# Reaction direction question

1. Nov 2, 2004

### Mathman23

Hi

Here is a reaction: Fe^(3+) + SCN^(-) <---> FeSCN^(2+)

If I add some extra KSCN to the above equilibrium reaction, in which direction will it react according to the law of chemical equilibrium? left or right?

Sincerely Yours
Fred
Denmark

2. Nov 2, 2004

### siddharth

If i add extra moles at equilibirium the system would tend to move in a direction which would minimise the change. Since more moles of reactants are added at equilibirium the reaction will move to the product side by Le Chatilers principle

3. Nov 2, 2004

### Mathman23

I have a second question:

The mass action expression for the reaction is:

$K_{c} = \frac{[FeSCN^{2+}]}{[Fe^{3+}][SCN^{-}]}$

Which of these size increase, then the extra KSCN is added? The numerator or denumerator of the mass action expression?

Sincerely

Fred

4. Nov 2, 2004

### Gokul43201

Staff Emeritus
Mathman, if this is homework (which it looks like), post it in the Homework Help section. Also, the policy here is that we will not give you answers to your homework problems unless you show that you've made some effort towards solving it - which you have not done here.

I suggest you learn how le Chatelier's Principle works - that is going to be an important tool for your understanding chemical equilibria.

Siddharth, it's better to give hints or suggestions than actually feeding the complete answer...especially if the problem looks like it could be homework. That way, you help the poster think about the problem.

5. Nov 2, 2004

### Gokul43201

Staff Emeritus
Fred, if you understood why the previous answer was what it should be, you would not have this doubt.

What happens when you add KSCN - in what form will the KSCN be present ? What effect does it have on the concentrations of various species, and hence on the equilibrium between these species ?

What do you know about the equilibrium constant K, and what does it depend on ? Does it, or does it not change with a change in the concentrations (activities) of the reactants/products?

6. Nov 2, 2004

### Mathman23

Hi its added in a solid-form.

It increases the concentration of SCN in the denumerator.

/Fred