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Flashcards in Equilibrium Deck (70)
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1
Define:

the law of mass action

The law of mass action states that, at equilibrium, the composition of the reaction mixture can be expressed in terms of an ideal equilibrium constant, keq.

For the chemical reaction

aA (g) + bB (g) ⇔ cC (g)

if [C]eq is the concentration of C at equilibrium,

2

For the chemical reaction

aA (g) + bB (g) ⇔ cC (g)

what is the value of the reaction quotient Q?

The expression for Q is very similar to the expression for keq. The only difference is that Q can apply to a chemical system at any concentrations, while keq specifically refers to the concentrations at equilibrium.

3

For the chemical reaction

A (g) ⇔ B (g)

what does it mean about the equilibrium concentrations of A and B if:

  1. keq >> 1
  2. keq = 1
  3. keq << 1

For this system,

keq = [B]eq/[A]eq

  1. If keq >> 1, [B]eq >> [A]eq and, at equilibrium, B will be in excess.
  2. If keq = 1, [B]eq = [A]eq and, at equilibrium, A and B will be at equal concentration.
  3. If keq << 1, [A]eq >> [B]eq and, at equilibrium, A will be in excess.

4

What is the equilibrium constant formula for the reaction:

NO + NO⇔ 2NO2

The value of the equilibrium constant is calculated from the actual concentrations of the products and reactants at equilibrium. The general formula for this reaction is:

5

What is the equilibrium constant keq for the following reaction:

CaCO3(s)→CaO(s)+CO2(g)

keq=[CO2]

The value of the equilibrium constant (and reaction quotient) depends only on the concentration of reactants and products present in the aqueous or gaseous phases. Chemicals in the solid or liquid phase do not affect the equilibrium levels.

6

What is true of the relationship between Keq and Q for any chemical system which is at equilibrium?

Q = Keq.

Equilibrium occurs when the reactants and products are at concentrations such that the reaction quotient, Q, is equal to the equilibrium constant keq.

7

What can be said about the rates of the forward and reverse reactions for any chemical system which is at equilibrium?

When the system is at equilibrium, by definition, the rates of the forward and reverse reactions are equal.

8
If the chemical system

NO + NO3 ⇔ 2NO2

is at dynamic equilibrium, what can be said about the rates of the forward and reverse reactions?

The rates of the forward and reverse reactions are equal.

For every molecule of NO that comes together with a molecule of NO3 to make 2 molecules of NO2, 2 molecules of NO2 will also come together to make a molecule of NO and NO3.

9
Define:

Le Chatelier's Principle

Le Chatelier's Principle states that when a system in equilibrium is placed under stress, the system adjusts to restore equilibrium.

There are three kinds of stress a chemical system can be placed under:

  • Concentration 
  • Temperature
  • Pressure

10
If the chemical system

aA (g) + bB (g) ⇔ cC (g)

is in equilibrium, and additional A is added, what does Le Chatelier's Principle predict will occur?

More C will be created.

According to Le Chatelier's Principle, the system will respond to relieve any stress placed on one side of the system. 

In this case, the reactant side is the one placed under stress by the addition. As such, the system will respond by shifting to the right, favoring the forward reaction, and more products will be created.

11
If the chemical system

aA (g) + bB (g) ⇔ cC (g)

is in equilibrium, and additional A is added, what happens to the reaction quotient Q?

Q decreases.

This circumstance favors the forward reaction, or the creation of more products.

As a general rule of reactions: when Q < keq, the forward reaction is favored and more products are created. 

12
If the chemical system

aA (g) + bB (g) ⇔ cC (g)

is in equilibrium, and additional C is added, what does Le Chatelier's Principle predict will occur?

More A and B will be created.

In this case, the product side is the one placed under stress by the addition. As such, the system will respond by shifting to the left, favoring the reverse reaction, and more reactants will be created.

13
If the chemical system

aA (g) + bB (g) ⇔ cC (g)

is in equilibrium, and is endothermic, what happens if the temperature is increased?

More C will be created.

For an endothermic reaction, heat must be added; hence it can be thought of as a reactant. Increasing the temperature is akin to adding more reactant, and therefore shifts the reaction to the right.

14
If the chemical system

aA (g) + bB (g) ⇔ cC (g)

is in equilibrium, and is exothermic, what happens if the temperature is increased?

More A and B will be created.

For an exothermic reaction, heat is given off; hence it can be thought of as a product. Increasing the temperature is akin to adding more product, and therefore shifts the reaction to the left.

15
If the chemical system

A (g) +  2 B (g) ⇔ C (g)

is in equilibrium, what happens if the pressure is increased?

More C will be created.

Increasing the pressure will add stress to the side which has more moles of gas. In this case, that is the reactant side, with 3 moles of gas vs. 1 mole of gas on the product side.

So, when the pressure is increased, the system will shift right, creating more products.

