Ch 16: Chemical Equilibrium

Question 1

What does Le’ Chatelier’s principle state?

Answer 1

When a chemical system at equilibrium is disturbed, the system shifts in a direction that minimizes the disturbance.

section 16.9
p. 711

Question 2

What happens if we push down on a piston (housing a system in equilibrium), lowering the volume and raising the pressure? How can the chemical system respond to bring the pressure back down?

Answer 2

From the ideal gas law, we know that decreasing the number of moles of a gas (n) results in a lower pressure (P). Therefore, the system shifts to the side of the reaction that has the fewer number of moles.

p. 717

Question 3

If a reaction has an equal number of moles of gas on both sides of the chemical equation, then a change in volume produces __ ______ on the equilibrium.

Adding an inert gas to the mixture at a fixed volume has no effect on the equilibrium.

Answer 3

no effect

(Don’t forget that we only consider the moles of GAS particles. If there are solids in the equation, we can ignore those.)

Question 4

Changing the temperature does change the value of the __________ constant.

Answer 4

equilibrium

Question 5

Remember, that in exothermic and endothermic reactions, respectively, heat is a product in the former, and a reactant in the latter. Therefore, if you add or remove heat, the result will be….

Answer 5

….. the same as if you removed products or reactants. Removing products shifts the equation to the right (toward the products).
Removing reactants shifts the equation to the left (toward the reactants).

Question 6

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.

Question 7

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 ________.

Decreasing the temperature causes an endothermic reaction to shift left (in the direction of the reactants); the equilibrium constant ________.

Answer 7

increases
————> increasing K

decreases

Question 8

A large value of K means that the reaction lies far to the right at equilibrium – a high concentration of ________, and a low concentration of _________.

Answer 8

products

reactants

Question 9

A small value of K means that the reaction lies far to the left at equilibrium – a high concentration of ________, and a low concentration of _________.

Answer 9

reactants

products

Question 10

To maintain equilibrium, hemoglobin binds oxygen when the surrounding oxygen concentration is high, but it releases oxygen when the surrounding oxygen concentration is low.

Interestingly, the equilibrium constant for fetal hemoglobin is larger than the equilibrium constant for adult hemoglobin, implying that….

Answer 10

the reaction tends to go farther in the direction of the product.

Question 11

Define dynamic equilibrium for a chemical reaction.

Answer 11

The condition in which the rate of the forward reaction equals the rate of the reverse reaction.

Question 12

When a chemical reaction reaches dynamic equilibrium, the concentrations of reactants and products becomes _______, but not necessarily _____.

Answer 12

constant

equal

Question 13

The equilibrium constant is defined as the ratio – at equilibrium – of the….

Answer 13

….concentrations of the products raised to their stoichiometric coefficients, divided by the concentrations of the reactants raised to their stoichiometric coefficients.

The coefficients in the chemical equation become the exponents.

Question 14

The equilibrium constant is always the same at a given temperature, regardless of the _______ __________.

Answer 14

initial concentrations

Question 15

16.7

If Q is = K, what does this imply?

Answer 15

That the reaction is at equilibrium.

Question 16

16.7

If Q < K, to which direction will the reaction proceed? What does this imply?

Answer 16

The reactants > products.
The reaction will proceed to the right (toward the products), meaning reactants will decrease, and product will increase until Q = K.

Question 17

16.7

If Q > K, to which direction will the reaction proceed? What does this imply?

Answer 17

The reactants < products.
The reaction will shift to the left toward the reactants, meaning reactants will increase and product will decrease until Q = K.

Question 18

Le Châtelier’s principle states that a change in volume causes a change in pressure so that the reaction that favors the opposite in this change would be predicted. For example, if volume is increased, the pressure in the system will decrease and concentration for each species in the system will decrease, therefore the equilibrium would shift to the side yielding more moles of gas.

Question 19

Heat is the product of an exothermic reaction. So, adding heat to the reaction causes it to shift to the left (toward the reactants). Upon this shift, the numerator of the equilibrium constant K decreases and the denominator increases, thus __________ the overall value of K.

Answer 19

decreasing

Question 20

Consider this reaction:

If you were trying to maximize the amount of C2H4I2 produced, which of the following might you try? Assume that the reaction mixture reaches equilibrium.

Check all that apply.

__adding C2H4 to the reaction mixture
__raising the reaction temperature
__decreasing the reaction volume
__removing I2 from the reaction mixture

Answer 20

_X_adding C2H4 to the reaction mixture
_X_raising the reaction temperature
_X_decreasing the reaction volume
__removing I2 from the reaction mixture

Question 21

The x is small approximation is most likely to apply to a reaction with a _____ equilibrium constant and an initial concentration of reactant that is not too small. The larger the equilibrium constant and the smaller the initial concentration of reactant, the ____ likely it is that the x is small approximation will apply.

Answer 21

small

less