Chem II Chapter 14
Px or Partial Pressure units
atm (atmospheres)
dynamic
chemical reactions can proceed in both the forward and reverse direction
Intermediate value of K
comparable amounts of products and reactants at equilibrium
For an exothermic reaction, K _________ as T __________, so that the reaction must shift left until the lower value of K is achieved
decrease; increases
if heat is a reactant, the reaction is __________
endothermic
The amount of solid/liquid product or reaction does not affect the _________.
equilibrium
Kc
equilibrium constant in concentration units
Kp
equilibrium constant in partial pressure units
If Q=K, the system/reaction is at ____________.
equilibrium.
Mass action expression: definition
equivalent to equilibrium constant expression, but applied to reaction mixtures that may or may not be at equilibrium; the ratio of concentrations or partial pressures of products over reactants, each raised to a power equal to the stoichiometric coefficient for that substance in the balanced chemical equation
if heat is a product, the reaction is __________
exothermic
Very Large K
favors formation of products
Very Small K
favors formation of reactants
Density
g/L
Molar Mass
g/mol
Given the equation: N2 + 3H2 <-> 2NH3 and enthalpy = -92.2 kJ/mol. Which way will the reaction shift?
heat is added to the products side because the reaction is exothermic (negative enthalpy) therefore the reaction will shift left, toward the reactants
the magnitude of K indicates
how far a reaction proceeds to achieve equilibrium
For an endothermic reaction, K ________ as T ___________, so the the reaction must shift right.
increases; increases
Effects of Temperature on Reactions: how to think of heat
increasing temperature adds heat to a certain side of the equation; think of heat as a reactant or product; adding heat to the left side shifts it right, adding it to the right side, it shifts left
decreasing temperature shifts a reaction to the
left
If Q>K, the reaction proceeds to the _______, favoring the _________.
left; reactants
Density x Molar Mass =
mol/L (Molarity)
Another way to establish which way a reaction will shift when increasing/decreasing pressure/volume, determine which side has more _______ in the gas phase.
moles
concentration units
moles/liter
n/V
moles/liter (Molarity)
How to calculate K when 2 equations have been added together to get a net equation?
multiply the two K's together to get the overall K, because each reaction will have a different equilibrium constant K (K3=K1 x K2)
If a catalyst is used, is the position of equilibrium effected? Is the value of K affected?
no; no
delta n; R; T
number of moles of gaseous products minus the number of moles of gaseous reactants; Gas Constant; Temperature (K)
Changing volumes will change ________ __________, changing Q relative to K.
partial pressures
A large value of K (K > 1) indicates that the equilibrium reaction favors the _________
products (lies to the right)
For Heterogenous Equilibria, ___________ are not included in the mass action expression at a given temperature. Explain why/how.
pure solids and/or liquids; the concentrations are constant and the concentration will only depend on gaseous species
How to calculate K when the equation is multiplied by a number
raise K to the power of what the equation was multiplied by
Take the equation: H2O + CO <-> H2 + CO2. What would happen if CO2 was removed? What is the equilibrium constant K? What is Q compared to K after CO2 is removed? Which direction does the reaction shift?
rate of reveres reaction decreases; reaction proceeds in forward direction to establish new equilibrium; K = [CO2][H2]/[CO][H2O]; Removing CO2, Q<K, therefore the reaction shifts right, toward the products, to establish equilibrium.
take the reaction 2N02 <-> N204. what is the forward rate? reverse rate? equilibrium equation? equilibrium constant K?
ratef=kf[NO2}^2 rater=kr[N204] kf[NO2]^2=kr[N2O4] kf/kr = [N2O4]/[NO2]^2
equilibrium constant expression: definition
ratio of equilibrium concentrations or partial pressures of products to reactants, each term raised to a power equal to the coefficient of that substance in the balanced chemical equation
A small value of K (K < 1) indicates that the equilibrium reaction favors the __________
reactants (lies to the left)
Increasing temperature shifts a reaction to the
right
If Q<K, the reaction proceeds to the __________, favoring the ________.
right; products
When given multiple experiments and multiple initial concentrations, different experiments and concentrations can be put into the equilibrium constant K equation. When the system achieves equilibrium, regardless of the initial conditions, the value of the equilibrium constant is the ________ for all experiments.
same
chemical equilibrium
the condition when both the forward and reverse reactions are occurring at the same rate; the point in the reaction when the concentrations of products and reactants remain constant over time
Take the equation: N2+ 3H2 <-> 2NH3. If we increase the volume of the container on the left side, what happens to Q compared to K/which way will the reaction shift?
the decrease in the denominator is greater so that Q > K and the reaction must proceed to the left; if we increase the volume, the reaction will shift in a direction that will produce a greater number of moles of gas to occupy the extra volume. in this case, there are 4 moles on the left side and 2 moles on the right, therefore shifting left will produce more moles of gas.
Equilibrium constant K: definition
the numerical value of the equilibrium constant expression of a reversible chemical reaction at a particular temperature
Law of Mass Action: definition
the principle relating the balanced chemical equation of a reversible reaction to its mass action expression or equilibrium constant expression
Homogeneous equilibria
when all products and reactants are in the same phase
how to tell equilibrium on a reaction rate vs time graph
when the forward rate and reverse rate equal each other and form a horizontal line
how to tell equilibrium on a concentration vs. time graph
when the products and reactants are both in a horizontal line
Heterogeneous equilibria
when the reactants and products are in more than one phase; these are treated differently than homogeneous equilibria
Le Chatelier's Principle
"A system at equilibrium responds to stress in a such a way that is relieves its stress."; Factors that will change the relative rates of the forward/reverse reactions or change the value of Q compared to K will cause a shift in the position of equilibrium.
