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Chemical Equilibrium

Introduction

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One very interesting question is: can a chemical reaction go backwards? Certain reactions we commonly see don´t seem to be able to return to its initial state, like  combustions or explosions for instance. But the answer to this question is also very interesting: reactions can go forward and backwards at the same time! This is what we know by chemical equilibrium.

 

Example: A reaction between two gases:

N204 ⇌ 2NO2 

 

This is a very interesting reaction because it can be followed visually. N204 is colourless and NO2 is red-brown. It is possible to force the equlibrium to shift one way or the other by varying the temperature (Le Châtelier´s Principle), and them we see the red-brown colour disappering and reappearing (there are lots of youtube videos that show that, for instance look at that one).

So, imagine that we start the reaction (at 25 °C) by inserting 2 mol of N204 in a 1 l reaction vessel (initially empty). After equilbrium is achieved, there will be 0.1 mol of NO2 and 1.95 mol of N204 . If instead, we insert 4 mol of NO2, the result is exactly the same. Those are the equilibrium concentrations for this reaction (at this temperature) and it will be achieved no matter it starts with NO2 or N204 (these calculations were realized using an equilibrium constant of 4,6 x 10-3 at 25 °C).

 

The equilibrium constant

Equilibrium is achieved when the rate (or velocity) of the forward reaction equals that of the reverse reaction.

Rates are given by the following expressions (where k, the rate constant, depends on the particular reaction) :

Forward reaction:

N204 → 2NO2

rate = kf [N204]

 

Reverse reaction:

2NO2 → N204

rate = kr [NO2]2

 

At equilibrium:

kf [N204] = kr [NO2]2

kf / kr = [NO2]2 / [N204] = Kc (equilibrium constant)

 

- Important points:

- it is a dynamical equilibrium:  Although the concentrations of each substance remain constant, the reactions are at work, e.g., products are forming and then serving as reagents all the time. The constant concentrations is just the net result !

- The  substances involved have constant concentrations but it doesn´t mean that they are all the same. The proportion between amount of products and reagents at equilibrium is given by the equilibrium constant, which may depend on various factors, including temperature off course.

- A catalyst doesn´t change the equilibrium. The equilibrium constant will be the same. The difference is that reactions will be accelerated so that equilibrium is reached quicker.


 

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