[Toxage]

I have to determine the balanced equation for a lab assignment. There is a fictitious element called "Silver Exide" (Fictitious Formula Ag"X"). We used 100 ml of "Silver Exide". The lab takes place at 18.5 celcius.

The following equation represents the equilibrium in this saturated sol'n:

AgX (s) <-----> Ag+ (aq) + X- (aq)

I know that 0.006 Moles of Copper reacted with the Silver Exide.

How do I write the balanced equation? I don't know how many moles of Silver Exide are used in the equation to write it.

So far I have:

0.006 Cu(s) + AgX (s) ----> Ag+(aq) + CuX-(s)

I am pretty sure that the Cu reacted with the X. Cu is positive and Ag is positive so the Cu had to react with the X. I observed a "fuzzy" product formed, which I believe would be a solid.

How can I determine the moles of "Silver Exide" to write the balanced equation?
 

[Borek]

Describe experiment you did.

[Arkcon]

Describe experiment you did.

Yeah, with the information you've given, your problem can't really be solved.  One thing, we don't usually put the moles of reactant into the balanced equation as a coefficient, that's just not how a balanced equation is used -- often one reagent is present in excess, if we put them all in, it would unbalance the equation.

[Toxage]

Describe experiment you did.

I can copy down the lab sheet for you.

Objective: To calculate the solubility product constant from experimental data to solve a theoretical KSP problem.

Dicussion: In this lab, you will use a saturated sol'n of a slightly soluble silver salt. This salt has the fictitious name "Silver Exide" (Fictitious Formula Ag"X"). The follow equation represents the equilibrium in this saturated sol'n:

AgX (s) <--> Ag+ (aq) + X- (aq)

You will immerse in this sol'n a copper wire, as was done in lab 14. By determining the mas sof the copper that reacts, you can calculate the [Ag+] and the [X-] in the sol'n. This data will enable you to determine the solubility product constant "AgX".

Procedure: Dispense 100ml of "Silver Exide" to a 100ml graduated cylinder. Obtain a 30cm length of copper wire and clean the surface of the wire with the steel wool. Free the wire of steel wool particles by wiping it with a clean paper towel. Coil the wire around a test tube, leaving a "handle" at one end.

Mass the wire using the Mettler, and place it in a 250ml beaker containing the saturated sol'n of "Silver Exide". Bend the "handle" over the lip of the beaker to prevent immersion of that end of the wire. Allow the system to stand for a period of one or more days. This will give all dissolved silver ions time to react. Since KSP is temp dependent, recored the temp of the system.

After one or more days have elapsed, shake the silver crystals free from the copper wire. Wipe off any Ag that clings to the wire using a paper towel. Rinse the wire in acetone and mass when dry. Capture Ag crystals in filter paper as directed by teacher.


I am confused on what is going on in the lab. Copper is used in the reaction (The mass of the starting copper was higher than after the reaction). So that copper must react with X (Since Ag and Cu are both positive). What does that leave the Ag to react with?

[Borek]

Have you heard about reactivity series?

You are explicitly told what happened to silver.

[Toxage]

I don't get it...

1) How does Ag react with Cu when they are both positive?

2) How do I determine the moles of AgX to write the balanced equation?

[Borek]

1) How does Ag react with Cu when they are both positive?

They are not both positive. You start with Ag ions and copper metal. Besides, even if they were both in form of cations, it is not a reason to not react.

Read about reactivity series.

2) How do I determine the moles of AgX to write the balanced equation?

First, you write balanced reaction equation. Then, you calculate amounts of substances reacting using information about change of the wire mass.

[Arkcon]

I don't get it...

1) How does Ag react with Cu when they are both positive?

The answer to this question is ... reactivity series.  You will have to learn this concept, and it will all become clear.  FWIW, copper doesn't react with silver, it reacts with the "exide" in solution, and leaves the silver behind.  If they weren't both positive, there would be no reaction with the "exide."  So your objection, is incorrect.

2) How do I determine the moles of AgX to write the balanced equation?

