# Homework Help: Equilibrium constant varying with temperature but confused!

1. Jul 30, 2009

### pyridine

1. The problem statement, all variables and given/known data
OK. so i have a separation science lab report write up to do which involves the equilibrium of the tri-iodide ion dissociating into iodine and iodide. I know the concentrations at room temperature (which is very cold, about 10 degrees C, where i come from!), and thus the equilibrium constant. I then am asked to work out the equilibrium constant, K, at 25 degrees C. I know no other data, except a table of logK values for 0, 10, 20, 30, and 40 degrees C.

2. Relevant equations
equilibrium for I3-:

I2+I- <---> I3-

3. The attempt at a solution

I think you use the Van't Hoff equation, but i don't know the enthalpy....
Van't Hoff equation= Log(K1/K2) = enthalpy/R(1/T1-1/T2)
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jul 30, 2009

### Ygggdrasil

Many times you can calculate the change in enthalpy for a reaction by using Hess's Law. You would just need to look up the standard enthalpies of formation for the products and reactants (which can usually be found in the appendices of most general chemistry text books).

Also, I assume you mean the product is I3- or else the equation is not balanced.

3. Jul 30, 2009

### pyridine

Yep, the product is tri iodide...i just put the superscript in the wrong place....
It's complex because I can't find the standard enthalpy of formation of iodine in the aqueous state, or the standard enthalpy of formation of tri iodide.

4. Jul 30, 2009

### Ygggdrasil

Actually, you don't need to look up the enthalpy of formation. You can use your experimental data and the Van't Hoff equation to solve for the enthalpy of formation. If you plot ln K versus 1/T, the slope of the line will give you a value related to ΔH (I'm not going to specify how it's related, because you should derive the relationship yourself).