6: Equilibrium

Question 1

dynamic equilibrium

Answer 1

forward and reverse reactions occur at the same rate… no net change in concentrations of products or reactants

Question 2

entropy

Answer 2

measure of the distribution of energy throughout a system or between a system and its environment

Question 3

at equilibrium…

Answer 3

reaction will reach equilibrium when the system’s entropy is at a maximum or ΔG is at a minimum

Question 4

law of mass action

Answer 4

if a system is at equilibrium at a constant temperature, then following ratio is constant

K_f/K_r = K_c = K_eq

Question 5

equilibrium expression

Answer 5

for aA + bB ⇔ cC + dD,

K_c= [C]^c[D]^d / [A]^a[B]^b

*exponents equal stoichiometric coefficients UNLIKE in rate laws

Question 6

reaction quotient, Q

Answer 6

“timer” to indicate how far the reaction has proceeded toward equilibrium… utility of Q is comparison that can be made between Q at any moment and known K_eq for reaction at a particular temperature

Question 7

Q and K_eq

Answer 7

• Q < K_eq: ΔG -, forward reaction has not yet reached equilibrium
  
  • forward rate of reaction is increased
• Q > Keq: ΔG +, forward reation has exceeded equilibrium
  
  • rate of reverse reaction increased to restore equilibrium
• Q = K_eq: ΔG=0, reaction is at dynamic equilibrium

Question 8

things to remember about equlibrium/law of mass action

Answer 8

• concentration of pure solids and pure liquids do not appear in the equilibrium constant expression
• K_eq is temperature dependent
• larger K_eq leads to farther right equilibrium position
• if K_eq is for forward reaction, 1/K_eq is for reverse reaction

Question 9

equilibrium equations

Answer 9

• K_eq=[products]/[reactants]
• K_eq with a large positive exponent indicates a reaction that goes almost to completion
• K_eq with a large negative exponent indicates that the amount of reactant that has reacted is neglibible compared to the amount of reactant that remains *use this instead of ICE

Question 10

le chatelier’s principle

Answer 10

if a stress if applied to a system, the system shifts to relieve that applied stress

• stress can be change in concentration, pressure, volume, temperature, etc.

Question 11

change in concentration

Answer 11

changing concentration of either product or reactant results in Q_c ≠ K_eq

Question 12

changes in pressure (and volume)

Answer 12

only chemical reactions w/ at least one gaseous species will be affected by pressure and volume

• when a system is compressed, increase in total pressure is correlated with increase in partial pressures of each gas in the system
• when pressure is increased, system moves toward whichever side has the lower total number of moles of gas… due to ideal gas law, which states that there is direct relationship between number of moles of ags and pressure of gas

Question 13

changes in temperature

Answer 13

changing temperature changes K_eq, not Q_c (like pressure and concentration)

• direction system moves to restore equilibrium is determined by enthaply of reaction…
• if reaction is endothermic (∆H>0), heat functions as reactant
• if reaction is exothermic (∆<0), heat functions as a product

Question 14

kinetic product

Answer 14

forms at lower tempertaures (with smaller heat transfer)

• less free energy needed for kinetic product to form
• kinetic product forms faster and is less stable

Question 15

thermodynamic product

Answer 15

forms at higher temperatures (with larger heat transfer)

• greater stability and more negative ∆G
• forms slower and higher stability