Let’s be disruptive: What happens when you disturb the equilibrium of a chemical reaction? Our episode talks you through several different scenarios and their effect on the reaction as well as K and Q.
Let’s be disruptive: What happens when you disturb the equilibrium of a chemical reaction? Our episode talks you through several different scenarios and their effect on the reaction as well as K and Q: We can add reactant and product (2:01) or remove reactant and products (2:58). We can also change the temperature of our reaction and either add heat (4:29) or lower the temperature (5:35). If we have gaseous, we can also change the pressure by changing the volume of the container (5:59).
Question (7:48): Which reaction is favored if you decrease the volume and have the same number of gaseous particles on reactant and product side?
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap Le Chatelier’s Principle.
Lets Zoom Out:
Unit 7 - Equilibrium
Topic 7.9 and 7.10
Big idea - Transformations
In episode 32, we distinguished between equilibrium and being in balance. As a reminder: When a system is in equilibrium, the rate of the forward and the rate of the reverse reaction is equal, the concentration of reactants and products stays constant - but is not necessarily equal. What if I disturb the balance? What if you are balancing on a log and your sibling gives you a shove? How are YOU getting back into balance? This is what we are recapping today: Le Chatelier’s Principle.
Let’s zoom in:
Le Chatelier’s Principle focuses on what happens when you disturb a dynamic equilibrium: It counteracts those changes to reestablish an equilibrium. How can a dynamic equilibrium be disturbed? There are several ways: We can change the concentration of reactants or products by either adding more or removing them. We can also change the temperature by either heating our reaction or cooling it. For reactions that involve gases, we can increase or decrease the pressure by changing the volume. So many options! In all these cases, we are disturbing the equilibrium. We can therefore use our reaction quotient Q and compare it to K to determine in which direction the reaction has to shift to counterbalance the disturbance and reestablish equilibrium. So, let’s discuss them one-by-one.
Scenario 1: We add more reactant. To counterbalance this, the reaction will proceed to increase the rate of the forward reaction. It uses up the added reactant until we are back in balance and have the same ratio of particles of product and reactant. In terms of K and Q, adding more reactant will increase the denominator and therefore make Q smaller than K. As we’ve discussed in episode 32, if Q is smaller than K, the reaction increases the rate of the forward reaction. Similarly, if we add more product, we have to use up those additional particles and produce more reactants to get back to our equilibrium ratio. If you prefer thinking about it in terms of K and Q, we are increasing the numerator when increasing the concentration of products. Therefore, Q is larger than K and the reaction increases the rate of the reverse reaction.
Scenario 2: We remove product, which leads to an increase in the rate of the forward reaction to make up for it! By removing the product, Q is smaller than K, so, again, we are increasing the rate of the forward reaction. Same, but different: If we remove reactant after the reaction has reached dynamic equilibrium, the rate of the reverse reaction will increase to produce more reactant. In both cases, the concentration of reactant and product in the newly established equilibrium will be lower than before.
Scenario 3: We heat our reaction. Our equilibrium constant K is temperature dependent. So, changing temperature will also change the value of K. But in which direction? It depends on if your reaction is exothermic - in which heat is being released, or endothermic - in which heat is being absorbed. In exothermic reactions, an increase in temperature decreases the value of K. In an endothermic reaction increasing the temperature increases K. To simplify this, we can treat heat as a product in an exothermic reaction and as a reactant in an endothermic reaction. Therefore, if I heat my exothermic reaction, I am, so to speak, adding a product. To counteract this, the reaction will increase the rate of the reverse reaction, which absorbs the added heat and therefore shifts towards reactants. Similarly, if I heat an endothermic reaction, I am, kind of, adding a reactant. The reaction will therefore increase the rate of the forward reaction until we have our dynamic equilibrium again.
Scenario 4: Cooling our reaction. In a similar manner, cooling our reaction is like removing a product in an exothermic reaction or removing a reactant in an endothermic reaction. As shown in scenario 2, cooling exothermic reactions therefore leads to an increase of the rate of the forward reaction, whereas cooling an endothermic reaction increases the rate of the reverse reaction.
Scenario 5: We change the volume and therefore the pressure in a reaction that involves gases. We can write our equilibrium expression in terms of partial pressures, and would still have the partial pressure of the products over the partial pressures of the reactants. This is all about how many gaseous particles the forward and reverse reaction are producing. Therefore, when dealing with volume changes, you should first count how many moles of gas you have on the reactant and product side by taking into account the stoichiometric coefficients. And careful: We only care about gaseous substances! When we increase the pressure of a reaction by decreasing the volume of the reaction vessel, the system counteracts this disturbance by lowering the pressure and favoring the reaction that produces LESS gas particles. On the other hand, when we increase the volume, the system favors the reaction that produces MORE gaseous particles.
To recap:
Le Chatelier’s Principle helps us to predict how a system reestablishes its dynamic equilibrium after a disturbance. Increasing the concentration of reactants as well as decreasing the concentration of products increases the rate of the forward reaction. Decreasing the concentration of reactants as well as increasing the concentration of products increases the rate of the reverse reaction. Change in temperature changes the value of K. Heat can be treated as a reactant or product. We have to take into account the number of gaseous molecules to make predictions when changing the volume of a reaction vessel for a reaction that involves gases.
Coming up next on the APsolute RecAP Chemistry Edition: Solubility Equilibria
Today’s Question of the day is about the Gas equilibria.
Question: Which reaction is favored if you decrease the volume and have the same number of gaseous particles on reactant and product side?