Try to keep your balance! The episode starts with an analogy of villagers commuting from Moleville to Chemtown.
Try to keep your balance! The episode starts with an analogy of villagers commuting from Moleville to Chemtown (0:59). Tying it to a chemical reaction, we define equilibrium as the point where the rate of the forward and the rate of the reverse reaction are the same (1:43). To determine if, at equilibrium, a reaction has more reactants or products, we can calculate the equilibrium constant, capital K (3:59) and write the equilibrium expression (4:34). The magnitude of K tells us if a reaction favors the products or the reactants (5:35). To determine if a reaction is at equilibrium, we can calculate Q (6:12).
Question (8:14): The equilibrium constant for a known reaction is K = 150. What would the value for K’ be if you’d reverse the reaction at the same temperature?
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap the equilibrium constant.
Lets Zoom Out:
Unit 7 - Equilibrium
Topic 7.1 - 7.5
Big idea - Transformations
Introduction:
When we hear the term equilibrium, we often use “balance” as a synonym. But what is balanced during equilibrium? In chemistry, using “balance” can actually mislead us and cause us to lose our footing! Listen to this episode to make sure you keep one foot in front of the other! Chemically speaking that is.
Let’s zoom in:
Imagine two villages: Village “Moleville” has 100 residents, village “Chemtown” has 200 residents. Every day, 50 villagers leave Moleville to drive to Chemtown. It starts slowly, with some commuters at 06:45 am. By 08:30 am, we are at rush hour! A continuous stream of 30 residents drives from Moleville to Chemtown, but also 30 residents return and drive from Chemtown to Moleville, throughout the entire day. The amount of people that are in Village Moleville and in Chemtown is constant: 70 in Moleville and 170 in Chemtown. This is equilibrium! What?
Okay, let’s look at it from a chemical perspective: Many chemical reactions are reversible and occasionally the forward and the reverse reaction happen at the same time. But let’s start at the beginning: The reactants react to form products. So the concentration of our reactants, our Moleville, slowly decreases. But, as soon as some of the product is formed, the products react to re-form the reactants. Our villagers turn around and drive back from Chemtown to Moleville. Eventually, we reach equilibrium: the rate of the forward and the rate of the reverse reaction are the same! The amount of people that are on the road between Moleville and Chemtown and the number of people that are driving from Chemtown to Moleville are the same. The concentration of products and reactants can be, but in most cases, isn’t the same! And this is one pitfall in understanding equilibrium: We often think the concentrations are the same, but it is the rate of the forward and reverse reaction. The concentrations, however, remain constant. The number of people in either village at rush hour isn’t the same either, but, the number of people remains constant - even though they are different individuals.
To determine if, at equilibrium, a reaction has more reactants or products, we can look at the ratio between the concentrations of products and reactants: the equilibrium constant, capital K. Often, it is also followed by a subscript c or p, indicating either the use of concentration in terms of mols/liter for solutions or the use of partial pressures for gases. Reminder: pure liquids and solids are NOT part of the equilibrium constant, since they assume the value of 1. The equilibrium constant is dependent on temperature as well as the stoichiometric coefficients. Therefore, the equilibrium expression for a general balanced equation like aA + bB yields cC + dD would be: Kc equals the concentration of substance C to the coefficient c times concentration of substance D to the coefficient d divided by the product of the concentration of A to the coefficient a and the concentration of B to the coefficient b. Sounds complicated? Simplified it is the concentration of products to their coefficients divided by the concentration of reactants to their coefficients. Note: if you are using concentrations, you write the expression - the chemical formula - in brackets. If you are using pressures, you use parentheses.
If you have experimentally determined the concentration of all species at equilibrium, then you can plug and play: You can put them in the equilibrium expression and determine the equilibrium constant. Since you are dividing the products by the reactants, an equilibrium constant with a value greater than 1 means you have a higher concentration of products than reactants at equilibrium and vice-versa. Generally speaking, if you have a reaction with a very large K, the reaction usually goes to completion. And if you have a reaction with a tiny K, the reaction barely proceeds.
Have you ever been on a road trip with a younger sibling? The most common (and most annoying) question is: ARE WE THERE YET? We can also ask this for chemical reactions: Knowing the equilibrium constant for a reaction at a certain temperature also allows us to determine if the reaction has reached equilibrium yet. By measuring the concentrations or partial pressures we can determine the reaction quotient Q for the reaction. Whereas for K we plug in the concentrations or partial pressures AT equilibrium, for Q we can perform the same calculations, products over reactants taking into account the coefficients, at ANY TIME of the reaction. By comparing Q to K, we can now determine if the reaction is already at equilibrium or moving towards equilibrium. If Q equals K, we are at equilibrium. If the value of Q is larger than K it indicates we have more products at that point than we have at equilibrium, since we are dividing products by reactants. That means, our reaction proceeds to form more reactants and use up some of the product. Vice-versa, if our Q is smaller than our K we do not have enough product yet and the reaction proceeds in the forward direction.
To recap:
Many chemical reactions are reversible and occasionally the forward and the reverse reaction happen at the same time. A reaction reaches dynamic equilibrium when the rate of the forward reaction and the rate of the reverse reaction are the same. To determine if, at equilibrium, a reaction has more reactants or products, we can look at the ratio between the concentrations of products and reactants: the equilibrium constant, capital K. An equilibrium constant with a value greater than 1 means you have a higher concentration of products than reactants at equilibrium. Calculate Q to determine if the reaction has reached equilibrium yet.
Coming up next on the APsolute RecAP Chemistry Edition: Le Chatelier's Principle
Today’s Question of the day is about the equilibrium constant.
Question: The equilibrium constant for a known reaction is K = 150. What would the value for K be if you’d reverse the reaction at the same temperature?