You should be used to the “why” question when I ask you about chemistry things. Mr. LeChâtelier is the “put it back the way it was” guy.
The direction of the arrow tells you a lot. The arrow always points to products. Notice that we are using arrows two ways!!!
So what we normally call products (the right side of the equation), actually become reactants when we are shifting to the left (arrow points the other way). WOW!
Here are the why’s of LeChâtelier’s Principle for 3 important factors that affect systems at equilibrium: pressure changes, amount of reactant / product change, heat change.
1. Pressure Changes – after the reaction is at equilibrium
To make sense of these changes we must realize that all gases take up exactly 22.4 L/mol at STP. So the mole ratios of the balanced equation tell us which side of the equation is going to make more pressure. We’ll use the following equation to demonstrate increases and decreases:
N2 + 3 H2 <------> 2 NH3
more moles fewer moles
more molecules fewer molecules
MORE MOLECULES MEANS MORE PRESSURE
a. When pressure increases the reaction will shift towards the side with fewer atoms / molecules. Why? Because fewer molecules take up less space and will reduce the pressure.
b. When pressure decreases the reaction will shift towards the side with more atoms / molecules. Why? Because more molecules take up more space and will increase the pressure.
2. Change amount of gas – after the reaction is at equilibrium
We can use the same equation for this example.
N2 + 3 H2 <------> 2 NH3
a. When we add more reactants the equilibrium will shift away from reactants to reduce the reactant.
added more reactants
Now we ned more product which will use up reactants. This will balance the teeter-totter by taking away from the left and add to the right. So we shift to the right.
b. When we take away reactants the equilibrium will shift in such a way that it will make more of the reactants.
took away reactants
Now we need more reactants (more stuff on the left of` the teeter-totter). This will balance the teeter-totter by taking away from the right and adding to the left at the same time. This is a shift to the left.
3. Heat or cool a reaction that is exothermic or endothermic – after the reaction is at equilibrium
Here we will have to use an exo- or endothermic reaction.
C2H2 + 2 Br2 <-------> C2H2Br4 + heat
If we add heat to this reaction we do the teeter-totter does this
This is just like adding more product above. The only was to take away the heat is to make the reaction go to the left, since that will use up heat.
Now that you have the explanation, look at a wonderful animation to show you how this stuff works on the molecular level. This one is really gorgeous!!! LeChatelier Principle – animation allowing conc., pressure & temp changes – really pretty .
Demonstration of LeChâtelier’s Principle using nitrogen dioxide – see the same demonstration you saw on video in class, showing how pressure changes affect the equilibrium between N2O4 (g) colorless <-----> 2 NO2 (g) brown and you will see a shift in the macro world.