# Finding an equilibrium constant

1. Mar 30, 2012

### shaa

Hello, I'm new to the forums and did in fact search for my thread before posting. I couldn't find anything that helped me with what I'm looking for. With that said, the problem I'm having is:

1) The value of Keq for the equilibrium H2+I2 → 2 HI is 794 at 25°C. What is the value of Keq for the equilibrium below?
1/2 H2+1/2 I2 → HI

2) The equilibrium constant for reaction 1 is K. The equilibrium constant for reaction 2 is _____.
1: SO2+1/2 O2 → SO3
2: 2SO3 → 2SO2+O2

I apologize for not denoting an equation at equilibrium, but I couldn't find the right arrow in the "Quick Symbols" box. Both equations are at equilibrium and involve only matter in the gaseous state.

2. Mar 30, 2012

### shaa

I've gotten the first question, considering that the second reaction is half of the first. Resulting in: x1/2, and x being the original Kc.

I know what the answer to the first question is, but I'm having difficulty understanding why it is what it is.

Last edited: Mar 30, 2012
3. Mar 30, 2012

### Staff: Mentor

This is a simple math. Write reaction quotient for each reaction.

4. Mar 30, 2012

### shaa

It states that the equilibrium constant for reaction 1 is = k. The equilibrium constant for reaction 2 is = 1/k2.

The answer makes complete sense given a general form of an equilibrium reaction, but why wouldn't the first reaction be = 1/k or the second be = -1/k2?

5. Mar 30, 2012

### Staff: Mentor

I don't understand why you think it should be. If you write reaction quotients for each reaction and compare them, it is obvious what value the new equilibrium constant has.

You can as well ask why $\sqrt 2$ doesn't equal $\pi$.

6. Mar 30, 2012

### shaa

Hahahah. Of course. It's the inverse reaction and it double. Thus 1/k2.
Thanks? :P

7. Mar 30, 2012

### Redbelly98

Staff Emeritus
One of these might help; feel free to copy-and-paste for future use, or bookmark the links in my sig:
← or ↔

p.s. Welcome to Physics Forums.

Last edited: Mar 30, 2012