Download Chemistry 12 Tutorial 4 and more Exams Chemistry in PDF only on Docsity!
Chemistry 12
Tutorial 4 - LeChatelier's Principle
In Tutorial 4 you will be shown:
- What LeChatelier's Principle is.
- How LeChatelier's Principle can be used to explain or predict shifts in equilibrium caused by changes in: - temperature - pressure or volume - concentration or partial pressure of a reactant
Do you remember getting somewhat "bogged down" in Tutorial 3, trying to always predict what will happen to an equilibrium when a change in temperature, volume, pressure etc. was made? In Tutorial 3, you figured out which rate (forward or reverse) was affected by the change, then you predicted what should happen. Tutorial 4 and LeChatelier's Principle will change all that! It gives you a simpler way to look at these concepts.
Let's look at LeChatelier's Principle first:
LeChatelier's Principle:
If a closed system at equilibrium is subjected to a change, processes will occur that tend to counteract that change.
- " Processes " usually mean that the equilibrium will shift to the left or right.
- " Counteract " means that if you do something to a system at equilibrium, the system will shift in such a way as to try to " undo " what you did.
- Remember, equilibrium only occurs in a closed system.
Examples of Counteracting :
- If you add heat to a system, it will shift in a way that it tends to " use up " the added heat.
- If you remove heat from a system, it will shift in a way that it tends to " produce heat "
- If you increase the concentration of a certain substance in an equilibrium mixture, the system will shift so as to reduce the concentration of that substance.
- If you decrease the concentration of a certain substance in an equilibrium mixture, the system will shift so as to increase the concentration of that substance.
- If you increase the total pressure on an equilibrium system involving gases , the system will shift in a way that will reduce the total pressure.
- If you decrease the total pressure on an equilibrium system involving gases, the system will shift in a way that will increase the total pressure.
Effect of Changes in Temperature
NOTE: When using LeChatelier's Principle, it is easier to have all reactions with "heat" in " Thermochemical Form ". eg.) The reaction: A + B C ∆H = -56 kJ is exothermic (∆H is negative), so heat is given off or written on the right. Thermochemical form: A + B C + 56 kJ The reaction: D + E F + G ∆H = 43 kJ is endothermic (∆H is positive), so heat is absorbed, or written on the left Thermochemical form: D + E + 43 kJ F + G
Okay, let's find out how LeChatelier's Principle applies here: Let's say we have an endothermic reaction: A + B + heat C + D If we increase the temperature of this system, we are adding heat. In order to counteract our change, the equilibrium will move in such a way as to use up heat.
Which means a new equilibrium will be established in which there is more C and D and less A and B than in the original equilibrium.
To summarize: When the temperature is increased , the equilibrium will shift away from the side with the heat term.
Now, if the temperature was decreased , the equilibrium would shift in such a way that would produce heat (to counteract the change). To do this, it would shift toward the side with the heat term. (in other words, produce heat)
To summarize: When the temperature is decreased , the equilibrium will shift toward the side with the heat term.
Here are a couple of questions:
- Given the reaction at equilibrium: A(g) ?+ B(g) + 32.5 kJ C(g) a) If the temperature was increased , which way would this equilibrium shift _________ b) If the temperature was decreased , which way would this equilibrium shift: _______
- Given the reaction: X(g) + Y(g) W(g) + Z(g) ∆H = -75 kJ a) Rewrite this as a thermochemical reaction ___________________________________________________________ b) If the temperature was increased , which way would this equilibrium shift: ____ c) If the temperature was decreased , which way would this equilibrium shift: ____ Check your answers for these on Page 1 of
Effect of Changes in Concentration or Partial Pressure
Consider the equilibrium equation: H2(g)? + I2(g) 2HI(g) If we add some H 2 to a flask containing this mixture at equilibrium, [H 2 ] will immediately increase. In order to counteract this change, the equilibrium will shift to the right in order to "use up" some of the extra H 2. (In other words to decrease the [H 2 ]).
Consider the equilibrium equation: H2(g) ?+ I2(g) 2HI(g)
Let's say now that we somehow take away some I 2. [I 2 ] will immediately decrease. In order to counteract this change, the equilibrium will shift to the left in order to increase [I 2 ] again. If we were to add some HI , the [HI] would immediately _____________________crease. In order to counteract this change, the equilibrium would shift to the __________________. To answer the last question, adding HI will increase [HI]. In order to counteract this change the equilibrium will shift to the left. In shifting to the left, [H 2 ] and [I 2 ] will go up and [HI] will go down. We can summarize the effects of changing concentrations by saying:
If the concentration of a substance in an equilibrium system is increased by us, the equilibrium will shift toward the other side of the equation, in order to counteract the change.
or
If the concentration of a substance in an equilibrium system is decreased by us, the equilibrium will shift toward the side of the equation with that substance , in order to counteract the change.
