Which Ion is Never in Excess in the FeSCN2+ Equilibrium Chem Lab?

[mill0132]

Homework Statement

Chem lab on equilibrium. We added the same amount of Fe(NO3)2 (5ml) to varying amounts of KSCN and used a given Beer's law and absorption data to calculate concentration of FeSCN2+ and then calculate the Eq. constant (K).

I calculated the Eq. constant, but I'm stuck on two follow up questions:

Consider the equation Fe+3(aq) + SCN(-)(aq) <---> FeSCN 2+

Which ion (Fe3+) or SCN (-) is never in excess? Why?
What experimental conditions are needed to ensure "all" SCN(-) is present as FeSCN2+

Homework Equations

The intro we were given in our lab manual described the instability of Fe2+ in water (how it forms Fe3+). Also described the colors of Fe and FeSCN2+. Also defined complex ions and oxidation states.

The Attempt at a Solution

I don't understand how I would know which ion is limiting. We used varying amounts of KSCN and kept the amount of FeNO3 consistent. The stoichiometry would indicate it's a 1:1 ratio. According to the calculations I did for one trial, I end up with fewer moles of SCN(-) at equilibrium, so does that mean it's limiting?

Any guidance on how to figure this out would be appreciated.

 

[nate808]

it is important to understand the concept of limiting reagents in a chemical reaction. In this case, the limiting reagent is the reactant that will be completely consumed in the reaction, thus limiting the amount of product that can be formed. In order to determine which ion is limiting in this reaction, you need to compare the initial amounts of Fe(NO3)2 and KSCN that were added to the reaction.

In your experiment, you added the same amount of Fe(NO3)2 (5ml) to varying amounts of KSCN. This means that the initial amount of Fe(NO3)2 is the same for all trials, but the initial amount of KSCN is different. The stoichiometry of the reaction is 1:1, meaning that for every 1 mole of Fe(NO3)2, you need 1 mole of KSCN to form the product FeSCN2+.

In order to determine which ion is limiting, you need to calculate the moles of each reactant present in the reaction. If the moles of Fe(NO3)2 is greater than the moles of KSCN, then Fe(NO3)2 is the limiting reagent. If the moles of KSCN is greater than the moles of Fe(NO3)2, then KSCN is the limiting reagent.

To ensure that all of the SCN(-) is present as FeSCN2+, you need to make sure that the reaction has reached equilibrium. This means that the forward and reverse reactions are occurring at the same rate, and there is no net change in the concentrations of reactants and products. In order to reach equilibrium, you need to maintain a constant temperature and pressure, and allow enough time for the reaction to reach equilibrium. You also need to make sure that the concentrations of reactants are not too high, as this can cause the reaction to go to completion before reaching equilibrium.

In summary, to determine which ion is limiting in the reaction, you need to compare the initial amounts of Fe(NO3)2 and KSCN. To ensure that all of the SCN(-) is present as FeSCN2+, you need to maintain constant experimental conditions and allow enough time for the reaction to reach equilibrium.