how do we know gas evolved?

[unsung]

Say HCl reacts with some kind of carbonate salt, like Na2CO3. How do we know the gas that's evolved is CO2?

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[ThePandaFactor]

Probably by lighting it on fire.

 

[Quantum Chem]

2HCl(aq) + Na2CO3(aq) ---> H2O(l) + 2NaCl(aq) + ?

Since HCl is an acid and Na2CO3 is a base you know that an acid/base reaction is going to take place. When an acid/base reaction happens you know that water is always formed and some sort of salt is produced. You know know that you need 2 HCl to react per 1 Bicarb because two H are needed in water and 2 Cl are needed for 2 Na+. Now you can see that the only atoms not taken into account in the reactants side is a carbon and two oxygen, hence CO2. Its got to be a gas because CO2 is a gas at room temp (assuming it took place at RT).

So overall Eqn: 2HCl(aq) + Na2CO3(aq) ---> H2O(l) + 2NaCl(aq) + CO2(g)

 

[unsung]

2HCl(aq) + Na2CO3(aq) ---> H2O(l) + 2NaCl(aq) + ?

Since HCl is an acid and Na2CO3 is a base you know that an acid/base reaction is going to take place. When an acid/base reaction happens you know that water is always formed and some sort of salt is produced. You know know that you need 2 HCl to react per 1 Bicarb because two H are needed in water and 2 Cl are needed for 2 Na+. Now you can see that the only atoms not taken into account in the reactants side is a carbon and two oxygen, hence CO2. Its got to be a gas because CO2 is a gas at room temp (assuming it took place at RT).

So overall Eqn: 2HCl(aq) + Na2CO3(aq) ---> H2O(l) + 2NaCl(aq) + CO2(g)

Thanks so much! btw, this may be a stupid Q, but how do you tell whether something's a base or a salt? Like with Na2CO3, that would be considered a basic salt, right? Since CO3 is derived from H2CO3 a weak acid, and Na is from NaOH a strong base, so the salt is more basic than acidic. That always kind of confused me.

 

[Quantum Chem]

A salt is an anion bonded to a cation, so not all bases are salts and not all salts are bases. Salts dissociate in water, allowing the ions to do whatever they are good for. Yes, Na2CO3 is a basic salt, but in water it would be Na+ CO3-2 so in an acid/base reaction Na+ is a spectator ion and not really payed attention to, so CO3-2 is the base since it is an H acceptor, electron donor, or whatever flavor of the definition of base you like. Also you should remember that not all weak acids produce strong bases. But I do believe that CO3-2 is a strong base if I remember correctly.