Equilibrium Constant: Kc in a Snap!

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The key points covered in this video include:
1. Recap dynamic equilibria
2. Decomposition of nitrogen tetroxide
3. Homogeneous and heterogeneous reactions
4. Equilibrium constant (Kc)
5. Calculating the equilibrium constant

Dynamic Equilibrium

Not all reactions happen in a one way direction going to completion. Some processes have a reverse reaction which converts products back to reactants. When the rate of the backward and forward reaction are the same, the concentration of products and reactants remains constant and we have Dynamic Equilibrium. Can we imagine a macroscopic dynamic equilibrium? At a certain point, we begin sending the green balls back the other way. When the rates of forward and backward movement are equal the buckets no longer appear to change. It’s important to remember that nothing has stopped happening the balls still move both ways, we have just reached a point where no more changes happen to the buckets.

Decomposition of Nitrogen Tetroxide

Dinitrogen tetroxide is a colourless gas that decomposes into nitrogen dioxide which is a brown gas. As the nitrogen dioxide becomes more concentrated it begins to combine back into dinitrogen tetroxide. Let’s take a look at how equilibrium is established when dinitrogen tetroxide decomposes. A: Initially we start with no nitrogen dioxide and a high concentration of dinitrogen tetroxide. B: As the reaction takes place the concentration of N2O4 falls and the concentration of NO2 increases giving the mixture a light brown colour. C: Once we reach equilibrium the concentration of each chemical will no longer change - the mixture has it’s darkest brown colour. If we change the conditions the equilibrium will need to be re-established - remember Le Chatelier’s principle from last year.

Homogeneous and Heterogeneous Reactions

The decomposition of dinitrogen tetroxide only involves gases. We call reactions where all the reactants and products are the same state Homogeneous. Another example is the reaction between Iodin gas and Hydrogen. Homogenous Equilibrium - all chemicals are in the same state. There a reaction where different states of matter are present: Heterogenous Equilibrium - more than one state is present.

The Equilibrium Constant

You should remember meeting the equilibrium constant, Kc last year. Consider the general representation for an equilibrium reaction: The Equilibrium Law states that Kc can be calculated from. For a heterogeneous reaction where solids or pure liquids are present we note that concentration of the solids and pure liquids doesn’t change. So we do not include solids or pure liquids in the expression for Kc. To calculate units of Kc we replace the terms in the Kc expression with their units.

Calculating the Equilibrium Constant

SO2(g) and O2(g) were mixed in a sealed conical flask of volume 3dm^3 and react as follows. At equilibrium it was found that there were. Calculate Kc and work out the units. Firstly we must find the concentrations. Now we use our expression for Kc. Now we work out the units. Firstly we must find the concentrations. Now we use our expression for Kc. Now we work out the units. Final answer.

Summary

We now understand dynamic equilibria has the same forward and backward rate of reaction
We are familiar with the expression for the equilibrium constant
We can calculate the units of Kc
We understand the terms heterogeneous and homogeneous