16
If the chemical system

A (g) +  B (g) ⇔ 3 C (g)

is in equilibrium, what happens if the pressure is decreased?

More C will be created.

Decreasing the pressure lessens the inhibition for the side creating more moles of gas. In this case, the product side has 3 moles of gas vs. 2 total moles of gas on the reactant side.

So, when the pressure is decreased, the system will shift right, creating more products.

17

What is the relationship between a substance's Gibbs' Free Energy of Formation and the equilibrium constant of the substance's formation reaction?

ΔGº = -RT ln(Keq)

Where:

  • R = the ideal gas constant in l-atm/K-mol
  • T = the absolute temperature in K

18
Define:

Kc, the molar concentration equilibrium constant

Kc is the value of Keq when all reactant and product concentrations are given in concentration units, such as mol/l.

When reactant and product concentrations are given in these units, Kc will be the calculated value of equilibrium.

19
Define:

Kp, the partial pressure equilibrium constant

Kp is the value of Keq when all reactant and product concentrations are given in pressure units, such as atm.

When reactant and product concentrations are given in these units, Kp will be the calculated value of equilibrium.

20

How can the value of Kp be calculated, given the value of Kc?

The equation relating Kp to Kc is:

Kp = Kc (RT)Δn

where:

  • R = the ideal gas constant in l-atm/K-mol
  • T = the absolute temperature in K
  • Δn = the change in moles of gas (moles product gas - moles reactant gas)

21

At STP, if the number of moles of gas decreases in a reaction, what will be the relationship between Kp and Kc?

Kc will be greater than Kp.

The equation relating Kp to Kc is:

Kp = Kc (RT)Δn

Since at STP, R*T is greater than 1, and since Δn is negative if the number of moles of gas decreases, this gives :

Kp = Kc (1 / [number greater than 1]), hence Kp is a fraction of the size of Kc.

22

At STP, if the number of moles of gas remains the same in a reaction, what will be the relationship between Kp and Kc?

Kc will be equal to Kp.

The equation relating Kp to Kc is:

Kp = Kc (RT)Δn

If the number of moles of gas is unchanged, Δn must equal zero, this gives:

Kp = Kc ([RT]0)=Kc * 1,
hence Kp = Kc.

23
Define:

the solubility product constant

The solubility product constant (Ksp) is the equilibrium constant for a solvation reaction.
Ksp = [anions]a[cations]c

Just like other equilibrium constants, Ksp is equal to the concentration of products over reactants raised to the power of their coefficients. However, since pure liquids and solids are omitted from equilibrium calculations, the equation simplifies to just the concentration of ions in solution, raised to the power of their coefficients.

24

If the temperature of a solution is increased, what happens to the solute's Ksp?

Ksp increases.

A warmer solvent can dissolve more solutes. This leads to a higher concentration of ions in solution, thus Ksp increases.

25

What is the solubility product constant formula for AgCl, which dissociates as below:

AgCl(s) ⇔ Ag+(aq) + Cl-(aq)

Ksp = [Ag+][Cl-]

The coefficient in front of both silver and chloride is 1, so the exponents will be 1.

26

What is the solubility product constant formula for BaF2, which dissociates as below:

BaF2(s) ⇔ Ba++ (aq) + 2F-(aq)

Ksp= [Ba++][F-]2

The coefficient in front of barium is 1 so the exponent will be 1 over [Ba++]. The coefficient in front of fluoride is 2, so the exponent will be 2 over [F-].

27
Define:

the common ion effect

The common ion effect states that a salt will be less soluble in a solution containing product ions than it would be in a pure solvent. A solution containing ions that a salt would separate into when dissolved, is referred to as having common ions.  

Ex: AgCl dissolved into chlorinated water (Cl- ions present) will be less soluble than AgCl in pure water. The added Cl- ion (product) shifts equilibrium to the side containing the solid salt (reactant).

28

Suppose NaCl were added to a saturated aqueous solution of AgCl. What will happen?

Note: NaCl has a higher solubility than AgCl.

  1. NaCl will still dissolve.
    The presence of Cl- ions from AgCl will not prevent the highly soluble NaCl from dissociating.
  2. AgCl will precipitate.
    The additional Cl- ions now present from NaCl exceed the saturation levels for AgCl in solution, and will force those ions back into solid AgCl salt form.

29

A given solution contains PbCl2 and PbSO4.  How could lead (II) chloride be selectively harvested from the solution?

If a highly soluble chloride salt such as NaCl were added to the solution, the common ion effect of Cl- ions would lead to the selective precipitation of lead (II) chloride.

30
Define:

complex (or coordination complex)

A complex is formed when a metallic atom or ion is bonded to a surrounding array of molecules or anions; usually written with brackets as: [complex]. Complexes cannot be solvated back into their original atoms/ions. 

Ex: Cisplatin [PtCl2(NH3)2] is formed when the Pt++ cation is bonded to two Cl- anions and two NH3 groups.