When does a reaction equation require the reversible sign and when does it not?

Sometimes my professor puts the $\to$ symbol and sometimes he uses $\leftrightharpoons$. How do I know which to use? I get that one means the reaction only heads one way while the other means that the reaction holds at an "equilibrium" where the reactants are converting into products and the products are converting into reactants. But if I'm only given a word problem that lists 2 reactants and their concentrations, how do I know which to use?

• Probably you should ask some guys who know teacher's preferences, which may be quite artificial. – Mithoron Jul 3 '16 at 21:43
• @Mithoron That would connote a teacher the needs to be educated. ;-) – Karl Jul 4 '16 at 3:01

2 Answers

You just need to learn and know. Luckily, there is a certain logic to the question which reactions can be reversible and which cannot. However, a full list is impossible and would exceed the space of this margin.

For most mechanisms, there is (at least) one point-of-no-return step. Learn to identify this step. All other arrows should probably be $\ce{<=>}$, if you’re strict. These non-reversible steps are often (but not always!) carbon-carbon bond forming reactions, as a very rough guideline on which is which.

• Hmm ok. But there seems to be a particular brand of equations for which my professor always uses the single arrow. I think they were called dissolving/ionization equations? I may be wrong though. – whatwhatwhat Jul 3 '16 at 21:22
• I wonder who downvoted me and why? – Jan Jul 5 '16 at 0:40

Ask yourself if it is reasonable to give thought to the backward reaction. If not, because it is not relevant for the particular question, use the single arrow.

E.g. for partial equations, usually the single arrow is used, because statements about the equillibrium only make sense for the full reaction.