Book: UTC: 17 Flashcards
What is the distinction between the speed (rate) and the extent of a reaction?
While the kinetics, or rate of reaction, assesses the change in concentration of a species over unit time, equilibrium assesses the concentration of species after an infinite amount of time, or when no more change is occurring.
Under what conditions does a system attain dynamic equilibrium?
At equilibrium, reactant and product concentrations are constant because a change in one direction is balanced exactly by a change in the other as the forward and reverse rates are equal. No overall change in concentration is occurring, but the system is considered dynamic as the processes to convert reactant into product as well as the reverse process are ongoing.
What is the equilibrium constant equal to?
K = k_fwd / k_rev. Since both of these reaction rates are proportional to the concentration of the species respectively, K = [products at eq.] / [reactants at eq.]
How is the magnitude of K related to the extent of the reaction?
K is a measure of reaction extent because it indicates how far a reaction proceeds towards product at a given temperature. e.g., K small if reaction yields very little product, and it is large if the reaction yields mostly product.
Is the same equilibrium state reached regardless of the starting concentrations of the reacting system? Explain.
Yes, the reaction quotient (ratio between concentrations of products and reactants at a given instant of time) depends on the rate of change for the system as a whole. As a differential equation, it is best viewed as having a horizontal asymptote at t = infinity, and the value of Q at that limit K. This value will not end up depending on the particular initial conditions.
How does the reaction quotient (Q) change continuously until the system reaches equilibrium, at which point Q = K?
If Q > K, then the rate of the forward reaction will be higher than the reverse reaction, so the reaction will shift to lower Q. Conversely, if Q < K, the reverse reaction will be faster than the forward reaction, and Q will increase. This will happen until Q = K.
Why is the form of Q based exactly on the balanced equation as written?
Since the exponents of the components of Q are dependent on the coefficients of its corresponding chemical equation, altering these coefficients, even by multiplying both sides by the same factor, for instance, will alter the value of Q.
Why do terms for pure solids and liquids not appear in Q?
These factors don’t interact with the system in the same way, but they do “appear” in the equation. They are just always equal to 1.
How is the overall Q found from a number of Q’s from equations added together to give an overall equation?
Multiply all the Q’s together to get Q overall.
How does the interconversion of K_c and K_p work?
K_p = K_c (RT)^(∆n_gas)
How does the reaction direction depend on the relative values of Q and K?
A net reaction will occur to shift Q in the direction of K as long as the two values are not equal.
How is reaction table used to find an unknown quantity (concentration or pressure)?
Write the equation at the top as a row with each of its components as columns. The next row is [ ] initial. “Let x moles dissociate” until equilibrium is reached and write the changes in concentration in the next row as [ ] change. The final value will be initial + change and written in the next row as [ ] equilibrium.
How does assuming that the change in [reactant] is relatively small simplify finding the equilibrium quantities?
It makes a quadratic expression linear, allowing for such quantities to be solved for without use of the quadratic formula.
How does Le Chatelier’s principle explain the effects of a change in concentration, pressure (volume), or temperature on a system at equilibrium and on K?
Changes in concentration shift the equilibrium position but they do not alter K. Temperature changes will alter K, and pressure changes will not affect K either. Although K remains unchanged, however, the values of individual concentrations at equilibrium are capable of altering in any dimension so long as K stays the same.
Why a change in temperature does affect K.
A change in temperature is best viewed as addition of another component (heat) which creates an excess either on the left side (endothermic reaction) or on the right side (exothermic).
