kimi8 said:
Hey everyone,
I was wondering if someone could help explain to me how you get these chemical equations?
Propane: C3H8 + 5O2 --> 3CO2 + 4H2O + energy (heat)
Corrosion (rust): 4Fe + 3O2 --> 2FeO3
How does 4 molecules of iron and 3 molecules of oxygen form 2 molecules of iron oxide? How does this work out? (I hope this question makes sense to you guys)
Lastly, how do you get chemical equations in general?
Thanks in advance :)
C_{3}H_{8} + 5 O_{2} \rightarrow 3 CO_{2} + 4 H_{2}O + \text{energy}
This formula is balanced by ensuring all atoms and charges are present in equal amounts on both sides of the equation. Going atom-by-atom (letter-by-letter), you can see that everything matches up. There are 3 carbons in propane, just as there are 3 carbons in 3 moles of carbon dioxide. The coefficients (numbers in front of each molecule) are used to balance everything.
If you have learned the mole in school, you'll quickly see why this is the case:
We know from Avogadro's number that there are 6.022\, \text{x} \, 10^{23} particles in one mole of a substance (be it atoms, molecules, etc.). Looking at propane, we see there are 3 carbon atoms in propane. There are, therefore, 3 (6.022\, \text{x} \, 10^{23}) carbon atoms per mole of propane. Since matter is conserved in all reactions, there must be exactly as many particles of each product as there were in the reactants. Since we cannot change the chemical structure of carbon dioxide, we know there must be 3 moles of carbon dioxide to balance the carbons found in propane. You keep doing this until everything balances.
The nice thing about balancing chemical formulas is that you know at the end if you're right or not. You can simply add up each element and see if it all balances properly.
Energy on the products side tells us that energy is released (so the reaction is, by definition, exothermic).
As for your rust example, you're dealing with Fe^{3+}. (If you don't know what that means, let us know and someone can explain it further.) Remember that charges must balance. Since Iron (III) oxide is a neutral molecule, the charges of each ion within it must balance.
2 iron ions, each with a charge of 3+ = 6+
3 oxygen ions, each with a charge of 2- = 6-
Combined, you have a charge of zero.
If anything here has confused you, let me know.