Equilibrium Flashcards
(27 cards)
Reversible Reactions
Some reactions go in one direction to COMPLETION, and are not reversible
Some reactions are reversible, and both reactants and products are formed simultaneously
What is chemical equilibrium??
Mix reactants A and B
With time C and D will start to form A + B C + D
As more C and D form, reverse reaction will start up. A + B C + D and More A and B will form
Eventually, an equilibrium will be reached (if all other conditions remain the same ) A+B C+D
of products and reactants reach levels where the rate of the forward reaction equals the rate of the reverse reaction.
Equilibrium is defined as when rate of forward reaction = rate of reverse reaction
As a result, the equilibrium constant, K, can be calculated from the rate constants, i.e., K=kf/kr
where kf is rate constant for forward reaction and kr is rate constant of reverse reaction
Equilibrium Does NOT Mean Equal Amounts of Reactants and Products Exist
The rates of the forward and reverse reactions are equal at equilibrium.
But that does not mean the concentrations of reactants and products are equal.
Some reactions reach equilibrium only after almost all the reactant molecules are consumed: We say the position of equilibrium favors the products.
Other reactions reach equilibrium when only a small percentage of the reactant molecules are consumed: We say the position of equilibrium favors the reactants.
Favored Direction of equilibrium
If the reaction favors the products within the chemical reaction, we say that the equilibrium position lies to the right, (because the products are written on the right side of the arrow).
Conversely, the reaction favors the reactants, one can say that the equilibrium position lies to the left.
The Equilibrium Constant
The relationship between the chemical equation and the concentrations of reactants and products is called the law of mass action.
Consider the following reaction at equilibrium:
jA + kB ⇋ lC + mD
Kc = [C]^l [D]^m / [A]j [B]k
A, B, C, and D = chemical species
Square brackets = concentrations of species at equilibrium
j, k, l, and m = coefficients in the balanced equation
Kc = equilibrium constant
SOLIDS and LIQJIDS are NOT ever included in a K expression.. So, always pay attention to states of matte when setting up a calculation for K.
Equilibrium constant important details
- If the reaction is reversed, the reciprocal of the old K value is taken to be the new K value.
- If the balanced equation for a reaction is multiplied by a new factor X, the new K value will be (Kold)^x.
Additional Relationships Between K and Chemical Equations
When you add equations to get a new equation, the equilibrium constant of the new equation is the product of the equilibrium constants of the old equations.
For the reactions (1) aA bB and (2) bB cC, the equilibrium constant expressions are as follows:
K1 = [B]^b / [A]^a K2 = [C]^c / [B]^b
For the reaction aA cC, the equilibrium constant expression is as follows:
or
K1 = [B]^b / [A]^a times [C]^c / [B]^b K3 = [C]^c / [A]^a
Therefore, K3 = K1 × K2
Calculate Kc for 2HI(g) ⇌ H2(g) + I2(g)
Given that at equilibrium: [HI] = 0.777M, [H2] = 0.233M, and [I2] = 0.233M
Kc = [H2] [I2] = (0.233) (0.233) = 0.0899
(0.777)2
Calculate Kc for 4HI(g) ⇌ 2H2(g) + 2I2(g), given the Kc for the reaction in the previous example was equal to 0.12.
Kc can be obtained by raising the old Kc to the second power.
Kc =(0.12)2 = .014
Calculate Kc for H2(g) + I2(g) ⇌ 2HI(g), given the Kc for the reaction in the first example was equal to 0.12.
Kc can be obtained by taking the reciprocal of the original equilibrium constant.
Kc = 1 = 8.3
0.12
Equilibrium expression for gases
- Kc involves concentrations
- Kp involves pressure
- Use Kp for gases
Gaseous Equilibria Constant Example: N2(g) + 3H2(g) ⇌ 2NH3(g)
Kp = (PNH3)^2 / (PN2)(PH2)3
The Relationship Between Kc and Kp
Kp = Kc(RT)^n
n = sum of the coefficients of the gaseous products minus the sum of the coefficients of the gaseous reactants
R = 0.08206 L/atm/mol/K
T = temperature in kelvin
What does the size of k mean?
