2. Standards
The state standards we will be covering in this lesson are:
HS-PS1-5. Apply scientific principles and evidence to provide an explanation about the effects of
changing the temperature or concentration of the reacting particles
on the rate at which a reaction occurs.
HS-PS1-6. Refine the design of a chemical system by specifying a change in conditions that would
produce increased amounts of products at equilibrium.
3. Let’s Get Started
Before we start learning about equilibrium, we are going to try a little
demonstration. Hold out both of your arms to your sides. Now, in one
hand, hold something heavy like a book or a backpack. Notice that
the weight on one side makes holding your arms out more
uncomfortable. Wouldn’t it feel better if we could move some of that
weight to the other arm?
4. What is Equilibrium?
● Equilibrium is when the forward reaction and the reverse reaction
happen at equal rates.
● Not every reaction has an equilibrium, some go to completion.
● The reactants and products may reach equal amounts when
at equilibrium, but not always.
5. “Stress”
● Reactions like to stay at equilibrium
● If an outside factor affects something in the equilibrium equation, this
would be considered a “Stress”
● “Stress” will cause the reaction to shift in order to reduce the stress
6. Le Chatelier’s Principle
● Le Chatelier’s Principle states that if an equilibrium changes due to
“stress” that equilibrium will shift its reaction in order to reduce that
“stress”.
● In the chart to the right, you can see that NO had its concentration
increased. In order to account for the “stress”, you can see the
concentrations of other compounds change.
7. Brainstorm!!
List some factors you believe would put “stress” on an equilibrium equation.
Share these with your neighbor and see if the two of you can come to a
consensus.
8. A Change in Concentration
● One aspect of an equilibrium you can change to introduce stress would be to change the
concentration of molecules in the reaction
● If you introduce other molecules into the system that aren’t in the equilibrium equation,
then they may not have an effect.
● Can you list a reason why a molecule not found in the equation would have an effect?
9. 2NO(g) + H2(g) <-> N2O(g) +H2O(g)
When 2NO(g) is increased in the system, the system experiences stress on the reactants side.
Because this stress is increasing the reactants side, the
reaction will shift towards the products to compensate
for the gain
When H2O(g) is decreased in the system, the system experiences stress on the products side.
Because this stress is decreasing the products side, the
reaction will shift towards the products to compensate
for the loss
10. Example: Fe3+
(aq) + SCN-
(aq) <-> FeSCN2+
(aq)
If Fe3+
(aq) is added to this reaction, you are adding stress
to the reactants side of this equilibrium. This stress is an
increase on the reactants side, therefore the reaction will
want to move away from this and shift to the products.
If SCN-
(aq) is removed from the reaction, you are again
adding stress to the reactants side. The stress on the
reactants side this time is a decrease. In order to go back
to equilibrium, the reaction will shift back to the
reactants side in compensate for the lost reactant.
11. Now You Try
2NO + H2 <-> N2O + H2O
1) What direction would the reaction shift if N2O was decreased in the system? Explain?
1) What direction would the reaction shift if H2 was decreased in the system? Explain?
1) What direction would the reaction shift if H2O was increased in the system? Explain?
12. Temperature and Equilibrium
2NO + H2 <-> N2O + H2O + 36 kJ
Changing the temperature can also affect Equilibrium.
In the reaction above, you can see heat given off as a product, or an exothermic reaction.
Another way it could be written would be H= -36 kJ. The negative indicates that the
system is losing the energy, therefore an exothermic reaction.
We can deal with the stress of changing the temperature the same way we have been
with concentration.
If Temperature is increased, the system experiences stress on the products side.
Therefore, because the product side is increasing in
heat, the reaction will shift to the left.
13. Pressure and Equilibrium
2NO(g) + H2(g) <-> N2O(g) + H2O(g)
Changing the pressure can also affect Equilibrium.
Pressure is a measure of how often gas molecules hit their container. If there are more
gas molecules, there is a higher pressure.
In the reaction above, there are three gas molecules on the reactants side, 2 NO
molecules and 1 H2 molecule. On the right side, there are only 2 gas molecules, N2O and
H2O. Therefore, the higher pressure is on the reactants side.
If we were to increase pressure, the stress stress on the system would want it to move
towards a lower pressure state.
If Pressure is increased, the system experiences stress on the reactants side.
Therefore, in order to move to a state with less
pressure the reaction will shift towards the products.
14. Pressure and Equilibrium cont.
● If a reaction has some elements in the gas state, and others not, only the gaseous
compounds count towards the pressure of the system.
● If a system has an equal number of gas molecules on each side of the reaction, pressure
won’t do anything to the system
● You can use whatever side has more gas molecules as the side that will be affected by a
change in pressure
● Volume and Pressure are inversely proportional, therefore, if you change volume, you get
the opposite change for pressure.
○ An increase in volume is a decrease in pressure and vice versa
15. Create a Tree Map
2NO(g) + H2(g) <-> N2O(g) + H2O(g) + 36 kJ
Create a Tree Map using these questions as the split in the tree:
Where is the stress happening?
Is the stress increasing or decreasing?
Toward where will the shift happen?
