Equilibrium Flashcards
(23 cards)
Equilibrium characteristics
- Reverse rate is equal to forward rate
- concentrations of products and reactants are constant
- rates are constant
- no macroscopic (visible) changes occur
- Only temperature affects equilibrium, no concentration of reactants/products
Ratio of moles and reaction rates
The ratio of total products to reactants does not change when the system is at equilibrium, but the rate must be in the mole ratio
Spontaneous change
When a change occurs by itself without outside assistance. Favors the side with minimum energy (minimum enthalpy) because molecules tend to go to or remain at minimum energy.
Trial keq
if Q<equilibrium concentrations, then there are too many reactants and equilibrium shifts to making more products
If Q>equilibrium concentration, then there are too many products and equilibrium shifts towards reactants
Le chatalier principle
if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change to reestablish an equilibrium.
If forward rate decreases, reverse rate will increase and vice versa.
equation constant expression
[products]/[reactants]
Pressure equilibrium
(pressure product)/(pressure reactants)
Solids and pure liquids
solids are not included in equilibrium expressions because they have constant concentration. Pure liquids are the same, but they must appear on both sides of the entire equilibrium equation. Water is never included in equilibrium equations.
Effect of temperature changes on equilibrium
Removing heat = the reaction shifts to produce more heat on the side that contains heat term. Adding heat causes the equilibrium to use up more heat on the side that contains heat.
Effect of temp changes on rate
If the system is at equilibrium, increasing the temperature will increase the rate of both reactions (forward and reverse) but the rate of the endothermic reaction will be increased by a greater extent (LeChatelier’s Principle). The system will, therefore, temporarily shift in the direction of the endothermic reaction until a new equilibrium is reached with a new Ke for the new temperature.
The opposite is true for a decrease in temperature. Reaction rates, generally, slow down at lower temperatures. In a reversible system, when the temperature is reduced, the rate of the endothermic reaction will be more dramatically impacted than that of the exothermic reaction (as a result, even though it was slowed down, the rate of the exothermic reaction will be temporarily faster). So, the system will temporarily shift in the direction of the exothermic reaction to establish a new equilibrium.
Effect of concentration changes
When the forward reaction is favored (aka reactants are added), the concentrations of products increase, while the concentrations of reactants decrease and vice versa
Effect of pressure changes
Decreasing volume increases pressure and concentration of all gases, and the reaction will lower the pressure by shifting to the side with the least number of gaseous molecules.
effect of a catalyst
speeds up forward and reverse rates by equal amount so that reaction at equilibrium remains at equilibrium
graphical effects of changing stuff
temp: no sudden change in concentrations, concentrations change slowly to new value
concentration: very sudden jumps in concentration
changing pressure: all species simultaneously jump up or down
changing keq
if exothermic: decreasing temp = increase in products = decrease in reactants = increase in keq bc equilibrium shifts to produce more heat on side with heat term (products)
if endothermic: decreasing temp = decrease in products = increase in reactants = decrease in keq bc equilibrium shifts to produce more heat on side with heat term (reactants)
Large keq = large amount of products
small keq = small products
Adding molarity doesn’t change keq, only changing temp does
2 central tendencies in a chem system
- Tendency to minimum enthalpy, aka tendency to release heat energy, favors side of the reaction containing the heat term (the heat is always on the downhill side)
- The tendency to maximum entropy, aka the tendency to increased disorder. The most random side (if both sides have equally random phases, the side with the most molecules is more random) is the favoured side
Entropy phase considerations
gas»solutions>liquid»solid
Randomness
The spreading out of energy
how to predict an equilibrium
an equilibrium is formed when enthalpy and entropy favor opposite sides of the reaction. Use the minimum enthalpy (side with heat term) and maximum entropy (most particles of most random phase) to determine this.
If enthalpy and entropy favour the same side, the reaction is spontaneous
how to predict a reaction
Negative entropy = no reaction
Decrease in enthalpy = reaction
Increase in entropy = reaction
entropy
measure of how spread out energy is/the amount of randomness in a system
endothermic vs exothermic energy requirements
Exothermic reactions occur spontaneously as long as activation energy is supplied, but they do not always go to completion (equilibrium will occur)
Endothermic reactions, however, cannot occur spontaneously if sufficient heat is not available, and must be forced.
hill analogy for spontaneous
Molecules on the downhill side of the energy hill will stay unless extra energy is given, and molecules on the uphill side will roll down the hill with enough activation energy.
