# 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

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.