Like I said, we don't use moles reacted as coefficients (the big number, not a subscript, before the molecules) to write balanced equations.

What you think you know, about this problem, will not help you solve it.  And is getting in the way of what Borek: and I are telling you.  You're going to have to read your lab explanation and textbook some more, and see if anything clicks.

[Toxage]

1) How does Ag react with Cu when they are both positive?

They are not both positive. You start with Ag ions and copper metal. Besides, even if they were both in form of cations, it is not a reason to not react.

Read about reactivity series.

2) How do I determine the moles of AgX to write the balanced equation?

First, you write balanced reaction equation. Then, you calculate amounts of substances reacting using information about change of the wire mass.

How are they both not positive? I am looking at my periodic table right here : Cu +1 , +2 Ag +1. I thought that in order for something to react the had to balance each other out.

Balanced Equation:

Cu(s) + AgX (s) ----> Ag+(aq) + CuX(s)

Starting Copper Mass = 3.361g
Ending Copper Mass = 2.951g
Copper Used In Reaction= 0.41g

I don't understand how I use the grams to balance the equation. I was taught that moles must be used to balance an equation. But I guess I'll try grams.

Balanced Equation:
0.41 Cu(s) + 0.41 AgX (s) ---->  0.41 Ag+(aq) + 0.41 CuX(s)

Is that right?

[Toxage]

I think I got, Cu is more reactive than Ag, so it combines with the X.


Cu(s) + AgX (s) ----> Ag+(aq) + CuX(s)

According to http://en.wikipedia.org/wiki/Reactivity_series Copper has a charge of +2.

Balanced Equation:


Cu(s) + 2AgX (s) ----> Ag+(aq) + CuX2(s)

[Arkcon]

Holy crap on a shingle!  You solved it, yourself, and balanced the charge without us hinting about it.  *Phew*  I almost gave up on you Toxage:, well done.  I need a cigarette ...

OK, now, you know the before and after weight of your copper, how much silver "exide" is that equivalent to?

[Toxage]

0.41 is the Copper

0.41 * 2 = 0.82g "Silver Exide"

Thanks for the help, I did the first half of my assignment =D

But I got stuck on tis one: What is he concentration of X- ion in moles/liter?

I assume this question refers to molarity. The balanced equation I did it based on .1L or 100Ml so do I just multiply the 2 by 10 to get it to 20 which is what it would be at under 1 Liter?

20 / 1 = 20 M for X- ions in the solution.

[Borek]

I think I got, Cu is more reactive than Ag, so it combines with the X.


Cu(s) + AgX (s) ----> Ag+(aq) + CuX(s)

Closer but still wrong Copper gets oxidized by silver. You were told to clean your copper wire from silver crystals, yet in your reaction silver in products is in ionic form. It should be metallic.

[Toxage]

I think I got, Cu is more reactive than Ag, so it combines with the X.


Cu(s) + AgX (s) ----> Ag+(aq) + CuX(s)

Closer but still wrong Copper gets oxidized by silver. You were told to clean your copper wire from silver crystals, yet in your reaction silver in products is in ionic form. It should be metallic.

So the silver should be in the solid form?

Yeh, the silver had to be in sold form because we put it through filter paper, which means that the silver was in solid form and the CuX went through the filter paper with the water which meant it had to be dissolved into the water which means it must be in (aq) form.

Unbalanced Equation:
Cu(s) + AgX (s) ----> Ag+(s) + CuX(aq)

Balanced Equation:

Cu(s) + 2AgX (s) ----> 2Ag+(s) + CuX2(aq)

I hope that is right.

[Borek]

I am afraid you are still spinning in the same place. X^- is only a spectator. There is a redox reaction taking place, with copper being oxidized and silver being reduced. In other words - silver is being replaced in the solution by more reactive copper.

No such thing as Ag⁺(s) - Ag⁺ is an ion and can be present either in the solution, or in some ionic crystal. That'll be Ag⁺(aq) or AgX(s).