Lecture Notes
These notes should act as supplements to the lecture material presented during weekly meetings. Great for catching up on make up work or reviewing previous material.
Equilibrium, as defined by the Webster Dictionary, is "a state in which opposing forces or actions are balanced so that one is not stronger or greater than the other." This same definition applies to chemical equilibrium systems. For any chemical reaction, reactants react to form products. However, those same products can also react to recreate the reactants. Both of these reactions - the original forward reaction and reverse reaction - have some amount of "chemical force of favorability," meaning that due to the nature of the bonds between the molecular species, the formations of either the products or the reactants both have some sort of drive to try to achieve the state of lowest potential energy, also known as balance or equilibrium. Chemical equilibrium is also called a dynamic equilibrium. This means that after the state of lowest potential energy, the molecules don't just stop reacting. The forward and the reverse reactions are just occurring at equal kinetic rates such that there is no net change in the concentration of any of the molecular species. aA + bB ⇌ cC + dD where A, B, C, and D are the molecular species and where a, b, c, and d are the respective coefficients. This notation actually represents two different reactions occurring at once:
Combustion reactions are very common. The combustion of propane, a hydrocarbon, is a common reaction, as propane is common fuel used for BBQs. The reaction can be represented as an equilibrium system: C3H8 + 5O2 ⇌ 3CO2 + 4H2O This means that the combustion of propane (C3H8) does occur, but theoretically, the reverse reaction, where carbon dioxide and water vapor react to form propane and oxygen gas, does occur, too. However, this reverse reaction occurs to such a small, infinitesimal extent that it has pretty much no impact on the concentrations of species, and the reaction occurs in the forward direction basically to completion. That is why the reaction is most commonly (and should be) represented as C3H8 + 5O2 → 3CO2 + 4H2O. However, even though the reaction occurs to completion (meaning all of the reactants react completely following stoichiometric laws), this can still be argued to be an equilibrium system. This combustion reaction demonstrates a few very important point in chemistry:
For the reaction aA + bB ⇌ cC + dD, the Law of Mass Action is the Keq mathematical expression shown above. Essentially, its a ratio between the product of concentrations of the products raised to their respective coefficients and the the product of concentrations of the reactants raised to their respective coefficients. The concentrations of reactants and products that you plug into the Keq Law of Mass Action expression are the concentrations at the equilibrium position. The law of mass action gives chemists a lot of information about equilibrium systems. For example, if the calculated Keq expression gives a number greater than 1, then the forward reaction to produce the products C and D are chemically favored. If the calculated Keq expression gives a number less than 1, then the reverse reaction to produce the reactants A and B are chemically favored. What determines chemical favorability? The answer literally lies within the bonds of the molecular species. Each bond between two atoms has a characteristic average amount of potential energy. The equilibrium state occurs when the bonds of the reactants and the bonds of the products have the same amount of potential energy stored. In this sense, a reaction is chemically favorable if it is conducive towards reaching that state of equal potential energy. A common misconception is that equilibrium always occurs when the number of products equals the number of reactants. Nothing could be further from the truth. Equilibrium is a measure of potential energy state, meaning that at equilibrium, potential energy is equal. The products and reactants will almost always have differing concentrations to achieve this state of total minimization of potential energy within the chemical system. Le Chatelier's Principle is very important when it comes to equilibrium. Its states that whenever one applies stress to an equilibrium system, the system will move in the direction to relieve the stress. This means that if you add some additional molecular species, the reaction will either move in the forward or reverse direction by either consuming or producing products to reestablish the same ratio of reactants and products. In essence, the Keq value always remains the same no matter what, except for temperature changes. For example, let's say I have a machine that can turn pens into pencils and vice versa. In my school supplies case, I am very picky and like to always have a constant ratio of 2 pencils per every 3 pens in my case. Currently, I have 10 pencils and 15 pens. Now, one day, my parents give me a birthday present of 10 more pencils. Now I have 20 pencils and 15 pens, but this upsets me because the ratio is now imbalanced. Because I have received an influx of pencils, the additional number of pencils places stress on my case/system, which can be relieved by consuming some pencils and producing pens. In this case, if I use my machine to convert 6 pencils into 6 pens, I will now have 14 pencils and 21 pens, thereby reestablishing my desired ratio and making me happy again! Le Chatelier's Principle is all about preserving a constant ratio between species. The amounts of products and reactants are always subject to change in a volatile environment, but what's important is preserving the ratio, or Keq value, of an equilibrium system. That's all of the conceptual background behind equilibrium and Le Chatelier's Principle. Remember, all reactions are systems of dynamic equilibrium that strive to preserve balance through a constant ratio of products and reactants and a drive to find the point of lowest potential energy.
More information about the math behind all of this can be found in this week's handout or at this link: Equilibrium Calculations.
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