<p>Hi, I'm new to this forum so sorry if this is in the wrong section or if there's another similar post XD</p>
<p>Basically, I'm in AP Chem this year and I'm having quite a bit of trouble predicting reaction products. I've memorized the solubility rules, a lot of the exceptions, important oxidizers/reducers, what certain ions produce, etc, etc. but if I see something that isn't in any of the rules, I completely blank. </p>
<p>If you've taken AP Chemistry before, do you have any methods that you went about predicting products or advice on reactions?</p>
<p>And how do you know if you why one compound is produced but not another?
For example in the equation: PbO + NH3 ----> N2 + H2O + Pb , why is nitrogen gas and lead produced but not lead nitride or lead nitrate? </p>
<p>This example is not an easy problem. The only real information you have to work with is that both Pb and N have other common oxidation states, so it is likely a redox problem. Nitrogen is already -3, so it can only go up. If N goes up, Pb must go down, and become Pb(0).</p>
<p>Why not form lead nitrate? Because that would mean Pb remains Pb(II), and N goes from N(-3) to N(+5). But N can’t lose 8 electrons unless something else gains them, and there’s nothing else in the reaction that can gain that many electrons (each H can pick up one electron and form H2 as a product, but that’s only 3 electrons per N atom, and you need 8). You’ll also find that you can’t balance an equation that forms lead nitrate. In this reaction PbO + NH3 –> Pb(NO3)2 + H2 you always have the same number of Pb atoms as O atoms on the reactant side, but you need 6 O for every Pb in the product.</p>
<p>Forming lead nitride as a product seems perfectly reasonable:</p>
<p>3 PbO + 2 NH3 –> Pb3N2 + 3 H2O</p>
<p>On paper there’s nothing wrong with that and no reason to suspect it’s wrong. You would get some partial credit for it on the AP exam. The only reason it’s not correct is that Pb3N2 isn’t stable, but you wouldn’t ordinarily know that. And a metal oxide reacting with ammonia to form the metal is a general reaction that works for many metal oxides, but again that’s pretty obscure information, and you can’t be faulted for not knowing it.</p>
<p>The good news is that to get a 5 on the AP Chem exam you only need about 57% correct. So don’t worry if you see an unfamiliar reaction – you’ll see lots of things on the test that you don’t recognize. If you give them something that’s reasonable and balanced, you’ll get some credit, and if not, just try to make up the points elsewhere.</p>
<p>I would say that you have to ask the atoms quietly where they want to go, and give them enough time to answer so you don’t mistake your own noisy thoughts for their gospel, and then go from there, but be gentle. I think this is where chemical intuition comes from. It only got me to a 4, but results will probably vary depending on the person.</p>