Fireworks and Extinguishers

Dr. Christoph

SC 2201

facebook.com/up.chemistry Nov. 2015

What will we learn ?

• How does FIRE work • How ROCKETS work • Colours of FIREWORK • How Fire EXTINGUISHERS work

Demo

Thailand Yasothon Rocket Festival

https://www.youtube.com/watch?v=1BB7ZGRm0xI

Demo

• Burn tissue paper: (a) in beaker – then close the beaker -> what is necessary to get a fire ? (3 things)

Fire Triangle

Demo 2

Paper impregnated with KNO3: Burn in beaker and close it -> why does it continue to burn ?

Redox Reactions

In the paper reaction, what is reducing and what is oxidizing agent ?

Oxi

diz

ing

agen

ts

Electron acceptors

Demo 3 Permanganate has Mn in oxidation state +7 (highest known ox.number !) -> it wants to have electrons to reduce Mn Mix with Glycerol ignites by itself: (Check the ox.numbers of Mn and C on both sides ) KMnO4 + CH2(OH)-CH(OH)-CH2(OH) MnO2 + CO2 + H2O + K2CO3

Use redox reaction for rockets

For a rocket motor we need to produce a lot of gas that expands quickly, so it can give thrust at the outlet. For example: Sugar + KNO3 C12H22O11 + KNO3 ---> K2CO3 + H2O + N2 + CO2

Demo Video

Easy KNO3_Sugar Rocket Tutorial

https://www.youtube.com/watch?v=-LI_GmG9x8M

Balanced equation: 5 C12H22O11 + 48 KNO3 ---> 24 K2CO3 + 55 H2O + 24 N2 + 36 CO2

So from 1 mol sugar = 342 g we get: H2O 11 mols = 200 g N2 5 mols = 140 g CO2 7 mols = 336 g Total 23 mols of gas = 680 g

Common gases produced At the end of a burning reaction, molecules with high stability are produced, mainly: CO2 (from C) H2O (from C-H) N2 (from NO2)

The volume of 1 mol of gas can be estimated by the ideal gas law: R = 8.31 N m mol−1 K−1

P in Pascal:

Suppose the pressure is 1 bar (105 N/m2) and the temperature 25 C Calculate the volume of 1 mol of gas : 24.8 L

So the 23 mol gives about 570 L at r.t. but the higher the temperature is, the more volume is produced Adding some heat generating material like Al or Mg powder to the mixture will increase the thrust.

Calculate the volume of 1 mol gas at 500 C : 64.3 L what is more than double compared to room temperature

Difference to explosives

Compared to KNO3: nitroglycerine and TNT decompose very rapidly on ignition. Here oxidizer and fuel are in the same molecule

Nitrocellulose

It is easy to produce and is used in gun ammunition -> explosives are molecules containing fuel and oxygen in one - so only heat is necessary to ignite

Fireworks

The color of fireworks come from the “Emission spectra” of metal salts

Flame emission Each element produces a typical color

when heated

Na K Li Ba

See: https://www.youtube.com/watch?v=q06GmNvxaik

Emission Spectra heating metal salts causes them to send light with typical wavelengths.

Origin of emission lines

Electrons falling back to their ground state result in emission of light

When an element emits a light line at 700 nm (red), what is the energy difference in the atom between the electron shells ? E = h c/λ = 1239 eV nm λ (nm)

FIRE EXTINGUISHERS Dr. Chris, UP May 2015

Stop a fire -> we have to remove at least one of the three components: WATER: cools down and covers the surface CO2: replaces oxygen FOAM: cools and covers from oxygen POWDER: replaces oxygen COMMON MISTAKE: running around when fire is present causes more oxygen to reach the fire !

Questions

What is easier to extinguish - burning wood or burning liquid fuel ? How to extinguish – burning oil ? burning body ? How to treat burn wounds ?

Types of extinguishers

http://www.fire-extinguisher101.com/agents.html

Extinguisher Classes

http://www.fireextinguisherrecharge.net/class-b-fire-extinguisher/

https://www.youtube.com/watch?v=GjSoxJF3RD4

What can be used to extinguish burning oil ( and what should be avoided) ?

What can be used to extinguish electric fire ( and what should be avoided) ?

What is the advantage of CO2 extinguishers over wet chemical and dry powder ?