fireworks
TRANSCRIPT
What will we learn ?
• How does FIRE work • How ROCKETS work • Colours of FIREWORK • How Fire EXTINGUISHERS work
Demo
• Burn tissue paper: (a) in beaker – then close the beaker -> what is necessary to get a fire ? (3 things)
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
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
Flame emission Each element produces a typical color
when heated
Na K Li Ba
See: https://www.youtube.com/watch?v=q06GmNvxaik
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)
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 ?