Chemical Equation

Hello. I'm new at here...
A question need help. I just move to Australia. N i had picked the subject of chemistry. Is quite hard for me.
Em.... my question is, can u guys teach me tat how to done the chemical equation correctly? Know wat i means?
For example, Aluminium + oxygen = aluminium oxide. Y the equation of it become 4Al + 3O2 = 2AL2O3.
Y will get 4 for aluminium?
My english not very good,.... Can someone use some simply english explain to me ???
Please... Thanks...



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Please check our lectures on

Please check our lectures on equation balancing, they should be helpfull. Start here:


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In a chemical equation you need to use the right "stoichiometry". In simple english, you need to get the amounts right, like in a recipe.

Aluminium + Oxygen → Aluminium Oxide.

The Chemical formula for Aluminium Oxide is Al[size=8]2[/size]O[size=8]3[/size].

So to make one of these, you need 2 Al molecules, and 3 O molecules.

BUT, Oxygen naturally occurs as O[size=8]2[/size]. In chemical equations we like to put in the reagents in their orignal forms. So you want to mix Al and O[size=8]2[/size].

So you need to change the ratios. If you double the amounts of everything, then you get this:
Product - 2 molecules of Al[size=8]2[/size]O[size=8]3[/size]
Reagents - 4 molecules of Al (there are 2 in 1 molecule of Al[size=8]2[/size]O[size=8]3[/size], and you now making 2 molecules of Al[size=8]2[/size]O[size=8]3[/size]. 2*2 = 4)
6 molecules of O, in it's natural form O[size=8]2[/size], which is therefore 3 molecules of O[size=8]2[/size].

Now that you have everything in it's natural form, you can make a balanced equation:

4Al + 2O[size=8]2[/size] → 2Al[size=8]2[/size]O[size=8]3[/size]

As Gauss observed, arithmetic is the business of accountants; clearly chemists don't do arithmetic very well.

Allan's explanation is otherwise right on the mark; like a recipe you have to have the right proportions for everything to come out right, so aluminum oxide is Al[sub]2[/sub]O[sub]3[/sub]; however, you can only buy O in packets of O[sub]2[/sub] (called a molecule) and any leftover O spoils if you break the molecule open; therefore if you bought only one packet (molecule) of O[sub]2[/sub] you wouldn't have enough because you need 3 and a second packet (molecule) would give a total of 4 O atoms and leave one O going to waste, so if you bought a third, you come out right, but with a total of 6 O's (3O[sub]2[/sub]); but 6 is twice what you need, you would have to double the rest of the recipe, which means that instead of 2 Al you use 4.

Put all of this together and you have 4 Al + 3 O[sub]2[/sub] > 2 Al[sub]2[/sub]O[sub]3[/sub].

To check your answer observe that on the left you have a total of 4 Al; when you multiply the 2 (of Al[sub]2[/sub]) by 2 (the total number of molecules of aluminum oxide) you get 4; when you multiple 3 (number of oxygen molecules) by 2 (number of oxygen atoms in a molecule) you get 6; on the right side if you multiply 2 (number of aluminum oxide molecules) by 3 (number of oxygen atoms in a molecule of aluminum oxide you also get 6; the equation is balanced, and you'vve done everything right.

Later things will become more complex because you will also have to balance electrical charges on the atoms, but time enough for that.

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