Topic 7: Equilibrium

Question 1

What is chemical equilibrium?

Answer 1

A state in which the rate of the forward reaction equals the rate of backward reaction. Both reactions continue to occur, but the concentrations of products/reactants stays the same.

Question 2

What are five characteristics of equilibrium?

Answer 2

Dynamic, closed system, concentrations stay the same, no change in macroscopic properties, can be reached from either direction

Question 3

What does it mean that equilibrium is dynamic?

Answer 3

The reaction has stopped by the forward and reverse reactions still occur at the same rate.

Question 4

What does it mean that equilibrium is achieved in a closed system?

Answer 4

No matter can escape the system

Question 5

What does it mean that concentrations of products and reactants are the same for equilibrium?

Answer 5

Forward and reverse reactions occur at the same rate, so there is an equal concentration of products and reactants

Question 6

What does it mean that a reaction at equilibrium has no change in macroscopic properties?

Answer 6

Visible properties, like color and density, do not change

Question 7

What does it mean that equilibrium can be reached from either direction?

Answer 7

Backward and forward reactions can both arrive to equilibrium

Question 8

What is the equilibrium constant represented as?

Answer 8

K subscript c

Question 9

How can an equilibrium reaction be represented?

Answer 9

aA + bB <—-> cC + dD. The lowercase letters represent the molar coefficients, the uppercase letters represent the element symbol

Question 10

How do you calculate Kc?

Answer 10

If aA + bB <—-> cC + dD, then Kc=
([C]^c x [D]^d) / ([A]^a x [B]^b). In other words, Kc is the concentration of the products raised to their molar coefficients over the concentration of reactants raised to their molar coefficients.

Question 11

What does [X] represent?

Answer 11

The concentration of element X

Question 12

Which elements/states should be included in a Kc calculation? Which elements/states should not

Answer 12

Only use gases and aqueous solutions. Liquids and solids don’t count!

Question 13

How do you interpret Kc values?

Answer 13

If Kc > 1, reaction favors products. If Kc < 1, reaction favors reactants.
If Kc = 1, reaction is at equilibrium

Question 14

What is the reaction quotient? How is it different from Kc?

Answer 14

It measures the relative amount of products/reactants at a certain time in the reaction. It is calculated the same way as Kc but Q is for reactions NOT at equilibrium.

Question 15

How do you interpret Q in the context of Kc?

Answer 15

If Q=Kc, equilibrium.
If Q > Kc, Q will decrease to produce more reactants
If Q < Kc, Q will increase to produce more products

Question 16

What do the expressions “lying to left” or “lying to right” mean?

Answer 16

Left=reaction favors reactants
Right=reaction favors products

Question 17

What happens to Q as time passes?

Answer 17

It will eventually equal Kc as the reaction reaches equilibrium

Question 18

What is Le Chatelier’s Principle?(which I will refer to as LCP for ease :)

Answer 18

If a dynamic equilibrium is disturbed by changing conditions, the position of equilibrium shifts to counteract the change and reestablish equilibrium

Question 19

According to LCP, how does increasing or decreasing concentration shift the reaction?

Answer 19

Increasing the concentration shifts the reaction to favor the side with fewer moles of solute.
Decreasing the concentration shifts the reaction to favor the side with more moles of solute.

Question 20

How does increasing or decreasing pressure shift the reaction according to LCP?

Answer 20

Increasing pressure shifts rxn to favor side with fewer moles of gas.
Decreasing pressure shifts rxn to favor side with more moles of gas.

Question 21

What do you need to know about a reaction to determine how increasing or decreasing temp affects it?

Answer 21

If it’s endo/exo.

Question 22

What is unique about temperature in terms of LCP and Kc?

Answer 22

It is the only factor that can change Kc and the position of equilibrium

Question 23

How does increasing temperature affect an EXOthermic reaction?

Answer 23

In an exo rxn, heat is a product, so increasing heat shifts reaction to favor reactants

Question 24

How does increasing temp affect an ENDOthermic rxn?

Answer 24

In endo, heat is a reactant, so increasing heat shifts rxn to favor products

Question 25

Do catalysts have any effect on Kc?

Answer 25

No