Question

when writing complete ionic equations, how do i know what molecules dissassociate?

Also, when predicting chemical equations, how do i determine if the products are aqueous, solid, liquid, or gas?

Answer 1

when writing complete ionic equations, how do i know what molecules dissociate?

Hi consider small examples below

HCl (aq) + NaOH (aq) --> H₂O (l) + NaCl (aq)

HCl, and NaOH are in aq state, now product NaCl also in aq state - so Na⁺, Cl^- are said to be spectator ions, we should omit them during net ionic equations (NIE) writing.

1. Only solid (s) forming ions will be present at reactant side and solid will be at product side.

2. If reactant side ions forms a new product then we should mention it NIE, like reaction below

Molecular equation: 2HCl (aq) + Na₂CO₃(aq) ==> 2NaCl(aq) + CO₂ (g) + H₂O(l)

NIE: 2H⁺(aq) + CO₃^2-(aq) ==> CO₂ (g) + H₂O(l)

3. Dissociation can be observed by looking at products only, no theory exist to predict how molecules dissociate, all are be practical only.

Also, when predicting chemical equations, how do i determine if the products are aqueous, solid, liquid, or gas?

This requires compound specifications, go to product specification page say Millipore-Sigma website and search for a specific compound then look at solubility. If the value is very low in water then it appears to be solid, if this information not available then we to look at melting point if this value is lower than STP then it exist as gaseous state. If it is higher than 25 ^oC then it exist as solid.

The general species encounter in our regular laboratory are as follows

H⁺, OH^-, H₃O⁺, Na⁺,Cl^-,CO₃^2-,K⁺,Ca²⁺, Br^-, I^- ----- all are in aqueous state (aq)

H₂O only one exist as liquid state ---- liquid state (l)

CO₂, steam water H₂O, CH₄, SO₂, NO₂ --- few examples which exist as gaseous state (g)

AgCl, AgBr, AgI --- all are insoluble almost in water, so we write them as solid (s) it is also used for precipitate.

Hope this helped you!

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