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CHEM 330 > Chemical Equilibrium > Flashcards

Chemical Equilibrium Flashcards

(57 cards)

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1
Q

What is chemical equilibrium?

A

The state where the concentrations of all reactants and products remain constant with time

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2
Q

When does a reaction reach equilibrium?

A

When it has the lowest possible free energy (not going to completion)

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3
Q

When the reaction mixture has not produced enough products to have reached equilibrium, the spontaneous direction of change is towards ___

A

More products

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4
Q

When excess products are present (e.g. pure ammonia), the reverse reaction is ___

A

Spontaneous

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5
Q

Graph showing changes in ∆G with excess reactants or products

A
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6
Q

What is ∆G?

A

The reaction free energy (at certain reaction points, fixed composition of the reaction mixture)

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7
Q

Equation for ∆G

A
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8
Q

What is ∆G°

A

The difference in molar free energy of the products and reactants in their standard states

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9
Q

What is Q?

A

Reaction quotient

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10
Q

What is R?

A

Universal gas constant

R = 8.315 J/mol*K

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11
Q

What is T?

A

Reaction temperature

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12
Q

Equation for reaction quotient (Q)

A
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13
Q

Does Q have units?

A

No

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14
Q

Q = ____/____

A

Reactants/products

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15
Q

Example of equation for Q in solution vs. gaseous state

A
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16
Q

What substances are included and not included in the equation for reaction quotient Q?

A

Gases and aqueous solutions are included, but pure liquids and solids are never included

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17
Q

What are ∆G and Q at equilibrium?

A

At equilibrium ∆G = 0 and Q = K (equilibrium constant)

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18
Q

Equation for ∆G°

A

∆G° = -RT lnK

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19
Q

What is K?

A
  • Equilibrium constant
  • Capital K, vs. lowercase k is rate constant
  • K has the same form as Q, but only uses the amount of products and reactants at equilibrium
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20
Q

What is the difference between K and Q?

A

K has the same form as Q, but only uses the amount of products and reactants at equilibrium

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21
Q

Example of equation for K in solution and gaseous phase

A
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22
Q

Table showing relationship between ∆G° and K

A
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23
Q

What is the position of equilibrium?

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A

A set of reactant & product concentrations that satisfies ∆G° = -RT lnK

24
Q

At one temperature (T), there is one corresponding ___

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A

Equilibrium constant K

25
What does K tell us about the mixture of products and reactants at equilibrium?
Whether we can expect high or low concentrations of products at equilibrium
26
27
Summary of the difference between K and Q
28
The relationship between Q and K determines the ___
Reaction direction
29
What happens to the reaction when Q
- The system shifts to the right - Consuming reactants and forming products, to attain equilibrium
30
What happens to the reaction when Q>K?
- The system shifts to the left - Consuming products and forming reactants, until equilibrium is achieved
31
What happens to the reaction when Q=K?
The system is at equilibrium, so no shift will occur
32
Explanation of what happens when K>1
33
Relationship between ∆G° and the magnitude of K
34
When is ∆G° a negative number?
If K is large (>1), more product
35
When is ∆G° a positive number?
If K is small (<1), less product
36
Le Chatelier's Principle
- Can be used to qualitatively predict the direction of change for a system under an external perturbation (external effects on K and Q) - If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change to re-establish an equilibrium - If a chemical reaction is at equilibrium and experiences a change in pressure, temperature, or concentration, the equilibrium shifts in the opposite direction to offset the change - The reaction will go to the direction that minimizes the stress
37
Le Chatelier's Principle - adding more reactants (e.g. H2)
- If you are at equilibrium and then add more hydrogen, according to Le Chatelier's principle, the system will tend to minimize the increase in number of hydrogen molecules - The reaction shifts to the right toward 2NH3 - This can be explained in terms of Q and K. When reactants are added, Q falls below K momentarily, because the reactant concentration term appears in the denominator
38
Le Chatelier's Principle - adding more products (e.g. NH3)
- Q rises above K - Q is larger because product term is in numerator (K is unchanged) - With Q>K, ∆G is positive, and the reaction goes toward the reactants (reaction proceeds in the reverse direction) Recall: ∆G = RT ln(Q/K)
39
Le Chatelier's Principle - removing product (e.g. NH3)
Q > K. The reaction shifts to the right toward product
40
Explain the effect of volume (V) on the chemical equilibrium of a reaction
- A decrease in the volume of a gaseous system causes an increase in the total pressure - Ideal gas law: PV = nRT - Where: P: pressure V: volume T: temperature n: number of mole R: ideal gas constant - Le Chatelier's principal predicts that the system would respond in a way as to reduce the total pressure
41
Le Chatelier's principle predicts that a decrease in the volume of a gaseous system would cause the system to respond in a way as to ___ the total pressure
Reduce Because a derease in violume causes an increase in pressure, the system will act to offset this increased pressure
42
Example of effect of volume on equilibrium
- A decrease in volume shifts the reaction to the right (toward product, P4 gas) - Thus, a shift of the reaction to the right reduces the total pressure Consider in terms of Q and K: - If Q decreases by a factor of 2, and Q is less than K, then ∆G is a negative value - Since the volume is decreased by a factor of 2, the partial pressure is increased by a factor of 2 - So, Q decreases by a factor of 2, and Q
43
If Q decreases by a factor of 2, and Q is less than K, then ∆G is ___
A negative value
44
How would an increase in volume affect chemical equilibrium?
- An increase in volume shifts the reaction to the left (toward reactants, P2 gas) - Thus, a shift of the reaction to the right increases the total pressure (minimize the stress)
45
What will happen if an inert gas is added to the container, increasing the total pressure at constant temperature?
Nothing changes At constant volume, adding an inert gas only increases total pressure, NOT the partial pressures of reactants/products - When adding an inert gas, Ptotal increases, partial pressure is unchanged. - When partial pressure is unchanged, Q is unchanged!
46
What will happen if an inert gas is added to the container, but both the total pressure and temperature are kept constant?
_____ - If you add gas but keep total pressure constant, the volume must increase to compensate for the added gas (according to the ideal gas law 𝑃𝑉=𝑛𝑅𝑇 - Increasing the volume means decreasing the partial pressures of all gases involved (reactants and products).
47
Effect of increasing temperature on equilibrium
This causes the reaction to shift such that some of the heat is absorbed
48
Effect of decreasing temperature on equilibrium
This causes the reaction to shift such that some of the heat is released
49
Increasing the temperature of an exothermic reaction favors the formation of ___
Reactants
50
Explain the effect of increasing the temperature of an exothermic reaction
- This favors the formation of reactants
51
Increasing the temperature of an endothermic reaction favors the formation of ___
Products
52
Explain the effect of increasing the temperature of an endothermic reaction
- Favors the formation of products
53
How to change ∆G° equation to solve for K
54
Temperature dependence of K
- K (equilibrium constant) can change with temperature - Since it is reasonable to assume that ∆H° and ∆S° are independent of temperature, K changes with a change in T
55
Van't Hoff Equation
56
Which of the following are true for a reaction where AH > 0? 1. The reaction is endothermic. 2. The equilibrium constant is larger at higher temperatures. 3. When T2 < T1, K1 > K2. 4. There are fewer products at equilibrium when the temperature is decreased. 5. 1, 2, and 3 are true. 6. All of the above are true.
57
Applications of Le Chatelier's Principle

CHEM 330 (18 decks)

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