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For CHEM4  there are 3 main topics that scream maths and all things numbery (yes it's a word). These are Kinetics (rates of reaction), Acids and Bases (and buffers) and Equilibrium.

Today we are covering Equalibrium. Mainly:

  • Recaping AS stuff.
  • What is Kc and how you calculate it.
  • How Le Chateir is used to estimate wether or not Kc will increase or decrease with a change.

So firstly we need to revise some basic stuff from AS.

Let's let take the reaction below:

H2 + I2 --> 2HI   H= -20 kj mol

A dynamic equilibrium in this case will occur when the rate of the docompostion of HI is the same as rate of Hydrogen and Iodine reacting.

However let's say you want to make alot of methamphetamine to counteract obesity, you want as much HI as possible. Now you know from AS kinetics that an increase in temperature, increases the number of kinetic energy and thus the rate of reaction, however this is an exothermic reaction and therefore a higher temperature will decrease the amount of product made. So compramises in industry have to be made. So with that let's take some baby steps into A2 land.

Let's say that we decomposed 0.8 moles of  Hydrogen Iodide in 5.00 dm in the laboratory. We then found that there was 0.3 moles of Iodine in the flask at equalibrium. Our teacher then asked to work out Kc, how do you do it?

Right the first thing you're going to think is that constants are always the same and therefore you can just spew an answer from a previous practical. Sorry guys but that's Wrong!!!! Instead the equlibrium constant depends of what the reaction is and the conditions it happens in (more on that later). We normally call this Kc. The general equation for this is:

Kc= [Product 1]a[Product 2]b/[Reactant 1]c[Reactant 2]d.

The small letters (a,b,c,d) refer the number of moles of each species. So for our lab example:

Kc= [H2][I2]/[HI] 2 

So if we look


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