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Unit 7

7.0 Equilibrium

5 min readjune 1, 2021

fiveable-dylan

Dylan Black


7.0: Unit Overview

What Is This Unit All About?

In unit 5 we brought up the idea of kinetics, the study of the rate of a reaction. We determined that there are many factors such as concentration and temperature that can increase or decrease the rate of a reaction. In unit 7 which focuses on a new topic called equilibrium, we’ll look at the rate of a reaction going forwards ➡️ and backwards ⬅️. In equilibrium, we discuss reactions that are reversible meaning that not only are products created from reactants but reactants can also be created from products! In fact, nearly every reaction is reversible. The study of equilibrium describes how product favored or reactant favored these reversible reactions are. 
Our essential question of this unit is “how can we determine, both qualitatively and quantitatively, how far forward a reaction goes and how we can impact its equilibrium?”. This question will be answered by looking into theories such as Le Chatelier’s Principle and mathematically by calculating equilibrium constants and equilibrium concentrations. By looking at both the qualitative and quantitative aspects of equilibrium, we’ll be able to get a better view of what actually happens during a reaction and why some reactions may seem not to happen but in fact, only happen a little bit.

So What Exactly Is Equilibrium?

Let’s start this unit out with a broaaaad overview of what equilibrium is all about. In chemistry, we define equilibrium as the point at which the rate of the forward reaction is equal to the rate of the reverse reaction. We learned about rates in unit 5 as how quickly a reaction proceeds. We can also think of this rate as a change in concentration of the products (or reactants) over a change in time: r = Δ[P]/ΔT = -Δ[R]/ΔT. Because of this, our study of equilibrium will primarily deal with changes in concentration up to equilibrium. 
With reverse reactions, we see the same just in the opposite direction! Reversible reactions can be described as two reactions in one: one going forward and the other going backwards. We notate this as reactants ⇌ products, the double arrow indicating to us that both reactions are occurring. Therefore, we have two reactions actually occurring: reactants → products and products → reactants. At equilibrium as we mentioned, the rates of both of these reactions are the same meaning that the change in concentration of the products over time equals the change in concentration of the reactants over time. This tells us two things: first that there are no more changes in concentrations of either (because we are losing and gaining at the same rate) and that equilibrium is not when a reaction “stops!" Equilibrium is a dynamic process by which rates of reactions settle, not stop!
https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-q6lN8cItU4cS.png?alt=media&token=7cd737f6-f0c3-44b0-bb59-4dd67d26aa7c

Image From Siyavula

Take a look at the above image. We see two rates: the rate of the forward reaction in dashed lines and the reverse reaction in the solid line. Over time we can see that these rates become equal. Because they are equal we say that our reaction is in equilibrium. This idea of equal forward and backwards rates drives all of our discussion in this unit! 

Some Key Topics Of This Unit

Let’s take a look at some of the major topics of unit 7 chunked up by section. Note that these chunks are arbitrary and written by myself, not the CollegeBoard!

Unit 7.1 - 7.6: Introduction and Equilibrium Constants

We’ll begin unit 7 with an introduction to the overall idea of equilibrium and what it means for the direction and extent of a reaction. We’ll start by discussing this qualitatively and theoretically by talking about rates of reactants similar to what we just discussed in the last section and then move into calculations of equilibrium constants. These are numbers that help us describe how far forward a reaction goes. A reaction can be product favored or reactant favored (or there is the option of neither but that only happens in a very specific case. Stay tuned!). You’ll learn how to calculate the constant itself and how to manipulate reactions to find them. Another major topic you’ll be introduced to is the reaction quotient, Q, which helps describe how a reaction will adjust back to equilibrium.

Unit 7.7 - 7.10: Equilibrium Concentrations and Shifting Equilibrium

Once we have a strong foundation on equilibrium as a whole, we can start discussing how to find concentrations at equilibrium. This is where the unit gets fun! In 7.7 you’ll start using ICE Boxes, a tool that you’ll carry with you for the rest of your chemistry career (which of course will last a lifetime because you’re going to study chemistry forever, right? right?). These will be used to find what the concentrations of both reactants and products are at equilibrium by using equilibrium constants. You’ll also learn about Le Chatelier’s Principle, which helps describe how reactions return to equilibrium from an external change like changes in concentrations or temperature. Le Chatelier’s acts similar to a seesaw!
https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-jIvH5JZf3Z4B.png?alt=media&token=acda5ab4-7dd3-4fbc-b624-2691093e5e27

Image From SlideShare

Unit 7.11 - 7.14: Solubility Equilibria

Finally, we’ll apply our newfound expertise on equilibrium to a topic that we learned all the way back in unit 4: solubility! In unit 4 we discussed precipitation reactions and that certain compounds are insoluble. We typically describe insolubility as the quality of a substance to not breakdown in a solvent at all. However, we’ll find that insoluble compounds do dissolve, it’s just that their equilibrium constants are suuuper small so very little ends up dissolving. We’ll discuss solubility and how the equilibrium between a dissolved solute and the solid precipitate can be impacted by things such as common ions and pH.

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