law is PV = nRT,where P is the pressure in atmospheres, V is the volume in liters, n is the number of moles, R is the gas constant in L·atm/mol·K (R = 0.08205 L·atm/mol·K), and T is the temperature in Kelvin.) A certain pressure is required to fill the airbag within milliseconds. Once this pressure has been determined, the ideal-gas law can be used to calculate the amount of N2 that must be generated to fill the airbag to this pressure. The amount of NaN3 in the gas generator is then carefully chosen to generate this exact amount of N2 gas. Summary

Airbags have been shown to significantly reduce the number and severity of injuries, as well as the number of deaths, in head-on automobile collisions. Airbags protect us in collisions by providing a cushion to decrease the force on the body from hitting the steering wheel, and by distributing the force over a larger area. The cushion is generated by rapidly inflating the airbag with N2 gas (from the explosive decomposition of NaN3 triggered by a collision sensor), and then allowing the airbag to deflate.

Fundamental chemical and physical concepts underly the design of airbags, as well as our understanding of how airbags work. The pressure in the airbag, and hence the amount of NaN3 needed in order for the airbag to be filled quickly enough to protect us in a collision, can be determined using the ideal-gas laws, and the kinetic theory of gases allows us to understand, at the molecular level, how the gas is responsible for the pressure inside the airbag. Newton's laws enable us to compute the force (and hence the pressure) required to move the front of the airbag forward during inflation, as well as how the airbag protects us by decreasing the force on the body. Estimating the Pressure Required to Fill the Airbag

An estimate for the pressure required to fill the airbag in milliseconds can be obtained by simple mechanical analysis. Assume the front face of the airbag begins at rest (i.e., initial velocity vi = 0.00 m/s), is traveling at 2.00x102 miles per hour by the end of the inflation (i.e., final velocity vf = 89.4 m/s), and has traveled 30.0 cm (the approximate thickness of a fully-inflated airbag).

The airbag's acceleration (a) can be computed from the velocities and distance moved (d) by the following formula encountered in any basic physics text:
vf2 - vi2 = 2ad.

The force exerted on an object is equal to the mass of the object times its acceleration (F = ma) ; therefore, we can find the force with which the gas molecules push a 2.50-kg airbag forward to inflate it so rapidly. 2.5 kg is a fairly heavy bag, but if you consider how much force the bag has to withstand, it becomes apparent that a lightweight-fabric bag would not be strong enough. Note: In the calculation below, we are assuming that the airbag is supported in the back (i.e., all the expansion is forward), and that the mass of the airbag is all contained in the front face of the airbag.

F = ma

Pressure is defined as the force exerted by a gas per unit area (A) on the walls of the container (P = F/A), so the pressure (in Pascals) in the airbag immediately after inflation can easily be determined using the force calculated above and the area of the front face of the airbag (the part of the airbag that is pushed forward by this force). Note: The pressure calculated is gauge pressure.

The amount of gas needed to fill the airbag at this pressure is then computed by the ideal-gas law (see Questions below). Note: the pressure used in the ideal gas equation is absolute pressure. Gauge pressure + atmospheric pressure = absolute pressure. Deflation of the Airbag

When N2 generation stops, gas molecules escape the bag through vents. The pressure inside the bag decreases and the bag deflates slightly to create a soft cushion. By 2 seconds after the initial impact, the pressure inside the bag has reached atmospheric pressure.
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