Let's have you do some examples:
- Given the equilibrium equation: PCl5(g) PCl3(g)? + Cl2(g) a) If the [PCl 5 ] is increased , the equilibrium will shift to the ........... _________________ b) If the [PCl 5 ] is decreased , the equilibrium will shift to the ........... _________________ c) If the [PCl 3 ] is increased , the equilibrium will shift to the ........... _________________ d) If the [PCl 3 ] is decreased , the equilibrium will shift to the ........... _________________ e) If the [Cl 2 ] is increased , the equilibrium will shift to the .............. _________________ f) If the [Cl 2 ] is decreased , the equilibrium will shift to the .............. _________________
Check page 1 of TuTuttoorriiaall 44 -- SSoolluuttiioonnss for the answers to these.
To review: A shift to the right in this case would mean that once the new equilibrium is established, the [NH 3 ] would be higher than before, the [N 2 ] and the [H 2 ] would be lower than before.
If the total pressure on this system is decreased , the equilibrium would shift to the left (the side with more moles of gas). This counteracts the imposed change by increasing the pressure. To see what happens when we change the total volume of the container we must remember that:
Increasing the volume of a closed system with gases will decrease the pressure. (the molecules have more room so they exert less pressure on the sides of the container)
So, given changes in volume of the container, just remember that the changes in pressure are just the opposite. For example: Given the equilibrium equation: N2(g) + 3H2(g) 2NH3(g)
1 + 3 = 4 moles ofgas on this side. 2moles of gas on thisside.
If the total volume of the container is increased , this means that the total pressure is decreased. The equilibrium will then shift to the side with more moles (to the left in this case), in order to counteract the change and try to increase the pressure again. If the total volume of the container is decreased , this means that the total pressure is increased. The equilibrium will then shift to the side with less moles (to the right in this case), in order to counteract the change and try to decrease the pressure again. Try the following:
- Given the equilibrium equation: N 2 O4(g) 2O2(g) ?+ N2(g) a) If the total pressure of this system is increased, the equilibrium will shift __________ b) If the total pressure of this system is decreased, the equilibrium will shift __________ c) If the total volume of this system is increased, the equilibrium will shift __________ d) If the total volume of this system is decreased, the equilibrium will shift __________
- Given the equilibrium equation: NO(g) + CO2(g) CO(g) + NO2(g) a) If the total pressure of this system is increased, the equilibrium will ______________ b) If the total pressure of this system is decreased, the equilibrium will ______________ c) If the total volume of this system is increased, the equilibrium will _______________ d) If the total volume of this system is decreased, the equilibrium will ________________
Check your answers on page 2 of TuTuttoorriiaall 44 -- SSoolluuttiioonnss.
Shifting Equilibrium and Rate of Reaction
Just a little note here about the difference between rate of reaction and the equilibrium shifting right or left! First of all, a " shift to the left “means that once the new equilibrium is reached, there will be more
reactants and less products. It does not say anything about the rate of the reaction!!
For example, consider the reaction: A + B C + heat If the temperature of this system was increased , the equilibrium would shift to the left.
This does not mean that the rate will be slower! It simply means that a new equilibrium will be reached
which has more A and B and less C. In fact, as we might recall from Unit 1, increasing the temperature always increases the rate of a reaction. (more molecules have the minimum energy necessary for an effective collision.) Increasing the temperature just causes equilibrium to be reached faster. Try these:
- Given the equilibrium equation: X + Y + heat Z a) Increasing the temperature will ______________________ the rate of reaction. b) Increasing the temperature will cause the equilibrium to shift ___________________ c) Decreasing the temperature will ______________________ the rate of reaction. d) Decreasing the temperature will cause the equilibrium to shift ___________________
- Given the equilibrium equation: N 2 O(g) ?+ NO2(g) + heat 3NO(g) What effect will each of the following changes have on the equilibrium partial pressure of NO? (The first question is done as an example.) a) the [N 2 O] is increased ......... The partial pressure of NO increases because the equilibrium shifts to the right. b) the total pressure of the system is decreased .............. _________________________ c) the temperature is decreased...................................... _________________________ d) the partial pressure of NO 2 is decreased................... _________________________ e) more NO 2 is added ................................................... _________________________ f) a catalyst is added ..................................................... _________________________ g) the temperature is increased ...................................... _________________________ h) the total volume of the container is increased ............. _________________________
Check the answers for the Self-Test on page 3 of TuTuttoorriiaall -- 44 SSoolluuttiioonnss..
