Mastering Chemical Equilibrium: Chapter Review and Test Prep

[intellekt]

[CHEM] Equlibrium Help please!

Hi, I am having trouble with a few questions on my chapter review and I have a test tomorrow. I wasn't at school today so I didn't get the chance to ask questions.

1. The oxidation of sulfur dioxide to sulfur trioxide is an important reaction. At 1000K, the value of Kc is 3.6x10^-3.
2 SO2 + O2 <-> 2 SO3
A closed flask originally contains 1.7 mol/L SO2 and 1.7 mol/L O2. What is [SO3] at equilibrium when the reaction vessel is maintained at 1000K?
Ok So I set up the ice table and for equilbrium i get 1.7-2x [SO2], 1.7-2x [O2], and 2x for [SO3]. Then...

Kc = -____[SO3]^2____
-------[SO2]^2[O2]
3.6x10^-3 = ____(2x)^2____
------------(1.7-2x)^2 (1.7-x)
(3.6x10^-3)(2.89-6.8x-4x^2)(1.7-x) = 4x^2
(3.6x10^-3)(4.913-14.45x+13.6x^2-4x^3)= 4x^2

I didn't go any further because I don't know what to do now that there is an x cubed. Unless I'm doing it wrong, could you please correct me.

2. Write the chemical equation for the reversible reaction that has the following equilibrium expression.
Kc = blah blah blah
Now, the eqn would be: 4 NH3 + 5 O2 <-> 4 NO + 6 H2O
Assume that, at a certain temperature, [NO] and [NH3] are equal. Also assume that [H2O] = 2.0 mol/L and [O2] = 3.0 mol/L. What is the value of Kc at this temperature.

What do I do for this question?

 

[Physics Monkey]

The equation is cubic and would generally be solved numerically (though a nasty formula, analogous to quadratic formula, does exist for cubic polynomials). There are ways to find a good approximate answer which is still correct to several decimal places.

What would you naively expect the value of x to be? In other words, will x be large (close to 1.7 say), will x be small (near zero), or something more in the middle? To rephrase it one more way, do you expect a lot or a little S03 to be produced?

 

[Blueshift5]

Hi there,

It seems like you have a good understanding of the concepts involved in chemical equilibrium. To solve the first question, you can use the quadratic formula to solve for x. Remember to convert your units to mol/L before plugging them into the equation. Also, make sure you are using the correct values for the concentrations of SO2 and O2 at equilibrium.

For the second question, you can use the given information to set up an expression for Kc. Remember that the equilibrium expression for a reaction is the ratio of the products to the reactants, each raised to their respective coefficients. So in this case, Kc would be [NO]^4[H2O]^6/[NH3]^4[O2]^5. Then, plug in the given concentrations and solve for Kc.

I hope this helps! Good luck on your test tomorrow. Remember to review all the concepts and equations before the exam.