How to write the equilibrium constant expression for Kp when given Kc, a balanced chemical equation, and each component in the gas phase.
1) Use law of mass action to write equilibrium constant expression as partial pressure of products over the partial pressure of reactants, each raised to the stoichiometric coefficient. Kp = (Pressure of products)^coefficient / (pressure of reactants)^coefficients. 2) Use equation Kp=Kc(RT)^ delta n. delta n is total number of product moles - total number of reactant moles. R = 0.0821 L atm/mol K. 3) Solve for K, and remember no units. 4) figure out which side is favored based on the K value. (K>1 favors products, K<1 favors reactants).
How to determine if a reaction is at equilibrium when given a balanced chemical equation, the value of K, and some initial concentrations of products and reactants?
1) derive the mass action expression from the balanced reaction and substitute the initial concentrations of products and reactants into the expression. 2) solve for Q. 3) Compare Q to K. If Q>K, the reaction favors the reactants. If Q<K, the reaction favors the products. If Q=K, the reaction is at equilibrium.
Given the equation 2HI <-> H2 + I2, initial concentration of HI = 0.100 M, and Kc = 0.11, calculate the concentration fo all species at equilibrium.
1) set up a rice table with initial concentrations, change (negative for reactants, positive for products + number of moles/coefficient), write out equilibrium equation from the change equation. 2) write out Kc = x^2/(0.100-2x)^2 = 0.11, solve for x to get 0.01994. 3) use the value of x to plug into the equilibrium equations for each reactant/products, to get H2 = I2 = 0.01994 M and Hi = 0.06012 M
How to Calculate K when given a balanced chemical equation, temperature, and equilibrium concentrations of products and reactants
1) substitute concentrations into the equilibrium constant equation K=[products]^coefficient / [reactants]^coefficient. 2) solve for K and determine whether reactants or products are favored.
Calculating K, we can determine what two things
1) whether a reaction has reached equilibrium (Q vs. K). 2) what the equilibrium concentrations of the reactants/products will be.
Decreasing the volume by a factor of 2 will increase the partial pressure by _____.
2
Response of the Reaction 2A<->B: How does the system respond if you Add B and what direction does it shift?
Consumes B and shifts left
Response of the Reaction 2A<->B: How does the system respond if you Add A and what direction does it shift?
It consumes A and shifts to the Right
Response of the Reaction 2A<->B: How does the system respond when you increase the volume of the reaction mixture?
It increases the moles of gas, shifting to the side with more moles, which is left.
Response of the Reaction 2A<->B: How does the system respond when you compress the reaction mixture?
It reduces the moles of gas, shifting to the side with less moles, which is right
The value of Q indicates how a reaction is proceeding. What does this mean?
It tells if the equation is at equilibrium or not.
At equilibrium, Q = what?
K
Changing the Temperature also changes ______
K
K for reverse reactions
K forward = 1/K reverse
given the equation H2 + I2 <-> 2HI, with initial concentration of H2 = 1.00 atm, I2 = 1.03 atm, and HI = 0.0 and Kp=50.2; write out the Kp equation, determine the partial pressures of reactants and products when the system achieves equilibrium?
Kp = (P HI)^2/(P H2)(P I2); initial conditions (I) - H2 = 1.00, I2 = 1.03, HI = 0.0; changes (C) = H2 = -x, I2 = -x, HI = +2x; Equilibrium equation (E) = H2 = 1.00 -x; I2 = 1.03 -x; HI = 0.0 + 2x; plus these values into the Kp equation, use the quadratic equation and solve for two answers, usually the answer that is higher than the initial conditions to get P H2 = 0.21, P I2 = 0.24 and P HI = 1.58.
Take the equation: 2NO2 <-> N2O4. what happens to Q if we decrease the volume by a factor of two, increasing the pressure by 2? Write out the equation for Kp and Qp.
Kp = (PN2O4)/(PNO2)^2 = y/x^2 Qp = 2Y/4X^2 = 1/2Kp. The net result is that the value of Q is now less than K, so that the reaction must proceed to the right to establish a new equilibrium position.
Equilibrium Constant Equation in the Gas Phase (determined from Kc)
Kp=Kc(RT)^delta n
units of gas constant R
L atm/mol K
Reaction quotient (Q): definition
Numerical value of the mass action expression for any values of concentrations (or partial pressures) of the reactants and products
Ideal Gas Law
PV=nRT
Response of the Reaction 2A<->B: How does the system respond if you Remove A and what direction does it shift?
Produces A and shifts to the left
Response of the Reaction 2A<->B: How does the system respond if you Remove B and what direction does it shift?
Produces B and shifts right
Take the equation: N2+ 3H2 <-> 2NH3. If we decrease the volume of the container on the left side, what happens to Q compared to K?
Q<K, the reaction will shift to the right; decreasing the volume on the left will cause the reaction to shift right in order to make less moles of gas.
RICE table: What does R, I, C, E stand for?
R = balanced chemical reaction equation; I = initial conditions (concentrations or pressures); C = changes as a system moves to equilibrium; E = equilibrium values
If the reactants are very small, the value of K is extremely small. Using this assumption, for the RICE table, how can you solve it more simply?
You can ignore the x's in the denominator (the reactants) and solve for x, assuming the the initial concentrations of reactants have not changed significantly
equilibrium constant K equation
[products] / [reactants]
Adding a catalyst decreases the _______ _________ for both the forward and reverse reactions, increasing the overall _________.
activation energy; rate
Stress: definition
any condition that changes the relatives rate of the forward/reverse reactions, or changes the value of K, so that the reaction must proceed in either the forward or reverse direction to reestablish equilibrium.