1
$\begingroup$

Like, in the equation between rubidium and water:

2Rb(s) + 2H2O(l) → 2RbOH(?) + H2(?)

How do we know what state symbol to use in 2RbOH and H2? I'd like to do this just by looking at the chemical formula, but it's OK if I have to memorize a list of stuff to be able to do this.

$\endgroup$

closed as too broad by Mithoron, Tyberius, paracetamol, ron, Todd Minehardt Nov 21 '17 at 23:44

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ If you don't get that compounds can be in different states depending on temperature... $\endgroup$ – Mithoron Nov 21 '17 at 16:02
7
$\begingroup$

You can't do this by:

just by looking at the chemical formula

But instead look up the properties of the compounds in question (Wikipedia is a good source of data).

$\text{RbOH}$ is highly soluble in water, so assuming there's an excess of water present, it will end up being dissolved in the water. So its state symbol will be $\text{(aq)}$.

Hydrogen on the other hand is a gas (at room temperature and above) and highly insoluble in water: it will leave the reaction as gas and its state symbol will be $\text{(g)}$.

So without knowing the physical/chemical properties of the reagents/reaction products, the state symbols cannot be determined.

Note that the state symbols are often idealisations: solid or liquid compounds are rarely completely soluble or completely insoluble. The same is true for gases. Even water is a case in point: liquid, $\text{(l)}$, at room temperature but with significant vapour pressure.

$\endgroup$
  • $\begingroup$ In many cases, it helps to know either something about the type of reaction occurring, or the temperature at which the reaction is taking place. For example: water is generally a product of combustion reactions, but since these take place at high temperature (we are burning something after all) the water will be present as a vapor (g). $\endgroup$ – Michael Lautman Nov 21 '17 at 15:53

Not the answer you're looking for? Browse other questions tagged or ask your own question.