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I am having trouble understanding how to create equations which can then be balanced. For example, if I am told that a freshly scraped piece of aluminum reacts with oxygen from the air to form a protective coating - where does the unbalanced equation $\ce{Al_{(s)} + O_{2(g)}-> Al_2O_{3(s)}}$ come from? In that, where does the $\ce{O_3}$ come from in the product? Following a synthesis $\ce{A + B -> AB}$, how am I to know I get 3 oxygens within the product of aluminum oxide, and why can't I say $\ce{Al + O_2 -> AlO_2}$?

Then take the reaction of fresh scraped aluminum with hot water, where the products of reaction are aluminum hydroxide and hydrogen gas: $\ce{Al + H2O -> Al(OH)_3 + H_2}$. How do I actually determine that these are the products in the first place, and how do we get the $\ce{(OH)_3}$ in the reaction equation? Following a single replacement $\ce{A + BC -> AC + B}$, what is wrong with saying $\ce{Al + H2O -> AlO + H_2}$, and then balancing?

Sorry if this is preschooler stuff around here, but will appreciate any guidance.

Thomas Prévost
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bleuofblue
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    It is like asking how to learn chemistry from reading few paragraphs. There is no such a shortcut to learn it. // See also Chemistry SE: resources-for-learning-chemistry – Poutnik Mar 28 '22 at 19:49
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    "Why Al2O3" — first, you need to know that aluminium usually comes with a 3+ charge, and oxygen usually with a 2− charge. Al2O3 is just the result of combining the two in an electronically neutral fashion. Why those charges, though? It's a result of their location in the Periodic Table, and their electronic configurations. This is very basic chemistry, if you haven't learnt it yet, I'm sure you will very soon, so just keep at it. And if you aren't taught this in whatever course you're doing, get yourself a textbook (it doesn't have to be a big fancy one to explain electronic configurations). – orthocresol Mar 28 '22 at 19:54
  • Thanks for the reply. So would the expectation be if only given a word problem "aluminum reacts with oxygen from the air" that I can read the periodic table, and look at aluminum and oxygen to discern their ion charges and determine that $Al_2O_3$ is the only logical configuration? – bleuofblue Mar 28 '22 at 19:59
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    In the simple cases, yes, that's pretty much it. Sometimes it's more complicated. Explaining the more complicated stuff, though, is the purpose of a full chemistry course, as Poutnik's comment suggested. – orthocresol Mar 28 '22 at 20:06
  • Also could you recommend a good (free) beginner textbook to get these concepts down properly? I have never taken a chemistry course, and am trying to learn on my own from high school resources I am finding online. Edit: just saw Poutnik's link, going to check out that list of books. Thank you again. – bleuofblue Mar 28 '22 at 20:06

1 Answers1

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Posting my findings for any other noobs that might be lurking and just starting out.

Special thanks to user orthocresol for guiding me where to start looking.

For aluminum reacting with oxygen from the air, the wording "forms a protective coating" suggests a synthesis reaction where aluminum and oxygen combine. The charge of the compound must be neutral, so this is how we determine the number of each atom in the product. Aluminum carries the +3, and oxygen the -2, so we need 2 aluminum atoms and 3 oxygen atoms to satisfy this neutrality. This leads to $\ce{Al + O_2 \rightarrow Al_2O_3}$, which balances to $\ce{4Al +3O_2 \rightarrow 2Al_2O_3}$

Likewise with the reaction with water, we can look at the form of the single replacement $\ce{A +BC \rightarrow AC +B}$ and look at $\ce{H_2O}$ as $\ce{H(OH)}$, we can find that $\ce{Al +H_2O\rightarrow Al(OH)_3 + H_2}$, when we consider the aluminum +3 charge, and hydroxide -1, so we need 3 hydroxide to 1 aluminum. And $\ce{H_2}$ as hydrogen gas is a diatomic molecule in nature so will normally always carry this form. This leads to the balanced equation $\ce{2Al +6H_2O\rightarrow 2Al(OH)_3 +3H_2}$

bleuofblue
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