I had an equilibrium laboratory and had to answer a few questions after the lab. Here are 2 questions I can't seem to know how to answer.

1. Explain in your own words why a saturated solution in contact with some undissolved solid is considered to be an equilibrium situation?

2. Rationalize how the addition of the silver nitrate to the Fe/SCN equilibrium system had an effect even though neither Ag nor NO3 appears in the equation shown in the text.

Attempt:

For number 2 I was going to write that the Ag ion will replace the Fe in FeSCN and the NO3 will do the same but with the Fe ion?

#1. In as few words as possible, the solid is continuing to dissolve to form a solution and material in the solution is continuing to ppt; therefore, the material in solution is constant and the solid is constant. That's equilibrium.

#2. You didn't write the equations in your text but I assume it is
Fe^3+ + SCN^-= ==> FeSCN^2+

The reason equilibrium is upset is because Ag^+ reacts with SCN^-. You can look up what it forms. If you pull SCN^- out then the Fe/SCN equilibrium shifts to the right.

1. To understand why a saturated solution in contact with undissolved solid is considered an equilibrium situation, we need to understand the concept of dynamic equilibrium. In a saturated solution, the rate at which solute molecules dissolve is equal to the rate at which they precipitate back into the solid form. This occurs because after some time, the concentration of solute molecules becomes constant due to this balance between dissolution and precipitation. At this point, we say the solution has reached a state of dynamic equilibrium.

In contact with undissolved solid, the solution continuously interacts with the solid particles. Some solute molecules will dissolve into the solution while an equal number of molecules will precipitate back onto the solid. This continuous process results in a constant concentration of solute in the solution, which is the defining characteristic of equilibrium. Thus, a saturated solution in contact with undissolved solid is considered to be an equilibrium situation.

2. Your attempt at explaining the effect of silver nitrate (AgNO3) on the Fe/SCN equilibrium system is partially correct. However, let's dive deeper into the explanation.

In the Fe/SCN equilibrium system, Fe(SCN)2+ ion is formed by the reaction of iron (Fe3+) ions and thiocyanate (SCN-) ions. The equilibrium equation for this reaction can be written as:

Fe3+ + SCN- ⇌ Fe(SCN)2+

When silver nitrate (AgNO3) is added to this system, it introduces silver ions (Ag+) into the solution. Although silver ions don't appear in the equilibrium equation, they can have an effect on the equilibrium system through a process called ion exchange.

Silver ions (Ag+) have a stronger affinity for thiocyanate ions (SCN-) than iron ions (Fe3+). As a result, the silver ions tend to replace the iron ions in the Fe(SCN)2+ complex, leading to the formation of silver thiocyanate (AgSCN).

The reaction can be represented as:

Fe(SCN)2+ + Ag+ ⇌ AgSCN + Fe3+

This reaction disrupts the balance of the original equilibrium system. As silver ions react with the Fe(SCN)2+ complex, some of the Fe(SCN)2+ dissociates, causing a shift in the equilibrium towards the formation of more Fe3+ and SCN- ions. This shift occurs to counterbalance the disturbance caused by the addition of silver nitrate.

In summary, the addition of silver nitrate to the Fe/SCN equilibrium system affects the equilibrium by favoring the formation of more Fe3+ and SCN- ions, leading to a shift in the equilibrium position.