[t.s.12]

I have this lab, and we have to predict whether the reaction between aqueous sodium sulfate and aqueous calcium chloride is quantitative (reaction in which all of the limiting reagent is consumed).

CaCl_{2 (aq)} + Na₂SO_{4 (aq)} ---> CaSO_{4 (s)} + 2NaCl _(aq)

I predicted that it is not, because I thought that since the reactants are aqueous and they would be placed in a beaker, a closed system would be formed; no matter would be exchanged with the surroundings.  There would be a competition between collisions of reactants to form products and collisions of products to form reactants, resulting in equilibrium, so both reactants will be present at the end of the reaction. However, after performing the lab and testing the filtrate both with Ba(NO₃)₂ and Na₂CO₃, to check for presence of excess sulfate ions or calcium ions, precipitate was formed with Ba(NO₃)₂ only, indicating that one reactant is in excess and the other one has been consumed completely.

It turned out the reaction is quantitative, which I did not expect. Could somebody please let me know how would we be able to predict that it is quantitative without carrying out an experiment???

[Donaldson Tan]

The ionic net reaction is
Ca²⁺ + SO₄^2- -> CaSO₄ (s)

If both calcium and sulphate ions are equimolar, then there can be no limiting reagent.

[t.s.12]

Ok, do you mean that there should be no limiting reagent that is completely consumed in this reaction?

Based on 3 trials, with diagnostic tests, precipitate was formed each time only with Ba(NO₃)_{2 (aq)}, which tells me that sodium sulfate is the excess reagent, and calcium chloride has been completely consumed.

Diagnostic Test: Ba²⁺_(aq) + SO₄^2-_(aq) ---> BaSO_4(s)

I'm sorry, but could you please elaborate on your previous point?

[Borek]

Define "quantitative reaction".

[t.s.12]

A quantitative reaction is one in which all of the limiting reagent is consumed.

[Donaldson Tan]

A quantitative reaction is one in which all of the limiting reagent is consumed.

no reactant can be considered as a limiting reagent if all reactants are consumed.

if a chemical reaction involves A reacting with B, and after the reaction, there are some B left-over, then reactant A is the limiting reagent. If there are some A left-ove (instead of B), then reactant B is the limiting reagent. If there are no left-overs of both A and B, then there is no limiting reagent.

[Borek]

CaSO₄ is very weakly soluble, so it precipitated almost completely - thus the reaction was quantitative.

However, every reaction has some equilibrium. Ksp of CaSO₄ is  10^-4.3, so saturated solution is about 0.005M. As you have used some excess of SO₄^2- final concentration of Ca²⁺ was lower - probably in the range of 10^-4/10^-5M - so reaction was not quantitative.

As you see - depending on the approach - same reaction is at the same time quantitative and not quantitative. Definition you use is not precise thus it is hard to answerthe question.

[t.s.12]

Alright..thanks to both of you.  

[Borek]

no reactant can be considered as a limiting reagent if all reactants are consumed.

I have missed your answer, as it took me much more time to answer then it should and later I had to leave, so I was not aware of the fact you answered too up to now.

The problem is, definition of quantitative reaction is bad. Mix 10 mL 0.01M CaCl₂ and 10 mL 0.01M Na₂SO₄. What will happen?

Now mix 10 mL 1M CaCl₂ and 10 mL 1M Na₂SO₄. Any difference?

So is CaSO₄ precipitation quantitative reaction, or not?