Predicting products of chemical equations? [duplicate]

Consider the equation $\ce{2 NaOH (aq) +Cl2 (g) —> NaOCl (aq) + NaCl (aq) + H2O (l)}$. How would one know the products of this reaction? Is there a general way of doing this? I would have no earthly knowledge of predicting this. Is it that we would look at each molecule and decide which goes with which on the basis of octet? Also, why does it just so happen that 2 moles of $\ce{NaOH}$ react with 1 mole of chlorine? What would happen if it was a 1:1 ratio?

• Welcome to Chemistry.SE. We’d like you to take the Tour to get familiarized with the site and to understand our policy towards homework questions. Meantime, let us know what you have done so far to solve this specific problem as starting point. – Mathew Mahindaratne May 22 '18 at 3:34
• Yes there is a way of doing this. It is called chemistry. It does not boil down to one or two simple rules, otherwise it would never be called a science in its own right. As to the ratio, that's quite simple: take any other ratio, and one of the reagents will be in excess. – Ivan Neretin May 22 '18 at 4:55
• I agree with Ivan Neretin. In addition to that, in general, knowing molecular orbital diagrams of individual reactants and possible product candidates will help quite much to guess the outcome. I did not memorize any reaction in inorganic chemistry for instance, but you can attain this kind of comprehension mostly at the end of the undergraduate program. – Güray Hatipoğlu May 22 '18 at 10:32

2 Answers

One would not "know" what the products of the reaction are, a priori. As mentioned by @IvanNerenten, the way you know is by studying chemistry. In fact, the reaction is different if you use cold NaOH vs hot NaOH. The reaction as written is with cold NaOH and is a disproportionation reaction.This is where the chlorine is both oxidized and reduced in the same reaction.

How would one know the products of this reaction? Is there a general way of doing this?

It's not easy at all, in chemistry, there are patterns that are valid in many cases (e.g. you know that you are adding a strong acid with a strong base you can predict that you will have a slat and water as products).

For more complex reactions it's harder to find valid patterns. There are some techniques that use ab-initio calculations and some softwares but in many cases, they are not so accurate.

I find useful these rules of thumbs not always valid:

• Get a good knowledge of electrochemistry.
• Write the equation highlighting the ionic compounds (e.g. $\ce{Na+ + Cl-}$ ).
• Check the possible products in agreement with the stoichiometry.
• Stable compounds (e.g. water) are more likely to occur compared to unstable compounds.

Why does it just so happen that 2 moles of NaOH react with 1 mole of chlorine? What would happen if it was a 1:1 ratio?

A chemical formula it's a model a representation of the expected behaviour. If you had 1:1 ratio you would have a limiting reagent. So you will end with 1 mole of NaOH reacting with half mole of $\ce{Cl2}$.