A value of K much larger than 1 means that at equilibrium the reaction system will consist of mostly products - the equilibrium lies to the right
(reaction goes essentially to completion)
A very small value of K means that the system at equilibrium will consist of mostly reactants - the equilibrium position is far to the left
(reaction does not occur to any significant extent)
What is the reaction quotient q??
The reaction quotient, Q, is used to predict which way the reaction will proceed.
Use initial concentrations instead of equilibrium concentrations in the Q expression.
Q is set up the same way as K.
What information can you find from q??
If Q = K; The system is at equilibrium. No shift will occur.
If Q > K; The system shifts to the left.
Consuming products and forming reactants, until equilibrium is achieved.
If Q < K; The system shifts to the right.
Consuming reactants and forming products, to attain equilibrium.
If a reaction mixture contains just reactants, then Q = 0, and the reaction will proceed in the forward direction.
If a reaction mixture contains just products, then Q = ∞, and the reaction will proceed in the reverse direction.
Chemical Equilibrium and Free Energy
The magnitude of K gives us the same information as the sign of ΔG°.
A reaction with K»_space; 1 has a negative ΔG° implying spontaneity for the forward reaction.
A reaction with K «_space;1 has a positive ΔG° implying non-spontaneity for the forward reaction.
The magnitude of K and sign of ΔG° enable us to solve a wide variety of problems.
Le Châtelier’s Principle: Disturbing and Restoring Equilibrium
Once a reaction is at equilibrium, the concentrations of all the reactants and products remain the same.
However, if the conditions are changed, the concentrations of all the chemicals will change until equilibrium is restored.
The new concentrations will be different, but the equilibrium constant will be the same, unless you change the temperature.
Le Châtelier’s principle guides us in predicting the effects various changes in conditions have on the position of equilibrium.
It says that if a system at equilibrium is disturbed, the position of equilibrium will shift to minimize the disturbance.
Disturbances all involve making the system open.
Le Chatelier’s Principle
When any system at equilibrium is subjected to change inconcentration,temperature, volume, orpressure, then the system readjusts itself to (partially) counteract the effect of the applied change and a new equilibrium is established.
Le chatelier’s Principle: Change in Concentration
The system will shift away from the added component. If a component is removed, the opposite effect occurs.
H2O is removed from CO2(g) + H2(g) ⇌ H2O(g) + CO(g)
Removal of a product - shifts right
Some iodine is added to H2(g) + I2(s) ⇌ 2HI(g)
No change, I2 is a solid!
The Effect of Temperature Changes on Equilibrium Position: Exothermic Reactions
Le Châtelier’s principle predicts the effect of temperature changes, even though heat is not matter and not written in a proper equation.
Exothermic reactions release energy.
Heat is “acting” as a product.
Increasing the temperature is like adding heat.
According to Le Châtelier’s principle, the equilibrium will shift away from the added heat.
exothermic reaction: A + B —-> C + D + heat
The Effect of Temperature Changes on Equilibrium and the K Value for Exothermic Reactions
Adding heat to an exothermic reaction will decrease the concentrations of products and increase the concentrations of reactants.
Adding heat to an exothermic reaction will decrease the value of K.
exothermic reaction: A + B —-> C + D + heat
The Effect of Temperature Changes on Equilibrium
In an exothermic chemical reaction, heat is a product.
Increasing the temperature causes an exothermic reaction to shift left (in the direction of the reactants); the value of the equilibrium constant decreases.
Decreasing the temperature causes an exothermic reaction to shift right (in the direction of the products); the value of the equilibrium constant increases.
The Effect of Temperature Changes on Equilibrium
In an endothermic chemical reaction, heat is a reactant.
Increasing the temperature causes an endothermic reaction to shift right (in the direction of the products); the equilibrium constant increases.
Decreasing the temperature causes an endothermic reaction to shift left (in the direction of the reactants); the equilibrium constant decreases.
